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9 pags., The use of polyproline II (PPII) helices in protein design is currently hindered by limitations in our understanding of their conformational stability and folding. Recent studies of the snow flea antifreeze protein (sfAFP), a useful model system composed of six PPII helices, suggested that a low denatured state entropy contributes to folding thermodynamics. Here, circular dichroism spectroscopy revealed minor populations of PPII like conformers at low temperature. To get atomic level information on the conformational ensemble and entropy of the reduced, denatured state of sfAFP, we have analyzed its chemical shifts and {H}-N relaxation parameters by NMR spectroscopy at four experimental conditions. No significant populations of stable secondary structure were detected. The stiffening of certain N-terminal residues at neutral versus acidic pH and shifted pK values leads us to suggest that favorable charge-charge interactions could bias the conformational ensemble to favor the formation the C1-C28 disulfide bond during nascent folding, although no evidence for preferred contacts between these positions was detected by paramagnetic relaxation enhancement under denaturing conditions. Despite a high content of flexible glycine residues, the mobility of the sfAFP denatured ensemble is similar for denatured α/β proteins both on fast ps/ns as well as slower μs/ms timescales. These results are in line with a conformational entropy in the denatured ensemble resembling that of typical proteins and suggest that new structures based on PPII helical bundles should be amenable to protein design., NMR experiments were performed in the “Manuel Rico” NMR laboratory (LMR) of the Spanish National Research Council (CSIC), a node of the Spanish Large-Scale National Facility (ICTS R-LRB). We acknowledge funding from the following grants: PID2019-109306RB-I00 from MCIN/AEI/10.13039/501100011033 (to D.V.L.) and PID2020-113907RA-I00 from MCIN/AEI/10.13039/501100011033 (to M.M.), and RYC2019-026574-I from MCIN/AEI/10.13039/501100011033 and “ESF Investing in Your Future” (to M.M.).

11 pags., 5 figs., Understanding early amyloidogenesis is key to rationally develop therapeutic strategies. Tau protein forms well-characterized pathological deposits but its aggregation mechanism is still poorly understood. Using single-molecule force spectroscopy based on a mechanical protection strategy, we studied the conformational landscape of the monomeric tau repeat domain (tauRD244-368). We found two sets of conformational states, whose frequency is influenced by mutations and the chemical context. While pathological mutations Δ280K and P301L and a pro-amyloidogenic milieu favored expanded conformations and destabilized local structures, an anti-amyloidogenic environment promoted a compact ensemble, including a conformer whose topology might mask two amyloidogenic segments. Our results reveal that to initiate aggregation, monomeric tau-RD244-368 decreases its polymorphism adopting expanded conformations. This could account for the distinct structures found in vitro and across tauopathies., Spanish Agency for Research (AEI). Grant Numbers: SAF2016-76678-C2-1-R, SAF2016-76678-C2-2-R, BFU2015-70072-R

13 pags., 7 figs., The mediation of liquid-liquid phase separation (LLPS) for fused in sarcoma (FUS) protein is generally attributed to the low-complexity, disordered domains and is enhanced at low temperature. The role of FUS folded domains on the LLPS process remains relatively unknown since most studies are mainly based on fragmented FUS domains. Here, we investigate the effect of metabolites on full-length (FL) FUS LLPS using turbidity assays and differential interference contrast (DIC) microscopy, and explore the behavior of the folded domains by nuclear magnetic resonance (NMR) spectroscopy. FL FUS LLPS is maximal at low concentrations of glucose and glutamate, moderate concentrations of NaCl, Zn2+ , and Ca2+ and at the isoelectric pH. The FUS RNA recognition motif (RRM) and zinc-finger (ZnF) domains are found to undergo cold denaturation above 0°C at a temperature that is determined by the conformational stability of the ZnF domain. Cold unfolding exposes buried nonpolar residues that can participate in LLPS-promoting hydrophobic interactions. Therefore, these findings constitute the first evidence that FUS globular domains may have an active role in LLPS under cold stress conditions and in the assembly of stress granules, providing further insight into the environmental regulation of LLPS., Fundaçao para a Ciência e a Tecnologia, ˜ Grant/Award Numbers: LA/P/0140/2020 PD/BD/148028/2019, ROTEIRO/0031/2013-PINFRA/22161/2016, UIDP/04378/ 2020, UIDB/04378/2020; Fundacion BBVA, Grant/Award Number: BBM_TRA_0203; Ministerio de Ciencia e Innovacion, Grant/Award Numbers: MCIN/AEI/10.13039/501100011033, PID2019-109276RA-I00, PID-2019-109306RBI00, RYC2018-026042-I

Intrinsically disordered proteins (IDPs) play essential roles in regulating physiological processes in eukaryotic cells. Many viruses use their own IDPs to “hack” these processes to deactivate host defenses and promote viral growth. Thus, viral IDPs are attractive drug targets. While IDPs are hard to study by X-ray crystallography or cryo-EM, atomic level information on their conformational preferences and dynamics can be obtained using NMR spectroscopy. SARS-CoV-2 Nsp2, whose C-terminal region (CtR) is predicted to be disordered, interacts with human proteins that regulate translation initiation and endosome vesicle sorting. Molecules that block these interactions could be valuable leads for drug development. The 13Cβ and backbone 13CO, 1HN, 13Cα, and 15N nuclei of Nsp2’s 45-residue CtR were assigned and used to characterize its structure and dynamics in three contexts; namely: (1) retaining an N-terminal His tag, (2) without the His tag and with an adventitious internal cleavage, and (3) lacking both the His tag and the internal cleavage. Two five-residue segments adopting a minor extended population were identified. Overall, the dynamic behavior is midway between a completely rigid and a fully flexible chain. Whereas the presence of an N-terminal His tag and internal cleavage stiffen and loosen, respectively, neighboring residues, they do not affect the tendency of two regions to populate extended conformations., Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. MM is a Ramón y Cajal Fellow of the Spanish AEI-Ministry of Science and Innovation (RYC2019-026574-I), and a “La Caixa” Foundation (ID 100010434) Junior Leader Fellow (LCR/BQ/PR19/11700003). Funded by project COV20/00764 from the Carlos III Institute of Health and the Spanish Ministry of Science and Innovation to MM and DVL.

10 pags., 2 figs., 1 tab., Biomolecular condensates are microdroplets that form inside cells and serve to selectively concentrate proteins, RNAs and other molecules for a variety of physiological functions, but can contribute to cancer, neurodegenerative diseases and viral infections. The formation of these condensates is driven by weak, transient interactions between molecules. These weak associations can operate at the level of whole protein domains, elements of secondary structure or even moieties composed of just a few atoms. Different types of condensates do not generally combine to form larger microdroplets, suggesting that each uses a distinct class of attractive interactions. Here, we address whether polyproline II (PPII) helices mediate condensate formation. By combining with PPII-binding elements such as GYF, WW, profilin, SH3 or OCRE domains, PPII helices help form lipid rafts, nuclear speckles, P-body-like neuronal granules, enhancer complexes and other condensates. The number of PPII helical tracts or tandem PPII-binding domains can strongly influence condensate stability. Many PPII helices have a low content of proline residues, which hinders their identification. Recently, we characterized the NMR spectral properties of a Gly-rich, Pro-poor protein composed of six PPII helices. Based on those results, we predicted that many Gly-rich segments may form PPII helices and interact with PPII-binding domains. This prediction is being tested and could join the palette of verified interactions contributing to biomolecular condensate formation., JO is a Ramon y Cajal Fellow of the Spanish AEI- Ministry of Science and Innovation (RYC2018- 026042-I) and a Leonardo Fellow from the BBVA Foundation (Grant Number BBM-TRA-0203). MM is a Ramon y Cajal Fellow of the Spanish AEI-Ministry of Science and Innovation (RYC2019-026574-I) and a ‘La Caixa’ Foundation Junior Leader Fellow (LCR/ BQ/PR19/11700003). This study was supported by Grant BTC-PID2019-109306RB-I00 (DVL) from the Spanish Ministry of Science and Innovation.

