Complement Components C5 and C7: Recombinant Factor I Modules of C7 Bind to the C345C Domain of C5

Discovery of functionally distinct anti-C7 monoclonal antibodies and stratification of anti-nicotinic AChR positive Myasthenia Gravis patients

Myasthenia Gravis (MG) is mediated by autoantibodies against acetylcholine receptors that cause loss of the receptors in the neuromuscular junction. Eculizumab, a C5-inhibitor, is the only approved treatment for MG that mechanistically addresses complement-mediated loss of nicotinic acetylcholine receptors. It is an expensive drug and was approved despite missing the primary efficacy endpoint in the Phase 3 REGAIN study. There are two observations to highlight. Firstly, further C5 inhibitors are in clinical development, but other terminal pathway proteins, such as C7, have been relatively understudied as therapeutic targets, despite the potential for lower and less frequent dosing. Secondly, given the known heterogenous mechanisms of action of autoantibodies in MG, effective patient stratification in the REGAIN trial may have provided more favorable efficacy readouts. We investigated C7 as a target and assessed the in vitro function, binding epitopes and mechanism of action of three mAbs against C7. We found the mAbs were human, cynomolgus monkey and/or rat cross-reactive and each had a distinct, novel mechanism of C7 inhibition. TPP1820 was effective in preventing experimental MG in rats in both prophylactic and therapeutic dosing regimens. To enable identification of MG patients that are likely to respond to C7 inhibition, we developed a patient stratification assay and showed in a small cohort of MG patients (n=19) that 63% had significant complement activation and C7-dependent loss of AChRs in this in vitro set up. This study provides validation of C7 as a target for treatment of MG and provides a means of identifying patients likely to respond to anti-C7 therapy based on complement-activating properties of patient autoantibodies.

Tracing and exploring the evolutionary origin and systematic function of fish complement C9

Complement C9, as a member of terminal complement component (TCC) protein, plays important roles in innate immunity. However, some complement components appear to show difference and evolutionary complexity between higher and lower vertebrates. Hence, it is essential to carry on a study of evolutionary origin and systematic function of C9 in fish and non-fish vertebrates. This study aims to explore the complement gene evolution and potential function in fish based on molecular and structural biology. Herein, we found complete divergence of C9 throughout the gene evolution. The optimal codons of C9 sequences tended to be closer to the genomes of lower vertebrates compared to higher vertebrates. Further, conserved amino acids in the C9 TMH1 region were identified, implying their potential functional association with MAC growth and pore formation. Transposons and simple repeats, as gene elements, exhibited a differential distribution in the genomic regions in different animal groups but were sparsely scattered around the sixth exon (TMH1 region). Notably, this demonstrated the regulatory complexity of the C9 gene in higher vertebrates. The negative selection pressures on fish and non-fish groups improved both the sequence conservation and similarity. Through gene/protein regulatory network and pathway analyses, the systematic function of C9 protein was showcased; thus, we could reveal the divergence of the systematic function of C9 across species from different evolutionary positions. In addition, more complicated functions of C9 in higher vertebrates could established by the altered spatial conformation of the protein. Collectively, the present study illustrates the C9 gene evolutionary process and the difference in its systematic function across multiple species. Such advances provide new insights for understanding the evolutionary and potential functions of complement C9.

Fish complement C4 gene evolution and gene/protein regulatory network analyses and simulated stereo conformation of C4-MASP-2 protein complex

Complement C4 is a central protein by acting as pivotal molecule in the activation of the complement system. More than a decade ago, C4 gene duplication had been found in several species including fish, revealing the evolutionary origin of C4 gene. However, the evolutionary pattern and systematic function of C4 are still limited. In this study, C4 D and H types in different species groups were completely diverged. The codon usage of C4 H type in higher vertebrates were much closer to their own genome environment, in contrast to lower vertebrates, suggesting that the evolution may provide the dynamic for homogeneous codon usage between specific gene and genome. Multiple C4 sequence alignment showed that the sequences were conserved among different species. However, sequence similarity was obviously different between species C4 D and H type. Negative selection pressure was found on C4 gene evolution and it may be one of the possible reasons for the sequence broad similarity and conservation among interspecies. Proteins from C4 protein-protein interaction (PPI) network were enriched in more hematopoiesis, infections, diseases and immune-related pathways in human than zebrafish. The result suggested that the functional complexities of C4 isotypes are distinct in species from different evolutionary positions. The simulated C4 protein structures between human and grass carp shared structural similarity and the stereo structures of grass carp C4-MASP-2 protein complexes were further simulated according to a study of human. These results suggested that the interaction between C4 and MASP-2 proteins may also exist in grass carp. Our results can provide an insight for the evolutionary process of C4 and better understanding to the potential mechanism of interaction between C4 and MASP-2 in fish species.

