Optimization of quenched fluorescent peptide substrates of SARS-CoV-2 3CLpro main protease (Mpro) from proteomic identification of P6-P6' active site specificity.

SARS-CoV-2 3C-like main protease (3CLpro) is essential for protein excision from the viral polyprotein. 3CLpro inhibitor drug development to block SARS-CoV-2 replication focuses on the catalytic non-prime (P) side for specificity and potency, but the importance of the prime (P') side in substrate specificity and for drug development remains underappreciated. We determined the P6-P6' specificity for 3CLpro from >800 cleavage sites that we identified using Proteomic Identification of Cleavage site Specificity (PICS). Cleavage occurred after the canonical P1-Gln and non-canonical P1-His and P1-Met residues. Moreover, P3 showed a preference for Arg/Lys and P3' for His. Essential H-bonds between the N-terminal Ser1 of protomer-B in 3CLpro dimers form with P1-His, but not with P1-Met. Nonetheless, cleavage occurs at P1-Met456 in native MAP4K5. Elevated reactive oxygen species in SARS-CoV-2 infection oxidize methionines. Molecular simulations revealed P1-MetOX forms an H-bond with Ser1 and notably, strong positive cooperativity between P1-Met with P3'-His was revealed, which enhanced peptide-cleavage rates. The highly plastic S3' subsite accommodates P3'-His that displays stabilizing backbone H-bonds with Thr25 lying central in a "'threonine trio" (Thr24-Thr25-Thr26) in the P'-binding domain I. Molecular docking simulations unveiled structure-activity relationships impacting 3CLpro-substrate interactions, and the role of these structural determinants was confirmed by MALDI-TOF-MS cleavage assays of P1'- and P3'-positional scanning peptide libraries carrying a 2nd optimal cut-site as an internal positive control. These data informed the design of two new and highly soluble 3CLproquenched-fluorescent peptide substrates for improved FRET monitoring of 3CLpro activity with 15× improved sensitivity over current assays.IMPORTANCEFrom global proteomics identification of >800 cleavage sites, we characterized the P6-P6' active site specificity of SARS-CoV-2 3CLpro using proteome-derived peptide library screens, molecular modeling simulations, and focussed positional peptide libraries. In P1', we show that alanine and serine are cleaved 3× faster than glycine and the hydrophobic small amino acids Leu, Ile, or Val prevent cleavage of otherwise optimal non-prime sequences. In characterizing non-canonical non-prime P1 specificity, we explored the unusual P1-Met specificity, discovering enhanced cleavage when in the oxidized state (P1-MetOX). We unveiled unexpected amino acid cooperativity at P1-Met with P3'-His and noncanonical P1-His with P2-Phe, and the importance of the threonine trio (Thr24-Thr25-Thr26) in the prime side binding domain I in defining prime side binding in SARS-CoV-2 3CLpro. From these analyses, we rationally designed quenched-fluorescence natural amino acid peptide substrates with >15× improved sensitivity and high peptide solubility, facilitating handling and application for screening of new antiviral drugs.

Mechanistic insights into COVID-19 by global analysis of the SARS-CoV-2 3CLpro substrate degradome.

The main viral protease (3CLpro) is indispensable for SARS-CoV-2 replication. We delineate the human protein substrate landscape of 3CLpro by TAILS substrate-targeted N-terminomics. We identify more than 100 substrates in human lung and kidney cells supported by analyses of SARS-CoV-2-infected cells. Enzyme kinetics and molecular docking simulations of 3CLpro engaging substrates reveal how noncanonical cleavage sites, which diverge from SARS-CoV, guide substrate specificity. Cleaving the interactors of essential effector proteins, effectively stranding them from their binding partners, amplifies the consequences of proteolysis. We show that 3CLpro targets the Hippo pathway, including inactivation of MAP4K5, and key effectors of transcription, mRNA processing, and translation. We demonstrate that Spike glycoprotein directly binds galectin-8, with galectin-8 cleavage disengaging CALCOCO2/NDP52 to decouple antiviral-autophagy. Indeed, in post-mortem COVID-19 lung samples, NDP52 rarely colocalizes with galectin-8, unlike in healthy lungs. The 3CLpro substrate degradome establishes a foundational substrate atlas to accelerate exploration of SARS-CoV-2 pathology and drug design.

