Complement genes C1r and C1s feature an intronless serine protease domain closely related to haptoglobin

Serum zonulin as a marker of intestinal mucosal barrier function: May not be what it seems

The protein, zonulin, has emerged as a popular serological marker to assess the integrity of the intestinal mucosal barrier. However, there is limited information on the utility of serum zonulin to indicate gastrointestinal disease and the validity of zonulin detection in widely-used commercial assays. The current study reports differences in zonulin levels across patient groups with gastrointestinal dysfunction compared with healthy individuals, though methodological inconsistencies indicated that actual zonulin protein was not detected by the commercial assays applied. The nature of the assays’ detected antigen was investigated using immunoprecipitation followed by mass spectrometric analysis and sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) followed by protein staining. Top matches of the assays’ detected antigen included haptoglobin and complement C3 for the assay manufactured by CUSABIO (Wuhan, China) and complement C3 for the assay manufactured by Immundiagnostik AG (Bensheim, Germany). These findings confirm that current commercial zonulin assays are not detecting the actual protein as prehaptoglobin-2. Until assay methodology is improved, we advise the greater scientific and medical community to exercise caution in considering the measurement of serum zonulin as a marker of mucosal barrier integrity.

Haptoglobin Is a Divergent MASP Family Member That Neofunctionalized To Recycle Hemoglobin via CD163 in Mammals

In mammals, haptoglobin (Hp) is an acute-phase plasma protein that binds with high affinity to hemoglobin (Hb) released by intravascular hemolysis. The resultant Hp-Hb complexes are bound and cleared by the scavenger receptor CD163, limiting Hb-induced oxidative damage. In this study, we show that Hp is a divergent member of the complement-initiating MASP family of proteins, which emerged in the ancestor of jawed vertebrates. We demonstrate that Hp has been independently lost from multiple vertebrate lineages, that characterized Hb-interacting residues of mammals are poorly conserved in nonmammalian species maintaining Hp, and that the extended loop 3 region of Hp, which mediates CD163 binding, is present only in mammals. We show that the Hb-binding ability of cartilaginous fish (nurse shark, Ginglymostoma cirratum; small-spotted catshark, Scyliorhinus canicula; and thornback ray, Raja clavata) and teleost fish (rainbow trout, Oncorhynchus mykiss) Hp is species specific, and where binding does occur it is likely mediated through a different structural mechanism to mammalian Hp. The continued, high-level expression of Hp in cartilaginous fishes in which Hb binding is not evident signals that Hp has (an)other, yet unstudied, role(s) in these species. Previous work indicates that mammalian Hp also has secondary, immunomodulatory functions that are independent of Hb binding; our work suggests these may be remnants of evolutionary more ancient functions, retained after Hb removal became the primary role of Hp in mammals.

Haptoglobin

Haptoglobin (Hp) is an abundant human plasma protein that tightly captures hemoglobin (Hb) during hemolysis. The Hb-Hp complex formation reduces the oxidative properties of heme/Hb and promotes recognition by the macrophage scavenger receptor CD163. This leads to Hb-Hp breakdown and heme catabolism by heme oxygenase and biliverdin reductase. Gene duplications of a part of or the entire Hp gene in the primate evolution have led to variant Hp gene products that collectively may be designated "the haptoglobins (Hps)" as they all bind Hb. These variant products include the human-specific multimeric Hp phenotypes in individuals, which are hetero- or homozygous for an Hp² gene allele. The Hp-related protein (Hpr) is another Hp duplication product in humans and other primates. Alternative functions of the variant Hps are indicated by numerous reports on association between Hp phenotypes and disease as well as the elucidation of a specific role of Hpr in the innate immune defense. Recent Advances: Recent functional and structural information on Hp and receptor systems for Hb removal now provides insight on how Hp carries out essential functions such as the Hb detoxification/removal, and how Hpr, by acting as an Hp-lookalike, can sneak a lethal toxin into trypanosome parasites that cause mammalian sleeping sickness. Critical Issues and Future Directions: The new structural insight may facilitate ongoing attempts of developing Hp derivatives for prevention of Hb toxicity in hemolytic diseases such as sickle cell disease and other hemoglobinopathies. Furthermore, the new structural knowledge may help identifying yet unknown functions based on other disease-relevant biological interactions involving Hps. Antioxid. Redox Signal. 26, 814-831.