Two sides of a coin: GG genotype of C7 provides protection against fibrosis severity while showing a higher risk for hepatocellular carcinoma in patients with hepatitis C

The complement system (CS) is a key element of immunity against pathogens but also seems to influence other events, such as tumorigenesis and tissue repair. Complement component 7 (C7) is a key component of the lytic pathway of CS, leading to the formation of the membrane attack complex (MAC). This study aimed to investigate the existence of the association of a polymorphism in the C7 gene, rs1063499, with hepatic fibrosis and the occurrence of hepatocellular carcinoma (HCC) in patients with hepatitis C. We analyzed 456 samples from patients with chronic hepatitis C. Real-time PCR was used for allelic discrimination. Patients were classified by their METAVIR score as F1 (n = 100), F2 (n = 83), F3 (n = 101) or F4 (n = 66); 106 patients were diagnosed with HCC. Patients carrying the G/G genotype of C7 had a lower chance of developing severe fibrosis in the recessive model (p = 0.042; OR: 0.65 95% CI 0.41-1.02). However, the G/G genotype frequency was higher in patients with HCC (P = 0.01; OR: 2.07 95% CI 1.20-3.53) and in those with larger tumors (p = 0.04). The G/G C7 genotype seems to be a protective factor against advanced fibrosis; however, it was associated with a higher risk of HCC and the occurrence of larger hepatic nodules, suggesting the involvement of C7 in the physiopathogenesis of HCC and fibrosis in patients with hepatitis C virus (HCV).

Structural basis of complement membrane attack complex formation

In response to complement activation, the membrane attack complex (MAC) assembles from fluid-phase proteins to form pores in lipid bilayers. MAC directly lyses pathogens by a 'multi-hit' mechanism; however, sublytic MAC pores on host cells activate signalling pathways. Previous studies have described the structures of individual MAC components and subcomplexes; however, the molecular details of its assembly and mechanism of action remain unresolved. Here we report the electron cryo-microscopy structure of human MAC at subnanometre resolution. Structural analyses define the stoichiometry of the complete pore and identify a network of interaction interfaces that determine its assembly mechanism. MAC adopts a 'split-washer' configuration, in contrast to the predicted closed ring observed for perforin and cholesterol-dependent cytolysins. Assembly precursors partially penetrate the lipid bilayer, resulting in an irregular β-barrel pore. Our results demonstrate how differences in symmetric and asymmetric components of the MAC underpin a molecular basis for pore formation and suggest a mechanism of action that extends beyond membrane penetration.

Structural biology of the membrane attack complex

The complement system is an intricate network of serum proteins that mediates humoral innate immunity through an amplification cascade that ultimately leads to recruitment of inflammatory cells or opsonisation or killing of pathogens. One effector arm of this network is the terminal pathway of complement, which leads to the formation of the membrane attack complex (MAC) composed of complement components C5b, C6, C7, C8 and C9. Upon formation of C5 convertases via the classical or alternative pathways of complement activation, C5b is generated from C5 by proteolytic cleavage, nucleating a series of association and polymerisation reactions of the MAC-constituting complement components that culminate in pore formation of pathogenic membranes. Recent structures of MAC components and homologous proteins significantly increased our understanding of oligomerisation, membrane association and integration, shedding light onto the molecular mechanism of this important branch of the innate immune system.

