The Structure of MBL-associated Serine Protease-2 Reveals that Identical Substrate Specificities of C1s and MASP-2 are Realized Through Different Sets of Enzyme–Substrate Interactions

Granzyme K drives a newly-intentified pathway of complement activation

Granzymes are a family of serine proteases mainly expressed by CD8+ T cells, natural killer cells, and innate-like lymphocytes1,2. Although their major role is thought to be the induction of cell death in virally infected and tumor cells, accumulating evidence suggests some granzymes can regulate inflammation by acting on extracellular substrates2. Recently, we found that the majority of tissue CD8+ T cells in rheumatoid arthritis (RA) synovium, inflammatory bowel disease and other inflamed organs express granzyme K (GZMK)3, a tryptase-like protease with poorly defined function. Here, we show that GZMK can activate the complement cascade by cleaving C2 and C4. The nascent C4b and C2a fragments form a C3 convertase that cleaves C3, allowing further assembly of a C5 convertase that cleaves C5. The resulting convertases trigger every major event in the complement cascade, generating the anaphylatoxins C3a and C5a, the opsonins C4b and C3b, and the membrane attack complex. In RA synovium, GZMK is enriched in areas with abundant complement activation, and fibroblasts are the major producers of complement C2, C3, and C4 that serve as targets for GZMK-mediated complement activation. Our findings describe a previously unidentified pathway of complement activation that is entirely driven by lymphocyte-derived GZMK and proceeds independently of the classical, lectin, or alternative pathways. Given the widespread abundance of GZMK-expressing T cells in tissues in chronic inflammatory diseases and infection, GZMK-mediated complement activation is likely to be an important contributor to tissue inflammation in multiple disease contexts.

Synergy of protease-binding sites within the ecotin homodimer is crucial for inhibition of MASP enzymes and for blocking lectin pathway activation

Ecotin is a homodimeric serine protease inhibitor produced by many commensal and pathogenic microbes. It functions as a virulence factor, enabling survival of various pathogens in the blood. The ecotin dimer binds two protease molecules, and each ecotin protomer has two protease-binding sites: site1 occupies the substrate-binding groove, whereas site2 engages a distinct secondary region. Owing to the twofold rotational symmetry within the ecotin dimer, sites 1 and 2 of a protomer bind to different protease molecules within the tetrameric complex. Escherichia coli ecotin inhibits trypsin-like, chymotrypsin-like, and elastase-like enzymes, including pancreatic proteases, leukocyte elastase, key enzymes of blood coagulation, the contact and complement systems, and other antimicrobial cascades. Here, we show that mannan-binding lectin-associated serine protease-1 (MASP-1) and MASP-2, essential activators of the complement lectin pathway, and MASP-3, an essential alternative pathway activator, are all inhibited by ecotin. We decipher in detail how the preorganization of site1 and site2 within the ecotin dimer contributes to the inhibition of each MASP enzyme. In addition, using mutated and monomeric ecotin variants, we show that site1, site2, and dimerization contribute to inhibition in a surprisingly target-dependent manner. We present the first ecotin:MASP-1 and ecotin:MASP-2 crystal structures, which provide additional insights and permit structural interpretation of the observed functional results. Importantly, we reveal that monomerization completely disables the MASP-2-inhibitory, MASP-3-inhibitory, and lectin pathway-inhibitory capacity of ecotin. These findings provide new opportunities to combat dangerous multidrug-resistant pathogens through development of compounds capable of blocking ecotin dimer formation.

Directed Evolution-Driven Increase of Structural Plasticity Is a Prerequisite for Binding the Complement Lectin Pathway Blocking MASP-Inhibitor Peptides

MASP-1 and MASP-2 are key activator proteases of the complement lectin pathway. The first specific mannose-binding lectin-associated serine protease (MASP) inhibitors had been developed from the 14-amino-acid sunflower trypsin inhibitor (SFTI) peptide by phage display, yielding SFTI-based MASP inhibitors, SFMIs. Here, we present the crystal structure of the MASP-1/SFMI1 complex that we analyzed in comparison to other existing MASP-1/2 structures. Rigidified backbone structure has long been accepted as a structural prerequisite for peptide inhibitors of proteases. We found that a hydrophobic cluster organized around the P2 Thr residue is essential for the structural stability of wild-type SFTI. We also found that the same P2 Thr prevents binding of the rigid SFTI-like peptides to the substrate-binding cleft of both MASPs as the cleft is partially blocked by large gatekeeper enzyme loops. Directed evolution removed this obstacle by replacing the P2 Thr with a Ser, providing the SFMIs with high-degree structural plasticity, which proved to be essential for MASP inhibition. To gain more insight into the structural criteria for SFMI-based MASP-2 inhibition, we systematically modified MASP-2-specific SFMI2 by capping its two termini and by replacing its disulfide bridge with varying length thioether linkers. By doing so, we also aimed to generate a versatile scaffold that is resistant to reducing environment and has increased stability in exopeptidase-containing biological environments. We found that the reduction-resistant disulfide-substituted l-2,3-diaminopropionic acid (Dap) variant possessed near-native potency. As MASP-2 is involved in the life-threatening thrombosis in COVID-19 patients, our synthetic, selective MASP-2 inhibitors could be relevant coronavirus drug candidates.

New Insights into the Role of the Complement System in Human Viral Diseases

The complement system (CS) is part of the human immune system, consisting of more than 30 proteins that play a vital role in the protection against various pathogens and diseases, including viral diseases. Activated via three pathways, the classical pathway (CP), the lectin pathway (LP), and the alternative pathway (AP), the complement system leads to the formation of a membrane attack complex (MAC) that disrupts the membrane of target cells, leading to cell lysis and death. Due to the increasing number of reports on its role in viral diseases, which may have implications for research on severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), this review aims to highlight significant progress in understanding and defining the role of the complement system in four groups of diseases of viral etiology: (1) respiratory diseases; (2) acute liver failure (ALF); (3) disseminated intravascular coagulation (DIC); and (4) vector-borne diseases (VBDs). Some of these diseases already present a serious global health problem, while others are a matter of concern and require the collaboration of relevant national services and scientists with the World Health Organization (WHO) to avoid their spread.

