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The ubiquitin E2 variant domain of TSG101 (TSG101‐UEV) plays a pivotal role in protein sorting and virus budding by recognizing PTAP motifs within ubiquitinated proteins. Disruption of TSG101‐UEV/PTAP interactions has emerged as a promising strategy for the development of host‐oriented broad‐spectrum antivirals with low susceptibility to resistance. TSG101 is a challenging target characterized by an extended and flat binding interface, low affinity for PTAP ligands, and complex binding energetics. Here, we assess the druggability of the TSG101‐UEV/PTAP binding interface by searching for drug‐like inhibitors and evaluating their ability to block PTAP recognition, impair budding, and inhibit viral proliferation. A discovery workflow was established by combining in vitro miniaturized HTS assays and a set of cell‐based activity assays including high‐content bimolecular complementation, virus‐like particle release measurement, and antiviral testing in live virus infection. This approach has allowed us to identify a set of chemically diverse molecules that block TSG101‐UEV/PTAP binding with IC50s in the low μM range and are able to disrupt the interaction between full‐length TSG101 and viral proteins in human cells and inhibit viral replication. State‐of‐the‐art molecular docking studies reveal that the active compounds exploit binding hotspots at the PTAP binding site, unlocking the full binding potential of the TSG101‐UEV binding pockets. These inhibitors represent promising hits for the development of novel broad‐spectrum antivirals through targeted optimization and are also valuable tools for investigating the involvement of ESCRT in the proliferation of different virus families and study the secondary effects induced by the disruption of ESCRT/virus interactions. [ABSTRACT FROM AUTHOR]

Sodium–calcium exchanger (NCX) proteins are ubiquitously expressed and play a pivotal role in cellular calcium homeostasis by mediating uphill calcium efflux across the cell membrane. Intracellular calcium allosterically regulates the exchange activity by binding to two cytoplasmic calcium‐binding domains, CBD1 and CBD2. However, the calcium‐binding affinities of these domains are seemingly inadequate to sense physiological calcium oscillations. Previously, magnesium binding to either domain was shown to tune their affinity for calcium, bringing it into the physiological range. However, while the magnesium‐binding site of CBD2 was identified, the identity of the CBD1 magnesium site remains elusive. Here, using molecular dynamics in combination with differential scanning fluorimetry and mutational analysis, we pinpoint the magnesium‐binding site in CBD1. Specifically, among four calcium‐binding sites (Ca1–Ca4) in this domain, only Ca1 can accommodate magnesium with an affinity similar to its free intracellular concentration. Moreover, our results provide mechanistic insights into the modulation of the regulatory calcium affinity by magnesium, which allows an adequate NCX activity level throughout varying physiological needs. [ABSTRACT FROM AUTHOR]

Lipopolysaccharide (LPS) synthesis in Gram‐negative bacteria is completed at the outer leaflet of the inner membrane (IM). Following synthesis, seven LPS transport (Lpt) proteins facilitate the movement of LPS to the outer membrane (OM), an essential process that if disrupted at any stage has lethal effects on bacterial viability. LptB2FG, the IM component of the Lpt bridge system, is a type VI ABC transporter that provides the driving force for LPS extraction from the IM and subsequent transport across a stable protein bridge to the outer leaflet of the OM. LptC is a periplasmic protein anchored to the IM by a single transmembrane (TM) helix intercalating within the lateral gate formed by LptF TM5 and LptG TM1. LptC facilitates the hand‐off of LPS from LptB2FG to the periplasmic protein LptA and has been shown to regulate the ATPase activity of LptB2FG. Here, using an engineered chromosomal knockout system in Escherichia coli to assess the effects of LptC mutations in vivo, we identified six partial loss of function LptC mutations in the first unbiased alanine screen of this essential protein. To investigate the functional effects of these mutations, nanoDSF (differential scanning fluorimetry) and site‐directed spin labeling (SDSL) electron paramagnetic resonance (EPR) spectroscopy in combination with an in vitro ATPase assay show that specific residues in the TM helix of LptC destabilize the LptB2FGC complex and regulate the ATPase activity of LptB. [ABSTRACT FROM AUTHOR]