Evolution of the complement system

The mammalian complement system constitutes a highly sophisticated body defense machinery comprising more than 30 components. Research into the evolutionary origin of the complement system has identified a primitive version composed of the central component C3 and two activation proteases Bf and MASP in cnidaria. This suggests that the complement system was established in the common ancestor of eumetazoa more than 500 million years ago. The original activation mechanism of the original complement system is believed to be close to the mammalian lectin and alternative activation pathways, and its main role seems to be opsonization and induction of inflammation. This primitive complement system has been retained by most deuterostomes without major change until the appearance of jawed vertebrates. At this stage, duplication of the C3, Bf and MASP genes as well as recruitment of membrane attack components added the classical and lytic pathways to the primitive complement system, converting it to the modern complement system. In contrast, the complement system was lost multiple times independently in the protostome lineage.

¹H, ¹³C and ¹⁵N resonance assignments of the complement control protein modules of the complement component C7

Human C7 is one of four homologous complement proteins that self-assemble on the nascent activation-specific fragment, C5b, thus forming the cytolytic membrane attack complex (MAC). In addition to the conserved modular core of the MAC/perforin protein family, C7 has four C-terminal domains comprising a pair of complement control protein modules (CCPs) preceding two Factor-I like modules (FIMs). It is proposed that the C7-CCPs might serve as a molecular arm for delivery of C7-FIMs to their binding site on C5b. Here we present the NMR chemical shift assignments for the C7-CCPs produced as a 14-kDa recombinant protein. Based upon triple-resonance experiments, 98 and 94 % of the backbone and side-chain ((1)H, (13)C and (15)N) assignments, respectively, have been completed. The chemical shifts and assignments have been deposited in the BioMagResBank database under accession number 18530.

Thrombin generates previously unidentified C5 products that support the terminal complement activation pathway

The coagulation and complement pathways simultaneously promote homeostasis in response to injury but cause tissue damage when unregulated. Mechanisms by which they cooperate are poorly understood. To delineate their interactions, we studied the effects of thrombin and C5 convertase on C5 in purified and plasma-based systems, measuring release of the anaphylatoxin C5a, and generation of C5b, the initial component of the lytic membrane attack complex. Thrombin cleaved C5 poorly at R751, yielding minimal C5a and C5b. However, thrombin efficiently cleaved C5 at a newly identified, highly conserved R947 site, generating previously undescribed intermediates C5(T) and C5b(T). Tissue factor-induced clotting of plasma led to proteolysis of C5 at a thrombin-sensitive site corresponding to R947 and not R751. Combined treatment of C5 with thrombin and C5 convertase yielded C5a and C5b(T), the latter forming a C5b(T)-9 membrane attack complex with significantly more lytic activity than with C5b-9. Our findings provide a new paradigm for complement activation, in which thrombin and C5 convertase are invariant partners, enhancing the terminal pathway via the generation of newly uncovered C5 intermediates. Delineating the molecular links between coagulation and complement will provide new therapeutic targets for diseases associated with excess fibrin deposition and complement activation.

Crystal structure of C5b-6 suggests structural basis for priming assembly of the membrane attack complex

The complement membrane attack complex (MAC) forms transmembrane pores in pathogen membranes. The first step in MAC assembly is cleavage of C5 to generate metastable C5b, which forms a stable complex with C6, termed C5b-6. C5b-6 initiates pore formation via the sequential recruitment of homologous proteins: C7, C8, and 12-18 copies of C9, each of which comprises a central MAC-perforin domain flanked by auxiliary domains. We recently proposed a model of pore assembly, in which the auxiliary domains play key roles, both in stabilizing the closed conformation of the protomers and in driving the sequential opening of the MAC-perforin β-sheet of each new recruit to the growing pore. Here, we describe an atomic model of C5b-6 at 4.2 Å resolution. We show that C5b provides four interfaces for the auxiliary domains of C6. The largest interface is created by the insertion of an interdomain linker from C6 into a hydrophobic groove created by a major reorganization of the α-helical domain of C5b. In combination with the rigid body docking of N-terminal elements of both proteins, C5b becomes locked into a stable conformation. Both C6 auxiliary domains flanking the linker pack tightly against C5b. The net effect is to induce the clockwise rigid body rotation of four auxiliary domains, as well as the opening/twisting of the central β-sheet of C6, in the directions predicted by our model to activate or prime C6 for the subsequent steps in MAC assembly. The complex also suggests novel small molecule strategies for modulating pathological MAC assembly.

