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Two modes of motility have been reported for bi-directional kinesin-5 motors: (a) context-dependent directionality reversal, a mode in which motors undergo persistent minus-end directed motility at the single-molecule level and switch to plus-end directed motility in different assays or under different conditions, such as during MT gliding or antiparallel sliding or as a function of motor clustering; and (b) bi-directional motility, defined as movement in two directions in the same assay, without persistent unidirectional motility. Here, we examine how modulation of motor-microtubule (MT) interactions affects these two modes of motility for the bi-directional kinesin-5, Cin8. We report that the large insert in loop 8 (L8) within the motor domain of Cin8 increases the MT affinity of Cin8 in vivo and in vitro and is required for Cin8 intracellular functions. We consistently found that recombinant purified L8 directly binds MTs and L8 induces single Cin8 motors to behave according to context-dependent directionality reversal and bi-directional motility modes at intermediate ionic strength and according to a bi-directional motility mode in an MT surface-gliding assay under low motor density conditions. We propose that the largely unstructured L8 facilitates flexible anchoring of Cin8 to the MTs. This flexible anchoring enables the direct observation of bi-directional motility in motility assays. Remarkably, although L8-deleted Cin8 variants exhibit a strong minus-end directed bias at the single-molecule level, they also exhibit plus-end directed motility in an MT-gliding assay. Thus, L8-induced flexible MT anchoring is required for bi-directional motility of single Cin8 molecules but is not necessary for context-dependent directionality reversal of Cin8 in an MT-gliding assay.

Hearing depends on intricate morphologies and mechanical properties of diverse inner ear cell types. The individual contributions of various inner ear cell types into mechanical properties of the organ of Corti and the mechanisms of their integration are yet largely unknown. Using sub-100-nm spatial resolution atomic force microscopy (AFM), we mapped the Young’s modulus (stiffness) of the apical surface of the different cells of the freshly dissected P5–P6 cochlear epithelium from wild-type and mice lacking either Trio and F-actin binding protein (TRIOBP) isoforms 4 and 5 or isoform 5 only. Variants of TRIOBP are associated with deafness in human and in Triobp mutant mouse models. Remarkably, nanoscale AFM mapping revealed unrecognized bidirectional radial stiffness gradients of different magnitudes and opposite orientations between rows of wild-type supporting cells and sensory hair cells. Moreover, the observed bidirectional radial stiffness gradients are unbalanced, with sensory cells being stiffer overall compared to neighboring supporting cells. Deafness-associated TRIOBP deficiencies significantly disrupted the magnitude and orientation of these bidirectional radial stiffness gradients. In addition, serial sectioning with focused ion beam and backscatter scanning electron microscopy shows that a TRIOBP deficiency results in ultrastructural changes of supporting cell apical phalangeal microfilaments and bundled cortical F-actin of hair cell cuticular plates, correlating with messenger RNA and protein expression levels and AFM stiffness measurements that exposed a softening of the apical surface of the sensory epithelium in mutant mice. Altogether, this additional complexity in the mechanical properties of the sensory epithelium is hypothesized to be an essential contributor to frequency selectivity and sensitivity of mammalian hearing.

The conserved kinesin-5 bipolar tetrameric motors slide apart microtubules during mitotic spindle assembly and elongation. Kinesin-5 bipolar organization originates from its conserved tetrameric helical minifilament, which position the C-terminal tail domains of two subunits near the N-terminal motor domains of two anti-parallel subunits (Scholey et al, 2014). This unique tetrameric structure enables kinesin-5 to simultaneously engage two microtubules and transmit forces between them, and for multiple kinesin-5 motors to organize via tail to motor interactions during microtubule sliding (Bodrug et al, 2020). Here, we show how these two structural adaptations, the kinesin-5 tail-motor domain interactions and the length of the tetrameric minifilament, determine critical aspects of kinesin-5 motility and sliding mechanisms. An x-ray structure of the 34-nm kinesin-5 minifilament reveals how the dual dimeric N-terminal coiled-coils emerge from the tetrameric central bundle. Using this structure, we generated active bipolar mini-tetrameric motors from Drosophila and human orthologs, which are half the length of native kinesin-5. Using single-molecule motility assays, we show that kinesin-5 tail domains promote mini-tetramers static pauses that punctuate processive motility. During such pauses, kinesin-5 mini-tetramers form multi-motor clusters mediated via tail to motor domain cross-interactions. These clusters undergo slow and highly processive motility and accumulate at microtubule plus-ends. In contrast to native kinesin-5, mini-tetramers require tail domains to initiate microtubule crosslinking. Although mini-tetramers are highly strained in initially aligning microtubules, they slide microtubules more efficiently than native kinesin-5, due to their decreased minifilament flexibility. Our studies reveal that the conserved kinesin-5 motor-tail mediated clustering and the length of the tetrameric minifilament are key features for sliding motility and are critical in organizing microtubules during mitotic spindle assembly and elongation.

