Molecular Interactions Required for Activation of Complement Component C2 Include Exosites Located on the Serine Protease Domain of C1s and Mannose-Binding Lectin Associated Protease-2

The activation of the CP/LP C3 proconvertase complex is a key event in complement activation and involves cleavage of C4 and C2 by the C1s protease (classical pathway) or the mannose-binding lectin-associated serine protease (MASP)-2 (lectin pathway). Efficient cleavage of C4 by C1s and MASP-2 involves exosites on the complement control protein and serine protease (SP) domains of the proteases. The complement control protein domain exosite is not involved in cleavage of C2 by the proteases, but the role of an anion-binding exosite (ABE) on the SP domains of the proteases has (to our knowledge) never been investigated. In this study, we have shown that the ABE on the SP of both C1s and MASP-2 is crucial for efficient cleavage of C2, with mutant forms of the proteases greatly impaired in their rate of cleavage of C2. We have additionally shown that the site of binding for the ABE of the proteases is very likely to be located on the von Willebrand factor domain of C2, with the precise area differing between the enzymes: whereas C1s requires two anionic clusters on the von Willebrand factor domain to enact efficient cleavage of C2, MASP-2 apparently only requires one. These data provide (to our knowledge) new information about the molecular determinants for efficient activation of C2 by C1s and MASP-2. The enhanced view of the molecular events underlying the early stages of complement activation provides further possible intervention points for control of this activation that is involved in a number of inflammatory diseases.

Effect of a protease‐activated receptor‐2 antagonist ( GB88 ) on inflammation‐related loss of alveolar bone in periodontal disease

BACKGROUND AND OBJECTIVE

Protease-activated receptor-2 (PAR₂ ), a pro-inflammatory G-protein coupled receptor, has been associated with pathogenesis of periodontitis and the resulting bone loss caused by oral pathogens, including the keystone pathogen Porphyromonas gingivalis (P. gingivalis). We hypothesised that administration of a PAR₂ antagonist, GB88, might prevent inflammation and subsequent alveolar bone resorption in a mouse model of periodontal disease.

METHODS

Periodontitis was induced in mice by oral inoculations with P. gingivalis for a total of eight times over 24 days. The infected mice were treated with either GB88 or vehicle for the duration of the trial. Following euthanasia on day 56, serum was collected and used for the detection of mast cell tryptase. The right maxillae were defleshed and stained with methylene blue to measure the exposed cementum in molar teeth. The left maxillae were prepared for cryosections followed by staining for tartrate-resistant acid phosphatase to identify osteoclasts or with toluidine blue to identify mast cells. Reverse transcription quantitative PCR (RT-qPCR) was used to quantify the expression of inflammatory cytokines in the gingival tissue. Supernatants of T-lymphocyte cultures isolated from the regional lymph nodes were assayed using a cytometric bead array to measure the Th1/Th2/Th17 cytokine levels.

RESULTS

Measurement of the exposed cementum showed that GB88 reduced P. gingivalis-induced alveolar bone loss by up to 69%. GB88 also prevented the increase in osteoclast numbers observed in the infected mice. Serum tryptase levels were significantly elevated in both the infected groups, and not altered by treatment. RT-qPCR showed that GB88 prevented the upregulation of Il1b, Il6, Ifng and Cd11b. In T-lymphocyte supernatants, only IFNγ and IL-17A levels were increased in response to infection, but this was prevented by GB88 treatment.

CONCLUSIONS

GB88 significantly reduced osteoclastic alveolar bone loss in mice infected with P. gingivalis, seemingly by preventing the upregulation of several inflammatory cytokines. PAR₂ antagonism may be an effective treatment strategy for periodontal disease.

Mapping the binding site of C1-inhibitor for polyanion cofactors

The serpin, C1-inhibitor (also known as SERPING1), plays a vital anti-inflammatory role in the body by controlling pro-inflammatory pathways such as complement and coagulation. The inhibitor's action is enhanced in the presence of polyanionic cofactors, such as heparin and polyphosphate, by increasing the rate of association with key enzymes such as C1s of the classical pathway of complement. The cofactor binding site of the serpin has never been mapped. Here we show that residues Lys284, Lys285 and Arg287 of C1-inhibitor play key roles in binding heparin and delivering the rate enhancement seen in the presence of polyanions and thus most likely represent the key cofactor binding residues for the serpin. We also show that simultaneous binding of the anion binding site of C1s by the polyanion is required to deliver the rate enhancement. Finally, we have shown that it is unlikely that the two positively charged zones of C1-inhibitor and C1s interact in the encounter complex between molecules as ablation of the charged zones did not in itself deliver a rate enhancement as might have been expected if the zones interacted. These insights provide crucial information as to the mechanism of action of this key serpin in the presence and absence of cofactor molecules.