Targeting the Complement Serine Protease MASP-2 as a Therapeutic Strategy for Coronavirus Infections

MASP-2, mannose-binding protein-associated serine protease 2, is a key enzyme in the lectin pathway of complement activation. Hyperactivation of this protein by human coronaviruses SARS-CoV, MERS-CoV and SARS-CoV-2 has been found to contribute to aberrant complement activation in patients, leading to aggravated lung injury with potentially fatal consequences. This hyperactivation is triggered in the lungs through a conserved, direct interaction between MASP-2 and coronavirus nucleocapsid (N) proteins. Blocking this interaction with monoclonal antibodies and interfering directly with the catalytic activity of MASP-2, have been found to alleviate coronavirus-induced lung injury both in vitro and in vivo. In this study, a virtual library of 8736 licensed drugs and clinical agents has been screened in silico according to two parallel strategies. The first strategy aims at identifying direct inhibitors of MASP-2 catalytic activity, while the second strategy focusses on finding protein-protein interaction inhibitors (PPIs) of MASP-2 and coronaviral N proteins. Such agents could represent promising support treatment options to prevent lung injury and reduce mortality rates of infections caused by both present and future-emerging coronaviruses. Forty-six drug repurposing candidates were purchased and, for the ones selected as potential direct inhibitors of MASP-2, a preliminary in vitro assay was conducted to assess their interference with the lectin pathway of complement activation. Some of the tested agents displayed a dose-response inhibitory activity of the lectin pathway, potentially providing the basis for a viable support strategy to prevent the severe complications of coronavirus infections.

Surface loops of trypsin-like serine proteases as determinants of function

Trypsin and chymotrypsin-like serine proteases from family S1 (clan PA) constitute the largest protease group in humans and more generally in vertebrates. The prototypes chymotrypsin, trypsin and elastase represent simple digestive proteases in the gut, where they cleave nearly any protein. Multidomain trypsin-like proteases are key players in the tightly controlled blood coagulation and complement systems, as well as related proteases that are secreted from diverse immune cells. Some serine proteases are expressed in nearly all tissues and fluids of the human body, such as the human kallikreins and kallikrein-related peptidases with specialization for often unique substrates and accurate timing of activity. HtrA and membrane-anchored serine proteases fulfill important physiological tasks with emerging roles in cancer. The high diversity of all family members, which share the tandem β-barrel architecture of the chymotrypsin-fold in the catalytic domain, is conferred by the large differences of eight surface loops, surrounding the active site. The length of these loops alters with insertions and deletions, resulting in remarkably different three-dimensional arrangements. In addition, metal binding sites for Na+, Ca2+ and Zn2+ serve as regulatory elements, as do N-glycosylation sites. Depending on the individual tasks of the protease, the surface loops determine substrate specificity, control the turnover and allow regulation of activation, activity and degradation by other proteins, which are often serine proteases themselves. Most intriguingly, in some serine proteases, the surface loops interact as allosteric network, partially tuned by protein co-factors. Knowledge of these subtle and complicated molecular motions may allow nowadays for new and specific pharmaceutical or medical approaches.

Leishmania donovani Inhibitor of Serine Peptidases 2 Mediated Inhibition of Lectin Pathway and Upregulation of C5aR Signaling Promote Parasite Survival inside Host

Leishmania donovani, the causative agent of Indian visceral leishmaniasis has to face several barriers of the immune system inside the mammalian host for its survival. The complement system is one of the first barriers and consists of a well-balanced network of proteases including S1A family serine proteases (SPs). Inhibitor of serine peptidases (ISPs) is considered as inhibitor of S1A family serine peptidases and is reported to be present in trypanosomes, including Leishmania. In our previous study, we have deciphered the role of ISPs [LdISP1 and L. donovani inhibitor of serine peptidases 2 (LdISP2)] in the survival of L. donovani inside the sandfly midgut. However, the role of theses ISPs in the survival of L. donovani inside mammalian host still remains elusive. In the present study, we have deciphered the inhibitory effect of LdISPs on the host complement S1A serine peptidases, such as C1r/C1s and MASP1/MASP2. Our study suggested that although both rLdISP1 and rLdISP2 inferred strong interaction with C1complex and MBL-associated serine proteases (MASPs) but rLdISP2 showed the stronger inhibitory effect on MASP2 than rLdISP1. Moreover, we found that rLdISP2 significantly reduces the formation of C3, C5 convertase, and membrane attacking complex (MAC) by lectin pathway (LP) resulting in significant reduction in serum mediated lysis of the parasites. The role of LdISP2 on neutrophil elastase-mediated C5aR signaling was also evaluated. Notably, our results showed that infection of macrophages with ISP2-overexpressed Leishmania parasites significantly induces the expression of C5aR both at the transcript and translational level. Simultaneously, infection with ISP2KD parasites results in downregulation of host PI3K/AKT phosphorylation and increased in IL-12 production. Taken together, our findings clearly suggest that LdISP2 promotes parasite survival inside host by inhibiting MAC formation and complement-mediated lysis via LP and by upregulation of C5aR signaling.

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.

The emerging roles of mannose-binding lectin-associated serine proteases (MASPs) in the lectin pathway of complement and beyond

Mannose-binding lectin (MBL)-associated serine proteases (MASPs) are the enzymatic constituents of the lectin pathway of the complement system. They are complexed with large pattern recognition molecules (PRMs) such as MBL, other collectins, and ficolins. The main function of two of the three MASPs has crystallized lately: MASP-1 autoactivates first, then it activates MASP-2, and finally both participate in the formation of the C4b2a convertase. In addition to this, both enzymes are involved in several other processes which are subject to intense research nowadays. Notably, MASP-1, as a promiscuous enzyme, has been implicated in the coagulation cascade, in the kinin generating contact system, and in cellular activation through protease-activated receptor (PAR) cleavage on endothelial cells. The third protease MASP-3 has emerged recently as the protease responsible for pro-factor D activation in resting blood, providing a fundamental link between two complement pathways. At present all three MASPs have at least one well-defined role and several other possible functions were implicated. Defect or more likely over-activation of MASPs may culminate into diseases such as ischemia-reperfusion injury (IRI); hence, MASPs are all potential targets of drug development.

Insight into the intermolecular recognition mechanism involved in complement component 4 activation through serine protease-trypsin

Serine protease cleaved-complement component 4 (C4) at sessile loop, which is significant for completion of lectin and classical complement pathways at the time of infections. The co-crystalized structure of C4 with Mannose-binding protein-associated serine protease 2 (MASP2) provided the structural and functional aspects of its interaction and underlined the C4 activation by MASP2. The same study also revealed the significance of complement control protein (CCP) domain through mutational study, where mutated CCP domain led to the inhibition of C4 activation. However, the interaction of trypsin serine domain with C4α sessile loop revealed another aspect of C4 activation. The human C4 cleavage by Trypsin (Tryp) in a control manner was explored but not yet revealed the identification of cleaved fragments. Hence, the present study investigated the Tryp mediated C4 activation using computational approach (protein-protein docking and molecular dynamics simulation) by comparing with the co-crystalized structure of C4-MASP2. Docking result identified the crucial interacting residues Gly219, Gln178, and Asn102 of Tryp catalytic pocket which were interacting with Arg756 and Glu759 (sessile loop) of α-Chain (C4) in a similar manner to C4-MASP2 co-crystallized complex. Moreover, MD simulation results and mutational study underlined the conformational rearrangements in the C4 due to the Tryp interaction. Comparative analysis of C4 alone, C4-Tryp, and C4-MASP2 revealed the impact of Tryp on C4 was similar as MASP2. These studies designate the role of sessile loop in the interaction with serine domain, which could be useful to understand the various interactions of C4 with other complement components.