DIPPER, a spatiotemporal proteomics atlas of human intervertebral discs for exploring ageing and degeneration dynamics.

The spatiotemporal proteome of the intervertebral disc (IVD) underpins its integrity and function. We present DIPPER, a deep and comprehensive IVD proteomic resource comprising 94 genome-wide profiles from 17 individuals. To begin with, protein modules defining key directional trends spanning the lateral and anteroposterior axes were derived from high-resolution spatial proteomes of intact young cadaveric lumbar IVDs. They revealed novel region-specific profiles of regulatory activities and displayed potential paths of deconstruction in the level- and location-matched aged cadaveric discs. Machine learning methods predicted a 'hydration matrisome' that connects extracellular matrix with MRI intensity. Importantly, the static proteome used as point-references can be integrated with dynamic proteome (SILAC/degradome) and transcriptome data from multiple clinical samples, enhancing robustness and clinical relevance. The data, findings, and methodology, available on a web interface (http://www.sbms.hku.hk/dclab/DIPPER/), will be valuable references in the field of IVD biology and proteomic analytics.

The backbone of vertebrate animals consists of a series of bones called vertebrae that are joined together by disc-like structures that allow the back to move and distribute forces to protect it during daily activities. It is common for these intervertebral discs to degenerate with age, resulting in back pain and severely reducing quality of life. The mechanical features of intervertebral discs are the result of their proteins. These include extracellular matrix proteins, which form the external scaffolding that binds cells together in a tissue, and signaling proteins, which allow cells to communicate. However, how the levels of different proteins in each region of the disc vary with time has not been fully examined. To establish how protein composition changes with age, Tam, Chen et al. quantified the protein levels and gene activity (which leads to protein production) of intervertebral discs from young and old deceased individuals. They found that the position of different mixtures of proteins in the intervertebral disc changes with age, and that young people have high levels of extracellular matrix proteins and signaling proteins. Levels of these proteins decreased as people got older, as did the amount of proteins produced. To determine which region of the intervertebral disc different proteins were in, Tam, Chen et al. also performed magnetic resonance imaging (MRI) of the samples to correlate image intensity (which represents water content) with the corresponding protein signature. The data obtained provides a high-quality map of how the location of different proteins changes with age, and is available online under the name DIPPER. This database is an informative resource for research into skeletal biology, and it will likely advance the understanding of intervertebral disc degeneration in humans and animals, potentially leading to the development of new treatment strategies for this condition.

Proteomics of Campylobacter jejuni Growth in Deoxycholate Reveals Cj0025c as a Cystine Transport Protein Required for Wild-type Human Infection Phenotypes.

Campylobacter jejuni is a major cause of food-borne gastroenteritis. Proteomics by label-based two-dimensional liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) identified proteins associated with growth in 0.1% sodium deoxycholate (DOC, a component of gut bile salts), and system-wide validation was performed by data-independent acquisition (DIA-SWATH-MS). LC-MS/MS quantified 1326 proteins (∼82% of the predicted C. jejuni proteome), of which 1104 were validated in additional biological replicates by DIA-SWATH-MS. DOC resulted in a profound proteome shift with 512 proteins showing significantly altered abundance. Induced proteins were associated with flagellar motility and antibiotic resistance; and these correlated with increased DOC motility and resistance to polymyxin B and ciprofloxacin. DOC also increased human Caco-2 cell adherence and invasion. Abundances of proteins involved in nutrient transport were altered by DOC and aligned with intracellular changes to their respective carbon sources. DOC increased intracellular levels of sulfur-containing amino acids (cysteine and methionine) and the dipeptide cystine (Cys-Cys), which also correlated with reduced resistance to oxidative stress. A DOC induced transport protein was Cj0025c, which has sequence similarity to bacterial Cys-Cys transporters. Deletion of cj0025c (Δcj0025c) resulted in proteome changes consistent with sulfur starvation, as well as attenuated invasion, reduced motility, atypical morphology, increased antimicrobial susceptibility and poor biofilm formation. Targeted metabolomics showed Δcj0025c could use known C. jejuni amino and organic acid substrates commensurate with wild-type. Medium Cys-Cys levels however, were maintained in Δcj0025c relative to wild-type. A toxic Cys-Cys mimic (selenocystine) inhibited wild-type growth, but not Δcj0025c Provision of an alternate sulfur source (2 mm thiosulfate) restored Δcj0025c motility. Our data confirm that Cj0025c is a Cys-Cys transporter that we have named TcyP consistent with the nomenclature of homologous proteins in other species.