Seals, such as elastomer O-rings, are used to prevent the leakage of vapour from the ductwork or devices through which it flows. The temperature of the vapour that is often present in industrial processes continues to increase. Therefore, there is a requirement for sealing materials that are proven to work under conditions that involve high-temperature vapour. This article describes the development of a cross-linking agent for perfluoroelastomer that has two features – resistance to high-temperature vapour and cross-linking efficiency.

Due to their unique properties, tunable size, and ability to provide a near native lipid environment, nanodiscs have found widespread use for the structural and functional studies of reconstituted membrane proteins. They have also been developed, albeit in a few applications, for therapeutic and biomedical use. For these studies and applications, it is essential to characterize the nanodisc preparations in terms of their monodispersity, size, and composition, as these can influence the properties of the membrane protein of interest. Of the many biophysical methods utilized for the study and characterization of nanodiscs, we show that analytical ultracentrifugation is able to report on sample homogeneity, shape, size, composition, and membrane protein stoichiometry or oligomerization state in a direct and simple fashion. The method is truly versatile and does not require nanodisc modification or disassembly.

Kinesin-5 motors organize mitotic spindles by sliding apart microtubules. They are homotetramers with dimeric motor and tail domains at both ends of a bipolar minifilament. Here, we describe a regulatory mechanism involving direct binding between tail and motor domains and its fundamental role in microtubule sliding. Kinesin-5 tails decrease microtubule-stimulated ATP-hydrolysis by specifically engaging motor domains in the nucleotide-free or ADP states. Cryo-EM reveals that tail binding stabilizes an open motor domain ATP-active site. Full-length motors undergo slow motility and cluster together along microtubules, while tail-deleted motors exhibit rapid motility without clustering. The tail is critical for motors to zipper together two microtubules by generating substantial sliding forces. The tail is essential for mitotic spindle localization, which becomes severely reduced in tail-deleted motors. Our studies suggest a revised microtubule-sliding model, in which kinesin-5 tails stabilize motor domains in the microtubule-bound state by slowing ATP-binding, resulting in high-force production at both homotetramer ends.

Kinesin-5 motors organize mitotic spindles by sliding apart anti-parallel microtubules. They are homotetramers composed of two antiparallel dimers placing orthogonal motor and tail domains at opposite ends of a bipolar minifilament. Here, we describe a regulatory mechanism, involving direct binding of the tail to motor domain and reveal its fundamental role in microtubule sliding motility. Biochemical analyses reveal that the tail down-regulates microtubule-activated ATP hydrolysis by specifically engaging the motor in the nucleotide-free or ADP-bound states. Cryo-EM structures reveal that the tail stabilizes a unique conformation of the motor N-terminal subdomain opening its active site. Full-length kinesin-5 motors undergo slow motility and cluster together along microtubules, while tail-deleted motors exhibit rapid motility without clustering along microtubules. The tail is critical for motors to zipper together two microtubules by generating substantial forces within sliding zones. The tail domain is essential for kinesin-5 mitotic spindle localization in vivo, which becomes severely reduced when the tail is deleted. Our studies suggest a revised microtubule-sliding model, in which tail domains directly engage motor domains at both ends of kinesin-5 homotetramers enhancing stability of the dual microtubule-bound states leading to slow motility yet high force production.

G protein-coupled receptor kinases (GRKs) play an important role in the desensitization of G protein-mediated signaling of G protein-coupled receptors (GPCRs). The level of interest in mapping their phosphorylation sites has increased because recent studies suggest that the differential pattern of receptor phosphorylation has distinct biological consequences. In vitro phosphorylation experiments using well-controlled systems are useful for deciphering the complexity of these physiological reactions and understanding the targeted event. Here, we report on the phosphorylation of the class A GPCR neurotensin receptor 1 (NTSR1) by GRKs under defined experimental conditions afforded by nanodisc technology. Phosphorylation of NTSR1 by GRK2 was agonist-dependent, whereas phosphorylation by GRK5 occurred in an activation-independent manner. In addition, the negatively charged lipids in the immediate vicinity of NTSR1 directly affect phosphorylation by GRKs. Identification of phosphorylation sites in agonist-activated NTSR1 revealed that GRK2 and GRK5 target different residues located on the intracellular receptor elements. GRK2 phosphorylates only the C-terminal Ser residues, whereas GRK5 phosphorylates Ser and Thr residues located in intracellular loop 3 and the C-terminus. Interestingly, phosphorylation assays using a series of NTSR1 mutants show that GRK2 does not require acidic residues upstream of the phospho-acceptors for site-specific phosphorylation, in contrast to the β2-adrenergic and μ-opioid receptors. Differential phosphorylation of GPCRs by GRKs is thought to encode a particular signaling outcome, and our in vitro study revealed NTSR1 differential phosphorylation by GRK2 and GRK5.