Flexibility in Mannan-Binding Lectin-Associated Serine Proteases-1 and -2 Provides Insight on Lectin Pathway Activation

The lectin pathway of complement is activated by complexes comprising a recognition component (mannose-binding lectin, serum ficolins, collectin-LK or collectin-K1) and a serine protease (MASP-1 or MASP-2). MASP-1 activates MASP-2, and MASP-2 cleaves C4 and C4b-bound C2. To clarify activation, new crystal structures of Ca²⁺-bound MASP dimers were determined, together with their solution structures from X-ray scattering, analytical ultracentrifugation, and atomistic modeling. Solution structures of the CUB1-EGF-CUB2 dimer of each MASP indicate that the two CUB2 domains were tilted by as much as 90° compared with the crystal structures, indicating considerable flexibility at the EGF-CUB2 junction. Solution structures of the full-length MASP dimers in their zymogen and activated forms revealed similar structures that were much more bent than anticipated from crystal structures. We conclude that MASP-1 and MASP-2 are flexible at multiple sites and that this flexibility may permit both intra- and inter-complex activation.

A journey through the lectin pathway of complement-MBL and beyond

Mannose-binding lectin (MBL), collectin-10, collectin-11, and the ficolins (ficolin-1, ficolin-2, and ficolin-3) are soluble pattern recognition molecules in the lectin complement pathway. These proteins act as mediators of host defense and participate in maintenance of tissue homeostasis. They bind to conserved pathogen-specific structures and altered self-antigens and form complexes with the pentraxins to modulate innate immune functions. All molecules exhibit distinct expression in different tissue compartments, but all are found to a varying degree in the circulation. A common feature of these molecules is their ability to interact with a set of serine proteases named MASPs (MASP-1, MASP-2, and MASP-3). MASP-1 and -2 trigger the activation of the lectin pathway and MASP-3 may be involved in the activation of the alternative pathway of complement. Furthermore, MASPs mediate processes related to coagulation, bradykinin release, and endothelial and platelet activation. Variant alleles affecting expression and structure of the proteins have been associated with a variety of infectious and non-infectious diseases, most commonly as disease modifiers. Notably, the severe 3MC (Malpuech, Michels, Mingarelli, and Carnevale) embryonic development syndrome originates from rare mutations affecting either collectin-11 or MASP-3, indicating a broader functionality of the complement system than previously anticipated. This review summarizes the characteristics of the molecules in the lectin pathway.

Re-evaluation of low-resolution crystal structures via interactive molecular-dynamics flexible fitting (iMDFF): a case study in complement C4

While the rapid proliferation of high-resolution structures in the Protein Data Bank provides a rich set of templates for starting models, it remains the case that a great many structures both past and present are built at least in part by hand-threading through low-resolution and/or weak electron density. With current model-building tools this task can be challenging, and the de facto standard for acceptable error rates (in the form of atomic clashes and unfavourable backbone and side-chain conformations) in structures based on data with dmax not exceeding 3.5 Å reflects this. When combined with other factors such as model bias, these residual errors can conspire to make more serious errors in the protein fold difficult or impossible to detect. The three recently published 3.6-4.2 Å resolution structures of complement C4 (PDB entries 4fxg, 4fxk and 4xam) rank in the top quartile of structures of comparable resolution both in terms of Rfree and MolProbity score, yet, as shown here, contain register errors in six β-strands. By applying a molecular-dynamics force field that explicitly models interatomic forces and hence excludes most physically impossible conformations, the recently developed interactive molecular-dynamics flexible fitting (iMDFF) approach significantly reduces the complexity of the conformational space to be searched during manual rebuilding. This substantially improves the rate of detection and correction of register errors, and allows user-guided model building in maps with a resolution lower than 3.5 Å to converge to solutions with a stereochemical quality comparable to atomic resolution structures. Here, iMDFF has been used to individually correct and re-refine these three structures to MolProbity scores of <1.7, and strategies for working with such challenging data sets are suggested. Notably, the improved model allowed the resolution for complement C4b to be extended from 4.2 to 3.5 Å as demonstrated by paired refinement.

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.

The Role of the Lys628 (192) Residue of the Complement Protease, C1s, in Interacting with Peptide and Protein Substrates

The C1s protease of the classical complement pathway propagates the initial activation of this pathway of the system by cleaving and thereby activating the C4 and C2 complement components. This facilitates the formation of the classical pathway C3 convertase (C4bC2a). C1s has a Lys residue located at position 628 (192 in chymotrypsin numbering) of the SP domain that has the potential to partially occlude the S2–S2′ positions of the active site. The 192 residue of serine proteases generally plays an important role in interactions with substrates. We therefore investigated the role of Lys628 (192) in interactions with C4 by altering the Lys residue to either a Gln (found in many other serine proteases) or an Ala residue. The mutant enzymes had altered specificity profiles for a combinatorial peptide substrate library, suggesting that this residue does influence the active site specificity of the protease. Generally, the K628Q mutant had greater activity than wild type enzyme against peptide substrates, while the K628A residue had lowered activity, although this was not always the case. Against peptide substrates containing physiological substrate sequences, the K628Q mutant once again had generally higher activity, but the activity of the wild type and mutant enzymes against a C4 P4–P4′ substrate were similar. Interestingly, alteration of the K628 residue in C1s had a marked effect on the cleavage of C4, reducing cleavage efficiency for both mutants about fivefold. This indicates that this residue plays a different role in cleaving protein versus peptide substrates and that the Lys residue found in the wild type enzyme plays an important role in interacting with the C4 substrate. Understanding the basis of the interaction between C1s and its physiological substrates is likely to lead to insights that can be used to design efficient inhibitors of the enzyme for use in treating diseases caused by inflammation as result of over-activity of the classical complement pathway.

Functional and structural characterization of Vibrio cholerae extracellular serine protease B, VesB

The chymotrypsin subfamily A of serine proteases consists primarily of eukaryotic proteases, including only a few proteases of bacterial origin. VesB, a newly identified serine protease that is secreted by the type II secretion system in Vibrio cholerae, belongs to this subfamily. VesB is likely produced as a zymogen because sequence alignment with trypsinogen identified a putative cleavage site for activation and a catalytic triad, His-Asp-Ser. Using synthetic peptides, VesB efficiently cleaved a trypsin substrate, but not chymotrypsin and elastase substrates. The reversible serine protease inhibitor, benzamidine, inhibited VesB and served as an immobilized ligand for VesB affinity purification, further indicating its relationship with trypsin-like enzymes. Consistent with this family of serine proteases, N-terminal sequencing implied that the propeptide is removed in the secreted form of VesB. Separate mutagenesis of the activation site and catalytic serine rendered VesB inactive, confirming the importance of these features for activity, but not for secretion. Similar to trypsin but, in contrast to thrombin and other coagulation factors, Na(+) did not stimulate the activity of VesB, despite containing the Tyr(250) signature. The crystal structure of catalytically inactive pro-VesB revealed that the protease domain is structurally similar to trypsinogen. The C-terminal domain of VesB was found to adopt an immunoglobulin (Ig)-fold that is structurally homologous to Ig-folds of other extracellular Vibrio proteins. Possible roles of the Ig-fold domain in stability, substrate specificity, cell surface association, and type II secretion of VesB, the first bacterial multidomain trypsin-like protease with known structure, are discussed.