Assembly and regulation of the membrane attack complex based on structures of C5b6 and sC5b9

Activation of the complement system results in formation of membrane attack complexes (MACs), pores that disrupt lipid bilayers and lyse bacteria and other pathogens. Here, we present the crystal structure of the first assembly intermediate, C5b6, together with a cryo-electron microscopy reconstruction of a soluble, regulated form of the pore, sC5b9. Cleavage of C5 to C5b results in marked conformational changes, distinct from those observed in the homologous C3-to-C3b transition. C6 captures this conformation, which is preserved in the larger sC5b9 assembly. Together with antibody labeling, these structures reveal that complement components associate through sideways alignment of the central MAC-perforin (MACPF) domains, resulting in a C5b6-C7-C8β-C8α-C9 arc. Soluble regulatory proteins below the arc indicate a potential dual mechanism in protection from pore formation. These results provide a structural framework for understanding MAC pore formation and regulation, processes important for fighting infections and preventing complement-mediated tissue damage.

Structure of complement C6 suggests a mechanism for initiation and unidirectional, sequential assembly of membrane attack complex (MAC)

The complement membrane attack complex (MAC) is formed by the sequential assembly of C5b with four homologous proteins as follows: one copy each of C6, C7, and C8 and 12-14 copies of C9. Together these form a lytic pore in bacterial membranes. C6 through C9 comprise a MAC-perforin domain flanked by 4-9 "auxiliary" domains. Here, we report the crystal structure of C6, the first and longest of the pore proteins to be recruited by C5b. Comparisons with the structures of the C8αβγ heterodimer and perforin show that the central domain of C6 adopts a "closed" (perforin-like) state that is distinct from the "open" conformations in C8. We further show that C6, C8α, and C8β contain three homologous subdomains ("upper," "lower," and "regulatory") related by rotations about two hinge points. In C6, the regulatory segment includes four auxiliary domains that stabilize the closed conformation, inhibiting release of membrane-inserting elements. In C8β, rotation of the regulatory segment is linked to an opening of the central β-sheet of its clockwise partner, C8α. Based on these observations, we propose a model for initiation and unidirectional propagation of the MAC in which the auxiliary domains play key roles: in the assembly of the C5b-8 initiation complex; in driving and regulating the opening of the β-sheet of the MAC-performin domain of each new recruit as it adds to the growing pore; and in stabilizing the final pore. Our model of the assembled pore resembles those of the cholesterol-dependent cytolysins but is distinct from that recently proposed for perforin.

The molecular identification of factor H and factor I molecules in rainbow trout provides insights into complement C3 regulation

The complement system in vertebrates plays a crucial role in the elimination of pathogens. To regulate complement on self-tissue and to prevent spontaneous activation and systemic depletion, complement is controlled by both fluid-phase and membrane-bound inhibitors. One such inhibitor, complement factor I (CFI) regulates complement by proteolytic cleavage of components C3b and C4b in the presence of specific cofactors. Complement factor H (CFH), the main cofactor for CFI, regulates the alternative pathway of complement activation by acting in the breakdown of C3b to iC3b. To gain further insight into the origin of C3 regulation in bony fish we have cloned and characterized the CFI and CFH1 cDNAs in the rainbow trout (Oncorhynchus mykiss). In this study we report the primary sequence, the tissue expression profile, the polypeptide domain architecture and the phylogenetic analysis of trout CFI and CFH1 genes. The deduced amino acid sequences of trout CFI and CFH1 polypeptides exhibit 42% and 32% identity with human orthologs, respectively. RNA expression analysis showed that CFI is expressed differentially in trout tissues, while liver is the main source of CFH1 expression. Our data indicate that factor H and I genes have emerged during evolution as early as the divergence of teleost fish.