Membrane lipids have been implicated to influence the activity of G protein-coupled receptors (GPCRs). Almost all of our knowledge on the role of lipids on GPCR and G protein function comes from work on the visual pigment rhodopsin and its G protein transducin, which reside in a highly specialized membrane environment. Thus insight gained from rhodopsin signaling may not be simply translated to other non-visual GPCRs. Here, we investigated the effect of lipid head group charges on the signal transduction properties of the class A GPCR neurotensin receptor 1 (NTS1) under defined experimental conditions, using self-assembled phospholipid nanodiscs prepared with the zwitter-ionic lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), the negatively charged 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1′-rac-glycerol) (POPG), or a POPC/POPG mixture. A combination of dynamic light scattering and sedimentation velocity showed that NTS1 was monomeric in POPC-, POPC/POPG- and POPG-nanodiscs. Binding of the agonist neurotensin to NTS1 occurred with similar affinities and was essentially unaffected by the phospholipid composition. In contrast, Gq protein coupling to NTS1 in various lipid nanodiscs was significantly different and the apparent affinity of Gαq and Gβ1γ1 to activated NTS1 increased with increasing POPG content. NTS1-catalyzed GDP/GTPγS nucleotide exchange at Gαq in the presence of Gβ1γ1 and neurotensin was crucially affected by the lipid type, with exchange rates higher by one or two orders of magnitude in POPC/POPG- and POPG-nanodiscs, respectively, compared to POPC-nanodiscs. Our data demonstrate that negatively charged lipids in the immediate vicinity of a non-visual GPCR modulate the G protein-coupling step.

We report the solution NMR structures of the N-domain of the Menkes protein (ATP7A) in the ATP-free and ATP-bound forms. The structures consist of a twisted antiparallel six-stranded beta-sheet flanked by two pairs of alpha-helices. A protein loop of 50 amino acids located between beta 3 and beta 4 is disordered and mobile on the subnanosecond time scale. ATP binds with an affinity constant of (1.2 +/- 0.1) x 10(4) m(-1) and exchanges with a rate of the order of 1 x 10(3) s(-1). The ATP-binding cavity is considerably affected by the presence of the ligand, resulting in a more compact conformation in the ATP-bound than in the ATP-free form. This structural variation is due to the movement of the alpha1-alpha2 and beta2-beta 3 loops, both of which are highly conserved in copper(I)-transporting P(IB)-type ATPases. The present structure reveals a characteristic binding mode of ATP within the protein scaffold of the copper(I)-transporting P(IB)-type ATPases with respect to the other P-type ATPases. In particular, the binding cavity contains mainly hydrophobic aliphatic residues, which are involved in van der Waal's interactions with the adenine ring of ATP, and a Glu side chain, which forms a crucial hydrogen bond to the amino group of ATP.

CooA is a CO-dependent transcriptional activator and transmits a CO-sensing signal to a DNA promoter that controls the expression of the genes responsible for CO metabolism. CooA contains a b-type heme as the active site for sensing CO. CO binding to the heme induces a conformational change that switches CooA from an inactive to an active DNA-binding form. Here, we report the crystal structure of an imidazole-bound form of CooA from Carboxydothermus hydrogenoformans (Ch-CooA). In the resting form, Ch-CooA has a six-coordinate ferrous heme with two endogenous axial ligands, the alpha-amino group of the N-terminal amino acid and a histidine residue. The N-terminal amino group of CooA that is coordinated to the heme iron is replaced by CO. This substitution presumably triggers a structural change leading to the active form. The crystal structure of Ch-CooA reveals that imidazole binds to the heme, which replaces the N terminus, as does CO. The dissociated N terminus is positioned approximately 16 A from the heme iron in the imidazole-bound form. In addition, the heme plane is rotated by 30 degrees about the normal of the porphyrin ring compared to that found in the inactive form of Rhodospirillum rubrum CooA. Even though the ligand exchange, imidazole-bound Ch-CooA remains in the inactive form for DNA binding. These results indicate that the release of the N terminus resulting from imidazole binding is not sufficient to activate CooA. The structure provides new insights into the structural changes required to achieve activation.