Zonulin as prehaptoglobin2 regulates lung permeability and activates the complement system

Zonulin is a protein involved in the regulation of tight junctions (TJ) in epithelial or endothelial cells. Zonulin is known to affect TJ in gut epithelial cells, but little is known about its influences in other organs. Prehaptoglobin2 has been identified as zonulin and is related to serine proteases (MASPs, C1qrs) that activate the complement system. The current study focused on the role of zonulin in development of acute lung injury (ALI) in C57BL/6 male mice following intrapulmonary deposition of IgG immune complexes. A zonulin antagonist (AT-1001) and a related peptide with permeability agonist activities (AT-1002) were employed and given intratracheally or intravenously. Also, zonulin was blocked in lung with a neutralizing antibody. In a dose-dependent manner, AT-1001 or zonulin neutralizing antibody attenuated the intensity of ALI (as quantitated by albumin leak, neutrophil accumulation, and proinflammatory cytokines). A similar pattern was found using the bacterial lipopolysaccharide model of ALI. Using confocal microscopy on sections of injured lungs, staining patterns for TJ proteins were discontinuous, reduced, and fragmented. As expected, the leak of blood products into the alveolar space confirmed the passage of 3 and 20 kDa dextran, and albumin. In contrast to AT-1001, application of the zonulin agonist AT-1002 intensified ALI. Zonulin both in vitro and in vivo induced generation of complement C3a and C5a. Collectively, these data suggest that zonulin facilitates development of ALI both by enhancing albumin leak and complement activation as well as increased buildup of neutrophils and cytokines during development of ALI.

Evolutionary aspects of hemoglobin scavengers

With the evolution of fish, systems appeared for the disposal of the hemoglobin (Hb) that was inevitably released from erythrocytes. Thus, a plasma protein that bound free Hb with great affinity, haptoglobin (Hp), evolved from a protease of the innate immune system. In parallel, other proteins appeared (for example, hemopexin and alpha(1)-microglobulin), which bound and mediated the removal of free heme groups. Remarkably, Hp later disappeared in some vertebrate lineages, suggesting that it could also be disadvantageous. In the avian lineage, a soluble protein evolved, possibly from a scavenger receptor, which in some birds seems to have replaced Hp. Among mammals, multimeric forms of Hp appeared independently at two discrete times, suggesting that this form of the protein confers an advantage on the bearer, possibly by improving resistance to infection.

Genetic analysis of complement C1s deficiency associated with systemic lupus erythematosus highlights alternative splicing of normal C1s gene

Deficiencies of complement proteins of the classical pathway are strongly associated with the development of autoimmune diseases. Deficiency of C1r has been observed to occur concomitantly with deficiency in C1s and 9 out of 15 reported cases presented systemic lupus erythematosus (SLE). Here, we describe a family in which all four children are deficient in C1s but only two of them developed SLE. Hemolytic activity mediated by the alternative and the lectin pathways were normal, but classical pathway activation was absent in all children's sera. C1s was undetectable, while in the parents' sera it was lower than in the normal controls. The levels of C1r observed in the siblings and parents sera were lower than in the control, while the concentrations of other complement proteins (C3, C4, MBL and MASP-2) were normal in all family members. Impairment of C1s synthesis was observed in the patients' fibroblasts when analyzed by confocal microscopy. We show that all four siblings are homozygous for a mutation at position 938 in exon 6 of the C1s cDNA that creates a premature stop codon. Our investigations led us to reveal the presence of previously uncharacterized splice variants of C1s mRNA transcripts in normal human cells. These variants are derived from the skipping of exon 3 and from the use of an alternative 3' splice site within intron 1 which increases the size of exon 2 by 87 nucleotides.