Identifying the targets and functions of N-linked protein glycosylation in Campylobacter jejuni.

Campylobacter jejuni is a major cause of bacterial gastroenteritis in humans that is primarily associated with the consumption of inadequately prepared poultry products, since the organism is generally thought to be asymptomatic in avian species. Unlike many other microorganisms, C. jejuni is capable of performing extensive post-translational modification (PTM) of proteins by N- and O-linked glycosylation, both of which are required for optimal chicken colonization and human virulence. The biosynthesis and attachment of N-glycans to C. jejuni proteins is encoded by the pgl (protein glycosylation) locus, with the PglB oligosaccharyltransferase (OST) enabling en bloc transfer of a heptasaccharide N-glycan from a lipid carrier in the inner membrane to proteins exposed within the periplasm. Seventy-eight C. jejuni glycoproteins (represented by 134 sites of experimentally verified N-glycosylation) have now been identified, and include inner and outer membrane proteins, periplasmic proteins and lipoproteins, which are generally of poorly defined or unknown function. Despite our extensive knowledge of the targets of this apparently widespread process, we still do not fully understand the role N-glycosylation plays biologically, although several phenotypes, including wild-type stress resistance, biofilm formation, motility and chemotaxis have been related to a functional pgl system. Recent work has described enzymatic processes (nitrate reductase NapAB) and antibiotic efflux (CmeABC) as major targets requiring N-glycan attachment for optimal function, and experimental evidence also points to roles in cell binding via glycan-glycan interactions, protein complex formation and protein stability by conferring protection against host and bacterial proteolytic activity. Here we examine the biochemistry of the N-linked glycosylation system, define its currently known protein targets and discuss evidence for the structural and functional roles of this PTM in individual proteins and globally in C. jejuni pathogenesis.

Moonlighting matrix metalloproteinase substrates: Enhancement of proinflammatory functions of extracellular tyrosyl-tRNA synthetase upon cleavage.

Tyrosyl-tRNA synthetase ligates tyrosine to its cognate tRNA in the cytoplasm, but it can also be secreted through a noncanonical pathway. We found that extracellular tyrosyl-tRNA synthetase (YRS) exhibited proinflammatory activities. In addition to acting as a monocyte/macrophage chemoattractant, YRS initiated signaling through Toll-like receptor 2 (TLR2) resulting in NF-κB activation and release of tumor necrosis factor α (TNFα) and multiple chemokines, including MIP-1α/ß, CXCL8 (IL8), and CXCL1 (KC) from THP1 monocyte and peripheral blood mononuclear cell-derived macrophages. Furthermore, YRS up-regulated matrix metalloproteinase (MMP) activity in a TNFα-dependent manner in M0 macrophages. Because MMPs process a variety of intracellular proteins that also exhibit extracellular moonlighting functions, we profiled 10 MMPs for YRS cleavage and identified 55 cleavage sites by amino-terminal oriented mass spectrometry of substrates (ATOMS) positional proteomics and Edman degradation. Stable proteoforms resulted from cleavages near the start of the YRS C-terminal EMAPII domain. All of the MMPs tested cleaved at ADS386↓387LYV and VSG405↓406LVQ, generating 43- and 45-kDa fragments. The highest catalytic efficiency for YRS was demonstrated by MMP7, which is highly expressed by monocytes and macrophages, and by neutrophil-specific MMP8. MMP-cleaved YRS enhanced TLR2 signaling, increased TNFα secretion from macrophages, and amplified monocyte/macrophage chemotaxis compared with unprocessed YRS. The cleavage of YRS by MMP8, but not MMP7, was inhibited by tyrosine, a substrate of the YRS aminoacylation reaction. Overall, the proinflammatory activity of YRS is enhanced by MMP cleavage, which we suggest forms a feed-forward mechanism to promote inflammation.

Proteomic and N-Terminomic TAILS Analyses of Human Alveolar Bone Proteins: Improved Protein Extraction Methodology and LysargiNase Digestion Strategies Increase Proteome Coverage and Missing Protein Identification.