Nanodiscs are self-assembled discoidal phospholipid bilayers surrounded and stabilized by membrane scaffold proteins (MSPs), that have become a powerful and promising tool for the study of membrane proteins. Even though their reconstitution is highly regulated by the type of MSP and phospholipid input, a biophysical characterization leading to the determination of the stoichiometry of MSP, lipid and membrane protein is essential. This is important for biological studies, as the oligomeric state of membrane proteins often correlates with their functional activity. Typically combinations of several methods are applied using, for example, modified samples that incorporate fluorescent labels, along with procedures that result in nanodisc disassembly and lipid dissolution. To obtain a comprehensive understanding of the native properties of nanodiscs, modification-free analysis methods are required. In this work we provide a strategy, using a combination of dynamic light scattering and analytical ultracentrifugation, for the biophysical characterization of unmodified nanodiscs. In this manner we characterize the nanodisc preparation in terms of its overall polydispersity and characterize the hydrodynamically resolved nanodisc of interest in terms of its sedimentation coefficient, Stokes' radius and overall protein and lipid stoichiometry. Functional and biological applications are also discussed for the study of the membrane protein embedded in nanodiscs under defined experimental conditions.

CooA, a homodimeric haem-containing protein, is responsible for transcriptional regulation in response to carbon monoxide (CO). It has a b-type haem as a CO sensor. Upon binding CO to the haem, CooA binds promoter DNA and activates expression of genes for CO metabolism. CooA from Carboxydothermus hydrogenoformans has been overexpressed in Escherichia coli, purified and crystallized by the vapour-diffusion method. The crystal belongs to space group P2(1), with unit-cell parameters a = 61.8, b = 94.7, c = 92.8 angstroms, beta = 104.8 degrees. The native and anomalous difference Patterson maps indicated that two CooA dimers are contained in the asymmetric unit and are related by a translational symmetry almost parallel to the c axis.

The UV and visible resonance Raman spectra are reported for CooA from Rhodospirillum rubrum, which is a transcriptional regulator activated by growth in a CO atmosphere. CO binding to heme in its sensor domain causes rearrangement of its DNA-binding domain, allowing binding of DNA with a specific sequence. The sensor and DNA-binding domains are linked by a hinge region that follows a long C-helix. UV resonance Raman bands arising from Trp-110 in the C-helix revealed local movement around Trp-110 upon CO binding. The indole side chain of Trp-110, which is exposed to solvent in the CO-free ferrous state, becomes buried in the CO-bound state with a slight change in its orientation but maintains a hydrogen bond with a water molecule at the indole nitrogen. This is the first experimental data supporting a previously proposed model involving displacement of the C-helix and heme sliding. The UV resonance Raman spectra for the CooA-DNA complex indicated that binding of DNA to CooA induces a further displacement of the C-helix in the same direction during transition to the complete active conformation. The Fe-CO and C-O stretching bands showed frequency shifts upon DNA binding, but the Fe-His stretching band did not. Moreover, CO-geminate recombination was more efficient in the DNA-bound state. These results suggest that the C-helix displacement in the DNA-bound form causes the CO binding pocket to narrow and become more negative.

CooA is a CO-sensing transcriptional activator that contains a b-type heme as the active site for sensing its physiological effector, CO. In this study, the spectroscopic and redox properties of a new CooA homologue from Carboxydothermus hydrogenoformans (Ch-CooA) were studied. Spectroscopic and mutagenesis studies revealed that His-82 and the N-terminal alpha-amino group were the axial ligands of the Fe(III) and Fe(II) hemes in Ch-CooA and that the N-terminal alpha-amino group was replaced by CO upon CO binding. Two neutral ligands, His-82 and the N-terminal alpha-amino group, are coordinated to the Fe(III) heme in Ch-CooA, whereas two negatively charged ligands, a thiolate from Cys-75 and the nitrogen atom of the N-terminal Pro, are the axial ligands of the Fe(III) heme in Rr-CooA. The difference in the coordination structure of the Fe(III) heme resulted in a large positive shift of redox potentials of Ch-CooA compared with Rr-CooA. Comparing the properties of Ch-CooA and Rr-CooA demonstrates that the essential elements for CooA function will be: (i) the heme is six-coordinate in the Fe(III), Fe(II), and Fe(II)-CO forms; (ii) the N-terminal is coordinated to the heme as an axial ligand, and (iii) CO replaces the N-terminal bound to the heme upon CO binding.