The serine protease domain of MASP-3: enzymatic properties and crystal structure in complex with ecotin

Mannan-binding lectin (MBL), ficolins and collectin-11 are known to associate with three homologous modular proteases, the MBL-Associated Serine Proteases (MASPs). The crystal structures of the catalytic domains of MASP-1 and MASP-2 have been solved, but the structure of the corresponding domain of MASP-3 remains unknown. A link between mutations in the MASP1/3 gene and the rare autosomal recessive 3MC (Mingarelli, Malpuech, Michels and Carnevale,) syndrome, characterized by various developmental disorders, was discovered recently, revealing an unexpected important role of MASP-3 in early developmental processes. To gain a first insight into the enzymatic and structural properties of MASP-3, a recombinant form of its serine protease (SP) domain was produced and characterized. The amidolytic activity of this domain on fluorescent peptidyl-aminomethylcoumarin substrates was shown to be considerably lower than that of other members of the C1r/C1s/MASP family. The E. coli protease inhibitor ecotin bound to the SP domains of MASP-3 and MASP-2, whereas no significant interaction was detected with MASP-1, C1r and C1s. A tetrameric complex comprising an ecotin dimer and two MASP-3 SP domains was isolated and its crystal structure was solved and refined to 3.2 Å. Analysis of the ecotin/MASP-3 interfaces allows a better understanding of the differential reactivity of the C1r/C1s/MASP protease family members towards ecotin, and comparison of the MASP-3 SP domain structure with those of other trypsin-like proteases yields novel hypotheses accounting for its zymogen-like properties in vitro.

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.

Quantitative characterization of the activation steps of mannan-binding lectin (MBL)-associated serine proteases (MASPs) points to the central role of MASP-1 in the initiation of the complement lectin pathway

Mannan-binding lectin (MBL)-associated serine proteases, MASP-1 and MASP-2, have been thought to autoactivate when MBL/ficolin·MASP complexes bind to pathogens triggering the complement lectin pathway. Autoactivation of MASPs occurs in two steps: 1) zymogen autoactivation, when one proenzyme cleaves another proenzyme molecule of the same protease, and 2) autocatalytic activation, when the activated protease cleaves its own zymogen. Using recombinant catalytic fragments, we demonstrated that a stable proenzyme MASP-1 variant (R448Q) cleaved the inactive, catalytic site Ser-to-Ala variant (S646A). The autoactivation steps of MASP-1 were separately quantified using these mutants and the wild type enzyme. Analogous mutants were made for MASP-2, and rate constants of the autoactivation steps as well as the possible cross-activation steps between MASP-1 and MASP-2 were determined. Based on the rate constants, a kinetic model of lectin pathway activation was outlined. The zymogen autoactivation rate of MASP-1 is ∼3000-fold higher, and the autocatalytic activation of MASP-1 is about 140-fold faster than those of MASP-2. Moreover, both activated and proenzyme MASP-1 can effectively cleave proenzyme MASP-2. MASP-3, which does not autoactivate, is also cleaved by MASP-1 quite efficiently. The structure of the catalytic region of proenzyme MASP-1 R448Q was solved at 2.5 Å. Proenzyme MASP-1 R448Q readily cleaves synthetic substrates, and it is inhibited by a specific canonical inhibitor developed against active MASP-1, indicating that zymogen MASP-1 fluctuates between an inactive and an active-like conformation. The determined structure provides a feasible explanation for this phenomenon. In summary, autoactivation of MASP-1 is crucial for the activation of MBL/ficolin·MASP complexes, and in the proenzymic phase zymogen MASP-1 controls the process.

Structural basis for activation of the complement system by component C4 cleavage

An essential aspect of innate immunity is recognition of molecular patterns on the surface of pathogens or altered self through the lectin and classical pathways, two of the three well-established activation pathways of the complement system. This recognition causes activation of the MASP-2 or the C1s serine proteases followed by cleavage of the protein C4. Here we present the crystal structures of the 203-kDa human C4 and the 245-kDa C4·MASP-2 substrate·enzyme complex. When C4 binds to MASP-2, substantial conformational changes in C4 are induced, and its scissile bond region becomes ordered and inserted into the protease catalytic site in a manner canonical to serine proteases. In MASP-2, an exosite located within the CCP domains recognizes the C4 C345C domain 60 Å from the scissile bond. Mutations in C4 and MASP-2 residues at the C345C-CCP interface inhibit the intermolecular interaction and C4 cleavage. The possible assembly of the huge in vivo enzyme-substrate complex consisting of glycan-bound mannan-binding lectin, MASP-2, and C4 is discussed. Our own and prior functional data suggest that C1s in the classical pathway of complement activated by, e.g., antigen-antibody complexes, also recognizes the C4 C345C domain through a CCP exosite. Our results provide a unified structural framework for understanding the early and essential step of C4 cleavage in the elimination of pathogens and altered self through two major pathways of complement activation.

The modular serine proteases of the complement cascade

Modular serine proteases are central to the complement cascade of the mammalian humoral immune system. These proteases form protein complexes through multi-domain interactions to achieve their proteolytic activity. We review the structural insights into complement initiation by auto-activation of the hetero-tetrameric proteases of the large danger-recognition protein complexes, amplification and labelling of particles by the formation and activity of C3 convertases, and regulation by convertase dissociation and degradation to prevent 'bystander' damage to healthy host cells and tissues. The data reveal that complex formation and large domain-domain rearrangements underlie the proteolytic reactions of the complement cascade, which enables the host to recognize and clear invading microbes and host debris from its blood and fluids surrounding tissues.

Monospecific inhibitors show that both mannan-binding lectin-associated serine protease-1 (MASP-1) and -2 Are essential for lectin pathway activation and reveal structural plasticity of MASP-2

The lectin pathway is an antibody-independent activation route of the complement system. It provides immediate defense against pathogens and altered self-cells, but it also causes severe tissue damage after stroke, heart attack, and other ischemia reperfusion injuries. The pathway is triggered by target binding of pattern recognition molecules leading to the activation of zymogen mannan-binding lectin-associated serine proteases (MASPs). MASP-2 is considered as the autonomous pathway-activator, while MASP-1 is considered as an auxiliary component. We evolved a pair of monospecific MASP inhibitors. In accordance with the key role of MASP-2, the MASP-2 inhibitor completely blocks the lectin pathway activation. Importantly, the MASP-1 inhibitor does the same, demonstrating that MASP-1 is not an auxiliary but an essential pathway component. We report the first Michaelis-like complex structures of MASP-1 and MASP-2 formed with substrate-like inhibitors. The 1.28 Å resolution MASP-2 structure reveals significant plasticity of the protease, suggesting that either an induced fit or a conformational selection mechanism should contribute to the extreme specificity of the enzyme.