Haptoglobin, a hemoglobin-binding plasma protein, is present in bony fish and mammals but not in frog and chicken

Hemoglobin (Hb) released from erythrocytes may cause oxidation of lipids and proteins. Haptoglobin (Hp), which occurs in the plasma of all mammals, binds free Hb and inhibits its oxidative activity. It is not known whether this protective protein also exists in lower vertebrates. By analyzing available genomic sequences, we have found that bony fish, but not more primitive animals, have a gene coding for a protein homologous to mammalian Hp. Furthermore, we show that this protein is present in the plasma of Japanese pufferfish (Takifugu rubripes) and that it binds Hb. These results, together with a phylogenetic analysis, suggest that Hp evolved from a complement-associated protein (mannose-binding lectin-associated serine proteinase, MASP), with the emergence of fish. Surprisingly, we found that both chicken (Gallus gallus) and the Western clawed frog (Xenopus tropicalis) lack the Hp gene. In chicken plasma, however, we identified a different type of Hb-binding protein, PIT54, which has been reported to be a potent antioxidant. PIT54 is a soluble member of the family of scavenger receptor cysteine-rich proteins, and we found that its gene exists only in birds. We also show that the plasma of ostrich (Strutio camelus), a primitive bird, contains both PIT54 and Hp. Collectively, our data suggest that PIT54 has successively taken over the function of Hp during the evolution of the avian lineage and has completely replaced the latter protein in chicken.

Prohaptoglobin is proteolytically cleaved in the endoplasmic reticulum by the complement C1r-like protein

Many secretory proteins are synthesized as proforms that become biologically active through a proteolytic cleavage in the trans-Golgi complex or at a later stage in the secretory pathway. Haptoglobin (Hp) is unusual in that it is cleaved in the endoplasmic reticulum before it enters the Golgi. Here, we present evidence that the recently discovered complement C1r-like protein (C1r-LP) mediates this cleavage. C1r-LP has not previously been shown to possess proteolytic activity, despite its homology to trypsin-like Ser proteinases. We demonstrate that coexpression of the proform of Hp (proHp) and C1r-LP in COS-1 cells effected cleavage of proHp in the endoplasmic reticulum. This cleavage depended on proteolytic activity of C1r-LP because mutation of the putative active-site Ser residue abolished the reaction. Furthermore, incubation of affinity-purified C1r-LP and proHp led to the cleavage of the latter protein. ProHp appeared to be cleaved at the expected site because substitution of Gly for Arg-161 blocked the reaction. C1r-LP showed specificity for proHp, in that it did not cleave the proform of complement C1s, a protein similar to Hp particularly around the cleavage site. C1r-LP accounts for at least part of the endogenous proHp-cleavage activity because suppression of the C1r-LP expression by RNA interference reduced the cleavage of proHp by up to 45% in the cells of a human hepatoma cell line (HepG2).

The human complement component C1R gene: the exon-intron structure and the molecular basis of allelic diversity

Human C1r is a component of the complement system, which is a major mediator of innate immunity. In this study we investigated the exon-intron organization of the human C1R gene, which spans 11 kb from the initiation codon to the stop codon, and is very similar in exon-intron structure to the C1S gene. Six common and rare alleles, C1R*1, C1R*2, C1R*5, C1R*8, C1R*9, and C1R*13, were characterized by five mutations at amino acid positions 114, 135, 146, 167 and 244, in exons 4, 5 and 7 where the CUB1, EGF and CUB2 domains are encoded, respectively. A comparison with the cDNA of the mouse C1r gene showed that C1R*2is likely to be an ancestral allele. In addition, nine nucleotide substitutions and one length polymorphism were found in introns 2, 3, 4, 8 and 10.

Origin of mannose-binding lectin-associated serine protease (MASP)-1 and MASP-3 involved in the lectin complement pathway traced back to the invertebrate, amphioxus

