A mechanism of platelet integrin αIIbß3 outside-in signaling through a novel integrin αIIb subunit-filamin-actin linkage.

The communication of talin-activated integrin αIIbß3 with the cytoskeleton (integrin outside-in signaling) is essential for platelet aggregation, wound healing, and hemostasis. Filamin, a large actin crosslinker and integrin binding partner critical for cell spreading and migration, is implicated as a key regulator of integrin outside-in signaling. However, the current dogma is that filamin, which stabilizes inactive αIIbß3, is displaced from αIIbß3 by talin to promote the integrin activation (inside-out signaling), and how filamin further functions remains unresolved. Here, we show that while associating with the inactive αIIbß3, filamin also associates with the talin-bound active αIIbß3 to mediate platelet spreading. Fluorescence resonance energy transfer-based analysis reveals that while associating with both αIIb and ß3 cytoplasmic tails (CTs) to maintain the inactive αIIbß3, filamin is spatiotemporally rearranged to associate with αIIb CT alone on activated αIIbß3. Consistently, confocal cell imaging indicates that integrin α CT-linked filamin gradually delocalizes from the ß CT-linked focal adhesion marker-vinculin likely because of the separation of integrin α/ß CTs occurring during integrin activation. High-resolution crystal and nuclear magnetic resonance structure determinations unravel that the activated integrin αIIb CT binds to filamin via a striking α-helix→ß-strand transition with a strengthened affinity that is dependent on the integrin-activating membrane environment containing enriched phosphatidylinositol 4,5-bisphosphate. These data suggest a novel integrin αIIb CT-filamin-actin linkage that promotes integrin outside-in signaling. Consistently, disruption of such linkage impairs the activation state of αIIbß3, phosphorylation of focal adhesion kinase/proto-oncogene tyrosine kinase Src, and cell migration. Together, our findings advance the fundamental understanding of integrin outside-in signaling with broad implications in blood physiology and pathology.

Determination of Heat Transfer Coefficient for Air-Atomized Water Spray Cooling and Its Application in Modeling of Thermomechanical Controlled Processing of Die Forgings.

The paper presents the method and results of determination of heat transfer coefficient for air-atomized water spray cooling with consideration of infrastructural factors of industrial cooling conveyor, such as effect of accelerated air. The established values of heat transfer coefficient were implemented into a numerical model of cooling line, with special definition of sprayers and the movement of the part subjected to quenching. After quantitative validation on selected samples, the obtained coefficients were used for the solution of the technological problem by means of localized cooling rate enhancement, which forms a case study confirming reliability of the established water spray heat transfer functions and suitability of the determined models for design of thermomechanical controlled processing of complex-geometry parts.

Cryo-EM structure of a human prion fibril with a hydrophobic, protease-resistant core.

Self-templating assemblies of the human prion protein are clinically associated with transmissible spongiform encephalopathies. Here we present the cryo-EM structure of a denaturant- and protease-resistant fibril formed in vitro spontaneously by a 9.7-kDa unglycosylated fragment of the human prion protein. This human prion fibril contains two protofilaments intertwined with screw symmetry and linked by a tightly packed hydrophobic interface. Each protofilament consists of an extended beta arch formed by residues 106 to 145 of the prion protein, a hydrophobic and highly fibrillogenic disease-associated segment. Such structures of prion polymorphs serve as blueprints on which to evaluate the potential impact of sequence variants on prion disease.

Amino acid function relates to its embedded protein microenvironment: A study on disulfide-bridged cystine.