Structural basis of mannan-binding lectin recognition by its associated serine protease MASP-1: implications for complement activation

Complement activation contributes directly to health and disease. It neutralizes pathogens and stimulates immune processes. Defects lead to immunodeficiency and autoimmune diseases, whereas inappropriate activation causes self-damage. In the lectin and classical pathways, complement is triggered upon recognition of a pathogen by an activating complex. Here we present the first structure of such a complex in the form of the collagen-like domain of mannan-binding lectin (MBL) and the binding domain of its associated protease (MASP-1/-3). The collagen binds within a groove using a pivotal lysine side chain that interacts with Ca(2+)-coordinating residues, revealing the essential role of Ca(2+). This mode of binding is prototypic for all activating complexes of the lectin and classical pathways, and suggests a general mechanism for the global changes that drive activation. The structural insights reveal a new focus for inhibitors and we have validated this concept by targeting the binding pocket of the MASP.

Risk factors for infection after liver transplantation

Infection is a common cause of morbidity and mortality after liver transplantation. Risk factors relate to transplantation factors, donor and recipient factors. Transplant factors include ischaemia-reperfusion damage, amount of intra-operative blood transfusion, level and type of immunosuppression, rejection, and complications, prolonged intensive care stay with dialysis or ventilation, type of biliary drainage, repeat operations, re-transplantation, antibiotics, antiviral regimen, and environment. Donor risk factors include infection, prolonged intensive care stay, quality of the donor liver (e.g. steatosis), and viral status. For the recipient the most important are MELD score >30, malnutrition, renal failure, acute liver failure, presence of infection or colonisation, and immune status for viruses like cytomegalovirus. In recent years it has become clear that genetic polymorphisms in innate immunity, especially the lectin pathway of complement activation and in Toll-like receptors importantly contribute to the infection risk after liver transplantation. Therefore, the risk for infections after liver transplantation is a multifactorial problem and all factors need attention to reduce this risk.

Multiple domains of MASP-2, an initiating complement protease, are required for interaction with its substrate C4

The complement system is fundamental to both innate and adaptive immunity and can be initiated via the classical, lectin or alternative pathways. Cleavage of C4 by MASP-2, the initiating protease of the lectin pathway, is a crucial event in the activation of this pathway, preceding the eventual formation of the C3 convertase (C4bC2a) complex on the pathogen surface. Interactions required for the cleavage of C4 by MASP-2 are likely to be facilitated by the initial binding of C4 to an exosite on the protease. We have shown that both proteolytically active and catalytically inactive CCP1-CCP2-serine protease (CCP1-CCP2-SP) forms bind C4 with similar affinity. Interestingly, proteins containing the CCP1-CCP2 domains or the SP domain alone bound C4 with much lower affinity than the CCP1-CCP2-SP protein, suggesting that the CCP domains cooperate positively with the active site to mediate efficient binding and cleavage of C4. In addition, mutation of residue K342 to alanine in the CCP1 domain abolished binding to both C4 and C4b in its CCP1-CCP2 form, suggesting a key electrostatic role for this amino acid. The presented data indicates that all of the domains are required in order to mediate high affinity interaction with C4.

Disease-causing mutations in genes of the complement system

Recent studies have revealed profound developmental consequences of mutations in genes encoding proteins of the lectin pathway of complement activation, a central component of the innate immune system. Apart from impairment of immunity against microorganisms, it is known that hereditary deficiencies of this system predispose one to autoimmune conditions. Polymorphisms in complement genes are linked to, for example, atypical hemolytic uremia and age-dependent macular degeneration. The complement system comprises three convergent pathways of activation: the classical, the alternative, and the lectin pathway. The recently discovered lectin pathway is less studied, but polymorphisms in the plasma pattern-recognition molecule mannan-binding lectin (MBL) are known to impact its level, and polymorphisms in the MBL-associated serine protease-2 (MASP-2) result in defects of complement activation. Recent studies have described roles outside complement and immunity of another MBL-associated serine protease, MASP-3, in the etiology of 3MC syndrome, an autosomal-recessive disorder involving a spectrum of developmental features, including characteristic facial dysmorphism. Syndrome-causing mutations were identified in MASP1, encoding MASP-3 and two additional proteins, MASP-1 and MAp44. Furthermore, an association was discovered between 3MC syndrome and mutations in COLEC11, encoding CL-K1, another molecule of the lectin pathway. The findings were confirmed in zebrafish, indicating that MASP-3 and CL-K1 underlie an evolutionarily conserved pathway of embryonic development. Along with the discovery of a role of C1q in pruning synapses in mice, these recent advances point toward a broader role of complement in development. Here, we compare the functional immunologic consequences of "conventional" complement deficiencies with these newly described developmental roles.

Mannose-binding lectin serine proteases and associated proteins of the lectin pathway of complement: two genes, five proteins and many functions?

The lectin pathway of the complement system is activated following the binding of carbohydrate-based ligands by recognition molecules such as mannose-binding lectin (MBL) or ficolins. Engagement of the recognition molecules causes activation of associated MBL-associated serine proteases or MASPs, which in turn activate downstream complement molecules to activate the system. Two MASP genes are alternatively spliced during expression to yield 5 proteins, including three proteases (MASP-1, -2 and -3) and two truncated proteins, MAp19 and MAp44. Here we discuss what is currently known about these proteins in terms of their structure and function. MASP-2 is autoactivated following the initial binding events of the pathway and is able to subsequently activate the C4 and C2 substrates required to activate the rest of the pathway. MASP-1 is able to augment MASP-2 activation, but also appears to play other roles, although the physiological significance of these is not yet clear. The roles of the truncated Map19 and Map44 proteins and the MASP-3 protease are currently unknown. The proteases form an interesting sub-family of proteins that clearly should be the focus of future research in order to establish their biological roles. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

MASP1 mutations in patients with facial, umbilical, coccygeal, and auditory findings of Carnevale, Malpuech, OSA, and Michels syndromes

Distinctive facial features consisting of hypertelorism, telecanthus, blepharophimosis, blepharoptosis, epicanthus inversus, periumbilical defects, and skeletal anomalies are seen in autosomal-recessive Carnevale, Malpuech, Michels, and oculo-skeletal-abdominal (OSA) syndromes. The gene or genes responsible for these syndromes were heretofore unknown. We report on three individuals from two consanguineous Turkish families with findings characteristic of these syndromes, including facial dysmorphism, periumbilical depression, mixed hearing loss, radioulnar synostosis, and coccygeal appendage. Homozygosity mapping yielded an autozygous region on chromosome 3q27 in both families. In one family, whole exome sequencing revealed a missense mutation, MASP1 c.2059G>A (p.G687R), that cosegregated with the phenotype. In the second family, Sanger sequencing of MASP1 revealed a nonsense mutation, MASP1 c.870G>A (p.W290X), that also cosegregated with the phenotype. Neither mutation was found in 192 Turkish controls or 1200 controls of various other ancestries. MASP1 encodes mannan-binding lectin serine protease 1. The two mutations occur in a MASP1 isoform that has been reported to process IGFBP-5, thereby playing a critical role in insulin growth factor availability during craniofacial and muscle development. These results implicate mutations of MASP1 as the cause of a human malformation syndrome and demonstrate the involvement of MASP1 in facial, umbilical, and ear development during the embryonic period.