The characterization of protein stability is essential for understanding the functions of proteins. Hydroxysteroid dehydrogenase is involved in the biosynthesis of steroid hormones and the detoxification of xenobiotic carbonyl compounds. However, the stability of hydroxysteroid dehydrogenases has not yet been characterized in detail. Here, we determined the changes in Gibbs free energy, enthalpy, entropy, and heat capacity of unfolding for 3α‐hydroxysteroid dehydrogenase/carbonyl reductase (3α‐HSD/CR) by varying the pH and urea concentration through differential scanning fluorimetry and presented pH‐dependent protein stability as a function of temperature. 3α‐HSD/CR shows the maximum stability of 30.79 kJ mol−1 at 26.4°C, pH 7.6 and decreases to 7.74 kJ mol−1 at 25.7°C, pH 4.5. The change of heat capacity of 30.25 ± 1.38 kJ mol−1 K−1 is obtained from the enthalpy of denaturation as a function of melting temperature at varied pH. Two proton uptakes are linked to protein unfolding from residues with differential pKa of 4.0 and 6.5 in the native and denatured states, respectively. The large positive heat capacity change indicated that hydrophobic interactions played an important role in the folding of 3α‐HSD/CR. These studies reveal the mechanism of protein unfolding in HSD and provide a convenient method to extract thermodynamic parameters for characterizing protein stability using differential scanning fluorimetry. [ABSTRACT FROM AUTHOR]

The high‐temperature requirement A (HtrA) serine protease family presents an attractive target class for antibacterial therapeutics development. These proteins possess dual protease and chaperone functions and contain numerous binding sites and regulatory loops, displaying diverse oligomerization patterns dependent on substrate type and occupancy. HtrA proteins that are natively purified coelute with contaminating peptides and activating species, shifting oligomerization and protein structure to differently activated populations. Here, a redesigned HtrA production results in cleaner preparations with high yields by overexpressing and purifying target protein from inclusion bodies under denaturing conditions, followed by a high‐throughput screen for optimal refolding buffer composition using function‐agnostic biophysical techniques that do not rely on target‐specific measurements. We use Borrelia burgdorferi HtrA to demonstrate the effectiveness of our function‐agnostic approach, while characterization with both new and established biophysical methods shows the retention of proteolytic and chaperone activity of the refolded protein. This systematic workflow and toolset will translate to the production of HtrA‐family proteins in higher quantities of pure and monodisperse composition than the current literature standard, with applicability to a broad array of protein purification strategies. [ABSTRACT FROM AUTHOR]

The stability and aggregation of NIST monoclonal antibody (NISTmAb) were investigated by hydrogen/deuterium exchange mass spectrometry (HDX‐MS), differential scanning calorimetry (DSC), and nano‐differential scanning fluorimetry (nanoDSF). NISTmAb was prepared in eight formulations at four different pHs (pH 5, 6, 7, and 8) in the presence and absence of 150 mM NaCl and analyzed by the three methods. The HDX‐MS results showed that NISTmAb is more conformationally stable at a pH near its isoelectric point (pI) in the presence of NaCl than a pH far from its pI in the absence of NaCl. The stabilization effects were global and not localized. The midpoint temperature of protein thermal unfolding transition results also showed the CH2 domain of the protein is more conformationally stable at a pH near its pI. On the other hand, the onset of aggregation temperature results showed that NISTmAb is less prone to aggregate at a pH far from its pI, particularly in the absence of NaCl. These seemingly contradicting results, higher conformational stability yet higher aggregation propensity near the pI than far away from the pI, can be explained by intramolecular and intermolecular electrostatic repulsion using Lumry‐Eyring model, which separates folding/unfolding equilibrium and aggregation event. The further a pH from the pI, the higher the net charge of the protein. The higher net charge leads to greater intramolecular and intermolecular electrostatic repulsions. The greater intramolecular electrostatic repulsion destabilizes the protein and the greater intermolecular electrostatic repulsion prevents aggregation of the protein molecules at pH far from the pI. [ABSTRACT FROM AUTHOR]