Transactive response DNA-binding protein of 43 kDa (TDP-43) is a 414-residue protein whose aberrant aggregation is implicated in neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) or frontotemporal lobar degeneration (FTLD). Intriguingly, TDP-43 has also been shown to functionally oligomerize to carry out physiological functions. TDP-43 also exists in mixed condensates or granules with other proteins (e.g. neuronal or stress granules), and its large C-terminal domain (CTD, residues 267–414) seems responsible for TDP-43 both homo- and heterotypic interactions underlying such diverse functional and pathological aggregation events. A myriad of distinct triggers may drive TDP-43 oligomerization, including interaction partners or changes in pH or salinity. In this Assignment Note, we report the complete backbone and a wealth of side chain chemical shift assignments for the CTD of TDP-43 at pH 4. The assignments presented here provide a solid starting point to study the aggregation pathway of TDP-43 at pH values below those considered physiological but relevant in pathological settings, and to contrast the aggregation behaviour under distinct conditions and in the presence of interacting partners.

The use of PPII helices in protein design is currently hindered by limitations in our understanding of their conformational stability and folding. Recent studies of the snow flea antifreeze protein (sfAFP), a useful model system composed of six PPII helices, suggested that a low denatured state entropy contributes to folding thermodynamics. To get atomic level information on the conformational ensemble and entropy of the reduced denatured state of sfAFP, we have analyzed its chemical shifts and {1H}-15N relaxation parameters by NMR spectroscopy at three experimental conditions. No significant populations of preferred secondary structure were detected. The stiffening of certain N-terminal residues at neutral versus acidic pH leads us to suggest that favorable charge-charge interactions could bias the conformational ensemble to favor the formation of the two disulfide bonds during nascent folding. Despite a high content of flexible glycine residues, the mobility of the sfAFP denatured ensemble is similar for denatured α/β proteins both on fast ps/ns as well as slower μs/ms timescales. These results are in line with a conformational entropy in the denatured ensemble resembling that of typical proteins and suggest that new structures based on PPII helical bundles should be amenable to protein design.

The mediation of fused in sarcoma (FUS) protein liquid-liquid phase separation (LLPS) is generally attributed to the low-complexity and disordered domains, while the role of its folded domains remains unknown. In this work we questioned the role of the folded domains on the full-length (FL) FUS LLPS and studied the influence of several metabolites, ions and overall conditions on the LLPS process using turbidity assays, differential interference contrast microscopy and nuclear magnetic resonance spectroscopy. We demonstrate that FL FUS LLPS is highly responsive to the surrounding conditions, and that overall intrinsic disorder is crucial for LLPS. To promote such disorder, we reveal that the FUS RNA-recognition domain (RRM) and the zinc-finger motif (ZnF) undergo cold denaturation above 0ºC, at a temperature that is determined by the conformational stability of the ZnF domain. We hypothesize that, in cold shock conditions, cold denaturation might provide a pathway that exposes additional residues to promote FUS self-assembly. Such findings mark the first evidence that FUS globular domains may have an active role in stress granule formation in cold stress.

12 pags., 5 figs., The artificial intelligence program AlphaFold 2 is revolutionizing the field of protein structure determination as it accurately predicts the 3D structure of two thirds of the human proteome. Its predictions can be used directly as structural models or indirectly as aids for experimental structure determination using X-ray crystallography, CryoEM or NMR spectroscopy. Nevertheless, AlphaFold 2 can neither afford insight into how proteins fold, nor can it determine protein stability or dynamics. Rare folds or minor alternative conformations are also not predicted by AlphaFold 2 and the program does not forecast the impact of post translational modifications, mutations or ligand binding. The remaining third of human proteome which is poorly predicted largely corresponds to intrinsically disordered regions of proteins. Key to regulation and signaling networks, these disordered regions often form biomolecular condensates or amyloids. Fortunately, the limitations of AlphaFold 2 are largely complemented by NMR spectroscopy. This experimental approach provides information on protein folding and dynamics as well as biomolecular condensates and amyloids and their modulation by experimental conditions, small molecules, post translational modifications, mutations, flanking sequence, interactions with other proteins, RNA and virus. Together, NMR spectroscopy and AlphaFold 2 can collaborate to advance our comprehension of proteins., This study was supported by project PID 2019-109306RB-I00/AEI/10.13039/501100011033 from the Spanish Ministry of Science and Innovation. The author declares no competing interests.

5 pags., 4 figs., The C-terminal, intrinsically disordered, prion-like domain (PrLD) of TDP-43 promotes liquid condensate and solid amyloid formation. These phase changes are crucial to the normal biological functions of the protein but also for its abnormal aggregation, which is implicated in amyotrophic lateral sclerosis (ALS) and certain dementias. We and other previously found that certain amyloid forms emerge from an intermediate condensed state that acts as a nucleus for fibrillization. To quantitatively ascertain the role of individual residues within TDP-43's PrLD in its early self-assembly we have followed the kinetics of NMR H–N HSQC signal loss to obtain values for the lag time, elongation rate and extent of condensate formation at equilibrium. The results of this analysis represent a robust corroboration that aliphatic and aromatic residues are key drivers of condensate formation., This work has been funded by Grants CTQ2017-84371-P to D.P.-U. and BTC-PID2019-109306RB-I00 to D.V.L. from the Spanish Ministry of Science and Innovation; AriSLA (PATHENSTDP project)to E.B.; and Grant LCF/BQ/PR19/11700003 from La Caixa Foundation(ID 100010434) to M.M. M.M. is a Ramon y Cajal Fellow of theSpanish AEI-Ministry of Science and Innovation (RYC2019-026574-I). NMR experiments were performed in the“Manuel Rico”NMR Laboratory (LMR) of the Spanish National Research Council (CSIC), anode of the Spanish Large-Scale National Facility (ICTS R-LRB).

18 pags., 6 figs., 1 tab.-- This article belongs to the Special Issue Protein Oligomerization, Human Angiogenin (hANG, or ANG, 14.1 kDa) promotes vessel formation and is also called RNase 5 because it is included in the pancreatic-type ribonuclease (pt-RNase) super-family. Although low, its ribonucleolytic activity is crucial for angiogenesis in tumor tissues but also in the physiological development of the Central Nervous System (CNS) neuronal progenitors. Nev-ertheless, some ANG variants are involved in both neurodegenerative Parkinson disease (PD) and Amyotrophic Lateral Sclerosis (ALS). Notably, some pt-RNases acquire new biological functions upon oligomerization. Considering neurodegenerative diseases correlation with massive protein aggregation, we analyzed the aggregation propensity of ANG and of three of its pathogenic variants, namely H13A, S28N, and R121C. We found no massive aggregation, but wt-ANG, as well as S28N and R121C variants, can form an enzymatically active dimer, which is called ANG-D. By contrast, the enzymatically inactive H13A-ANG does not dimerize. Corroborated by a specific cross-linking analysis and by the behavior of H13A-ANG that in turn lacks one of the two His active site residues necessary for pt-RNases to self-associate through the three-dimensional domain swapping (3D-DS), we demonstrate that ANG actually dimerizes through 3D-DS. Then, we deduce by size exclusion chromatography (SEC) and modeling that ANG-D forms through the swapping of ANG N-termini. In light of these novelties, we can expect future investigations to unveil other ANG determinants possibly related with the onset and/or development of neurodegenerative pathologies., The research was funded by the Italian Ministry of University and Research (MUR) with the “FUR-GOTTE” years 2017–2019.