Selective inhibition of the lectin pathway of complement with phage display selected peptides against mannose-binding lectin-associated serine protease (MASP)-1 and -2: significant contribution of MASP-1 to lectin pathway activation

The complement system, an essential part of the innate immune system, can be activated through three distinct routes: the classical, the alternative, and the lectin pathways. The contribution of individual activation pathways to different biological processes can be assessed by using pathway-selective inhibitors. In this paper, we report lectin pathway-specific short peptide inhibitors developed by phage display against mannose-binding lectin-associated serine proteases (MASPs), MASP-1 and MASP-2. On the basis of the selected peptide sequences, two 14-mer peptides, designated as sunflower MASP inhibitor (SFMI)-1 and SFMI-2, were produced and characterized. SFMI-1 inhibits both MASP-1 and MASP-2 with a K(I) of 65 and 1030 nM, respectively, whereas SFMI-2 inhibits only MASP-2 with a K(I) of 180 nM. Both peptides block the lectin pathway activation completely while leaving the classical and the alternative routes intact and fully functional, demonstrating that of all complement proteases only MASP-1 and/or MASP-2 are inhibited by these peptides. In a C4 deposition inhibitor assay using preactivated MASP-2, SFMI-2 is 10-fold more effective than SFMI-1 in accordance with the fact that SFMI-2 is a more potent inhibitor of MASP-2. Surprisingly, however, out of the two peptides, SFMI-1 is much more effective in preventing C3 and C4 deposition when normal human serum containing zymogen MASPs is used. This suggests that MASP-1 has a crucial role in the initiation steps of lectin pathway activation most probably by activating MASP-2. Because the lectin pathway has been implicated in several life-threatening pathological states, these inhibitors should be considered as lead compounds toward developing lectin pathway blocking therapeutics.

Lectin complement pathway gene profile of donor and recipient determine the risk of bacterial infections after orthotopic liver transplantation

Infectious complications after orthotopic liver transplantation (OLT) are a major clinical problem. The lectin pathway of complement activation is liver-derived and a crucial effector of the innate immune defense against pathogens. Polymorphisms in lectin pathway genes determine their functional activity. We assessed the relationship between these polymorphic genes and clinically significant bacterial infections, i.e., sepsis, pneumonia, and intra-abdominal infection, and mortality within the first year after OLT, in relation to major risk factors in two cohorts from different transplant centers. Single-nucleotide polymorphisms in the mannose-binding lectin gene (MBL2), the ficolin-2 gene (FCN2), and the MBL-associated serine protease gene (MASP2) of recipients and donors were determined. Recipients receiving a donor liver in the principal cohort with polymorphisms in all three components i.e., MBL2 (XA/O; O/O), FCN2+6359T, and MASP2+371A, had a cumulative risk of an infection of 75% as compared to 18% with wild-type donor livers (P = 0.002), an observation confirmed in the second cohort (P = 0.04). In addition, a genetic (mis)match between donor and recipient conferred a two-fold higher infection risk for each separate gene. Multivariate Cox analysis revealed a stepwise increase in infection risk with the lectin pathway gene profile of the donor (hazard ratio = 4.52; P = 8.1 x 10(-6)) and the donor-recipient (mis)match genotype (hazard ratio = 6.41; P = 1.9 x 10(-7)), independent from the other risk factors sex and antibiotic prophylaxis (hazard ratio > 1.7 and P < 0.02). Moreover, patients with a lectin pathway gene polymorphism and infection had a six-fold higher mortality (P = 0.9 x 10(-8)), of which 80% was infection-related.

CONCLUSION

Donor and recipient gene polymorphisms in the lectin complement pathway are major determinants of the risk of clinically significant bacterial infection and mortality after OLT.

Calcium-dependent conformational flexibility of a CUB domain controls activation of the complement serine protease C1r

C1, the first component of the complement system, is a Ca(2+)-dependent heteropentamer complex of C1q and two modular serine proteases, C1r and C1s. Current functional models assume significant flexibility of the subcomponents. Noncatalytic modules in C1r have been proposed to provide the flexibility required for function. Using a recombinant CUB2-CCP1 domain pair and the individual CCP1 module, we showed that binding of Ca(2+) induces the folding of the CUB2 domain and stabilizes its structure. In the presence of Ca(2+), CUB2 shows a compact, folded structure, whereas in the absence of Ca(2+), it has a flexible, disordered conformation. CCP1 module is Ca(2+)-insensitive. Isothermal titration calorimetry revealed that CUB2 binds a single Ca(2+) with a relatively high K(D) (430 mum). In blood, the CUB2 domain of C1r is only partially (74%) saturated by Ca(2+), therefore the disordered, Ca(2+)-free form could provide the flexibility required for C1 activation. In accordance with this assumption, the effect of Ca(2+) on the autoactivation of native, isolated C1r zymogen was proved. In the case of infection-inflammation when the local Ca(2+) concentration decreases, this property of CUB2 domain could serve as subtle means to trigger the activation of the classical pathway of complement. The CUB2 domain of C1r is a novel example for globular protein domains with marginal stability, high conformational flexibility, and proteolytic sensitivity. The physical nature of the behavior of this domain is similar to that of intrinsically unstructured proteins, providing a further example of functionally relevant ligand-induced reorganization of a polypeptide chain.

Complement and its role in innate and adaptive immune responses

The complement system plays a crucial role in the innate defense against common pathogens. Activation of complement leads to robust and efficient proteolytic cascades, which terminate in opsonization and lysis of the pathogen as well as in the generation of the classical inflammatory response through the production of potent proinflammatory molecules. More recently, however, the role of complement in the immune response has been expanded due to observations that link complement activation to adaptive immune responses. It is now appreciated that complement is a functional bridge between innate and adaptive immune responses that allows an integrated host defense to pathogenic challenges. As such, a study of its functions allows insight into the molecular underpinnings of host-pathogen interactions as well as the organization and orchestration of the host immune response. This review attempts to summarize the roles that complement plays in both innate and adaptive immune responses and the consequences of these interactions on host defense.