In our previous study, we have shown that the microenvironments around conserved amino acids are also conserved in protein families (Bandyopadhyay and Mehler, Proteins 2008; 72:646-659). In this study, we have hypothesized that amino acids perform similar functions when embedded in a certain type of protein microenvironment. We have tested this hypothesis on the microenvironments around disulfide-bridged cysteines from high-resolution protein crystal structures. Although such cystines mainly play structural role in proteins, in certain enzymes they participate in catalysis and redox reactions. We have performed and report a functional annotation of enzymatically active cystines to their respective microenvironments. Three protein microenvironment clusters were identified: (i) buried-hydrophobic, (ii) exposed-hydrophilic, and (iii) buried-hydrophilic. The buried-hydrophobic cluster encompasses a small group of 22 redox-active cystines, mostly in alpha-helical conformations in a -C-x-x-C- motif from the Oxido-reductase enzyme class. All these cystines have high strain energy and near identical microenvironments. Most of the active cystines in hydrolase enzyme class belong to buried hydrophilic microenvironment cluster. In total there are 34 half-cystines detected in buried hydrophilic cluster from hydrolases, as a part of enzyme active site. Even within the buried hydrophilic cluster, there is clear separation of active half-cystines between surface exposed part of the protein and protein interior. Half-cystines toward the surface exposed region are higher in number compared to those in protein interior. Apart from cystines at the active sites of the enzymes, many more half-cystines were detected in buried hydrophilic cluster those are part of the microenvironment of enzyme active sites. However, no active half-cystines were detected in extremely hydrophilic microenvironment cluster, that is, exposed hydrophilic cluster, indicating that total exposure of cystine toward the solvent is not favored for enzymatic reactions. Although half-cystines in exposed-hydrophilic clusters occasionally stabilize enzyme active sites, as a part of their microenvironments. Analysis performed in this work revealed that cystines as a part of active sites in specific enzyme families or folds share very similar protein microenvironment regions, despite of their dissimilarity in protein sequences and position specific sequence conservations. Proteins 2016; 84:1576-1589. © 2016 Wiley Periodicals, Inc.

Conformational stability of mammalian prion protein amyloid fibrils is dictated by a packing polymorphism within the core region.

Mammalian prion strains are believed to arise from the propagation of distinct conformations of the misfolded prion protein PrP(Sc). One key operational parameter used to define differences between strains has been conformational stability of PrP(Sc) as defined by resistance to thermal and/or chemical denaturation. However, the structural basis of these stability differences is unknown. To bridge this gap, we have generated two strains of recombinant human prion protein amyloid fibrils that show dramatic differences in conformational stability and have characterized them by a number of biophysical methods. Backbone amide hydrogen/deuterium exchange experiments revealed that, in sharp contrast to previously studied strains of infectious amyloid formed from the yeast prion protein Sup35, differences in ß-sheet core size do not underlie differences in conformational stability between strains of mammalian prion protein amyloid. Instead, these stability differences appear to be dictated by distinct packing arrangements (i.e. steric zipper interfaces) within the amyloid core, as indicated by distinct x-ray fiber diffraction patterns and large strain-dependent differences in hydrogen/deuterium exchange kinetics for histidine side chains within the core region. Although this study was limited to synthetic prion protein amyloid fibrils, a similar structural basis for strain-dependent conformational stability may apply to brain-derived PrP(Sc), especially because large strain-specific differences in PrP(Sc) stability are often observed despite a similar size of the PrP(Sc) core region.

Crystal structure of a human prion protein fragment reveals a motif for oligomer formation.

The structural transition of the prion protein from α-helical- to ß-sheet-rich underlies its conversion into infectious and disease-associated isoforms. Here we describe the crystal structure of a fragment from human prion protein consisting of the disulfide-bond-linked portions of helices 2 and 3. Instead of forming a pair-of-sheets steric zipper structure characteristic of amyloid fibers, this fragment crystallized into a ß-sheet-rich assembly of hexameric oligomers. This study reveals a never before observed structural motif for ordered protein aggregates and suggests a possible mechanism for self-propagation of misfolded conformations by such nonamyloid oligomers.

A novel expression system for production of soluble prion proteins in E. coli.

Expression of eukaryotic proteins in Escherichia coli is challenging, especially when they contain disulfide bonds. Since the discovery of the prion protein (PrP) and its role in transmissible spongiform encephalopathies, the need to obtain large quantities of the recombinant protein for research purposes has been essential. Currently, production of recombinant PrP is achieved by refolding protocols. Here, we show that the co-expression of two different PrP with the human Quiescin Sulfhydryl OXidase (QSOX), a human chaperone with thiol/disulfide oxidase activity, in the cytoplasm of E. coli produces soluble recombinant PrP. The structural integrity of the soluble PrP has been confirmed by nuclear magnetic resonance spectroscopy, demonstrating that properly folded PrP can be easily expressed in bacteria. Furthermore, the soluble recombinant PrP produced with this method can be used for functional and structural studies.

Intermolecular alignment in Y145Stop human prion protein amyloid fibrils probed by solid-state NMR spectroscopy.