With 2129 proteins still classified by the Human Proteome Organisation Human Proteome Project (HPP) as "missing" without compelling evidence of protein existence (PE) in humans, we hypothesized that in-depth proteomic characterization of tissues that are technically challenging to access and extract would yield evidence for tissue-specific missing proteins. Paradoxically, although the skeleton is the most massive tissue system in humans, as one of the poorest characterized by proteomics, bone falls under the HPP umbrella term as a "rare tissue". Therefore, we aimed to optimize mineralized tissue protein extraction methodology and workflows for proteomic and data analyses of small quantities of healthy young adult human alveolar bone. Osteoid was solubilized by GuHCl extraction, with hydroxyapatite-bound proteins then released by ethylenediaminetetraacetic acid demineralization. A subsequent GuHCl solubilization extraction was followed by solid-phase digestion of the remaining insoluble cross-linked protein using trypsin and then 6 M urea dissolution incorporating LysC digestion. Bone extracts were digested in parallel using trypsin, LysargiNase, AspN, or GluC prior to liquid chromatography-mass spectrometry analysis. Terminal Amine Isotopic Labeling of Substrates was used to purify semitryptic peptides, identifying natural and proteolytic-cleaved neo N-termini of bone proteins. Our strategy enabled complete solubilization of the organic bone matrix leading to extensive categorization of bone proteins in different bone matrix extracts, and hence matrix compartments, for the first time. Moreover, this led to the high confidence identification of pannexin-3, a "missing protein", found only in the insoluble collagenous matrix and revealed for the first time by trypsin solid-phase digestion. We also found a singleton proteotypic peptide of another missing protein, meiosis inhibitor protein 1. We also identified 17 proteins classified in neXtprot as PE1 based on evidence other than from MS, termed non-MS PE1 proteins, including ≥9-mer proteotypic peptides of four proteins.

Simplified high yield TAILS terminomics using a new HPG-ALD 800K-2000 polymer with precipitation.

Identification of the N terminus of proteins in complex samples has been accelerated in recent years by the increased power of mass spectrometers, bioinformatic tools and biochemical methods. We developed Terminal Amine Isotopic Labeling of Substrates (TAILS), a powerful method for the isolation, identification and quantification of N terminal peptides comprising the N terminome from proteome in vitro analyses, cultured cells and tissues. TAILS depletes internal tryptic peptides so enriching for the natural and cleaved neo N-terminal peptides present in complex proteome samples. To do so N-terminal peptides are blocked at the protein level by isotopically labeled amine reactive reagents, then digested with trypsin. TAILS relies on a water-soluble 100-kDa highly-branched polyglycerol polymer (HPG-ALD) developed in our laboratories to bind and deplete the sample of tryptic peptides. Polymer-bound internal tryptic peptides are separated from the blocked N-terminal peptides by spin filters. Here we describe a revised TAILS protocol that uses a newly developed 800-kDa HPG-ALD polymer that allows precipitation of the polymer post-depletion. Precipitation has multiple advantages over spin filters including maximal N-terminal peptide recovery and faster high-throughput N-terminome identification and quantification in any sample type.

Matrix metalloproteinases inactivate the proinflammatory functions of secreted moonlighting tryptophanyl-tRNA synthetase.

Tryptophanyl-tRNA synthetase (WRS) is a cytosolic aminoacyl-tRNA synthetase essential for protein synthesis. WRS is also one of a growing number of intracellular proteins that are attributed distinct noncanonical "moonlighting" functions in the extracellular milieu. Moonlighting aminoacyl-tRNA synthetases regulate processes such as inflammation, but how these multifunctional enzymes are themselves regulated remains unclear. Here, we demonstrate that WRS is secreted from human macrophages, fibroblasts, and endothelial cells in response to the proinflammatory cytokine interferon γ (IFNγ). WRS signaled primarily through Toll-like receptor 2 (TLR2) in macrophages, leading to phosphorylation of the p65 subunit of NF-κB with associated loss of NF-κB inhibitor α (IκB-α) protein. This signaling initiated secretion of tumor necrosis factor α (TNFα) and CXCL8 (IL8) from macrophages. We also demonstrated that WRS is a potent monocyte chemoattractant. Of note, WRS increased matrix metalloproteinase (MMP) activity in the conditioned medium of macrophages in a TNFα-dependent manner. Using purified recombinant proteins and LC-MS/MS to identify proteolytic cleavage sites, we demonstrated that multiple MMPs, but primarily macrophage MMP7 and neutrophil MMP8, cleave secreted WRS at several sites. Loss of the WHEP domain following cleavage at Met48 generated a WRS proteoform that also results from alternative splicing, designated Δ1-47 WRS. The MMP-cleaved WRS lacked TLR signaling and proinflammatory activities. Thus, our results suggest that moonlighting WRS promotes IFNγ proinflammatory activities, and these responses can be dampened by MMPs.