Paths reunited: Initiation of the classical and lectin pathways of complement activation

Understanding the structural organisation and mode of action of the initiating complex of the classical pathway of complement activation (C1) has been a central goal in complement biology since its isolation almost 50 years ago. Nevertheless, knowledge is still incomplete, especially with regard to the interactions between its subcomponents C1q, C1r and C1s that trigger activation upon binding to a microbial target. Recent studies have provided new insights into these interactions, and have revealed unexpected parallels with initiating complexes of the lectin pathway of complement: MBL-MASP and ficolin-MASP. Here, we develop and expand these concepts and delineate their implications towards the key aspects of complement activation via the classical and lectin pathways.

Complement protease MASP-1 activates human endothelial cells: PAR4 activation is a link between complement and endothelial function

Activation of the complement system can induce and enhance inflammatory reaction. Mannose-binding lectin-associated serine protease-1 (MASP-1) is an abundant protease of the complement lectin pathway; however, its physiological function is unclear. In this study, we demonstrate for the first time that MASP-1 is able to activate Ca(2+) signaling, NF-kappaB, and p38 MAPK pathways in cultured HUVECs. Activation was initiated by MASP-1 only; the related protease, MASP-2, had no such effect. The phenomenon was dependent on the proteolytic activity of MASP-1, suggesting modulation of endothelial cell function through a protease-activated receptor (PAR). Using synthetic peptide substrates representing the protease-sensitive regions of PARs, we were able to demonstrate that PAR4 is a target of MASP-1. The presence of functionally active PAR4 in HUVECs was demonstrated using PAR4 agonist peptide and mRNA quantification. Finally, we showed that the amount of membrane-bound intact PAR4 decreases after MASP-1 treatment. All of these results provide a novel link between the regulation of endothelial cell function and complement system activation, and they suggest that MASP-1-induced PAR4 activation could contribute to the development of the inflammatory reaction.

Stringent regulation of complement lectin pathway C3/C5 convertase by C4b-binding protein (C4BP)

The complement lectin pathway, an essential component of the innate immune system, is geared for rapid recognition of infections as each C4b deposited via this pathway is capable of forming a C3/C5 convertase. In the present study, role of C4b-binding protein (C4BP) in regulating the lectin pathway C3/C5 convertase assembled on zymosan and sheep erythrocytes coated with mannan (E(Man)) was examined. While the C4BP concentration for inhibiting 50% (IC(50)) formation of surface-bound C3 convertase on the two surfaces was similar to that obtained for the soluble C3 convertase (1.05nM), approximately 3- and 41-fold more was required to inhibit assembly of the C5 convertase on zymosan (2.81nM) and E(Man) (42.66nM). No difference in binding interactions between C4BP and surface-bound C4b alone or in complex with C3b was observed. Increasing the C4b density on zymosan (14,000-431,000 C4b/Zym) increased the number of C4b bound per C4BP from 2.87 to 8.23 indicating that at high C4b density all seven alpha-chains of C4BP are engaged in C4b-binding. In contrast, the number of C4b bound per C4BP remained constant (3.79+/-0.60) when the C4b density on E(Man) was increased. The data also show that C4BP regulates assembly and decay of the lectin pathway C3/C5 convertase more stringently than the classical pathway C3/C5 convertase because of a approximately 7- to 13-fold greater affinity for C4b deposited via the lectin pathway than the classical pathway. C4BP thus regulates efficiently the four times greater potential of the lectin pathway than the classical pathway in generating the C3/C5 convertase and hence production of pro-inflammatory products, which are required to fight infections but occasionally cause pathological inflammatory reactions.

Killing of Gram-negative bacteria with normal human serum and normal bovine serum: use of lysozyme and complement proteins in the death of Salmonella strains O48

Serum is an environment in which bacterial cells should not exist. The serum complement system provides innate defense against microbial infections. It consists of at least 35 proteins, mostly in pre-activated enzymatic forms. The activation of complement is achieved through three major pathways: the classical, alternative, and lectin. Lysozyme, widely present in body fluids, catalyzes the hydrolysis of beta 1,4 linkage between N-acetyloglucosamine and N-acetylmuramic acid in the bacterial cell wall and cooperates with the complement system in the bactericidal action of serum. In this study, ten strains of serotype O48 Salmonella, mainly associated with warm-blooded vertebrates and clinically important causing diarrhea in infants and children, were tested. The results demonstrated that the most efficient killing of Salmonella O48 occurred when all the components of normal bovine serum (NBS) and normal human serum (NHS) cooperated. To prove the role of lysozyme in the bactericidal activity of bovine and human serum, the method of serum adsorption onto bentonite (montmorillonite, MMT) was used. In order to investigate structural transitions accompanying the adsorption of serum components, we applied X-ray diffraction methods. The results of this investigation suggested that apart from lysozyme, other proteins (as, e.g., C3 protein or IgG immunoglobulin) were adsorbed on MMT particles. It was also shown that Ca(2+) cations can be adsorbed on bentonite. This may explain the different sensitivities of the serovars belonging to the same O48 Salmonella serotype to NBS and NHS devoid of lysozyme.

MASP-1, a promiscuous complement protease: structure of its catalytic region reveals the basis of its broad specificity

Mannose-binding lectin (MBL)-associated serine protease (MASP)-1 is an abundant component of the lectin pathway of complement. The related enzyme, MASP-2 is capable of activating the complement cascade alone. Though the concentration of MASP-1 far exceeds that of MASP-2, only a supporting role of MASP-1 has been identified regarding lectin pathway activation. Several non-complement substrates, like fibrinogen and factor XIII, have also been reported. MASP-1 belongs to the C1r/C1s/MASP family of modular serine proteases; however, its serine protease domain is evolutionary different. We have determined the crystal structure of the catalytic region of active MASP-1 and refined it to 2.55 A resolution. Unusual features of the structure are an internal salt bridge (similar to one in factor D) between the S1 Asp189 and Arg224, and a very long 60-loop. The functional and evolutionary differences between MASP-1 and the other members of the C1r/C1s/MASP family are reflected in the crystal structure. Structural comparison of the protease domains revealed that the substrate binding groove of MASP-1 is wide and resembles that of trypsin rather than early complement proteases explaining its relaxed specificity. Also, MASP-1's multifunctional behavior as both a complement and a coagulation enzyme is in accordance with our observation that antithrombin in the presence of heparin is a more potent inhibitor of MASP-1 than C1 inhibitor. Overall, MASP-1 behaves as a promiscuous protease. The structure shows that its substrate binding groove is accessible; however, its reactivity could be modulated by an unusually large 60-loop and an internal salt bridge involving the S1 Asp.