Polyproline II (PPII) helices play vital roles in biochemical recognition events and structures like collagen and form part of the conformational landscapes of intrinsically disordered proteins (IDPs). Nevertheless, this structure is generally hard to detect and quantify. Here, we report the first thorough NMR characterization of a PPII helical bundle protein, the Hypogastrura harveyi ¿snow flea¿ antifreeze protein (sfAFP). J-couplings and nuclear Overhauser enhancement spectroscopy confirm a natively folded structure consisting of six PPII helices. NMR spectral analyses reveal quite distinct H¿2 versus H¿3 chemical shifts for 28 Gly residues as well as 13C¿, 15N, and 1HN conformational chemical shifts (¿¿) unique to PPII helical bundles. The 15N ¿¿ and 1HN ¿¿ values and small negative 1HN temperature coefficients evince hydrogen-bond formation. 1H¿15N relaxation measurements reveal that the backbone structure is generally highly rigid on ps¿ns time scales. NMR relaxation parameters and biophysical characterization reveal that sfAFP is chiefly a dimer. For it, a structural model featuring the packing of long, flat hydrophobic faces at the dimer interface is advanced. The conformational stability, measured by amide H/D exchange to be 6.24 ± 0.2 kcal·mol¿1, is elevated. These are extraordinary findings considering the great entropic cost of fixing Gly residues and, together with the remarkable upfield chemical shifts of 28 Gly 1H¿, evidence significant stabilizing contributions from C¿H¿ ||| O¿C hydrogen bonds. These stabilizing interactions are corroborated by density functional theory calculations and natural bonding orbital analysis. The singular conformational chemical shifts, J-couplings, high hNOE ratios, small negative temperature coefficients, and slowed H/D exchange constitute a unique set of fingerprints to identify PPII helical bundles, which may be formed by hundreds of Gly-rich motifs detected in sequence databases. These results should aid the quantification of PPII helices in IDPs, the development of improved antifreeze proteins, and the incorporation of PPII helices into novel designed proteins.

14 pags., 4 figs., Background: Amyloids are ordered, insoluble protein aggregates, characterized by a cross-β sheet quaternary structure in which molecules in a β-strand conformation are stacked along the filament axis via intermolecular interactions. While amyloids are typically associated with pathological conditions, functional amyloids have also been identified and are present in a wide variety of organisms ranging from bacteria to humans. The cytoplasmic polyadenylation element-binding (CPEB) prion-like protein is an mRNA-binding translation regulator, whose neuronal isoforms undergo activity-dependent aggregation, a process that has emerged as a plausible biochemical substrate for memory maintenance. CPEB aggregation is driven by prion-like domains (PLD) that are divergent in sequence across species, and it remains unknown whether such divergent PLDs follow a similar aggregating assembly pathway. Here, we describe the amyloid-like features of the neuronal Aplysia CPEB (ApCPEB) PLD and compare them to those of the Drosophila ortholog, Orb2 PLD. Results: Using in vitro single-molecule and bulk biophysical methods, we find transient oligomers and mature amyloid-like filaments that suggest similarities in the late stages of the assembly pathway for both ApCPEB and Orb2 PLDs. However, while prior to aggregation the Orb2 PLD monomer remains mainly as a random coil in solution, ApCPEB PLD adopts a diversity of conformations comprising α-helical structures that evolve to coiled-coil species, indicating structural differences at the beginning of their amyloid assembly pathways. Conclusion: Our results indicate that divergent PLDs of CPEB proteins from different species retain the ability to form a generic amyloid-like fold through different assembly mechanisms., The work was funded by two joint grants from the Ministry of Economy and Competitiveness to MCV (SAF2013-49179-C2-1-R and SAF2016-76678-C2-1-R) and DVL (SAF2013-49179-C2-2-R and SAF2016-76678-C2-2-R) and grants of the Ministry of Economy and Science (BFU2015-70072-R) and the CIBER de Enfermedades Respiratorias (CIBERES; ISCIII) to MM.

16 pags., 7 figs. -- This article belongs to the Special Issue Protein Oligomerization, Protein oligomerization is key to countless physiological processes, but also to abnormal amyloid conformations implicated in over 25 mortal human diseases. Human Angiogenin (h-ANG), a ribonuclease A family member, produces RNA fragments that regulate ribosome formation, the creation of new blood vessels and stress granule function. Too little h-ANG activity leads to abnormal protein oligomerization, resulting in Amyotrophic Lateral Sclerosis (ALS) or Parkinson’s disease. While a score of disease linked h-ANG mutants has been studied by X-ray diffraction, some elude crystallization. There is also a debate regarding the structure that RNA fragments adopt after cleavage by h-ANG. Here, to better understand the beginning of the process that leads to aberrant protein oligomerization, the solution secondary structure and residue-level dynamics of WT h-ANG and two mutants i.e., H13A and R121C, are characterized by multidimensional heteronuclear NMR spectroscopy under near-physiological conditions. All three variants are found to adopt well folded and highly rigid structures in the solution, although the elements of secondary structure are somewhat shorter than those observed in crystallography studies. R121C alters the environment of nearby residues only. By contrast, the mutation H13A affects local residues as well as nearby active site residues K40 and H114. The conformation characterization by CD and 1D 1H NMR spectroscopies of tRNAAla before and after h-ANG cleavage reveals a retention of the duplex structure and little or no G-quadruplex formation., This research was funded by the Italian GIANESINI FOUNDATION FELLOWSHIP, 3rd Ed, 2016 (AF), and grants BFU2017-89707-P (CGI) and SAF2016-76678-C2-2-R (DVL) from the Spanish Ministry of Economy and Competitivity and BTC_PID2019-109306RB-I00 (DVL) from the Ministry of Science and Innovation/AEI/10.13039/501100011033, UE

Supplementary data related to this article can be found at https://doi.org/10.1016/j.abb.2021.108867., Many intrinsically disordered proteins contain Gly-rich regions which are generally assumed to be disordered. Such regions often form biomolecular condensates which play essential roles in organizing cellular processes. However, the bases of their formation and stability are still not completely understood. Based on NMR studies of the Gly-rich H. harveyi "snow flea" antifreeze protein, we recently proposed that Gly-rich sequences, such as the third "RGG" region of Fused in Sarcoma (FUS) protein, may adopt polyproline II helices whose association might stabilize condensates. Here, this hypothesis is tested with a polypeptide corresponding to the third RGG region of FUS. NMR spectroscopy and molecular dynamics simulations suggest that significant populations of polyproline II helix are present. These findings are corroborated in a model peptide Ac-RGGYGGRGGWGGRGGY-NH2, where a peak characteristic of polyproline II helix is observed using CD spectroscopy. Its intensity suggests a polyproline II population of 40%. This result is supported by data from FTIR and NMR spectroscopies. In the latter, NOE correlations are observed between the Tyr and Arg, and Arg and Trp side chain hydrogens, confirming that side chains spaced three residues apart are close in space. Taken together, the data are consistent with a polyproline II helix, which is bent to optimize interactions between guanidinium and aromatic moieties, in equilibrium with a statistical coil ensemble. These results lend credence to the hypothesis that Gly-rich segments of disordered proteins may form polyproline II helices which help stabilize biomolecular condensates., This study was supported by projects: LCF/BQ/PR19/11700003 from “La Caixa Foundation” (ID 100010434) to M.M., BBSRC CASE award with UCB Pharma BB/L014734/1 to AJD, and SAF2016-76678-C2-2-R, 2019AEP121 and BTC-PID2019-109306RB-I00 to DVL from the Spanish Ministry of Economy and Competitivity, the Spanish National Research Council and the Spanish Ministry of Science and Innovation, respectively. The authors acknowledge FCT-Portugal for the PhD studentship attributed to Sara S. Félix (PD/BD/148028/2019). J.O. is a Ramón y Cajal Fellow of the Spanish AEI Ministry of Science and Innovation, and a Leondardo Fellow from the BBVA Foundation (grant number BBM-TRA-0203). NMR experiments were performed in the “Manuel Rico” NMR Laboratory (LMR) of the Spanish National Research Council (CSIC), a node of the Spanish Large-Scale National Facility (ICTS R-LRB). We are grateful to Dr. José Varela Espinosa, Emilia Aporta Sosa and Cristina Quevedo Sierra (MS-CIB/CSIC) for expert technical work with peptide synthesis.