Structure of human C8 protein provides mechanistic insight into membrane pore formation by complement

C8 is one of five complement proteins that assemble on bacterial membranes to form the lethal pore-like "membrane attack complex" (MAC) of complement. The MAC consists of one C5b, C6, C7, and C8 and 12-18 molecules of C9. C8 is composed of three genetically distinct subunits, C8α, C8β, and C8γ. The C6, C7, C8α, C8β, and C9 proteins are homologous and together comprise the MAC family of proteins. All contain N- and C-terminal modules and a central 40-kDa membrane attack complex perforin (MACPF) domain that has a key role in forming the MAC pore. Here, we report the 2.5 Å resolution crystal structure of human C8 purified from blood. This is the first structure of a MAC family member and of a human MACPF-containing protein. The structure shows the modules in C8α and C8β are located on the periphery of C8 and not likely to interact with the target membrane. The C8γ subunit, a member of the lipocalin family of proteins that bind and transport small lipophilic molecules, shows no occupancy of its putative ligand-binding site. C8α and C8β are related by a rotation of ∼22° with only a small translational component along the rotation axis. Evolutionary arguments suggest the geometry of binding between these two subunits is similar to the arrangement of C9 molecules within the MAC pore. This leads to a model of the MAC that explains how C8-C9 and C9-C9 interactions could facilitate refolding and insertion of putative MACPF transmembrane β-hairpins to form a circular pore.

Molecular cloning, characterization and expression analysis of the complement component C6 gene in grass carp

The complement system, as a representative of innate immunity, plays a key role in the host defense against infections. C6 is the member of complement components creating the membrane attack complex (MAC). In this study, we cloned and characterized the grass carp complement component C6 (gcC6) gene. Our data showed that gcC6 gene contained a 2724bp open reading frame (ORF), a 237bp 5'-untranslated region (UTR) and a 219bp 3'-UTR. The deduced amino acid sequence of gcC6 showed 77.6% and 58.9% identity to zebrafish C6 and rainbow trout C6, respectively. GcC6 gene was expressed in a wide range of grass carp tissues, and the highest expression level of gcC6 was detected in the spleen and liver. Upon challenge with Aeromonas hydrophila, its expression was significantly up-regulated in muscle, trunk kidney, liver, head kidney, spleen, heart and intestine, whereas it was down-regulated in the brain and skin. The expression level of gcC6 was high at the unfertilized egg stage. It was significantly increased at 1 day post-hatching, but it was decreased at 10 days post-hatching. This result suggested that the complement C6 transcripts in early embryos were of maternal origin.

Cloning of the sixth complement component and, spatial and temporal expression profile of MAC structural and regulatory genes in chicken

Humoral cytotoxicity results from the assembly of terminal components of complement, called membrane attack complex (MAC), which lead to the formation of pores on pathogen membranes. The complement components involved in MAC formation are C5b, C6, C7, C8alpha, C8beta, C8gamma and C9. Among them, C6 protein interacts with C5b through a metastable binding site to form a soluble C5b-6 dimer in the vicinity of the activating cell. Formation of the MAC is controlled by complement regulatory molecules, such as CD59, vitronectin and clusterin. Here, we report the molecular characterization of the C6 complement component, as well as the spatial and temporal expression profile of MAC structural (C6, C7, C8alpha, C8beta, C8gamma) and regulatory (CD59, vitronectin and clusterin) genes in chicken (Gallus gallus). The deduced polypeptide sequence of chicken C6 consists of 935 amino acid residues and exhibits 81%, 58%, 56% and 44% identity with zebra finch, human, frog and trout orthologs, respectively. The 'domain' architecture of chicken C6 resembles that of mammalian counterparts and the cysteine backbone is also conserved. MAC structural and regulatory genes are expressed in a wide range of adult chicken tissues, with the liver being the major source of their produced transcripts. The developmental expression profile of chicken MAC structural genes shows that their transcripts initially appear in the 12th embryonic day in the liver, exhibiting a pick in the 17th, while no expression was detected in the early whole embryo (day 4 and 6), as well as in the 2-day old neonate chicken liver. On the other hand, MAC regulatory genes are expressed in all the developmental stages investigated.