TAILS proteomics reveals dynamic changes in airway proteolysis controlling protease activity and innate immunity during COPD exacerbations.

Dysregulated protease activity is thought to cause parenchymal and airway damage in chronic obstructive pulmonary disease (COPD). Multiple proteases have been implicated in COPD, and identifying their substrates may reveal new disease mechanisms and treatments. However, as proteases interact with many substrates that may be protease inhibitors or proteases themselves, these webs of protease interactions make the wider consequences of therapeutically targeting proteases difficult to predict. We therefore used a systems approach to determine protease substrates and protease activity in COPD airways. Protease substrates were determined by proteomics using the terminal amine isotopic labeling of substrates (TAILS) methodology in paired sputum samples during stable COPD and exacerbations. Protease activity and specific protein degradation in airway samples were assessed using Western blotting, substrate assays, and ex vivo cleavage assays. Two hundred ninety-nine proteins were identified in human COPD sputum, 125 of which were proteolytically processed, including proteases, protease inhibitors, mucins, defensins, and complement and other innate immune proteins. During exacerbations, airway neutrophils and neutrophil proteases increased and more proteins were cleaved, particularly at multiple sites, consistent with degradation and inactivation. During exacerbations, different substrates were processed, including protease inhibitors, mucins, and complement proteins. Exacerbations were associated with increasing airway elastase activity and increased processing of specific elastase substrates, including secretory leukocyte protease inhibitor. Proteolysis regulates multiple processes including elastase activity and innate immune proteins in COPD airways and differs during stable disease and exacerbations. The complexity of protease, inhibitor, and substrate networks makes the effect of protease inhibitors hard to predict which should be used cautiously.

C-terminal truncation of IFN-γ inhibits proinflammatory macrophage responses and is deficient in autoimmune disease.

Controlled macrophage differentiation and activation in the initiation and resolution of inflammation is crucial for averting progression to chronic inflammatory and autoimmune diseases. Here we show a negative feedback mechanism for proinflammatory IFN-γ activation of macrophages driven by macrophage-associated matrix metalloproteinase 12 (MMP12). Through C-terminal truncation of IFN-γ at 135Glu↓Leu136 the IFN-γ receptor-binding site was efficiently removed thereby reducing JAK-STAT1 signaling and IFN-γ activation of proinflammatory macrophages. In acute peritonitis this signature was absent in Mmp12 -/- mice and recapitulated in Mmp12 +/+ mice treated with a MMP12-specific inhibitor. Similarly, loss-of-MMP12 increases IFN-γ-dependent proinflammatory markers and iNOS+/MHC class II+ macrophage accumulation with worse lymphadenopathy, arthritic synovitis and lupus glomerulonephritis. In active human systemic lupus erythematosus, MMP12 levels were lower and IFN-γ higher compared to treated patients or healthy individuals. Hence, macrophage proteolytic truncation of IFN-γ attenuates classical activation of macrophages as a prelude for resolving inflammation.

Granzyme B is elevated in autoimmune blistering diseases and cleaves key anchoring proteins of the dermal-epidermal junction.