The SARS-CoV-2 Nsp8 protein is a critical component of the RNA replicase, as its N-terminal domain (NTD) anchors Nsp12, the RNA, and Nsp13. Whereas its C-terminal domain (CTD) structure is well resolved, there is an open debate regarding the conformation adopted by the NTD as it is predicted as disordered but found in a variety of complex-dependent conformations or missing from many other structures. Using NMR spectroscopy, we show that the SARS CoV-2 Nsp8 NTD features both well folded secondary structure and disordered segments. Our results suggest that while part of this domain corresponding to two long α-helices forms autonomously, the folding of other segments would require interaction with other replicase components. When isolated, the α-helix population progressively declines towards the C-termini, and dynamics measurements indicate that the Nsp8 NTD behaves as a dimer under our conditions., MM is a Ramón y Cajal Fellow of the Spanish AEI-Ministry of Science and Innovation (RYC2019-026574-I), and a "La Caixa" Foundation (ID 100010434) Junior Leader Fellow (LCR/BQ/PR19/11700003). Funded by project COV20/00764 from the Carlos III Institute of Health and the Spanish Ministry of Science and Innovation to MM and DVL. NMR experiments were performed in the “Manuel Rico” NMR Laboratory (LMR) of the Spanish National Research Council (CSIC), a node of the Spanish Large-Scale National Facility for Biomolecular NMR (ICTS R-LRB).

12 pags., 5 figs., The recent structural elucidation of ex vivo Drosophila Orb2 fibrils revealed a novel amyloid formed by interdigitated Gln and His residue side chains belonging to the prion-like domain. However, atomic-level details on the conformational transitions associated with memory consolidation remain unknown. Here, we have characterized the nascent conformation and dynamics of the prion-like domain (PLD) of Orb2A using a nonconventional liquid-state NMR spectroscopy strategy based on C detection to afford an essentially complete set of Ca, Cb, Ha, and backbone CO and N assignments. At pH 4, where His residues are protonated, the PLD is disordered and flexible, except for a partially populated a-helix spanning residues 55–60, and binds RNA oligos, but not divalent cations. At pH 7, in contrast, His residues are predominantly neutral, and the Q/H segments adopt minor populations of helical structure, show decreased mobility and start to self-associate. At pH 7, the His residues do not bind RNA or Ca, but do bind Zn, which promotes further association. These findings represent a remarkable case of structural plasticity, based on which an updated model for Orb2A functional amyloidogenesis is suggested., This work was supported byLeonardo Grant BBM_TRA_0203 from the BBVA Foundation (toJ. O.) and by FCT-Portugal PhD studentship PD/BD/148028/2019(to S. S. F.). This study was also supported by Spanish Ministry of Economy and Competitivity project SAF2016-76678-C2-2-R (AEI/FEDER, UE to D. V. L.). This work was also partially supported by the Applied Molecular Biosciences Unit—UCIBIO, which isfinanced by national funds from the Portuguese FCT UIDB/04378/2020. J. O. is a Ramón y Cajal fellow from the Spanish AEI-Ministryof Science and Innovation

Intrinsically disordered proteins (IDPs) play essential roles in regulating physiological processes in eukaryotic cells. Many virus use their own IDPs to hack these processes to disactive host defenses and promote viral growth. Thus, viral IDPs are attractive drug targets. While IDPs are hard to study by X-ray crystallography or cryo-EM, atomic level information on their conformational perferences and dynamics can be obtained using NMR spectroscopy. SARS-CoV-2 Nsp2 interacts with human proteins that regulate translation initiation and endosome vesicle sorting, and the C-terminal region of this protein is predicted to be disordered. Molecules that block these interactions could be valuable leads for drug development. To enable inhibitor screening and to uncover conformational preferences and dynamics, we have expressed and purified the 13C,15N-labeled C-terminal region of Nsp2. The 13Cβ and backbone 13CO, 1HN, 13Cα and 15N nuclei were assigned by analysis of a series of 2D 1H-15N HSQC and 13C-15N CON as well as 3D HNCO, HNCA, CBCAcoNH and HncocaNH spectra. Overall, the chemical shift data confirm that this region is chiefly disordered, but contains two five-residue segments that adopt a small population of β-strand structure. Whereas the region is flexible on ms/ms timescales as gauged by T1ρ measurements, the {1H}-15N NOEs reveal a flexibility on ns/ps timescales that is midway between a fully flexible and a completely rigid chain., M. Mompeán is a “Caixa” Foundation Junior Group Leader. Funded by project COV20/00764 from the Carlos III Institute of Health and the Spanish Ministry of Science and Innovation. NMR experiments were performed in the “Manuel Rico” NMR Laboratory (LMR) of the Spanish National Research Council (CSIC), a node of the Spanish Large-Scale National Facility for Biomolecular NMR (ICTS R-LRB). Inhibitor screening is being conducted within the COVID19-NMR consortium by Dr. S. Sreeramula, Prof. Dr. H. Schwalbe and co-workers.

Transactive response DNA-binding protein of 43 kDa (TDP-43) is a 414-residue protein whose aberrant aggregation is implicated in neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) or frontotemporal lobar degeneration (FTLD). Intriguingly, TDP-43 has also been shown to functionally oligomerize to carry out physiological functions. TDP-43 also exists in mixed condensates or granules with other proteins (e.g. neuronal or stress granules), and its large C-terminal domain (CTD, residues 267-414) seems responsible for TDP-43 both homo- and heterotypic interactions underlying such diverse functional and pathological aggregation events. A myriad of distinct triggers may drive TDP-43 oligomerization, including interaction partners or changes in pH or salinity. In this Assignment Note, we report the complete backbone and a wealth of side chain chemical shift assignments for the CTD of TDP-43 at pH 4. The assignments presented here provide a solid starting point to study the aggregation pathway of TDP-43 at pH values below those considered physiological but relevant in pathological settings, and to contrast the aggregation behaviour under distinct conditions and in the presence of interacting partners.

Many intrinsically disordered proteins contain Gly-rich regions which are generally assumed to be disordered. Such regions often form biomolecular condensates which play essential roles in organizing cellular processes. However, the bases of their formation and stability are still not completely understood. Considering NMR studies of the Gly-rich H. harveyi "snow flea" antifreeze protein, we recently proposed that Gly-rich sequences, such as the third "RGG" region of Fused in Sarcoma (FUS) protein, may adopt polyproline II helices whose association might stabilize condensates. Here, this hypothesis is tested with a polypeptide corresponding to the third RGG region of FUS. NMR spectroscopy and molecular dynamics simulations suggest that significant populations of polyproline II helix are present. These findings are corroborated in a model peptide Ac-RGGYGGRGGWGGRGGY-NH2, where a peak characteristic of polyproline II helix is observed using CD spectroscopy. Its intensity suggests a polyproline II population of 40%. This result is supported by data from FTIR and NMR spectroscopies. In the latter, NOE correlations are observed between the Tyr and Arg, and Arg and Trp side chain hydrogens, confirming that side chains spaced three residues apart are close in space. Taken together, the data are consistent with a polyproline II helix, which is bent to optimize interactions between guanidinium and aromatic moieties, in equilibrium with a statistical coil ensemble. In cells, the polyproline II population of these peptides could be augmented by binding profilin protein or SH3, WW or OCRE domains, association with RNA or assembly into polyproline II helical bundles. These results lend credence to the hypothesis that Gly-rich segments of disordered proteins may form polyproline II helices which help stabilize biomolecular condensates.