The Y145Stop mutant of human prion protein, huPrP23-144, has been linked to PrP cerebral amyloid angiopathy, an inherited amyloid disease, and also serves as a valuable in vitro model for investigating the molecular basis of amyloid strains. Prior studies of huPrP23-144 amyloid by magic-angle-spinning (MAS) solid-state NMR spectroscopy revealed a compact ß-rich amyloid core region near the C-terminus and an unstructured N-terminal domain. Here, with the focus on understanding the higher-order architecture of huPrP23-144 fibrils, we probed the intermolecular alignment of ß-strands within the amyloid core using MAS NMR techniques and fibrils formed from equimolar mixtures of (15)N-labeled protein and (13)C-huPrP23-144 prepared with [1,3-(13)C(2)] or [2-(13)C]glycerol. Numerous intermolecular correlations involving backbone atoms observed in 2D (15)N-(13)C spectra unequivocally suggest an overall parallel in-register alignment of the ß-sheet core. Additional experiments that report on intermolecular (15)N-(13)CO and (15)N-(13)Cα dipolar couplings yielded an estimated strand spacing that is within ∼10% of the distances of 4.7-4.8 Å typical for parallel ß-sheets.

Prion protein and its conformational conversion: a structural perspective.

The key molecular event in the pathogenesis of prion diseases is the conformational conversion of a cellular prion protein, PrP(C), into a misfolded form, PrP(Sc). In contrast to PrP(C) that is monomeric and α-helical, PrP(Sc) is oligomeric in nature and rich in ß-sheet structure. According to the "protein-only" model, PrP(Sc) itself represents the infectious prion agent responsible for transmissibility of prion disorders. While this model is supported by rapidly growing experimental data, detailed mechanistic and structural aspects of prion protein conversion remain enigmatic. In this chapter we describe recent advances in understanding biophysical and biochemical aspects of prion diseases, with a special focus on structural underpinnings of prion protein conversion, the structural basis of prion strains, and generation of prion infectivity in vitro from bacterially-expressed recombinant PrP.

Atomic structures suggest determinants of transmission barriers in mammalian prion disease.

Prion represents a unique class of pathogens devoid of nucleic acid. The deadly diseases transmitted by it between members of one species and, in certain instances, to members of other species present a public health concern. Transmissibility and the barriers to transmission between species have been suggested to arise from the degree to which a pathological protein conformation from an individual of one species can seed a pathological conformation in another species. However, this hypothesis has never been illustrated at an atomic level. Here we present three X-ray atomic structures of the same segment from human, mouse, and hamster PrP, which is critical for forming amyloid and confers species specificity in PrP seeding experiments. The structures reveal that different sequences encode different steric zippers and suggest that the degree of dissimilarity of these zipper structures gives rise to transmission barriers in prion disease, such as those that protect humans from acquiring bovine spongiform encephalopathy (BSE) and chronic wasting disease (CWD).

Crystallographic studies of prion protein (PrP) segments suggest how structural changes encoded by polymorphism at residue 129 modulate susceptibility to human prion disease.

A single nucleotide polymorphism (SNP) in codon 129 of the human prion gene, leading to a change from methionine to valine at residue 129 of prion protein (PrP), has been shown to be a determinant in the susceptibility to prion disease. However, the molecular basis of this effect remains unexplained. In the current study, we determined crystal structures of prion segments having either Met or Val at residue 129. These 6-residue segments of PrP centered on residue 129 are "steric zippers," pairs of interacting ß-sheets. Both structures of these "homozygous steric zippers" reveal direct intermolecular interactions between Met or Val in one sheet and the identical residue in the mating sheet. These two structures, plus a structure-based model of the heterozygous Met-Val steric zipper, suggest an explanation for the previously observed effects of this locus on prion disease susceptibility and progression.

Molecular mechanisms for protein-encoded inheritance.

In prion inheritance and transmission, strains are phenotypic variants encoded by protein 'conformations'. However, it is unclear how a protein conformation can be stable enough to endure transmission between cells or organisms. Here we describe new polymorphic crystal structures of segments of prion and other amyloid proteins, which offer two structural mechanisms for the encoding of prion strains. In packing polymorphism, prion strains are encoded by alternative packing arrangements (polymorphs) of beta-sheets formed by the same segment of a protein; in segmental polymorphism, prion strains are encoded by distinct beta-sheets built from different segments of a protein. Both forms of polymorphism can produce enduring conformations capable of encoding strains. These molecular mechanisms for transfer of protein-encoded information into prion strains share features with the familiar mechanism for transfer of nucleic acid-encoded information into microbial strains, including sequence specificity and recognition by noncovalent bonds.

Coupling between substituents as a function of cage structure: synthesis and valence ionized states of bridgehead disubstituted parent and hexafluorinated bicyclo[1.1.1]pentane derivatives C5X6Y2.