Mannose-binding lectin-associated serine proteases (MASPs) are involved in complement activation through the lectin pathway. To elucidate the phylogenetic origin of MASP and a primordial complement system, we cloned two MASP cDNAs from amphioxus (Branchiostoma belcheri) of the cephalochordates, considered to be the closest relative of vertebrates. The two sequences, orthologues of mammalian MASP-1 and MASP-3, were produced by alternative processing of RNA from a single gene consisting of a common H chain-encoding region and two L chain-encoding regions, a structure which is similar to that of the human MASP1/3 gene. We also isolated two MASP genes from the ascidian Halocynthia roretzi (urochordates) and found that each of them consists simply of an H chain-encoding region and a single L chain-encoding region. The difference in structure between the ascidian MASP genes and the amphioxus/mammalian MASP genes suggests that a prototype gene was converted to the MASP1/3-type gene possessing two L chain-encoding regions at an early stage of evolution before the divergence of amphioxus. This conclusion is supported by the presence of MASP-1 and MASP-3 homologues in almost all vertebrates, as demonstrated by the cloning of novel cDNA sequences representing lamprey (cyclostomes) MASP-1 and Xenopus MASP-3. The ancient origin of MASP-1 and MASP-3 suggests that they have crucial functions common to all species which emerged after cephalochordates.

A novel murine complement-related gene encoding a C1r-like serum protein

C1r and C1s are highly specific serine proteases that initiate the classical pathway of complement activation. We recently demonstrated that, in the mouse, the genes encoding these proteins are duplicated. Analysis of the 5'-flanking region of the murine C1rA gene, the homologue of human C1r, revealed the presence of a novel gene encoding a C1r-like protein (c1r-LP). Although this gene carries a large deletion, it shows an overall structure similar to that of c1rA, suggesting that it may have arisen from a duplication of the C1r gene. The c1r-LP gene is expressed primarily in the liver, and is not regulated by lipopolysaccharide. The open reading frame of full-length cDNA clones encodes a pre-protein with a calculated molecular mass of 50.6 kDa which, except for an internal deletion of several modules, has a modular organization similar to that of C1r and shows 51% overall amino acid identity to corresponding regions of C1rA. Western blot analysis demonstrates the presence of C1r-LP in mouse serum. The serine protease domain of C1r-LP displays 60% amino acid residue identity to that of C1rA, however, certain atypical features of the active center, and primarily the absence of the activation/cleavage site, suggest that C1r-LP is either an atypical enzyme, or it lacks proteolytic activity, perhaps serving a regulatory function in the classical pathway.

Complement C1r and C1s genes are duplicated in the mouse: differential expression generates alternative isomorphs in the liver and in the male reproductive system

C1r and C1s are the serine proteases that form the catalytic unit of the C1 complex, the first component of complement. In the present study, we found that the genes encoding murine C1r and C1s are duplicated. One set of these genes, referred to as c1rA and c1sA, are primarily expressed in the liver and are therefore the homologues of the human C1r and C1s genes. The other two genes, termed c1rB and c1sB, are expressed exclusively in male reproductive tissues, specifically the coagulating gland and the prostate. The predicted C1rB and C1sB proteins share 96 and 93% amino acid identity with C1rA and C1sA respectively. Most of the substitutions are clustered in the serine protease domains, suggesting differences in catalytic efficiencies and/or substrate specificities or alternatively adaptation to different physiological environments. The high homology of C1rB and C1sB with C1rA and C1sA in the non-catalytic regions indicates that they are probably capable of assembling the C1 complex. The expression of alternative genes encoding isomorphs of activating components of complement in male reproductive tissues raises the possibility of new mechanisms of complement activation in the male genital tract or of novel functions for complement proteases in reproduction.

Evolution of the initiating enzymes of the complement system

Analysis of the human MASP-1/3 gene, which encodes two proteases of the lectin-triggered complement cascade, has revealed alternatively used serine-protease-encoding regions for the gene's two protein products. Phylogenetic studies indicate that one arose by retrotransposition early in vertebrate evolution, supporting the idea that the lectin branch of the complement cascade arose earlier than the 'classical' pathway.