Knowledge on the molecular bases of early amyloid assembly is fundamental to understand its structure-dysfunction relationship during disease progression. Tauopathies, a well-defined set of neurodegenerative disorders that includes Alzheimer’s disease, are characterized by the pathological amyloid aggregation of tau. However, the underlying molecular mechanisms that trigger tau aggregation and toxicity are poorly understood. Here, using a single-molecule approach, AFM-based single molecule-force spectroscopy (AFM-SMFS), combined with a protein-engineering mechanical protection strategy, we have analyzed the fluctuations of the conformational space of tau during the start of its pathological amyloid assembly. Specifically, we have analyzed the region that includes the four tau microtubule-binding repeats, known to play a key role on tau aggregation. We find that, unlike other amyloid-forming proteins, tau aggregation is accompanied by a decrease of conformational polymorphism, which is driven by amyloid-promoting factors, such as the Δ280K and P301L mutations, linked to Frontotemporal Dementia-17, or by specific chemical conditions. Such perturbations have distinct effects and lead to different tau (aggregate) structures. In addition to providing insight into how tau aggregates in a context dependent manner, these findings may help delve into how protein aggregation-based diseases, like Alzheimer’s, might be treated using monomer fluctuations as a pharmacological target.Abstract Figure

The cytoplasmic polyadenylation element-binding protein 3 (CPEB3), is an RNA-binding protein which in its soluble state is localized in membraneless neuronal RNA granules keeping target mRNAs in a repressed state. The stimulus-dependent aggregation of CPEB3 activates target mRNAs translation, a central event for the maintenance of long-term memory-related synaptic plasticity in mammals. To date, the molecular determinants that govern both connected events remain unclear. Here, to gain insight into these processes, the biophysical properties of the human CPEB3 (hCPEB3) are characterized. We found that hCPEB3 homotypic condensation is mainly driven by hydrophobic interactions and occurs under physiological conditions. Moreover, hCPEB3 biomolecular condensates are dynamic inside living cells, whose localization and stabilization are mediated by its RNA-recognition domains. In contrast, the hCPEB3 polar N-terminal region is crucial for hCPEB3 amyloid-like aggregationin vitro, which is disrupted by the polyglutamine binding peptide 1 (QBP1), Aβ42seeds and Hsp70, highlighting the importance of the Q4RQ4tract as well as the hydrophobic residues for hCPEB3 functional aggregation. Based on these findings, we postulate a model for hCPEB3’s role in memory persistence that advances a rather sophisticated control for hCPEB3 condensate dissociation and amyloid-like formation to achieve its physiological function.HighlightshCPEB3 forms toxic intermediates that persist longer than in other functional amyloids.RNA-recognition domains stabilize hCPEB3 granule formation and dynamics.Different segments within hCPEB3 promote amyloidogenesis and liquid demixing.hCPEB3 amyloid formation requires both hydrophobic and polyQ segments.Graphical Abstract

Amyloids play key pathological roles in a score of neurodegenerative diseases, but they are also essential to several physiological processes. Perhaps the most fascinating functional amyloid is formed by the Cytoplasmic Polyadenylation Element Binding protein (CPEB) protein family as its structural conversion is essential for memory consolidation in different animals. In vitro, CPEB amyloid formation is driven by the intrinsically disordered region (IDR) present in neuronal-specific isoforms. However, the underlying conformational transitions from the monomeric state to the amyloid state remain poorly understood. Here, we characterize the residue level conformational preferences and ps−ns dynamics for human CPEB3 (hCPEB3) by high field, heteronuclear NMR spectroscopy. At 426 residues, this is the second longest IDR characterized to date in such detail. We find that the residues 1−29: M1QDDLLMDKSKTQPQPQQQQRQQQQQPQP29, adopt an α−helical+disordered Q-rich motif. Similar helix + Q/N−rich motifs are observed in CPEB homologs as well as other RNA−binding proteins like TDP-43 that form pathological amyloids. Residues 86−93: P83QQPPPP93, and residues 166−175: P166PPPAPAPQP175 form polyproline−II (PPII) helices, and we propose NMR chemical shift-based criteria for identifying this conformation. We advance that the presence of helix and amyloid breaker residues, such as proline, in hCPEB3 and its absence in TDP−43 may be a key difference between the functional and pathological amyloids in higher eukaryotes. While the (VG)5 repeat motif (residues 272−282) appears to be completely disordered, residues S221−A235 form a highly populated, rather rigid α−helix and two nearby segments adopt partially populated α−helices. Residues 345−355 also form a partially populated α−helix. These residues comprise the nuclear export signal and border a putative phosphoTyr site which may mediate STAT5B binding. Thus, nascent hCPEB3 protein is not fully disordered like a blank sheet of paper; instead, it contains creases which guide the engraving of our memories. Based on these findings and previous results, a working model for hCPEB3 structural transitions in human memory consolidation is advanced.

29 p.-6 fig.-1 tab., How proteins with a stable globular fold acquire the amyloid state is still largely unknown. RepA, a versatile plasmidic DNA binding protein from Pseudomonas savastanoi, is functional as a transcriptional repressor or as an initiator or inhibitor of DNA replication, the latter via assembly of an amyloidogenic oligomer. Its N-terminal domain (WH1) is responsible for discrimination between these functional abilities by undergoing insufficiently understood structural changes. RepA-WH1 is a stable dimer whose conformational dynamics had not been explored. Here, we have studied it through NMR {1H}-15N relaxation and H/D exchange kinetics measurements. The N- and the C-terminal α-helices, and the internal amyloidogenic loop, are partially unfolded in solution. S4-indigo, a small inhibitor of RepA-WH1 amyloidogenesis, binds to and tethers the N-terminal α-helix to a β-hairpin that is involved in dimerization, thus providing evidence for a priming role of fraying ends and dimerization switches in the amyloidogenesis of folded proteins., This work was supported by grants SAF2016-76678-C2-2-R (DVL), CTQ2017-84371-P (Mª Ángeles Jiménez), BIO2015-68730-R (R.G.), BFU2015-72271-EXP (R.G.) and RTI2018-094549-B-I00 (R.G.) from the Spanish Ministry of Innovation and Competitiveness. NMR experiments were performed in the‘‘Manuel Rico’’ NMR laboratory (LMR) of the Spanish National Research Coun-cil (CSIC), a node of the Spanish Large-Scale National Facility (ICTS R-LRB)

TDP-43 assembles various aggregate forms, including biomolecular condensates or functional and pathological amyloids, with roles in disparate scenarios (e.g. muscle regeneration versus neurodegeneration). The link between condensates and fibrils remains unclear, just as the factors controlling conformational transitions within these aggregate species: salt- or RNA-induced droplets may evolve into fibrils or remain in the droplet form, suggesting distinct endpoint species of different aggregation pathways. Using microscopy and NMR methods, we unexpectedly observed in vitro droplet formation in the absence of salts or RNAs, and provided visual evidence for fibrillization at the droplet surface/solvent interface but not the droplet interior. Our NMR analyses unambiguously uncovered a distinct amyloid conformation in which Phe-Gly motifs are key elements of the reconstituted fibril form, suggesting a pivotal role for these residues in creating the fibril core. This contrasts the minor participation of Phe-Gly motifs in initiation of the droplet form. Our results point to an intrinsic (i.e., non-induced) aggregation pathway that may exist over a broad range of conditions, and illustrate structural features that distinguishes between aggregate forms.

48 p.-6 fig.-1 tab.-9 fig-supl.-3 tab.supl., The essential bacterial division protein FtsZ uses GTP binding and hydrolysis to assemble into dynamic filaments that treadmill around the Z-ring, guiding septal wall synthesis and cell division. FtsZ is a structural homolog of tubulin and a target for discovering new antibiotics. Here, using FtsZ from the pathogen S aureus (SaFtsZ), we reveal that, prior to assembly, FtsZ monomers require nucleotide binding for folding; this is possibly relevant to other mesophilic FtsZs. Apo-SaFtsZ is essentially unfolded, as assessed by nuclear magnetic resonance and circular dichroism. Binding of GTP (≥ 1mM) dramatically shifts the equilibrium towards the active folded protein. Supportingly, SaFtsZ refolded with GDP crystallizes in a native structure. Apo-SaFtsZ also folds with 3.4 M glycerol, enabling high-affinity GTP binding (KD 20 nM determined by isothermal titration calorimetry) similar to thermophilic stable FtsZ. Other stabilizing agents that enhance nucleotide binding include ethylene glycol, trimethylamine N-oxide (TMAO), and several bacterial osmolytes. High salt stabilizes SaFtsZ without bound nucleotide in an inactive twisted conformation. We identified a cavity behind the SaFtsZ-GDP nucleotide-binding pocket that harbors different small compounds, which is available for extended nucleotide-replacing inhibitors. Furthermore, we devised a competition assay to detect any inhibitors that overlap the nucleotide site of SaFtsZ, or Escherichia coli FtsZ, employing osmolyte-stabilized apo-FtsZs and the specific fluorescence anisotropy change of mant-GTP upon dissociation from the protein. This robust assay provides a basis to screening for high affinity GTP-replacing ligands, which combined with structural studies and phenotypic profiling should facilitate development of a next generation of FtsZ-targeting antibacterial inhibitors., This work was supported by grants BFU 2014-51823-R (JMA), BFU 2017-87387-P (CFT), SAF-2016-76678-C2-2-R (DVL) and a FPI fellowship (AJCV).