In healthy skin, epidermis and dermis are anchored together at the dermal-epidermal junction (DEJ), a specialized basement membrane pivotal for skin integrity and function. However, increased inflammation in the DEJ is associated with the disruption and separation of this junction and sub-epidermal blistering. Granzyme B (GzmB) is a serine protease secreted by immune cells. Dysregulated inflammation may lead to increased GzmB accumulation and proteolysis in the extracellular milieu. Although elevated GzmB is observed at the level of the DEJ in inflammatory and blistering skin conditions, the present study is the first to explore GzmB in the context of DEJ degradation in autoimmune sub-epidermal blistering. In the present study, GzmB induced separation of the DEJ in healthy human skin. Subsequently, α6/ß4 integrin, collagen VII, and collagen XVII were identified as extracellular substrates for GzmB through western blot, and specific cleavage sites were identified by mass spectrometry. In human bullous pemphigoid, dermatitis herpetiformis, and epidermolysis bullosa acquisita, GzmB was elevated at the DEJ when compared to healthy samples, while α6/ß4 integrin, collagen VII, and collagen XVII were reduced or absent in the area of blistering. In summary, our results suggest that regardless of the initial causation of sub-epidermal blistering, GzmB activity is a common final pathway that could be amenable to a single targeted treatment approach.

N-Terminomics TAILS Identifies Host Cell Substrates of Poliovirus and Coxsackievirus B3 3C Proteinases That Modulate Virus Infection.

Enteroviruses encode proteinases that are essential for processing of the translated viral polyprotein. In addition, viral proteinases also target host proteins to manipulate cellular processes and evade innate antiviral responses to promote replication and infection. Although some host protein substrates of enterovirus proteinases have been identified, the full repertoire of targets remains unknown. We used a novel quantitative in vitro proteomics-based approach, termed terminal amine isotopic labeling of substrates (TAILS), to identify with high confidence 72 and 34 new host protein targets of poliovirus and coxsackievirus B3 (CVB3) 3C proteinases (3Cpros) in HeLa cell and cardiomyocyte HL-1 cell lysates, respectively. We validated a subset of candidate substrates that are targets of poliovirus 3Cproin vitro including three common protein targets, phosphoribosylformylglycinamidine synthetase (PFAS), hnRNP K, and hnRNP M, of both proteinases. 3Cpro-targeted substrates were also cleaved in virus-infected cells but not noncleavable mutant proteins designed from the TAILS-identified cleavage sites. Knockdown of TAILS-identified target proteins modulated infection both negatively and positively, suggesting that cleavage by 3Cpro promotes infection. Indeed, expression of a cleavage-resistant mutant form of the endoplasmic reticulum (ER)-Golgi vesicle-tethering protein p115 decreased viral replication and yield. As the first comprehensive study to identify and validate functional enterovirus 3Cpro substrates in vivo, we conclude that N-terminomics by TAILS is an effective strategy to identify host targets of viral proteinases in a nonbiased manner.IMPORTANCE Enteroviruses are positive-strand RNA viruses that encode proteases that cleave the viral polyprotein into the individual mature viral proteins. In addition, viral proteases target host proteins in order to modulate cellular pathways and block antiviral responses in order to facilitate virus infection. Although several host protein targets have been identified, the entire list of proteins that are targeted is not known. In this study, we used a novel unbiased proteomics approach to identify ∼100 novel host targets of the enterovirus 3C protease, thus providing further insights into the network of cellular pathways that are modulated to promote virus infection.

Identification of Protease Cleavage Sites and Substrates in Cancer by Carboxy-TAILS (C-TAILS).

Determination of drug targets and development of novel therapeutics for the treatment of different cancers are actively ongoing areas of research. Proteases being the second largest group of enzymes in humans present themselves as attractive targets for blocking and activation to treat malignancies. However, determination of the protease cleavage substrates is often missed by utilizing conventional modern proteomic approaches. The relatively low abundance of proteolytically processed, and mostly semi-tryptic, peptides compared to tryptic peptides generated in shotgun proteomics compounded with their poorer identification rates makes the identification of such critical peptides challenging and so are mostly overlooked. Our laboratory introduced Terminal Amine Isotopic Labeling of Substrates (TAILS) to identify N-terminal peptides from cleavage events. In this chapter we present a protocol from our complementary method carboxy-TAILS (C-TAILS) to identify C-terminal peptides in metabolically labeled cancer cell lines.

Proteolytic Cleavage-Mechanisms, Function, and "Omic" Approaches for a Near-Ubiquitous Posttranslational Modification.