Determination of the crystal structure and substrate specificity of ananain

Ananain (EC 3.4.22.31) accounts for less than 10% of the total enzyme in the crude pineapple stem extract known as bromelain, yet yields the majority of the proteolytic activity of bromelain. Despite a high degree of sequence identity between ananain and stem bromelain, the most abundant bromelain cysteine protease, ananain displays distinct chemical properties, substrate preference and inhibitory profile compared to stem bromelain. A tripeptidyl substrate library (REPLi) was used to further characterize the substrate specificity of ananain and identified an optimal substrate for cleavage by ananain. The optimal tripeptide, PLQ, yielded a high k_cat/K_m value of 1.7 x 106 M^-1s^-1, with cleavage confirmed to occur after the Gln residue. Crystal structures of unbound ananain and an inhibitory complex of ananain and E-64, solved at 1.73 and 1.98 Å, respectively, revealed a geometrically flat and open S1 subsite for ananain. This subsite accommodates diverse P1 substrate residues, while a narrow and deep hydrophobic pocket-like S2 subsite would accommodate a non-polar P2 residue, such as the preferred Leu residue observed in the specificity studies. A further illustration of the atomic interactions between E-64 and ananain explains the high inhibitory efficiency of E-64 toward ananain. These data reveal the first in depth structural and functional data for ananain and provide a basis for further study of the natural properties of the enzyme.

The Structural Basis for Complement Inhibition by Gigastasin, a Protease Inhibitor from the Giant Amazon Leech

Complement is crucial to the immune response, but dysregulation of the system causes inflammatory disease. Complement is activated by three pathways: classical, lectin, and alternative. The classical and lectin pathways are initiated by the C1r/C1s (classical) and MASP-1/MASP-2 (lectin) proteases. Given the role of complement in disease, there is a requirement for inhibitors to control the initiating proteases. In this article, we show that a novel inhibitor, gigastasin, from the giant Amazon leech, potently inhibits C1s and MASP-2, whereas it is also a good inhibitor of MASP-1. Gigastasin is a poor inhibitor of C1r. The inhibitor blocks the active sites of C1s and MASP-2, as well as the anion-binding exosites of the enzymes via sulfotyrosine residues. Complement deposition assays revealed that gigastasin is an effective inhibitor of complement activation in vivo, especially for activation via the lectin pathway. These data suggest that the cumulative effects of inhibiting both MASP-2 and MASP-1 have a greater effect on the lectin pathway than the more potent inhibition of only C1s of the classical pathway.

Structural basis for substrate specificity of Helicobacter pylori M17 aminopeptidase

The M17 aminopeptidase from the carcinogenic gastric bacterium Helicobacter pylori (HpM17AP) is an important housekeeping enzyme involved in catabolism of endogenous and exogenous peptides. It is implicated in H. pylori defence against the human innate immune response and in the mechanism of metronidazole resistance. Bestatin inhibits HpM17AP and suppresses H. pylori growth. To address the structural basis of catalysis and inhibition of this enzyme, we have established its specificity towards the N-terminal amino acid of peptide substrates and determined the crystal structures of HpM17AP and its complex with bestatin. The position of the D-phenylalanine moiety of the inhibitor with respect to the active-site metal ions, bicarbonate ion and with respect to other M17 aminopeptidases suggested that this residue binds to the S1 subsite of HpM17AP. In contrast to most characterized M17 aminopeptidases, HpM17AP displays preference for L-Arg over L-Leu residues in peptide substrates. Compared to very similar homologues from other bacteria, a distinguishing feature of HpM17AP is a hydrophilic pocket at the end of the S1 subsite that is likely to accommodate the charged head group of the L-Arg residue of the substrate. The pocket is flanked by a sodium ion (not present in M17 aminopeptidases that show preference for L-Leu) and its coordinating water molecules. In addition, the structure suggests that variable loops at the entrance to, and in the middle of, the substrate-binding channel are important determinants of substrate specificity of M17 aminopeptidases.

Protein unfolding is essential for cleavage within the α-helix of a model protein substrate by the serine protease, thrombin

Proteolysis has a critical role in transmitting information within a biological system and therefore an important element of biology is to determine the subset of proteins amenable to proteolysis. Until recently, it has been thought that proteases cleave native protein substrates only within solvent exposed loops, but recent evidence indicates that cleavage sites located within α-helices can also be cleaved by proteases, despite the conformation of this secondary structure being generally incompatible with binding into an active site of a protease. In this study, we address the mechanism by which a serine endopeptidase, thrombin, recognizes and cleaves a target sequence located within an α-helix. Thrombin was able to cleave a model substrate, protein G, within its α-helix when a suitable cleavage sequence for the enzyme was introduced into this region. However, structural data for the complex revealed that thrombin was not perturbing the structure of the α-helix, thus it was not destabilizing the helix in order to allow it to fit within its active site. This indicated that thrombin was only cleaving within the α-helix when it was in an unfolded state. In support of this, the introduction of destabilizing mutations within the protein increased the efficiency of cleavage by the enzyme. Our data suggest that a folded α-helix cannot be proteolytically cleaved by thrombin, but the species targeted are the unfolded conformations of the native state ensemble.

Investigation of the mechanism of interaction between Mannose-binding lectin-associated serine protease-2 and complement C4

The interaction between mannose-binding lectin [MBL]-associated serine protease-2 (MASP-2) and its first substrate, C4 is crucial to the lectin pathway of complement, which is vital for innate host immunity, but also involved in a number of inflammatory diseases. Recent data suggests that two areas outside of the active site of MASP-2 (so-called exosites) are crucial for efficient cleavage of C4: one at the junction of the two complement control protein (CCP) domains of the enzyme and the second on the serine protease (SP) domain. Here, we have further investigated the roles of each of these exosites in the binding and cleavage of C4. We have found that both exosites are required for high affinity binding and efficient cleavage of the substrate protein. Within the SP domain exosite, we have shown here that two arginine residues are most important for high affinity binding and efficient cleavage of C4. Finally, we show that the CCP domain exosite appears to play the major role in the initial interaction with C4, whilst the SP domain exosite plays the major role in a secondary conformational change between the two proteins required to form a high affinity complex. This data has provided new insights into the binding and cleavage of C4 by MASP-2, which may be useful in the design of molecules that modulate this important interaction required to activate the lectin pathway of complement.