12 pags., 10 figs., 2 tabs., Microbial pathogens, such as Trypanosoma brucei, have an enormous impact on global health and economic systems. Protein kinase A of T. brucei is an attractive drug target as it is an essential enzyme which differs significantly from its human homolog. The hinge region of this protein's regulatory domain is vital for enzymatic function, but its conformation is unknown. Here, the secondary structure of this region has been characterized using NMR and CD spectroscopies. More specifically, three overlapping peptides corresponding to residues T187-I211, G198-Y223 and V220–S245 called peptide 1, peptide 2 and peptide 3, respectively, were studied. The peptide 1 and peptide 2 are chiefly unfolded; only low populations (, This research was supported by FONACIT, Caracas, Venezuela [grant numbers 2012000364 and 2013001659 to NAA]. We are grateful to the Spanish Ministry of Economy and Competitivity and Dr. Ma ́Angeles Jimenez L ́opez for additional support through project CTQ2017- 84371P. NMR experiments were performed in the “Manuel Rico” NMR laboratory (LMR) of the Spanish National Research Council (CSIC), a node of the Spanish Large-Scale National Facility (ICTS R-LRB).

15 pags., 11 figs., Protein self-assembly is a ubiquitous phenomenon, traditionally studied for its links to amyloid pathologies, which has also gained attention as its physiological roles and possible biotechnological applications emerged over time. It is also known that varying the conditions to which proteins are exposed can lead to aggregate polymorphism. To understand the factors that trigger aggregation and/or direct it toward specific outcomes, we performed a multifaceted structural characterization of the thermally induced self-assembly process of MNEI, a model protein able to form amyloid aggregates under nondenaturing conditions. MNEI is also known for its extreme sweetness which, combined with a considerable thermal stability, makes the protein a promising alternative sweetener. Fourier-transformed infrared spectroscopy and electron microscopy data showed that the presence of NaCl accelerates the kinetics of fibrillar aggregation, while disfavoring the population of off-pathway states that are instead detected by native gel electrophoresis at low ionic strength. NMR studies revealed how NaCl modulates the self-assembling mechanism of MNEI, switching the process from soluble oligomeric forms to fibrils. Comparative analysis demonstrated that the presence of NaCl induces local differences in the protein dynamics and surface accessibility, without altering the native fold. We identified the regions most affected by the presence of NaCl, which control the aggregation process, and represent hot spots on the protein surface for the rational design of new mutants with controlled aggregation propensity., This work was partially supported by Fondazione conil SUD (grant 2011-PDR-19) and SAF2016-76678-C2-2-R from the Spanish Ministry of Economy and Competitivity. We acknowledge the Horizon 2020 Pro-gramme (grant agreement No. 653706, iNEXT) and University of Milano-Bicocca (grant ‘Fondo di Ateneoper la Ricerca 2017-ATE-0339’). Some NMR experiments were performed in the ‘Manuel Rico’ NMR lab-oratory (LMR) of the Spanish National ResearchCouncil (CSIC), a node of the Spanish Large-ScaleNational Facility (ICTS R-LRB). FD was the recipientof a fellowship from Regione Campania (POR Campania FSE 2014–2020). MD was supported by PON CCI2014IT16M2OP005 from the Italian Ministry ofUniversity and Research

20 pags. 6 figs., Cellulose is the most abundant organic molecule on Earth and represents a renewable and practically everlasting feedstock for the production of biofuels and chemicals. Self-assembled owing to the high-affinity cohesin-dockerin interaction, cellulosomes are huge multi-enzyme complexes with unmatched efficiency in the degradation of recalcitrant lignocellulosic substrates. The recruitment of diverse dockerin-borne enzymes into a multicohesin protein scaffold dictates the three-dimensional layout of the complex, and interestingly two alternative binding modes have been proposed. Using single-molecule fluorescence resonance energy transfer and molecular simulations on a range of cohesin-dockerin pairs, we directly detect varying distributions between these binding modes that follow a built-in cohesin-dockerin code. Surprisingly, we uncover a prolyl isomerase-modulated allosteric control mechanism, mediated by the isomerization state of a single proline residue, which regulates the distribution and kinetics of binding modes. Overall, our data provide a novel mechanistic understanding of the structural plasticity and dynamics of cellulosomes., This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement no. 746635. Support by the DFG excellence cluster CIPSM is acknowledged. This work was supported by a Seventh Framework Programme in Nanosciences, Nanotechnologies, Materials & New Production Technologies (7PM -NMP 2013-17, 604530-2, CellulosomePlus) and the ERA-IB-ERANET-2013-16 (EIB.12.022, FiberFuel) through the Spanish MINECO (PCIN-2013-011-C02-01) to M.C.V. and by a MINECO/AEI/FEDER, UE SAF2016-76678-C2-2-R grant (D.V.L.). NMR experiments were performed in the Manuel Rico NMR laboratory (LMR) of the Spanish National Research Council (CSIC), a node of the Spanish Large-Scale National Facility (ICTS R-LRB). The Warsaw group has received support from the National Science Centre (NCN) under grant nos. 2018/31/B/NZ1/00047 (M.C. and M.W.) and 2016/21/B/NZ1/00006 (B.R. and M.C.).

Bacterial cellulases are drawing increased attention as a means to obtain plentiful chemical feedstocks and fuels from renewable lignocellulosic biomass sources. Certain bacteria deploy a large extracellular multi-protein complex, called the cellulosome, to degrade cellulose. Scaffoldin, a key non-catalytic cellulosome component, is a large protein containing a cellulose-specific carbohydrate-binding module and several cohesin modules which bind and organize the hydrolytic enzymes. Despite the importance of the structure and protein/protein interactions of the cohesin module in the cellulosome, its structure in solution has remained unknown to date. Here, we report the backbone 1H, 13C and 15N NMR assignments of the Cohesin module 5 from the highly stable and active cellulosome from Clostridium thermocellum. These data reveal that this module adopts a tightly packed, well folded and rigid structure in solution. Furthermore, since in scaffoldin, the cohesin modules are connected by linkers we have also characterized the conformation of a representative linker segment using NMR spectroscopy. Analysis of its chemical shift values revealed that this linker is rather stiff and tends to adopt extended conformations. This suggests that the scaffoldin linkers act to minimize interactions between cohesin modules. These results pave the way towards solution studies on cohesin/dockerin's fascinating dual-binding mode., This work was supported by grants from a Seventh Framework Programme in Nanosciences, Nanotechnologies, Materials & New Production Technologies (7PM -NMP 2013-17, 604530-2) and the ERA-IB-ERANET-2013-16 (EIB.12.022) through the Spanish MINECO (PCIN-2013-011-C02-01) to MC-V. and from grants SAF-2016-76678-C2-2-R (DVL) & CTQ-2014-52633-P (DPU). AGP acknowledges financial support from an FPU fellowship from Spanish MECD.

Transactive response DNA-binding protein 43 (TDP-43) performs multiple tasks in mRNA processing, transport, and translational regulation, but it also forms aggregates implicated in amyotrophic lateral sclerosis. TDP-43’s N-terminal domain (NTD) is important for these activities and dysfunctions; however, there is an open debate about whether or not it adopts a specifically folded, stable structure. Here, we studied NTD mutations designed to destabilize its structure utilizing NMR and fluorescence spectroscopies, analytical ultracentrifugation, splicing assays, and cell microscopy. The substitutions V31R and T32R abolished TDP-43 activity in splicing and aggregation processes, and even the rather mild L28A mutation severely destabilized the NTD, drastically reducing TDP-43’s in vitro splicing activity and inducing aberrant localization and aggregation in cells. These findings strongly support the idea that a stably folded NTD is essential for correct TDP-43 function. The stably folded NTD also promotes dimerization, which is pertinent to the protein’s activities and pathological aggregation, and we present an atomic-level structural model for the TDP-43 dimer based on NMR data. Leu-27 is evolutionarily well conserved even though it is exposed in the monomeric NTD. We found here that Leu-27 is buried in the dimer and that the L27A mutation promotes monomerization. In conclusion, our study sheds light on the structural and biological properties of the TDP-43 NTD, indicating that the NTD must be stably folded for TDP-43’s physiological functions, and has implications for understanding the mechanisms promoting the pathological aggregation of this protein.