Proteases enzymatically hydrolyze peptide bonds in substrate proteins, resulting in a widespread, irreversible posttranslational modification of the protein's structure and biological function. Often regarded as a mere degradative mechanism in destruction of proteins or turnover in maintaining physiological homeostasis, recent research in the field of degradomics has led to the recognition of two main yet unexpected concepts. First, that targeted, limited proteolytic cleavage events by a wide repertoire of proteases are pivotal regulators of most, if not all, physiological and pathological processes. Second, an unexpected in vivo abundance of stable cleaved proteins revealed pervasive, functionally relevant protein processing in normal and diseased tissue-from 40 to 70% of proteins also occur in vivo as distinct stable proteoforms with undocumented N- or C-termini, meaning these proteoforms are stable functional cleavage products, most with unknown functional implications. In this Review, we discuss the structural biology aspects and mechanisms of catalysis by different protease classes. We also provide an overview of biological pathways that utilize specific proteolytic cleavage as a precision control mechanism in protein quality control, stability, localization, and maturation, as well as proteolytic cleavage as a mediator in signaling pathways. Lastly, we provide a comprehensive overview of analytical methods and approaches to study activity and substrates of proteolytic enzymes in relevant biological models, both historical and focusing on state of the art proteomics techniques in the field of degradomics research.

Cell Shaving and False-Positive Control Strategies Coupled to Novel Statistical Tools to Profile Gram-Positive Bacterial Surface Proteomes.

A powerful start to the discovery and design of novel vaccines, and for better understanding of host-pathogen interactions, is to profile bacterial surfaces using the proteolytic digestion of surface-exposed proteins under mild conditions. This "cell shaving" approach has the benefit of both identifying surface proteins and their surface-exposed epitopes, which are those most likely to interact with host cells and/or the immune system, providing a comprehensive overview of bacterial cell topography. An essential requirement for successful cell shaving is to account for (or minimize) cellular lysis that can occur during the shaving procedure and thus generate data that is biased towards non-surface (e.g., cytoplasmic) proteins. This is further complicated by the presence of "moonlighting" proteins, which are proteins predicted to be intracellular but with validated surface or extracellular functions. Here, we describe an optimized cell shaving protocol for Gram-positive bacteria that uses proteolytic digestion and a "false-positive" control to reduce the number of intracellular contaminants in these datasets. Released surface-exposed peptides are analyzed by liquid chromatography (LC) coupled to high-resolution tandem mass spectrometry (MS/MS). Additionally, the probabilities of proteins being surface exposed can be further calculated by applying novel statistical tools.

Comparative analysis of Staphylococcus epidermidis strains utilizing quantitative and cell surface shaving proteomics.

Staphylococcus epidermidis is an opportunistic pathogen that is an emerging risk factor in hospitals worldwide and is often difficult to eradicate as virulent strains produce a protective biofilm matrix. We utilized cell shaving proteomics to profile surface-exposed proteins from two fully genome sequenced S. epidermidis strains: the avirulent, non-biofilm forming ATCC12228 and the virulent, strongly adherent biofilm forming ATCC35984 (RP62A). A false positive control strategy was employed to calculate the probabilities of proteins being truly surface-exposed. A total of 78 surface-exposed proteins were identified, of which only 19 proteins were common to ATCC12228 and RP62A, and which thus represents the core surfaceome. S. epidermidis RP62A displayed additional proteins involved in biofilm formation (cell wall-associated Bhp and intercellular adhesion protein IcaB), surface antigenicity, peptidoglycan biosynthesis and antibiotic resistance. We concurrently profiled whole cell proteomes of the two strains using iTRAQ quantitation and LC-MS/MS. A total of 1610 proteins were confidently identified (representing 64% of the theoretical S. epidermidis proteome). One hundred and ninety one proteins were differentially abundant between strains. Proteins associated with RP62A were clustered into functions including Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-mediated defense, sulfate assimilation, antibiotic resistance and biofilm formation. Validation of the sulfate assimilation and cysteine/methionine biosynthesis pathways showed RP62A contained elevated levels (~25% increase) of methionine that are likely linked to biofilm formation. BIOLOGICAL SIGNIFICANCE: Cell shaving and quantitative proteomics identified proteins associated with a biofilm-forming, virulent strain of S. epidermidis (RP62A). These proteins show RP62A maintains an active CRISPR-mediated defense, as well as heightened antibiotic resistance in comparison to a non-virulent, non-biofilm forming strain. Increased abundances of sulfate assimilation proteins lead to elevated intracellular methionine. Proteins and their exposed peptides identified on the surface of S. epidermidis RP62A may be useful vaccine antigens in clinical settings if administered in at-risk patients prior to surgical implantations.