Molecular cloning of the terminal complement components C6 and C8beta of cartilaginous fish

The terminal complement components (TCCs) of mammals, C6, C7, C8alpha, C8beta, and C9, are a group of serum proteins involved in the cytolytic killing of microbial pathogens. The mammalian TCCs share a unique core domain structure and were probably generated by the duplication of the ancestral TCC gene and subsequent addition and/or deletion of the N- and C-terminal domains. Proteins and genes for all the TCCs have been identified from bony fish. In contrast, no TCC gene has been identified from cyclostome lamprey using whole-genome shotgun-sequence analysis and liver EST analysis. To clarify the evolutionary origin of TCCs, we performed degenerate RT-PCR and RACE analyses of the cartilaginous fish liver and identified the C6 gene from a shark, Mustelus manazo, and the C8B gene from a chimaera, Chimaera phantasma. The presence of the C6 gene in shark suggests that one of the most crucial steps in the establishment of the cytolytic complement pathway, the addition of the FIM and CCP domains to the primitive TCC, occurred in a common ancestor of the jawed vertebrates. These results also indicate that the gene duplications among TCCs occurred at an early stage of the jawed vertebrate evolution.

Molecular and expression analysis of complement component C5 in the nurse shark (Ginglymostoma cirratum) and its predicted functional role

We present the complete cDNA sequence of shark (Ginglymostoma cirratum) pro-C5 and its molecular characterization with a descriptive analysis of the structural elements necessary for its potential functional role as a potent mediator of inflammation (fragment C5a) and initiator molecule (fragment C5b) for the assembly of the membrane attack complex (MAC) upon activation by C5 convertase. In mammals the three complement activation cascades, the classical, alternative and lectin pathways, converge at the activation of C3, a pivotal complement protein. It is, however, the subsequent activation of the next complement component, C5, which is the focal point at which the initiation of the terminal lytic pathway takes place and involves the stepwise assembly of the MAC. The effector cytolytic function of complement occurs with the insertion of MAC into target membranes causing dough-nut like holes and cell leakage. The lytic activity of shark complement results in structurally similar holes in target membranes suggesting the assembly of a shark MAC that likely involves a functional analogue of C5. The composition of shark MAC remains unresolved and to date conclusive evidence has been lacking for shark C5. The gene has not been cloned nor has the serum protein been characterized for any elasmobranch species. This report is the first to confirm the presence of C5 homologue in the shark. GcC5 is remarkably similar to human C5 in overall structure and domain arrangement. The GcC5 cDNA measured 5160-bp with 5' and 3' UTRs of 35 bp and 79 bp, respectively. Structural analysis of the derived protein sequence predicts a molecule that is a two-chain structure which lacks a thiolester bond and contains a C5 convertase cleavage site indicating that activation will generate two peptides, akin to C5b and C5a. The putative GcC5 molecule also contains the C-terminal C345C/Netrin module that characterizes C3, C4 and C5. Multiple alignment of deduced amino acid sequences shows that GcC5 shares more amino acid identities/similarities with mammals than that with bony fish. We conclude that at the time of emergence of sharks the elaborate mosaic structure of C5 had already evolved.

1H, 15N and 13C resonance assignment of the pair of Factor-I like modules of the complement protein C7

The carboxy terminus of human complement component C7 comprises two Factor I-like Modules (FIMs) which are essential for formation of the Membrane Attack Complex, the terminal pathway of the innate immune system. C7-FIMs is a 16.9 kDa, recombinant, disulphide-rich, protein encompassing this C-terminal domain. Using conventional triple resonance experiments 93% of the (1)H, (15)N and (13)C assignment has been achieved, accounting for all assignment apart from a flexible N-terminus cloning artefact and an undefined loop. The chemical shifts have been deposited in the BioMagResBank; Accession No. 15996.

Solution structure of factor I-like modules from complement C7 reveals a pair of follistatin domains in compact pseudosymmetric arrangement

Factor I-like modules (FIMs) of complement proteins C6, C7, and factor I participate in protein-protein interactions critical to the progress of a complement-mediated immune response to infections and other trauma. For instance, the carboxyl-terminal FIM pair of C7 (C7-FIMs) binds to the C345C domain of C5 and its activated product, C5b, during self-assembly of the cytolytic membrane-attack complex. FIMs share sequence similarity with follistatin domains (FDs) of known three-dimensional structure, suggesting that FIM structures could be reliably modeled. However, conflicting disulfide maps, inconsistent orientations of subdomains within FDs, and the presence of binding partners in all FD structures led us to determine the three-dimensional structure of C7-FIMs by NMR spectroscopy. The solution structure reveals that each FIM within C7 contains a small amino-terminal FOLN subdomain connected to a larger carboxyl-terminal KAZAL domain. The open arrangement of the subdomains within FIMs resembles that of first FDs within structures of tandem FDs but differs from the more compact subdomain arrangement of second or third FDs. Unexpectedly, the two C7-FIMs pack closely together with an approximate 2-fold rotational symmetry that is rarely seen in module pairs and has not been observed in FD-containing proteins. Interfaces between subdomains and between modules include numerous hydrophobic and electrostatic contributions, suggesting that this is a physiologically relevant conformation that persists in the context of the parent protein. Similar interfaces were predicted in a homology-based model of the C6-FIM pair. The C7-FIM structures also facilitated construction of a model of the single FIM of factor I.