Molecular basis of a selective C1s deficiency associated with early onset multiple autoimmune diseases

We have investigated the molecular basis of selective and complete C1s deficiency in 2-year-old girl with complex autoimmune diseases including lupus-like syndrome, Hashimoto's thyroiditis, and autoimmune hepatitis. This patient's complement profile was characterized by the absence of CH50 activity, C1 functional activity <10%, and undetectable levels of C1s Ag associated with normal levels of C1r and C1q Ags. Exon-specific amplification of genomic DNA by PCR followed by direct sequence analysis revealed a homozygous nonsense mutation in the C1s gene exon XII at codon 534, caused by a nucleotide substitution from C (CGA for arginine) to T (TGA for stop codon). Both parents were heterozygous for this mutation. We used the new restriction site for endonuclease Fok-1 created by the mutation to detect this mutation in the genomic DNA of seven healthy family members. Four additional heterozygotes for the mutation were identified in two generations. Our data characterize for the first time the genetic defect of a selective and complete C1s deficiency in a Caucasian patient.

The human gene for mannan-binding lectin-associated serine protease-2 (MASP-2), the effector component of the lectin route of complement activation, is part of a tightly linked gene cluster on chromosome 1p36.2-3

The proteases of the lectin pathway of complement activation, MASP-1 and MASP-2, are encoded by two separate genes. The MASP1 gene is located on chromosome 3q27, the MASP2 gene on chromosome 1p36.23-31. The genes for the classical complement activation pathway proteases, C1r and C1s, are linked on chromosome 12p13. We have shown that the MASP2 gene encodes two gene products, the 76 kDa MASP-2 serine protease and a plasma protein of 19 kDa, termed MAp19 or sMAP. Both gene products are components of the lectin pathway activation complex. We present the complete primary structure of the human MASP2 gene and the tight cluster that this locus forms with non-complement genes. A comparison of the MASP2 gene with the previously characterised C1s gene revealed identical positions of introns separating orthologous coding sequences, underlining the hypothesis that the C1s and MASP2 genes arose by exon shuffling from one ancestral gene.

Nicastrin, a presenilin-interacting protein, contains an aminopeptidase/transferrin receptor superfamily domain

Nicastrin, a protein implicated in Alzheimer's disease, has a domain that is found in the aminopeptidase/transferrin receptor superfamily. In nicastrin, this domain might possess catalytic activity (as observed with aminopeptidases) or it could serve merely as a binding domain (with analogy to the transferrin receptors) for the beta-amyloid precursor protein.

Frequent de novo mutations and exon deletions in the C1inhibitor gene of patients with angioedema

BACKGROUND

Cases of angioedema with no family history but with functionally low levels of C1 inhibitor and recurrent attacks are often observed. Clinical and biochemical data do not distinguish these cases from proven inherited forms of hereditary angioedema.

OBJECTIVE

We sought to test the hypothesis of de novo mutations in patients affected by angioedema without a family history of the disease.

METHODS

Among 137 independent kindreds followed for hereditary angioedema, 45 (32.8%) patients with early onset of the disease were registered as sporadic cases. Nineteen patients with unaffected parents were screened for point mutations and microdeletions-insertions by using fluorescence-assisted mismatch analysis. The biologic paternity of these patients was verified by determining their alleles at 4 microsatellite loci. Gross deletions were detected with Southern blot analysis.

RESULTS

C1 inhibitor plasma levels measured in both parents of 24 sporadic patients were normal in all but 3 patients. Among the 19 patients studied at the DNA level, 9 de novo single nucleotide substitutions and 6 de novo microdeletions were found. De novo exon deletions were detected in 3 additional patients with Southern blot analysis.

CONCLUSIONS

De novo C1inhibitor mutations and exon deletions account for at least 25% of all unrelated cases of angioedema. This finding has implications relevant to the genetic epidemiology and genetic counseling of this disease. The observation that 5 of the 9 de novo point mutations reproduce previously reported changes underlines the presence of multiple hot spots, two of which contain a CpG dinucleotide.

Crystal structure of the catalytic domain of human complement c1s: a serine protease with a handle

C1s is the highly specific modular serine protease that mediates the proteolytic activity of the C1 complex and thereby triggers activation of the complement cascade. The crystal structure of a catalytic fragment from human C1s comprising the second complement control protein (CCP2) module and the chymotrypsin-like serine protease (SP) domain has been determined and refined to 1.7 A resolution. In the areas surrounding the active site, the SP structure reveals a restricted access to subsidiary substrate binding sites that could be responsible for the narrow specificity of C1s. The ellipsoidal CCP2 module is oriented perpendicularly to the surface of the SP domain. This arrangement is maintained through a rigid module-domain interface involving intertwined proline- and tyrosine-rich polypeptide segments. The relative orientation of SP and CCP2 is consistent with the fact that the latter provides additional substrate recognition sites for the C4 substrate. This structure provides a first example of a CCP-SP assembly that is conserved in diverse extracellular proteins. Its implications in the activation mechanism of C1 are discussed.