He(I) photoelectron spectroscopy was used to examine the valence-shell electronic structure of three new and seven previously known bicyclo[1.1.1]pentane derivatives, 1,3-Y2-C5X6 (for X = H, Y = H, Cl, Br, I, CN; for X = F, Y = H, Br, I, CN). A larger series (X = H or F, Y = H, F, Cl, Br, I, At, CN) has been studied computationally with the SAC-CI (symmetry-adapted cluster configuration interaction) method. The outer-valence ionization spectra calculated by the SAC-CI method, including spin-orbit interaction, reproduced the experimental photoelectron spectra well, and quantitative assignments are given. When the extent of effective through-cage interaction between the bridgehead halogen lone-pair orbitals was defined in the usual way by orbital-energy splitting, it was found to be larger than that mediated by other cages such as cubane, and was further enhanced by hexafluorination. The origin of the orbital-energy splitting is analyzed in terms of cage structure, and it is pointed out that its relation to the degree of interaction between the bridgehead substituents is not as simple as is often assumed.

Atomic structures of amyloid cross-beta spines reveal varied steric zippers.

Amyloid fibrils formed from different proteins, each associated with a particular disease, contain a common cross-beta spine. The atomic architecture of a spine, from the fibril-forming segment GNNQQNY of the yeast prion protein Sup35, was recently revealed by X-ray microcrystallography. It is a pair of beta-sheets, with the facing side chains of the two sheets interdigitated in a dry 'steric zipper'. Here we report some 30 other segments from fibril-forming proteins that form amyloid-like fibrils, microcrystals, or usually both. These include segments from the Alzheimer's amyloid-beta and tau proteins, the PrP prion protein, insulin, islet amyloid polypeptide (IAPP), lysozyme, myoglobin, alpha-synuclein and beta(2)-microglobulin, suggesting that common structural features are shared by amyloid diseases at the molecular level. Structures of 13 of these microcrystals all reveal steric zippers, but with variations that expand the range of atomic architectures for amyloid-like fibrils and offer an atomic-level hypothesis for the basis of prion strains.

Sizing large proteins and protein complexes by electrospray ionization mass spectrometry and ion mobility.

Mass spectrometry (MS) and ion mobility with electrospray ionization (ESI) have the capability to measure and detect large noncovalent protein-ligand and protein-protein complexes. Using an ion mobility method of gas-phase electrophoretic mobility molecular analysis (GEMMA), protein particles representing a range of sizes can be separated by their electrophoretic mobility in air. Highly charged particles produced from a protein complex solution using electrospray can be manipulated to produce singly charged ions, which can be separated and quantified by their electrophoretic mobility. Results from ESI-GEMMA analysis from our laboratory and others were compared with other experimental and theoretically determined parameters, such as molecular mass and cryoelectron microscopy and X-ray crystal structure dimensions. There is a strong correlation between the electrophoretic mobility diameter determined from GEMMA analysis and the molecular mass for protein complexes up to 12 MDa, including the 93 kDa enolase dimer, the 480 kDa ferritin 24-mer complex, the 4.6 MDa cowpea chlorotic mottle virus (CCMV), and the 9 MDa MVP-vault assembly. ESI-GEMMA is used to differentiate a number of similarly sized vault complexes that are composed of different N-terminal protein tags on the MVP subunit. The average effective density of the proteins and protein complexes studied was 0.6 g/cm(3). Moreover, there is evidence that proteins and protein complexes collapse or become more compact in the gas phase in the absence of water.

Regulation by oligomerization in a mycobacterial folate biosynthetic enzyme.

Folate derivatives are essential cofactors in the biosynthesis of purines, pyrimidines and amino acids across all forms of life. Mammals uptake folate from their diets, whereas most bacteria must synthesize folate de novo. Therefore, the enzymes in the folate biosynthetic pathway are attractive drug targets against bacterial pathogens such as Mycobacterium tuberculosis, the cause of the world's most deadly infectious disease, tuberculosis (TB). M.tuberculosis 7,8-dihydroneopterin aldolase (Mtb FolB, DHNA) is the second enzyme in the folate biosynthetic pathway, which catalyzes the conversion of 7,8-dihydroneopterin to 6-hydroxymethyl-7,8-dihydropterin and glycoaldehyde. The 1.6A X-ray crystal structure of Mtb FolB complexed with its product, 6-hydroxymethyl-7,8-dihydropterin, reveals an octameric assembly similar to that seen in crystal structures of other FolB homologs. However, the 2.5A crystal structure of unliganded Mtb FolB reveals a novel tetrameric oligomerization state, with only partially formed active sites. A substrate induced conformational change appears to be necessary to convert the inactive tetramer to the active octamer. Ultracentrifugation confirmed that in solution unliganded Mtb FolB is mainly tetrameric and upon addition of substrate FolB is predominantly octameric. Kinetic analysis of substrate binding gives a Hill coefficient of 2.0, indicating positive cooperativity. We hypothesize that Mtb FolB displays cooperativity in substrate binding to regulate the cellular concentration of 7,8-dihydroneopterin, so that it may function not only as a precursor to folate but also as an antioxidant for the survival of M.tuberculosis against host defenses.