The structure of OMCI, a novel lipocalin inhibitor of the complement system

The complement (C) system is a potent innate immune defence system against parasites. We have recently characterised and expressed OmCI, a 16 kDa protein derived from the soft tick Ornithodoros moubata that specifically binds C5, thereby preventing C activation. The structure of recombinant OmCI determined at 1.9 A resolution confirms a lipocalin fold and reveals that the protein binds a fatty acid derivative that we have identified by mass spectrometry as ricinoleic acid. We propose that OmCI could sequester one of the fatty acid-derived inflammatory modulators from the host plasma, thereby interfering with the host inflammatory response to the tick bite. Mapping of sequence differences between OmCI and other tick lipocalins with different functions, combined with biochemical investigations of OmCI activity, supports the hypothesis that OmCI acts by preventing interaction with the C5 convertase, rather than by blocking the C5a cleavage site.

Hereditary complement C7 deficiency in nine families: Subtotal C7 deficiency revisited

Deficiencies in terminal complement components, including the component C7, are uncommon and associated with an increased risk of recurrent systemic neisserial infection. A total of 22 molecular defects have been reported in the C7 gene with both complete (C7Q0) and subtotal (C7SD) C7 deficiencies. In this study we report the molecular basis of nine new cases of C7 deficiencies that were characterized by exon-specific sequence analysis. Seven different C7 gene mutations were identified corresponding to small deletions (n=2), splice site changes (n=1) and single base pair substitutions leading to nonsense (n=1) or missense (n=3) mutations. Altogether, three changes of the C7 gene (G357R, R499S and 5' splice donor site of intron 16) account for half of the molecular defects which emphasize that a restricted number of molecular abnormalities are involved in this deficiency. We identified two patients with combined C7Q0/C7SD(R499S) and established the C7SD(R499S) frequency at about 1% in normal Caucasian population. We demonstrated that C7(R499S) mutant protein is retained in the endoplasmic reticulum whereas the wild-type C7 is located in the Golgi apparatus. Our results provide evidence that R499S represents a loss-of-function polymorphism of C7 due to a defective folding of the protein.

Genomic view of the evolution of the complement system

The recent accumulation of genomic information of many representative animals has made it possible to trace the evolution of the complement system based on the presence or absence of each complement gene in the analyzed genomes. Genome information from a few mammals, chicken, clawed frog, a few bony fish, sea squirt, fruit fly, nematoda and sea anemone indicate that bony fish and higher vertebrates share practically the same set of complement genes. This suggests that most of the gene duplications that played an essential role in establishing the mammalian complement system had occurred by the time of the teleost/mammalian divergence around 500 million years ago (MYA). Members of most complement gene families are also present in ascidians, although they do not show a one-to-one correspondence to their counterparts in higher vertebrates, indicating that the gene duplications of each gene family occurred independently in vertebrates and ascidians. The C3 and factor B genes, but probably not the other complement genes, are present in the genome of the cnidaria and some protostomes, indicating that the origin of the central part of the complement system was established more than 1,000 MYA.

cDNA cloning and phylogenetic analysis of the sixth complement component in rainbow trout

The sixth complement protein (C6) is an essential component of the membrane attack complex (MAC); the end product of the lytic pathway of complement activation. The MAC complex constitutes a supramolecular assembly containing the five precursor proteins C5b, C6, C7, C8, and C9. Once assembled on the target surface it forms transmembrane channels that cause membrane damage and cytolysis of complement-opsonized pathogens. Besides mediating direct pathogen elimination, exposure of cells to sublytic doses of MAC can trigger diverse cellular responses such as, cell activation, induction of apoptosis, cell cycle re-entry and proliferation in various biological settings. The terminal complement components (C6-C9) are structurally related proteins, differing in size and complexity. In order to study their evolution, we report here the cloning and molecular characterization of C6 component in rainbow trout. The deduced amino acid sequence of trout C6 exhibits 55 and 44% identity with zebra fish and human orthologs, respectively. The 'domain' architecture of trout C6 resembles that of mammalian counterparts, and the cysteine backbone is also conserved. Finally, trout C6 gene appears to exist as a single copy in the trout genome, and is expressed in a wide range of trout tissues.