Scabies mite inactive serine proteases are potent inhibitors of the human complement lectin pathway

Scabies is an infectious skin disease caused by the mite Sarcoptes scabiei and has been classified as one of the six most prevalent epidermal parasitic skin diseases infecting populations living in poverty by the World Health Organisation. The role of the complement system, a pivotal component of human innate immunity, as an important defence against invading pathogens has been well documented and many parasites have an arsenal of anti-complement defences. We previously reported on a family of scabies mite proteolytically inactive serine protease paralogues (SMIPP-Ss) thought to be implicated in host defence evasion. We have since shown that two family members, SMIPP-S D1 and I1 have the ability to bind the human complement components C1q, mannose binding lectin (MBL) and properdin and are capable of inhibiting all three human complement pathways. This investigation focused on inhibition of the lectin pathway of complement activation as it is likely to be the primary pathway affecting scabies mites. Activation of the lectin pathway relies on the activation of MBL, and as SMIPP-S D1 and I1 have previously been shown to bind MBL, the nature of this interaction was examined using binding and mutagenesis studies. SMIPP-S D1 bound MBL in complex with MBL-associated serine proteases (MASPs) and released the MASP-2 enzyme from the complex. SMIPP-S I1 was also able to bind MBL in complex with MASPs, but MASP-1 and MASP-2 remained in the complex. Despite these differences in mechanism, both molecules inhibited activation of complement components downstream of MBL. Mutagenesis studies revealed that both SMIPP-Ss used an alternative site of the molecule from the residual active site region to inhibit the lectin pathway. We propose that SMIPP-Ss are potent lectin pathway inhibitors and that this mechanism represents an important tool in the immune evasion repertoire of the parasitic mite and a potential target for therapeutics.

A molecular basis for the association of the HLA-DRB1 locus, citrullination, and rheumatoid arthritis

Rheumatoid arthritis (RA) is strongly associated with the human leukocyte antigen (HLA)-DRB1 locus that possesses the shared susceptibility epitope (SE) and the citrullination of self-antigens. We show how citrullinated aggrecan and vimentin epitopes bind to HLA-DRB1*04:01/04. Citrulline was accommodated within the electropositive P4 pocket of HLA-DRB1*04:01/04, whereas the electronegative P4 pocket of the RA-resistant HLA-DRB1*04:02 allomorph interacted with arginine or citrulline-containing epitopes. Peptide elution studies revealed P4 arginine-containing peptides from HLA-DRB1*04:02, but not from HLA-DRB1*04:01/04. Citrullination altered protease susceptibility of vimentin, thereby generating self-epitopes that are presented to T cells in HLA-DRB1*04:01(+) individuals. Using HLA-II tetramers, we observed citrullinated vimentin- and aggrecan-specific CD4(+) T cells in the peripheral blood of HLA-DRB1*04:01(+) RA-affected and healthy individuals. In RA patients, autoreactive T cell numbers correlated with disease activity and were deficient in regulatory T cells relative to healthy individuals. These findings reshape our understanding of the association between citrullination, the HLA-DRB1 locus, and T cell autoreactivity in RA.

The x-ray crystal structure of mannose-binding lectin-associated serine proteinase-3 reveals the structural basis for enzyme inactivity associated with the Carnevale, Mingarelli, Malpuech, and Michels (3MC) syndrome

The mannose-binding lectin associated-protease-3 (MASP-3) is a member of the lectin pathway of the complement system, a key component of human innate and active immunity. Mutations in MASP-3 have recently been found to be associated with Carnevale, Mingarelli, Malpuech, and Michels (3MC) syndrome, a severe developmental disorder manifested by cleft palate, intellectual disability, and skeletal abnormalities. However, the molecular basis for MASP-3 function remains to be understood. Here we characterize the substrate specificity of MASP-3 by screening against a combinatorial peptide substrate library. Through this approach, we successfully identified a peptide substrate that was 20-fold more efficiently cleaved than any other identified to date. Furthermore, we demonstrated that mutant forms of the enzyme associated with 3MC syndrome were completely inactive against this substrate. To address the structural basis for this defect, we determined the 2.6-Å structure of the zymogen form of the G666E mutant of MASP-3. These data reveal that the mutation disrupts the active site and perturbs the position of the catalytic serine residue. Together, these insights into the function of MASP-3 reveal how a mutation in this enzyme causes it to be inactive and thus contribute to the 3MC syndrome.