Differential scanning fluorimetry (DSF) is a widely used thermal shift assay for measuring protein stability and protein–ligand interactions that are simple, cheap, and amenable to high throughput. However, data analysis remains a challenge, requiring improved methods. Here, the program SimpleDSFviewer, a user‐friendly interface, is described to help the researchers who apply DSF technique in their studies. SimpleDSFviewer integrates melting curve (MC) normalization, smoothing, and melting temperature (Tm) analysis and directly previews analyzed data, providing an efficient analysis tool for DSF. SimpleDSFviewer is developed in Matlab, and it is freely available for all users to use in Matlab workspace or with Matlab Runtime. It is easy to use and an efficient tool for researchers to preview and analyze their data in a very short time. [ABSTRACT FROM AUTHOR]

Biophysical and biochemical studies on the lipoproteins and other periplasmic proteins from the spirochetal species Treponema pallidum have yielded numerous insights into the functioning of the organism's peculiar membrane organization, its nutritional requirements, and intermediary metabolism. However, not all T. pallidum proteins have proven to be amenable to biophysical studies. One such recalcitrant protein is Tp0309, a putative polar‐amino‐acid‐binding protein of an ABC transporter system. To gain further information on its possible function, a homolog of the protein from the related species T. vincentii was used as a surrogate. This protein, Tv2483, was crystallized, resulting in the determination of its crystal structure at a resolution of 1.75 Å. The protein has a typical fold for a ligand‐binding protein, and a single molecule of l‐arginine was bound between its two lobes. Differential scanning fluorimetry and isothermal titration calorimetry experiments confirmed that l‐arginine bound to the protein with unusually high selectivity. However, further comparison to Tp0309 showed differences in key amino‐acid‐binding residues may impart an alternate specificity for the T. pallidum protein. PDB Code(s): 6DET [ABSTRACT FROM AUTHOR]

Calcium ions are required for proper function of a wide spectrum of proteins within cells. X‐ray crystallography of human glutamate carboxypeptidase II (GCPII) revealed the presence of a Ca2+‐binding site, but its importance for the structure and function of this metallopeptidase has not been elucidated to date. Here, we prepared a panel of mutants targeting residues that form the Ca2+ coordination sphere of GCPII and analyzed their structural and enzymatic properties using an array of complementary biophysical and biochemical approaches. Our data unequivocally show that even a slight disruption of the Ca2+‐binding site destabilizes the three‐dimensional fold of GCPII and is associated with impaired secretion, a high propensity to form nonphysiological oligomers, and an inability to bind active site‐targeted ligands. Additionally, the Ca2+‐binding site is critical for maintenance of the native homodimeric quaternary arrangement of GCPII, which is indispensable for its enzymatic activity. Overall, our results offer a clear picture of the importance of Ca2+ for the structural integrity and hydrolytic activity of human GCPII and by extension homologous members of the M28 zinc‐dependent metallopeptidase family. [ABSTRACT FROM AUTHOR]

The spirochete Treponema pallidum is the causative agent of syphilis, a sexually transmitted infection of major global importance. Other closely related subspecies of Treponema also are the etiological agents of the endemic treponematoses, such as yaws, pinta, and bejel. The inability of T. pallidum and its close relatives to be cultured in vitro has prompted efforts to characterize T. pallidum's proteins structurally and biophysically, particularly those potentially relevant to treponemal membrane biology, with the goal of possibly revealing the functions of those proteins. This report describes the structure of the treponemal protein Tp0737; this polypeptide has a fold characteristic of a class of periplasmic ligand-binding proteins associated with ABC-type transporters. Although no ligand for the protein was observed in electron-density maps, and thus the nature of the native ligand remains obscure, the structural data described herein provide a foundation for further efforts to elucidate the ligand and thus the function of this protein in T. pallidum. [ABSTRACT FROM AUTHOR]