Recombinant C345C and factor I modules of complement components C5 and C7 inhibit C7 incorporation into the complement membrane attack complex

Complement component C5 binds to components C6 and C7 in reversible reactions that are distinct from the essentially nonreversible associations that form during assembly of the complement membrane attack complex (MAC). We previously reported that the approximately 150-aa residue C345C domain (also known as NTR) of C5 mediates these reversible reactions, and that the corresponding recombinant module (rC5-C345C) binds directly to the tandem pair of approximately 75-residue factor I modules from C7 (C7-FIMs). We suggested from these and other observations that binding of the C345C module of C5 to the FIMs of C7, but not C6, is also essential for MAC assembly itself. The present report describes a novel method for assembling a complex that appears to closely resemble the MAC on the sensor chip of a surface plasmon resonance instrument using the complement-reactive lysis mechanism. This method provides the ability to monitor individually the incorporation of C7, C8, and C9 into the complex. Using this method, we found that C7 binds to surface-bound C5b,6 with a K(d) of approximately 3 pM, and that micromolar concentrations of either rC5-C345C or rC7-FIMs inhibit this early step in MAC formation. We also found that similar concentrations of either module inhibited complement-mediated erythrocyte lysis by both the reactive lysis and classical pathway mechanisms. These results demonstrate that the interaction between the C345C domain of C5 and the FIMs of C7, which mediates reversible binding of C5 to C7 in solution, also plays an essential role in MAC formation and complement lytic activity.

Survey of the year 2004 commercial optical biosensor literature

The year 2004 represents a milestone for the biosensor research community: in this year, over 1000 articles were published describing experiments performed using commercially available systems. The 1038 papers we found represent an approximately 10% increase over the past year and demonstrate that the implementation of biosensors continues to expand at a healthy pace. We evaluated the data presented in each paper and compiled a 'top 10' list. These 10 articles, which we recommend every biosensor user reads, describe well-performed kinetic, equilibrium and qualitative/screening studies, provide comparisons between binding parameters obtained from different biosensor users, as well as from biosensor- and solution-based interaction analyses, and summarize the cutting-edge applications of the technology. We also re-iterate some of the experimental pitfalls that lead to sub-optimal data and over-interpreted results. We are hopeful that the biosensor community, by applying the hints we outline, will obtain data on a par with that presented in the 10 spotlighted articles. This will ensure that the scientific community at large can be confident in the data we report from optical biosensors.

Functional insights from the structure of the multifunctional C345C domain of C5 of complement

The complement protein C5 initiates assembly of the membrane attack complex. This remarkable process results in lysis of target cells and is fundamental to mammalian defense against infection. The 150-amino acid residue domain at the C terminus of C5 (C5-C345C) is pivotal to C5 function. It interacts with enzymes that convert C5 to C5b, the first step in the assembly of the membrane attack complex; it also binds to the membrane attack complex components C6 and C7 with high affinity. Here a recombinant version of this C5-C345C domain is shown to adopt the oligosaccharide/oligonucleotide binding fold, with two helices packed against a five-stranded beta-barrel. The structure is compared with those from the netrin-like module family that have a similar fold. Residues critical to the interaction with C5-convertase cluster on a mobile, hydrophobic inter-strand loop that protrudes from the open face of the beta-barrel. The opposite, helix-dominated face of C5-C345C carries a pair of exposed hydrophobic side chains adjacent to a striking negatively charged patch, consistent with affinity for positively charged factor I modules in C6 and C7. Modeling of homologous domains from complement proteins C3 and C4, which do not participate in membrane attack complex assembly, suggests that this provisionally identified C6/C7-interacting face is indeed specific to C5.