Thrombin inhibits osteoclast differentiation through a non-proteolytic mechanism

Thrombin stimulates expression of interleukin 6 and cyclooxygenase 2 by osteoblasts, both of which enhance osteoblast-mediated osteoclast differentiation by increasing the ratio of receptor activator of nuclear factor κB ligand (RANKL) expression to that of osteoprotegerin (OPG) in osteoblasts. We hypothesised that thrombin would also increase this ratio and thereby stimulate osteoclast differentiation in mixed cultures of osteoblastic cells and osteoclast precursors. In primary mouse osteoblasts, but not in bone marrow stromal cells, thrombin increased the ratio of RANKL to OPG expression. Thrombin inhibited differentiation of osteoclasts, defined as tartrate-resistant acid phosphatase (TRAP)-positive cells with three or more nuclei, in mouse bone marrow cultures treated with osteoclastogenic hormones; this effect was not mediated by the major thrombin receptor, protease-activated receptor 1, nor did it require thrombin's proteolytic activity. Thrombin also caused a decrease in the number of TRAP-positive cells with fewer than three nuclei. Thrombin (active or inactive) also inhibited osteoclast differentiation and bone resorption, respectively, in cultures of mouse spleen cells and human peripheral blood mononuclear cells induced to undergo osteoclastogenesis by treatment with RANKL and macrophage colony-stimulating factor. Osteoclast differentiation in spleen cells was inhibited when they were exposed to thrombin from days 0 to 3 or 3 to 5 of culture but not days 5 to 7 when most fusion occurred. Thrombin inhibited expression of RANK by spleen cells. These observations indicate that, although thrombin stimulates production of osteoclastogenic factors by osteoblastic cells, it inhibits the early stages of RANKL-induced osteoclast differentiation through a direct effect on osteoclast precursors that does not require thrombin's proteolytic activity.

Molecular determinants of the substrate specificity of the complement-initiating protease, C1r

The serine protease, C1r, initiates activation of the classical pathway of complement, which is a crucial innate defense mechanism against pathogens and altered-self cells. C1r both autoactivates and subsequently cleaves and activates C1s. Because complement is implicated in many inflammatory diseases, an understanding of the interaction between C1r and its target substrates is required for the design of effective inhibitors of complement activation. Examination of the active site specificity of C1r using phage library technology revealed clear specificity for Gln at P2 and Ile at P1', which are found in these positions in physiological substrates of C1r. Removal of one or both of the Gln at P2 and Ile at P1' in the C1s substrate reduced the rate of C1r activation. Substituting a Gln residue into the P2 of the activation site of MASP-3, a protein with similar domain structure to C1s that is not normally cleaved by C1r, enabled efficient activation of this enzyme. Molecular dynamics simulations and structural modeling of the interaction of the C1s activation peptide with the active site of C1r revealed the molecular mechanisms that particularly underpin the specificity of the enzyme for the P2 Gln residue. The complement control protein domains of C1r also made important contributions to efficient activation of C1s by this enzyme, indicating that exosite interactions were also important. These data show that C1r specificity is well suited to its cleavage targets and that efficient cleavage of C1s is achieved through both active site and exosite contributions.

A molecular switch governs the interaction between the human complement protease C1s and its substrate, complement C4

The complement system is an ancient innate immune defense pathway that plays a front line role in eliminating microbial pathogens. Recognition of foreign targets by antibodies drives sequential activation of two serine proteases, C1r and C1s, which reside within the complement Component 1 (C1) complex. Active C1s propagates the immune response through its ability to bind and cleave the effector molecule complement Component 4 (C4). Currently, the precise structural and biochemical basis for the control of the interaction between C1s and C4 is unclear. Here, using surface plasmon resonance, we show that the transition of the C1s zymogen to the active form is essential for C1s binding to C4. To understand this, we determined the crystal structure of a zymogen C1s construct (comprising two complement control protein (CCP) domains and the serine protease (SP) domain). These data reveal that two loops (492-499 and 573-580) in the zymogen serine protease domain adopt a conformation that would be predicted to sterically abrogate C4 binding. The transition from zymogen to active C1s repositions both loops such that they would be able to interact with sulfotyrosine residues on C4. The structure also shows the junction of the CCP1 and CCP2 domains of C1s for the first time, yielding valuable information about the exosite for C4 binding located at this position. Together, these data provide a structural explanation for the control of the interaction with C1s and C4 and, furthermore, point to alternative strategies for developing therapeutic approaches for controlling activation of the complement cascade.

Assembly of the type II secretion system such as found in Vibrio cholerae depends on the novel Pilotin AspS

The Type II Secretion System (T2SS) is a molecular machine that drives the secretion of fully-folded protein substrates across the bacterial outer membrane. A key element in the machinery is the secretin: an integral, multimeric outer membrane protein that forms the secretion pore. We show that three distinct forms of T2SSs can be distinguished based on the sequence characteristics of their secretin pores. Detailed comparative analysis of two of these, the Klebsiella-type and Vibrio-type, showed them to be further distinguished by the pilotin that mediates their transport and assembly into the outer membrane. We have determined the crystal structure of the novel pilotin AspS from Vibrio cholerae, demonstrating convergent evolution wherein AspS is functionally equivalent and yet structurally unrelated to the pilotins found in Klebsiella and other bacteria. AspS binds to a specific targeting sequence in the Vibrio-type secretins, enhances the kinetics of secretin assembly, and homologs of AspS are found in all species of Vibrio as well those few strains of Escherichia and Shigella that have acquired a Vibrio-type T2SS.