Analogous interactions in initiating complexes of the classical and lectin pathways of complement

The classical and lectin pathways of complement activation neutralize pathogens and stimulate key immunological processes. Both pathways are initiated by collagen-containing, soluble pattern recognition molecules associated with specific serine proteases. In the classical pathway, C1q binds to Ab-Ag complexes or bacterial surfaces to activate C1r and C1s. In the lectin pathway, mannan-binding lectin and ficolins bind to carbohydrates on pathogens to activate mannan-binding lectin-associated serine protease 2. To characterize the interactions leading to classical pathway activation, we have analyzed binding between human C1q, C1r, and C1s, which associate to form C1, using full-length and truncated protease components. We show that C1r and C1s bind to C1q independently. The CUB1-epidermal growth factor fragments contribute most toward binding, but CUB2 of C1r, but not of C1s, is also important. Each C1rs tetramer presents a total of six binding sites, one for each of the collagenous domains of C1q. We also demonstrate that subcomponents of the lectin and classical pathways cross-interact. Thus, although the stoichiometries of complexes differ, interactions are analogous, with equivalent contacts between recognition and protease subcomponents. Importantly, these new data are contrary to existing models of C1 and enable us to propose a new model using mannan-binding lectin-mannan-binding lectin-associated serine protease interactions as a template.

Early complement proteases: C1r, C1s and MASPs. A structural insight into activation and functions

C1r, C1s and the mannose-binding lectin-associated serine proteases (MASPs) are responsible for the initiation of the classical- and lectin pathway activation of the complement system. These enzymes do not act alone, but form supramolecular complexes with pattern recognition molecules such as C1q, MBL, and ficolins. They share the same domain organization but have different substrate specificities and fulfill different physiological functions. In the recent years the rapid progress of structural biology facilitated the understanding of the molecular mechanism of complement activation at atomic level. In this review we summarize our current knowledge about the structure and function of the early complement proteases, delineate the latest models of the multimolecular complexes and present the functional consequences inferred from the structural studies. We also discuss some open questions and debated issues that need to be resolved in the future.

Polymorphisms in mannan-binding lectin (MBL)-associated serine protease 2 affect stability, binding to MBL, and enzymatic activity

Mannan-binding lectin-associated serine protease 2 (MASP-2) is an enzyme of the innate immune system. MASP-2 forms complexes with the pattern recognition molecules mannan-binding lectin (MBL), H-ficolin, L-ficolin, or M-ficolin, and is activated when one of these proteins recognizes microorganisms and subsequently cleaves complement factors C4 and C2, thus initiating the activation of the complement system. Missense polymorphisms of MASP-2 exist in different ethnic populations. To further characterize the nature of these, we have produced and characterized rMASP-2s representing the following naturally occurring polymorphisms: R99Q, D120G, P126L, H155R, 156_159dupCHNH (CHNHdup), V377A, and R439H. Only very low levels of CHNHdup were secreted from the cells, whereas quantities similar to wild-type MASP-2 were found intracellularly, indicating that this mutation results in a misfolded protein. We found that D120G and CHNHdup could not associate with MBL, whereas R99Q, P126L, H155R, V377A, R439H, and wild-type MASP-2 bound equally well to MBL. Accordingly, when D120G and CHNHdup were mixed with MBL, no activation of complement factor C4 was observed, whereas R99Q, P126L, and V377A cleaved C4 with an activity comparable to wild-type MASP-2 and H155R slightly better. In contrast, the R439H variant was deficient in this process despite its normal binding to MBL. This variant was also not able to autoactivate in the presence of MBL and mannan. We find the R439H variant is common in Sub-Saharan Africans with a gene frequency of 10%. Our results indicate that individuals with different types of MASP-2 defects may be identified through genotyping.

C1, MBL-MASPs and C1-inhibitor: novel approaches for targeting complement-mediated inflammation

Complement activation is initiated by the pattern-recognition molecules complement component C1q, mannose-binding lectin (MBL) and ficolins (H-, L-, M-ficolin), which typically recognize antibody-antigen complexes or foreign polysaccharides. The associated proteases (C1r, C1s, MASP-1 and MASP-2) then activate the complement system. The serpin C1-inhibitor (C1-inh) blocks activity of all these complexes and has been successfully used in models of disease. Many structures of these components became available recently, including that of C1-inh, facilitating the structure-guided design of drugs targeting complement activation. Here, we propose an approach in which therapeutic proteins are made up of natural protein domains and C1-inh to allow targeting to the site of inflammation and more specific inhibition of complement activation. In particular, engineering a fast-acting C1-inh or fusing it to an 'aiming module' has been shown to be feasible and economical using a humanized yeast expression system. Complement-mediated inflammation has been linked to ischemia-reperfusion injury, organ graft rejection and even neurodegeneration, so targeting this process has direct clinical implications.

Purification, crystallization and preliminary X-ray analysis of human mannose-binding lectin-associated serine protease-1 (MASP-1) catalytic region

MASP-1, a multidomain serine protease, is a component of the lectin pathway of complement. Its precise function is unknown, although it seems to enhance the complement-activating capacity of MASP-2, a related enzyme. MASP-1 has also been implicated as playing a role in blood coagulation. It is mostly found associated with mannose-binding lectin (MBL) and ficolins. Early attempts to crystallize MASP-1 failed because of the inhomogeneity of the purified material. MASP-1 was shown by acidic nondenaturing PAGE to be composed of differently charged species, which are most likely to be the products of deamidation occurring during the refolding procedure. Sequential cation-exchange and anion-exchange chromatography resulted in a homogeneous material, which was successfully crystallized. The best crystal diffracted to 2.55 A resolution and belonged to space group P2(1)2(1)2(1), with unit-cell parameters a = 68.4, b = 70.4, c = 121.4 A. The crystal structure of MASP-1 may help in understanding the function of this mysterious serine protease.

Activation of complement component C5: comparison of C5 convertases of the lectin pathway and the classical pathway of complement

Although the initiating complex of lectin pathway (called M1 in this study) generates C3/C5 convertases similar to those assembled by the initiating complex (C1) of the classical pathway, activation of complement component C5 via the lectin pathway has not been examined. In the present study kinetic analysis of lectin pathway C3/C5 convertases assembled on two surfaces (zymosan and sheep erythrocytes coated with mannan (E(Man))) revealed that the convertases (ZymM1,C4b,C2a and E(Man)M1,C4b,C2a) exhibited a similar but weak affinity for the substrate, C5 indicated by a high K(m) (2.73-6.88 microm). Very high affinity C5 convertases were generated when the low affinity C3/C5 convertases were allowed to deposit C3b by cleaving native C3. These C3b-containing convertases exhibited K(m) (0.0086-0.0075 microm) well below the normal concentration of C5 in blood (0.37 microm). Although kinetic parameters, K(m) and k(cat), of the lectin pathway C3/C5 convertases were similar to those reported for classical pathway C3/C5 convertases, studies on the ability of C4b to bind C2 indicated that every C4b deposited on zymosan or E(Man) was capable of forming a convertase. These findings differ from those reported for the classical pathway C3/C5 convertase, where only one of four C4b molecules deposited formed a convertase. The potential for four times more amplification via the lectin pathway than the classical pathway in the generation of C3/C5 convertases and production of pro-inflammatory products, such as C3a, C4a, and C5a, implies that activation of complement via the lectin pathway might be a more prominent contributor to the pathology of inflammatory reactions.

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.