Apoptin is a nonstructural protein encoded by one of the three open reading frames of the chicken anemia virus genome. It has attracted a great deal of interest due to its ability to induce apoptosis in multiple transformed and malignant mammalian cell lines without affecting primary and non-transformed cells. However, the use of Apoptin as an anticancer drug is restricted by its strong tendency to aggregate. A number of methods to overcome this problem have been proposed, including transduction techniques to deliver the Apoptin gene into tumor cells, but all such methods have certain drawbacks. Here we describe that a truncated variant of Apoptin, lacking residues 1 to 43, is a soluble, non-aggregating protein that maintains most of the biological properties of wild-type Apoptin when transfected into cells. We show that the cytotoxic effect of this variant is also present when it is added exogenously to cancer cells, but not to normal cells. In addition to the interest this protein has attracted as a promising therapeutic strategy, it is also an excellent model to study the structural properties of Apoptin and how they relate to its mechanism of action.

7 pags., 9 figs., 2 tabs., Quick, inexpensive and accurate methods are needed for glucose determination in many biochemical and medical labs. The purpose of this study is to develop and test a novel inexpensive electrode based on nickel/gamma-aminopropyltriethoxysilane nanoparticles for measuring glucose concentrations. First, the Ni nanoparticles (Ni NPs) were synthesized by a bottom-up approach. Then, the Ni NPs and gamma-Aminopropyltriethoxysilane (KH550) were mixed at 60 °C under a nitrogen atmosphere to produce Ni/KH550 nanocomposite. Afterwards, the synthesized Ni/KH550 nanocomposites were characterized using different methods such as Fourier transform infrared (FT-IR), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Next, the surface of a glassy carbon electrode (GCE) was polished and washed with alumina powder and deionized water, respectively; and ultrasonicated in the H O/ethanol solution. Then, the nanocomposite suspension was pipetted dropwise on the GCE surface and dried to GCE-Ni/KH550 production. The newly prepared electrode was characterized and applied to glucose detection utilizing electrochemical methods. The cyclic voltammetry measurements showed that the oxidation peak of glucose appeared at a potential of about 0.53 V (vs. Ag/AgCl) on the surface of the modified GCE. Differential pulse voltammetry exhibited two wide linear dynamic ranges of 0.5–20 and 100–500 μM glucose with a detection limit of 0.043 μM. Finally, measurements of glucose in a real sample using our designed sensor indicated that GCE-Ni/KH550 is as good as those obtained with a high efficiency, commercial apparatus.

Transactive Response DNA-Binding Protein of 43 kDa (TDP-43) is an essential human protein implicated in Amyotrophic Lateral Sclerosis (ALS) and common dementias. Its C-terminal disordered region, composed of residues 264–414 includes a hydrophobic segment (residues 320–340), which drives physiological liquid/liquid phase separation and a Q/N-rich segment (residues 341–357), which is essential for pathological amyloid formation. Due to TDP-43's relevance for pathology, identifying inhibitors and characterizing their mechanism of action are important pharmacological goals. The Polyglutamine Binding Peptide 1 (QBP1), whose minimal active core is the octapeptide WGWWPGIF, strongly inhibits the aggregation of polyQ-containing amyloidogenic proteins such as Huntingtin. Rather promiscuous, this inhibitor also blocks the aggregation of other glutamine containing amyloidogenic proteins, but not Aβ, and its mechanism of action remains unknown. Using a series of spectroscopic assays and biochemical tests, we establish that QBP1 binds and inhibits amyloid formation by TDP-43's Q/N-rich region. NMR spectroscopic data evince that the aromatic rings of QBP1 accept hydrogen bonds from the HN groups of the Asn and Gln to block amyloidogenesis. This mechanism of blockage may be general to polyphenol amyloid inhibitors., This study was supported by a Junior Leader Project LCF/BQ/PR19/11700003 (to MMG) from the Caixa Foundation (CID-100010434),and projects SAF2016-76678-C2-1-R (MC-V) and SAF2016-76678-C2-2-R(DVL) from the Spanish Ministry of Economy and Competitivity.NMR experiments were performed in the “Manuel Rico” NMR Laboratory (LMR) of the Spanish National Research Council (CSIC), a node of the Spanish Large-Scale National Facility (ICTSR-LRB).

13 pags., 5 figs., 1 tab., The conformation and mechanism of proteins that degrade and bind RNA, which has provided key insights into post-transcriptional gene regulation, is explored here. During the twentieth century's last decades, the characterization of ribonucleases and RNA binding domains revealed the diversity of their reaction mechanisms and modes of RNA recognition, and the bases of protein folding, substrate specificity and binding affinity. More recent research showed how these domains combine through oligomerization or genetic recombination to create larger proteins with highly specific and readily programmable ribonucleolytic activity. In the last 15 years, the study of the capacity of proteins, usually disordered, to pool RNAs into discrete, non-aqueous microdroplets to facilitate their transport, modification and degradation - analogous to cowboys herding cattle - has advanced our comprehension of gene expression. Finally, the current uses of RNA binding proteins and the future applications of protein/RNA microdroplets are highlighted., This work was supported by grant SAF2016-76678-C2-2-R from the Spanish Ministry of Economy and Competitiveness.

17 pags., 8 figs., We investigated the Central Nervous System (CNS) and skeletal muscle tissue from A woman was clinically diagnosed with amyotrophic lateral sclerosis (ALS) at the age of 22. Neuropathologic evaluation showed upper and lower motor neuron loss, corticospinal tract degeneration and skeletal muscle denervation. Analysis of the patient's Deoxyribonucleic acid (DNA) revealed a AGT>GGT change resulting in an S375G substitution in the C-terminal region of TDP-43. This variant was previously reported as being benign. Considering the early onset and severity of the disease in this patient, we tested the effects of this genetic variant on TDP-43 localization, pre-mRNA splicing activity and toxicity, in parallel with the effects on known neighboring disease-associated mutations. In cell lines, expressed in culture, S375G TDP-43 appeared to be more significantly localized in the nucleus and to exert higher toxicity than wild-type TDP-43. Strikingly, a phosphomimic mutant at the same residue (S375E) showed a strong tendency to accumulate in the cytoplasm, especially under stress conditions, and molecular dynamics simulations suggest that phosphorylation of this residue can disrupt TDP-43 intermolecular interactions. The results of the current study highlight the importance of phosphorylation and regulation of TDP-43 nuclear-cytoplasmic shuttling/redistribution, in relation to the pathogenetic mechanisms involved in different forms of ALS., This work was supported by the International Scientific Co-operation Agreement between Italy and Israel (SCREENCELLS4ALS, Ministero Affari Esteri, MAE, Italy) and SAF-2016-76678-C2-2-R (DVL). BG is supported by grant P30 AG010133. KN is supported by grant P30 AG035982.