The type 2 secretion system (T2SS) is a sophisticated, multi-component molecular machine that drives the secretion of fully-folded protein substrates across the bacterial outer membrane. In Vibrio cholerae, for example, the T2SS mediates the secretion of cholera toxin. We find that there are three distinct forms of T2SS, based on the sequence characteristics of the secretin. A targeting paradigm, developed for the Klebsiella-type secretin PulD, could not previously be applied to the T2SS in Vibrio cholerae and many other bacterial species whose genomes encode no homolog of the crucial targeting factor PulS (also called OutS, EtpO or GspS). Using bioinformatics we find, remarkably, that these bacteria have instead evolved a structurally distinct protein to serve in place of PulS. We crystallized and solved the structure of this distinct factor, AspS, measured its activity in novel assays for T2SS assembly, and show that the protein is essential for the function of the Vibrio-type T2SS. A structural homolog of AspS found here in Pseudomonas suggests widespread use of the pilotin-secretin targeting paradigm for T2SS assembly.

Identification of a catalytic exosite for complement component C4 on the serine protease domain of C1s

The classical pathway of complement is crucial to the immune system, but it also contributes to inflammatory diseases when dysregulated. Binding of the C1 complex to ligands activates the pathway by inducing autoactivation of associated C1r, after which C1r activates C1s. C1s cleaves complement component C4 and then C2 to cause full activation of the system. The interaction between C1s and C4 involves active site and exosite-mediated events, but the molecular details are unknown. In this study, we identified four positively charged amino acids on the serine protease domain that appear to form a catalytic exosite that is required for efficient cleavage of C4. These residues are coincidentally involved in coordinating a sulfate ion in the crystal structure of the protease. Together with other evidence, this pointed to the involvement of sulfate ions in the interaction with the C4 substrate, and we showed that the protease interacts with a peptide from C4 containing three sulfotyrosine residues. We present a molecular model for the interaction between C1s and C4 that provides support for the above data and poses questions for future research into this aspect of complement activation.

Novel scabies mite serpins inhibit the three pathways of the human complement system

Scabies is a parasitic infestation of the skin by the mite Sarcoptes scabiei that causes significant morbidity worldwide, in particular within socially disadvantaged populations. In order to identify mechanisms that enable the scabies mite to evade human immune defenses, we have studied molecules associated with proteolytic systems in the mite, including two novel scabies mite serine protease inhibitors (SMSs) of the serpin superfamily. Immunohistochemical studies revealed that within mite-infected human skin SMSB4 (54 kDa) and SMSB3 (47 kDa) were both localized in the mite gut and feces. Recombinant purified SMSB3 and SMSB4 did not inhibit mite serine and cysteine proteases, but did inhibit mammalian serine proteases, such as chymotrypsin, albeit inefficiently. Detailed functional analysis revealed that both serpins interfered with all three pathways of the human complement system at different stages of their activation. SMSB4 inhibited mostly the initial and progressing steps of the cascades, while SMSB3 showed the strongest effects at the C9 level in the terminal pathway. Additive effects of both serpins were shown at the C9 level in the lectin pathway. Both SMSs were able to interfere with complement factors without protease function. A range of binding assays showed direct binding between SMSB4 and seven complement proteins (C1, properdin, MBL, C4, C3, C6 and C8), while significant binding of SMSB3 occurred exclusively to complement factors without protease function (C4, C3, C8). Direct binding was observed between SMSB4 and the complement proteases C1s and C1r. However no complex formation was observed between either mite serpin and the complement serine proteases C1r, C1s, MASP-1, MASP-2 and MASP-3. No catalytic inhibition by either serpin was observed for any of these enzymes. In summary, the SMSs were acting at several levels mediating overall inhibition of the complement system and thus we propose that they may protect scabies mites from complement-mediated gut damage.

S1 pocket of a bacterially derived subtilisin-like protease underpins effective tissue destruction

The ovine footrot pathogen, Dichelobacter nodosus, secretes three subtilisin-like proteases that play an important role in the pathogenesis of footrot through their ability to mediate tissue destruction. Virulent and benign strains of D. nodosus secrete the basic proteases BprV and BprB, respectively, with the catalytic domain of these enzymes having 96% sequence identity. At present, it is not known how sequence variation between these two putative virulence factors influences their respective biological activity. We have determined the high resolution crystal structures of BprV and BprB. These data reveal that that the S1 pocket of BprV is more hydrophobic but smaller than that of BprB. We show that BprV is more effective than BprB in degrading extracellular matrix components of the host tissue. Mutation of two residues around the S1 pocket of BprB to the equivalent residues in BprV dramatically enhanced its proteolytic activity against elastin substrates. Application of a novel approach for profiling substrate specificity, the Rapid Endopeptidase Profiling Library (REPLi) method, revealed that both enzymes prefer cleaving after hydrophobic residues (and in particular P1 leucine) but that BprV has more restricted primary substrate specificity than BprB. Furthermore, for P1 Leu-containing substrates we found that BprV is a significantly more efficient enzyme than BprB. Collectively, these data illuminate how subtle changes in D. nodosus proteases may significantly influence tissue destruction as part of the ovine footrot pathogenesis process.