LysargiNase mirrors trypsin for protein C-terminal and methylation-site identification.

To improve proteome coverage and protein C-terminal identification, we characterized the Methanosarcina acetivorans thermophilic proteinase LysargiNase, which cleaves before lysine and arginine up to 55 °C. Unlike trypsin, LysargiNase-generated peptides had N-terminal lysine or arginine residues and fragmented with b ion-dominated spectra. This improved protein C terminal-peptide identification and several arginine-rich phosphosite assignments. Notably, cleavage also occurred at methylated or dimethylated lysine and arginine, facilitating detection of these epigenetic modifications.

Comparative proteomics and glycoproteomics reveal increased N-linked glycosylation and relaxed sequon specificity in Campylobacter jejuni NCTC11168 O.

Campylobacter jejuni is a major cause of bacterial gastroenteritis. C. jejuni encodes a protein glycosylation (Pgl) locus responsible for the N-glycosylation of membrane-associated proteins. We examined two variants of the genome sequenced strain NCTC11168: O, a representative of the original clinical isolate, and GS, a laboratory-adapted relative of O. Comparative proteomics by iTRAQ and two-dimensional liquid chromatography coupled to tandem mass spectrometry (2D-LC-MS/MS) allowed the confident identification of 1214 proteins (73.9% of the predicted C. jejuni proteome), of which 187 were present at statistically significant altered levels of abundance between variants. Proteins associated with the O variant included adhesins (CadF and FlpA), proteases, capsule biosynthesis, and cell shape determinants as well as six proteins encoded by the Pgl system, including the PglK flippase and PglB oligosaccharyltransferase. Lectin blotting highlighted specific glycoproteins more abundant in NCTC11168 O, whereas others remained unaltered. Hydrophilic interaction liquid chromatography (HILIC) and LC-MS/MS identified 30 completely novel glycosites from 15 proteins. A novel glycopeptide from a 14 kDa membrane protein (Cj0455c) was identified that did not contain the C. jejuni N-linked sequon D/E-X-N-X-S/T (X ≠ Pro) but that instead contained a sequon with leucine at the -2 position. Occupied atypical sequons were also observed in Cj0958c (OxaA; Gln at the -2 position) and Cj0152c (Ala at the +2 position). The relative O and GS abundances of 30 glycopeptides were determined by label-free quantitation, which revealed a >100-fold increase in the atypical glycopeptide from Cj0455c in isolate O. Our data provide further evidence for the importance of the Pgl system in C. jejuni.

Staphylococcus aureus surface proteins involved in adaptation to oxacillin identified using a novel cell shaving approach.

Staphylococcus aureus is a Gram-positive pathogen responsible for a variety of infections, and some strains are resistant to virtually all classes of antibiotics. Cell shaving proteomics using a novel probability scoring algorithm to compare the surfaceomes of the methicillin-resistant, laboratory-adapted S. aureus COL strain with a COL strain in vitro adapted to high levels of oxacillin (APT). APT displayed altered cell morphology compared with COL and increased aggregation in biofilm assays. Increased resistance to ß-lactam antibiotics was observed, but adaptation to oxacillin did not confer multidrug resistance. Analysis of the S. aureus COL and APT surfaceomes identified 150 proteins at a threshold determined by the scoring algorithm. Proteins unique to APT included the LytR-CpsA-Psr (LCP) domain-containing MsrR and SACOL2302. Quantitative RT-PCR showed increased expression of sacol2302 in APT grown with oxacillin (>6-fold compared with COL). Overexpression of sacol2302 in COL to levels consistent with APT (+ oxacillin) did not influence biofilm formation or ß-lactam resistance. Proteomics using iTRAQ and LC-MS/MS identified 1323 proteins (∼50% of the theoretical S. aureus proteome), and cluster analysis demonstrated elevated APT abundances of LCP proteins, capsule and peptidoglycan biosynthesis proteins, and proteins involved in wall remodelling. Adaptation to oxacillin also induced urease proteins, which maintained culture pH compared to COL. These results show that S. aureus modifies surface architecture in response to antibiotic adaptation.