Gram-positive DsbE proteins function differently from Gram-negative DsbE homologs. A structure to function analysis of DsbE from Mycobacterium tuberculosis.

Mycobacterium tuberculosis, a Gram-positive bacterium, encodes a secreted Dsb-like protein annotated as Mtb DsbE (Rv2878c, also known as MPT53). Because Dsb proteins in Escherichia coli and other bacteria seem to catalyze proper folding during protein secretion and because folding of secreted proteins is thought to be coupled to disulfide oxidoreduction, the function of Mtb DsbE may be to ensure that secreted proteins are in their correctly folded states. We have determined the crystal structure of Mtb DsbE to 1.1 A resolution, which reveals a thioredoxin-like domain with a typical CXXC active site. These cysteines are in their reduced state. Biochemical characterization of Mtb DsbE reveals that this disulfide oxidoreductase is an oxidant, unlike Gram-negative bacteria DsbE proteins, which have been shown to be weak reductants. In addition, the pK(a) value of the active site, solvent-exposed cysteine is approximately 2 pH units lower than that of Gram-negative DsbE homologs. Finally, the reduced form of Mtb DsbE is more stable than the oxidized form, and Mtb DsbE is able to oxidatively fold hirudin. Structural and biochemical analysis implies that Mtb DsbE functions differently from Gram-negative DsbE homologs, and we discuss its possible functional role in the bacterium.

Structural genomics of Mycobacterium tuberculosis: a preliminary report of progress at UCLA.

The growing list of fully sequenced genomes, combined with innovations in the fields of structural biology and bioinformatics, provides a synergy for the discovery of new drug targets. With this background, the TB Structural Genomics Consortium has been formed. This international consortium is comprised of laboratories from 31 universities and institutes in 13 countries. The goal of the consortium is to determine the structures of over 400 potential drug targets from the genome of Mycobacterium tuberculosis and analyze their structures in the context of functional information. We summarize the efforts of the UCLA consortium members. Potential drug targets were selected using a variety of bioinformatics methods and screened for certain physical and species-specific properties to yield a starting group of protein targets for structure determination. Target determination methods include protein phylogenetic profiles and Rosetta Stone methods, and the use of related biochemical pathways to select genes linked to essential prokaryotic genes. Criteria imposed on target selection included potential protein solubility, protein or domain size, and targets that lack homologs in eukaryotic organisms. In addition, some protein targets were chosen that are specific to M. tuberculosis, such as PE and PPE domains. Thus far, the UCLA group has cloned 263 targets, expressed 171 proteins and purified 40 proteins, which are currently in crystallization trials. Our efforts have yielded 13 crystals and eight structures. Seven structures are summarized here. Four of the structures are secreted proteins: antigen 85B; MPT 63, which is one of the three major secreted proteins of M. tuberculosis; a thioredoxin derivative Rv2878c; and potentially secreted glutamate synthetase. We also report the structures of three proteins that are potentially essential to the survival of M. tuberculosis: a protein involved in the folate biosynthetic pathway (Rv3607c); a protein involved in the biosynthesis of vitamin B5 (Rv3602c); and a pyrophosphatase, Rv2697c. Our approach to the M. tuberculosis structural genomics project will yield information for drug design and vaccine production against tuberculosis. In addition, this study will provide further insights into the mechanisms of mycobacterial pathogenesis.

The TB structural genomics consortium: providing a structural foundation for drug discovery.

Structural genomics, the large-scale determination of protein structures, promises to provide a broad structural foundation for drug discovery. The tuberculosis (TB) Structural Genomics Consortium is devoted to encouraging, coordinating, and facilitating the determination of structures of proteins from Mycobacterium tuberculosis and hopes to determine 400 TB protein structures over 5 years. The Consortium has determined structures of 28 proteins from TB to date. These protein structures are already providing a basis for drug discovery efforts.