Nerve growth factor inhibits metalloproteinase-disintegrins and blocks ectodomain shedding of platelet glycoprotein VI

Nerve growth factor (NGF) plays an important role in regulating mammalian neuronal/embryonic development, angiogenesis, and other physiological processes and has recently been investigated as a potential treatment for the neurodegenerative disorder, Alzheimer disease. In this study, we provide evidence that human NGF may also function as a metalloproteinase inhibitor, based on studies of NGF from snake venom. Originally, our aim was to isolate snake venom metalloproteinases targeting platelet receptors and/or ligands relevant to hemostasis and thrombosis, using Ni(2+)-agarose as a purification step based on the conserved metal ion-coordination motif in venom metalloproteinases. However, subsequent analysis of cobra (Naja kaouthia) venom led to the unexpected discovery that cobra venom NGF bound to Ni(2+)-agarose, eluting at approximately 15 mm imidazole, enabling a one-step purification. The identity of the purified protein was confirmed by mass spectrometry and N-terminal sequence analysis. Partial co-purification of NGF within metalloproteinase-enriched venom fractions led us to test whether NGF affected metalloproteinase activity. Venom NGF potently inhibited metalloproteinases isolated from the same or different venom and specifically bound to purified Nk metalloproteinase immobilized on agarose beads. Human NGF also interacted with human metalloproteinases because it blocked metalloproteinase-mediated shedding of the platelet collagen receptor, glycoprotein (GP)VI, and associated with recombinant ADAM10 by surface plasmon resonance. Together, these results suggest that NGF can function as a metalloproteinase inhibitor.

Characterization of a serine protease homologous to house dust mite group 3 allergens from the scabies mite Sarcoptes scabiei

The scabies mite, Sarcoptes scabiei var. hominis, infests human skin, causing allergic reactions and facilitating bacterial infection by Streptococcus sp., with serious consequences such as rheumatic fever and rheumatic heart disease. To identify a possible drug target or vaccine candidate protein, we searched for homologues of the group 3 allergen of house dust mites, which we subsequently identified in a cDNA library. The native protein, designated Sar s 3, was shown to be present in the mite gut and excreted in fecal pellets into mite burrows within the upper epidermis. The substrate specificity of proteolytically active recombinant rSar s 3 was elucidated by screening a bacteriophage library. A preference for substrates containing a RS(G/A) sequence at the P1-P2' positions was revealed. A series of peptides synthesized as internally quenched fluorescent substrates validated the phage display data and high performance liquid chromatography/mass spectrometry analysis of the preferred cleaved substrate and confirmed the predicted cleavage site. Searches of the human proteome using sequence data from the phage display allowed the in silico prediction of putative physiological substrates. Among these were numerous epidermal proteins, with filaggrin being a particularly likely candidate substrate. We showed that recombinant rSar s 3 cleaves human filaggrin in vitro and obtained immunohistological evidence that the filaggrin protein is ingested by the mite. This is the first report elucidating the substrate specificity of Sar s 3 and its potential role in scabies mite biology.

A major cathepsin B protease from the liver fluke Fasciola hepatica has atypical active site features and a potential role in the digestive tract of newly excysted juvenile parasites

The newly excysted juvenile (NEJ) stage of the Fasciola hepatica lifecycle occurs just prior to invasion into the wall of the gut of the host, rendering it an important target for drug development. The cathepsin B enzymes from NEJ flukes have recently been demonstrated to be crucial to invasion and migration by the parasite. Here we characterize one of the cathepsin B enzymes (recombinant FhcatB1) from NEJ flukes. FhcatB1 has biochemical properties distinct from mammalian cathepsin B enzymes, with an atypical preference for Ile over Leu or Arg residues at the P(2) substrate position and an inability to act as an exopeptidase. FhcatB1 was active across a broad pH range (optimal activity at pH 5.5-7.0) and resistant to inhibition by cystatin family inhibitors from sheep and humans, suggesting that this enzyme would be able to function in extracellular environments in its mammalian hosts. It appears, however, that the FhcatB1 protease functions largely as a digestive enzyme in the gut of the parasite, due to the localization of a specific, fluorescently labeled inhibitor with an Ile at the P(2) position. Molecular modelling and dynamics were used to predict the basis for the unusual substrate specificity: a P(2) Ile residue positions the substrate optimally for interaction with catalytic residues of the enzyme, and the enzyme lacks an occluding loop His residue crucial for exopeptidase activity. The unique features of the enzyme, particularly with regard to its specificity and likely importance to a vital stage of the parasite's life cycle, make it an excellent target for therapeutic inhibitors or vaccination.

The gingipains: scissors and glue of the periodontal pathogen, Porphyromonas gingivalis

The anaerobic bacterium, Porphyromonas gingivalis, is associated with chronic periodontal disease (periodontitis or gum disease). The disease is not only the leading cause of tooth loss in the developed world, but is associated with a number of systemic diseases, such as cardiovascular disease and diabetes. The most potent virulence factors of this bacterium are the gingipains, three cysteine proteases that bind and cleave a wide range of host proteins. This article summarizes current knowledge of the structure and function of the enzymes, with a particular focus on what remains to be elucidated regarding the structure and function of the nonenzymatic adhesin domains of the high-molecular-weight forms of the proteases.