A novel PCR-based technique using expressed sequence tags and gene homology for murine genetic mapping: localization of the complement genes

The complement system is a cascade of serum proteins and receptors which forms a vital arm of innate immunity and enhances the adaptive immune response. This work establishes the chromosomal localization of four key genes of the murine complement system. Mapping was performed using a novel and rapid PCR restriction length polymorphism method which was developed to exploit the murine expressed sequence tag (EST) database. This technique circumvents the laborious cDNA or genomic cloning steps of other mapping methods by relying on EST data and the prediction of exon-intron boundaries. This method can be easily applied to the genes of other systems, ranging from the interests of the individual researcher to large-scale gene localization projects. Here the complement system, probably one of the most well-characterized areas of immunology, was used as a model system. It was shown that the C3a receptor C1r and C1s genes form an unexpected complement gene cluster towards the telomeric end of chromosome 6. The second mannose binding lectin-associated serine protease gene was mapped to the telomeric end of chromosome 4, which is distinct from other complement-activating serine proteases. These results provide new insights into the evolution of this group of proteins.

The Rat and Mouse Homologues of MASP-2 and MAp19, components of the Lectin Activation Pathway of Complement

Recently, we described two novel constituents of the multimolecular initiation complex of the mannan-binding lectin (MBL) pathway of complement activation, a serine protease of 76 kDa, termed MASP-2, and a MASP-2 related plasma protein of 19 kDa, termed MAp19. Upon activation of the MBL/MASPs/MAp19 complex, MASP-2 cleaves the fourth complement component C4, while the role of MAp19 within the MBL/MASP-1/MASP-2/MAp19 complex remains to be clarified. In humans, the mRNA species encoding MASP-2 (2.6 kb) and MAp19 (1.0 kb) arise by an alternative polyadenylation/splicing mechanism from a single structural MASP-2 gene. Here, we report the complete primary structures of the rat homologue of MASP-2 and of rat and mouse MAp19. We show that both MASP-2 and MAp19 are part of the rat MBL pathway activation complex and demonstrate their exclusively hepatic biosynthesis. Southern blot and PCR analyses of rat genomic DNA indicate that as in humans, rat MASP-2 and MAp19 are encoded by a single structural gene.

Activities of the MBL-associated serine proteases (MASPs) and their regulation by natural inhibitors

There has been rapid progress in determining the mechanism by which complement is activated by the complex formed between Mannose-Binding Lectin and its associated proteases (MASPs). MBL and the MASPs are of low abundance, but are similar to the more abundant C1q-C1r2s2 complex (C1), which has been extensively investigated. In this review we summarise recent findings on MBL-MASPs' structure. enzymic activity and regulation, and compare MBL-MASPs with C1.

Gene structure of the P100 serine-protease component of the human Ra-reactive factor

The Ra-reactive factor (RaRF) is a complement dependent anti-microbial factor that reacts with numerous microorganisms such as viruses, bacteria, fungi and protozoa. It is a complex of a mannan-binding lectin (MBL) and the serine protease, P100 (MASPI). P100 activates the C4 component of the complement system and its domain organization is similar to C1r and C1s. In this study, determination was made of the structure of the human P100 gene which was found longer than 67 kbp and to be comprised of 16 exons. Its non-protease region consisted of 10 exons, as in the case of C1r and C1s, and the introns were found present in the boundary separating two CUB domains, an EGF-like domain and two CCP domains and each CUB and CCP domain contained extra internal introns. The serine protease region was comprised of 6 exons in contrast to C1r and C1s, either of which consists of a single exon. The exon-intron structure was found to reflect the evolution of these molecules and P100 to have derived earlier in the stage of evolution than C1r or C1s.