17 pags., 4 figs. -- https://doi.org/10.1371/journal.pbio.3001198.g001 (fig. 1) ; https://doi.org/10.1371/journal.pbio.3001198.g002 (fig. 2) ; https://doi.org/10.1371/journal.pbio.3001198.g003 (fig. 3) ; https://doi.org/10.1371/journal.pbio.3001198.g004 (fig. 4) ; https://doi.org/10.1371/journal.pbio.3001198.s005 (numerical data), Transactive response DNA-binding Protein of 43 kDa (TDP-43) assembles various aggregate forms, including biomolecular condensates or functional and pathological amyloids, with roles in disparate scenarios (e.g., muscle regeneration versus neurodegeneration). The link between condensates and fibrils remains unclear, just as the factors controlling conformational transitions within these aggregate species: Salt- or RNA-induced droplets may evolve into fibrils or remain in the droplet form, suggesting distinct end point species of different aggregation pathways. Using microscopy and NMR methods, we unexpectedly observed in vitro droplet formation in the absence of salts or RNAs and provided visual evidence for fibrillization at the droplet surface/solvent interface but not the droplet interior. Our NMR analyses unambiguously uncovered a distinct amyloid conformation in which Phe-Gly motifs are key elements of the reconstituted fibril form, suggesting a pivotal role for these residues in creating the fibril core. This contrasts the minor participation of Phe-Gly motifs in initiation of the droplet form. Our results point to an intrinsic (i.e., non-induced) aggregation pathway that may exist over a broad range of conditions and illustrate structural features that distinguishes between aggregate forms., This work has been funded by Grants CTQ2017-84371-P to D.P.-U. and SAF2016-76678-C2-2-R to D.V.L., from the Spanish MINECO; Grant MCB1412253 from the U. S. National Science Foundation to A.E.M; AriSLA (PathensTDP project) to E.B.; and Grant LCF/BQ/PR19/11700003 from La Caixa Foundation (ID 100010434) to M.M. M.M. is a Ramón y Cajal Fellow of the Spanish AEI-Ministry of Science and Innovation (RYC2019-026574-I). NMR experiments were performed in the “Manuel Rico” NMR Laboratory (LMR) of the Spanish National Research Council (CSIC), a node of the Spanish Large-Scale National Facility (ICTS R-LRB).

Intrinsically disordered proteins (IDPs) lack a tertiary structure. Amyloidogenic IDPs (aIDPs) in particular have attracted great interest due to their implication in several devastating diseases as well as in critical biological functions. However, the conformational changes that trigger amyloid formation in aIDPs are largely unknown. aIDPs' conformational polymorphism at the monomer level encumbers their study using bulk techniques. Single-molecule techniques like atomic force microscopy-based single-molecule force spectroscopy represent a promising approach and a >carrier-guest> strategy, in which the protein of interest is mechanically protected, was developed to overcome the spurious signals from the noisy proximal region. However, since the carrier and single-molecule markers have similar mechanostabilities, their signals can intermingle in the force-extension recordings, making peak selection and analysis very laborious, cumbersome and prone to error for the non-expert. Here we have developed a new carrier, the c8C module from the CipC scaffoldin, with a higher mechanostability so that the signals from the protected protein will appear at the end of the recordings. This assures an accurate, more efficient and expert-independent analysis, simplifying both the selection and analysis of the single-molecule data. Furthermore, this modular design can be integrated into any SMFS polyprotein-based vector, thus constituting a useful utensil in the growing toolbox of protein nanomechanics.

Enzymes and structural proteins dominated thinking about protein structure and function for most of the twentieth century. In recent decades, however, we have begun to appreciate the significant physiological and pathological roles of nonglobular proteins. Amyloids first gained infamy from their implications in a score of human mortal diseases. However, they have recently been discovered to play vital physiological roles, such as memory consolidation in humans. This raises an important question: Can we inhibit pathological amyloids without affecting functional amyloids? Intrinsically disordered proteins (IDPs), many of which are prone to form amyloids, perform many essential functions, yet their importance has only been recognized in the last quarter century. A subclass of IDPs can form, under certain conditions, water immiscible liquid phases which serve to process, regulate, store or transport RNA. Perturbation of these remarkable liquid phases can lead to aggregates, such as those formed by the proteins TDP-43 and FUS, which are linked to ALS and other dementia. Here, we summarize our changing view of intrinsically disordered, liquid phase forming and amyloidogenic proteins and the uncertainties that will drive future research.

Apoptin is a 121 residue protein which forms large, soluble aggregates and possesses an exceptionally selectively cytotoxic action on cancer cells. In the accompanying paper, we described the design, production and initial characterization of an Apoptin truncated variant called H-ApopΔProΔLeu. Whereas both the variant and wild type protein possess similar selective cytotoxicity against cancer cells following transfection, only the variant is cytotoxic when added externally. Remarkably, as observed by gel filtration chromatography and dynamic light scattering, H-ApopΔProΔLeu lacks the tendency of wild type Apoptin to form large aggregates, which greatly facilitated the study of its biological properties. Here, we characterize the conformation and dynamics of H-ApopΔProΔLeu. Using a battery of 2D, 3D and (4,2)D NMR spectra, the essentially complete H, C and N resonance assignments of H-ApopΔProΔLeu were obtained. The analysis of these data shows that the variant is an intrinsically disordered protein, which lacks a preferred conformation. This conclusion is corroborated by a lack of protection against proteolytic cleavage and hydrogen/deuterium exchange. Moreover, the CD spectra are dominated by random coil contributions. Finally, H-N NOE ratios are low, which indicates flexibility on the ps-ns time scale. Interestingly, H-ApopΔProΔLeu's intrinsically disordered ensemble is not significantly altered by the redox state of its Cys residues or by Thr phosphorylation, which has been proposed to play a key role in Apoptin's selective cytotoxicity. These results serve to better comprehend Apoptin's remarkably selective anticancer action and provide a framework for the future design of improved Apoptin variants.

Bovine pancreatic ribonuclease A (RNase A) is the monomeric prototype of the so-called secretory ‘pancreatic-type’ RNase super-family. Like the naturally domain-swapped dimeric bovine seminal variant, BS-RNase, and its glycosylated RNase B isoform, RNase A forms N- and C-terminal 3D domain-swapped oligomers after lyophilization from acid solutions, or if subjected to thermal denaturation at high protein concentration. All mentioned RNases can undergo deamidation at Asn67, forming Asp or isoAsp derivatives that modify the protein net charge and consequently its enzymatic activity. In addition, deamidation slightly affects RNase B self-association through the 3D domain swapping (3D-DS) mechanism. We report here the influence of extensive deamidation on RNase A tendency to oligomerize through 3D-DS. In particular, deamidation of Asn67 alone slightly decreases the propensity of the protein to oligomerize, with the Asp derivative being less affected than the isoAsp one. Contrarily, the additional Asp and/or isoAsp conversion of residues other than N67 almost nullifies RNase A oligomerization capability. In addition, Gln deamidation, although less kinetically favorable, may affect RNase A self-association. Using 2D and 3D NMR we identified the Asn/Gln residues most prone to undergo deamidation. Together with CD spectroscopy, NMR also indicates that poly-deamidated RNase A generally maintains its native tertiary structure. Again, we investigated in silico the effect of the residues undergoing deamidation on RNase A dimers structures. Finally, the effect of deamidation on RNase A oligomerization is discussed in comparison with studies on deamidation-prone proteins involved in amyloid formation.

TDP-43 is a nuclear protein whose abnormal aggregates are implicated in ALS and FTLD. Recently, an Asn/Gln rich C-terminal segment of TDP-43 has been shown to produce aggregation in vitro and reproduce most of the protein's pathological hallmarks in cells, but little is known about this segment's structure. Here, CD and 2D heteronuclear NMR spectroscopies provide evidence that peptides corresponding to the wild type and mutated sequences of this segment adopt chiefly disordered conformations that, in the case of the wild type sequence, spontaneously forms a β-sheet rich oligomer. Moreover, MD simulation provides evidence for a structure consisting of two β-strands and a well-defined, yet non-canonical structural element. Furthermore, MD simulations of four pathological mutations (Q343R, N345K, G348V and N352S) occurring in this segment predict that all of them could affect this region's structure. In particular, the Q343R variant tends to stabilize disordered conformers, N345K permits the formation of longer, more stable β-strands, and G348V tends to shorten and destabilize them. Finally, N352S acts to alter the β-stand register and when S352 is phosphorylated, it induces partial unfolding. Our results provide a better understanding of TDP-43 aggregation process and will be useful to design effectors capable to modulate its progression. © 2014 Elsevier Inc. All rights reserved.