High molecular weight gingipains from Porphyromonas gingivalis induce cytokine responses from human macrophage-like cells via a nonproteolytic mechanism

Periodontal disease is an oral inflammatory disease affecting the supporting structures of teeth. Porphyromonas gingivalis, a major pathogenic agent for the disease, expresses a number of virulence factors, including cysteine proteases called the gingipains. The arginine- and lysine-specific gingipains, HRgpA and Kgp, respectively, are expressed as high molecular weight forms containing both catalytic and adhesin subunits. We examined the expression pattern of cytokines and their receptors in differentiated macrophages following exposure to active and inactive forms of the gingipains, using a cDNA array, quantitative PCR and ELISA analysis. Amongst other pro-inflammatory cytokines, results from the cDNA array suggested that interleukin-1beta, granulocyte-macrophage colony stimulatory factor and interferon-gamma were upregulated after exposure of the macrophages to the gingipains. Quantitative PCR analysis substantiated these observations and indicated that active or inactive forms of the high molecular weight gingipains were able to upregulate expression of transcripts for these cytokines. The strongly enhanced production of interleukin-1beta and granulocyte-macrophage colony stimulatory factor by differentiated macrophages in response to active or inactive forms of the high molecular weight gingipains was confirmed at the protein level by ELISA analysis. The results indicate that the adhesin subunits of the gingipains mediate strong upregulation of the expression of pro-inflammatory cytokines in macrophages.

Snake venom metalloproteinases, crotarhagin and alborhagin, induce ectodomain shedding of the platelet collagen receptor, glycoprotein VI

Glycoprotein (GP)VI, that binds collagen, together with GPIb-IX-V which binds vonWillebrand factor, forms an adheso-signalling complex on platelets that initiates thrombus formation in haemostasis and thrombosis. In this study, we show that two snake venom metalloproteinases, crotarhagin and alborhagin, induce ectodomain shedding of GPVI by a mechanism that involves activation of endogenous platelet metalloproteinases. Alborhagin is a viper venom metalloproteinase from Trimeresurus albolabris, while crotarhagin is a previously undescribed toxin from the rattlesnake Crotalus horridus horridus ( approximately 60-kDa non-reduced and reduced). Like alborhagin, crotarhagin induces aggregation in human platelet-rich plasma (maximal activity, approximately 0.3 microg/ml). Aggregation of washed platelets was inhibited by soluble GPVI ectodomain expressed as an Fc-fusion protein, confirming crotarhagin targeted GPVI. Treating washed platelets with crotarhagin or alborhagin resulted in time-dependent loss of surface GPVI and the appearance of an approximately 55-kDa soluble GPVI fragment in supernatants. Crotarhagin also induced shedding in GPVItransfected RBL-2H3 cells. Crotarhagin-induced shedding was metalloproteinase-dependent (inhibited by EDTA), but also blocked by inhibitors of GPVI signalling (Src kinase inhibitors, PP1 or PP2, or Syk inhibitor, piceatannol), indicating shedding required GPVI-dependent platelet activation. Together, the data suggest that the rattlesnake metalloproteinase, crotarhagin, and the viper toxin alborhagin, induce GPVI shedding by a mechanism involving activation of endogenous platelet metalloproteinases rather than direct cleavage of GPVI.

Elucidation of the substrate specificity of the MASP-2 protease of the lectin complement pathway and identification of the enzyme as a major physiological target of the serpin, C1-inhibitor

Complement is a central component of host defence, but unregulated activation can contribute to disease. The system can be initiated by three pathways: classical, alternative and lectin. The classical and lectin pathways are initiated by the C1 and mannose-binding lectin (MBL) or ficolin complexes, respectively, with C1s the executioner protease of the C1 complex and MASP-2 its counterpart in the lectin complexes. These proteases in turn cleave the C4 and C2 components of the system. Here we have elucidated the cleavage specificity of MASP-2 using a randomised substrate phage display library. Apart from the crucial P1 position, the MASP-2 S2 and S3 subsites (in that order) play the greatest role in determining specificity, with Gly residues preferred at P2 and Leu or hydrophobic residues at P3. Cleavage of peptide substrates representing the known physiological cleavage sequences in C2, C4 or the serpin C1-inhibitor (a likely regulator of MASP-2) revealed that MASP-2 is up to 1000 times more catalytically active than C1s. C1-inhibitor inhibited MASP-2 50-fold faster than C1s and much faster than any other protease tested to date, implying that MASP-2 is a major physiological target of C1-inhibitor.

The initiating proteases of the complement system: controlling the cleavage

The complement system is a vital component of the host immune system, but when dysregulated, can also cause disease. The system is activated by three pathways: classical, lectin and alternative. The initiating proteases of the classical and lectin pathways have similar domain structure and employ similar mechanisms of activation. The C1r, C1s and MASP-2 proteases have the most defined roles in the activation of the system. This review focuses on the mechanisms whereby their interaction with substrates and inhibitors is regulated.

Fractionation of snake venom metalloproteinases by metal ion affinity: a purified cobra metalloproteinase, Nk, from Naja kaouthia binds Ni2+-agarose

Snake venom metalloproteinases represent unique probes for analyzing platelet adhesion receptors regulating hemostasis and thrombosis. Snake venom metalloproteinase-disintegrins consist of a propeptide domain, a catalytic domain containing a metal ion-coordination sequence (HEXXHXXGXXH), a disintegrin domain, and a Cys-rich domain. Here, we investigate whether metal ion-affinity chromatography may be used to fractionate venom metalloproteinases based on the metal ion-coordination motif. First, we showed that a purified cobra metalloproteinase, Nk, from Naja kaouthia bound Ni(2+)-agarose, and was eluted by approximately 10mM imidazole, confirming the validity of the approach. Nk cleaved the platelet von Willebrand factor (VWF) receptor, glycoprotein (GP)Ibalpha, with similar activity to the previously reported cobra metalloproteinase, mocarhagin, as shown by EDTA-inhibitable Nk-dependent proteolysis of a purified GPIbalpha extracellular fragment (glycocalicin), and inhibition of (125)I-VWF binding to GPIbalpha on washed human or canine platelets. Second, crude venom from the viper, Trimeresurus albolabris, was fractionated on Ni(2+)-agarose. Samples of flow-through, wash, and imidazole-eluted (0-30mM gradient) fractions were analyzed by (i) SDS-polyacrylamide gel electrophoresis, (ii) immunoblotting with a rabbit anti-mocarhagin antibody, and (iii) assessing metalloproteinase activity using human fibrinogen as substrate. The combined results support the general concept of using Ni(2+)-agarose to fractionate snake venom metalloproteinases.

Snake venom probes of platelet adhesion receptors and their ligands

Snake venom proteins that modulate platelet adhesive interactions are chiefly from either of two main structural families: the C-type lectin-like family, or the metalloproteinase-disintegrins. Snake venom probes from both families selectively target platelet adhesion receptors, including glycoprotein (GP) Ib-IX-V, GP VI, alpha2beta1 and alphaIIbbeta3 (GP IIb-IIIa). These receptors act together to mediate platelet adhesion, activation and aggregation (thrombus formation) under hydrodynamic shear stress in flowing blood. The receptors are members of the leucine-rich repeat family (GP Ib-IX-V), the immunoglobulin superfamily (GP VI), or integrins (alpha2beta1, alphaIIbbeta3). In addition, adhesive glycoproteins in matrix and/or plasma such as von Willebrand factor (that binds GP Ibalpha and alphaIIbbeta3), collagen (that binds GP V, GP VI and alpha2beta1), or fibrinogen (that binds alphaIIbbeta3), are also targeted by C-type lectin family or metalloproteinase-disintegrin snake venom proteins. Emerging structural and functional evidence is beginning to explain how interactions between the conserved structural module-domains that make up these mammalian and snake proteins are regulated. Whether homologous adhesion/counter-receptors on platelets and other vascular cells are also potential snake venom targets is as yet largely unexplored.

Requirement of multiple phage displayed peptide libraries for optimal mapping of a conformational antibody epitope on CCR5

In the absence of information from crystallography, conformational epitopes can often be discerned by antibody screening of phage displayed random peptide libraries. However the context in which the peptide is displayed, and the number of copies displayed in the library, can influence results and interpretations. Here, the monoclonal antibodies 3A9 specific for the transmembrane chemokine receptor CCR5, and CII-C1 specific for type II collagen, were used to screen multiple phage-displayed peptide libraries in which peptides were displayed in either the pIII or pVIII coat proteins. ELISA was used to test for reactivity and cross-inhibitory activity of isolated phage clones. Based on sequences of reactive phage inserts, epitope motifs were initially inferred from a molecular model of CCR5 and subsequently confirmed experimentally using mutagenesis to alanine. For each mAb, phage sequences from pIII biopannings were more diverse than from pVIII biopannings. Notably, sequences from either biopanning were cross-inhibitory despite a lack of linear sequence homology. For CCR5, residues 88H and 94W in the first loop of CCR5 were identified by pIII biopannings, and 7S9IYD11 at the N-terminus by pVIII biopannings. Thus conformational epitopes can be identified using phage display, but optimal mapping of complex epitopes can require the use of multiple peptide libraries.

Prediction of the immunodominant epitope of the pyruvate dehydrogenase complex E2 in primary biliary cirrhosis using phage display

Primary biliary cirrhosis (PBC) is an autoimmune liver disease characterized by autoantibodies reactive with the pyruvate dehydrogenase complex. A conformational epitope has been mapped to aa 91-227 within the inner lipoyl domain of the E2 subunit (pyruvate dehydrogenase complex E2 (PDC-E2)). We have used phage display to further localize this epitope. A random heptapeptide library was screened using IgG from two patients with PBC, with negative selection using pooled normal IgG. Phage that contained peptide inserts (phagotopes) selected using PBC sera differed from those selected using IgG from patients with RA or polychondritis. Two motifs occurred only among the PBC-selected phagotopes; these were MH (13 sequences, 16 phagotopes) and FV (FVEHTRW, FVEIYSP, FVLPWRI). The phagotopes selected were tested for reactivity with anti-PDC-E2 affinity purified from four patients with PBC. Phagotopes that contained 1 of 15 different peptide sequences were reactive with one or more of these four anti-PDC-E2 preparations, whereas phagotopes that contained 1of the remaining 28 sequences were negative. The peptides (FVLPWRI, MHLNTPP, MHLTQSP) encoded by three phagotopes that were strongly reactive with all four preparations of anti-PDC-E2 were synthesized. Each of the selected peptides, but not an irrelevant peptide, inhibited the reactivity by ELISA of PBC serum with recombinant PDC-E2 and reduced the inhibition of the enzyme activity of PDC by a PBC serum. The peptide sequences, along with the known NMR structure of the inner lipoyl domain of PDC-E2, allow the prediction of nonsequential residues 131HM132 and 178FEV180 that contribute to a conformational epitope.