Two Constituents of the Initiation Complex of the Mannan-Binding Lectin Activation Pathway of Complement Are Encoded by a Single Structural Gene

Mannan-binding lectin (MBL) forms a multimolecular complex with at least two MBL-associated serine proteases, MASP-1 and MASP-2. This complex initiates the MBL pathway of complement activation by binding to carbohydrate structures present on bacteria, yeast, and viruses. MASP-1 and MASP-2 are composed of modular structural motifs similar to those of the C1q-associated serine proteases C1r and C1s. Another protein of 19 kDa with the same N-terminal sequence as the 76-kDa MASP-2 protein is consistently detected as part of the MBL/MASP complex. In this study, we present the primary structure of this novel MBL-associated plasma protein of 19 kDa, MAp19, and demonstrate that MAp19 and MASP-2 are encoded by two different mRNA species generated by alternative splicing/polyadenylation from one structural gene.

Molecular Basis of Human Complement C1s Deficiency

This is the first report on the molecular basis of human complement C1s deficiency. Two abnormalities in the C1s gene were identified in a Japanese family, including one patient, by using exon-specific PCR, single-strand conformation polymorphism analysis, and nucleotide sequencing. A deletion of 4 bp, TTTG, was identified in exon X when using genomic DNA from the patient, his father, and his paternal grandmother. They were all heterozygous for the mutation. The mutant gene encodes a truncated C1s from the N terminus to the short consensus repeat domain. By further sequencing the PCR products, a nonsense mutation from G to T was identified at codon 608 in exon XII in the patient, his mother, and his sister. They were all heterozygous for the nonsense mutation. The mutant gene encodes a truncated form of C1s that lacks the C-terminal 80 amino acids. These results indicate that the patient was a compound heterozygote with the 4-bp deletion on the paternal allele and the nonsense mutation on the maternal allele. The levels of serum C1s seem to be correlated to the genotypes of the C1s gene in which no C1s was detected in the patient, and one-half of the normal level in the family members who are heterozygous for either mutation. The present study demonstrates that the disease is inherited in an autosomal recessive mode.

Structure of the human beta2-glycoprotein I (apolipoprotein H) gene

The gene encoding the human plasma protein beta2-glycoprotein I or apolipoprotein H was cloned and its structure determined. The gene which consists of eight exons was shown to span 18 kb and was localized to chromosome 17q23-24. The transcriptional initiation site was assigned to a position 31 bp upstream of the start codon. Several consensus sequence elements relevant for regulation of transcription in liver were seen in the 5'-upstream region of the gene. Exon 1 contains the 5'-UTR together with the signal peptide coding sequences. Short consensus repeats (SCRs) 1, 3, 4, and 5 are encoded by single exons each while SCR2 is encoded by two exons. Exon 8 comprises the region encoding the C-terminal end of beta2-glycoprotein I (from His-310), the stop codon and the 3'-UTR.

Two Lineages of Mannose-Binding Lectin-Associated Serine Protease (MASP) in Vertebrates

Mannose-binding lectin-associated serine protease (MASP) is a newly identified member of the serine protease superfamily. MASP is involved in host defense against pathogens through a novel system of complement activation, designated the lectin pathway. To elucidate the origin of the lectin pathway and the molecular evolution of MASP, we cloned six MASP cDNAs from five vertebrate species going from mammal to cyclostome. An alignment of the amino acid sequences deduced from the cDNAs revealed the presence of two different lineages of the MASP gene. This classification was supported by the precise correlation with two types of exon organization for the protease domain. One of the two lineages is unique in that a single exon encodes the protease domain, unlike most other serine proteases. All members of this group, termed the AGY type, have an AGY codon at the active site serine. A phylogenetic tree suggests that the AGY type diverged from another lineage, termed the TCN type, before the emergence of primitive vertebrates. Furthermore, the presence of MASP or MASP-like sequences in most vertebrate species suggests that the lectin pathway functions extensively in vertebrates and that its origin is traced back to the invertebrate stage.

Evolutionary conserved rigid module-domain interactions can be detected at the sequence level: the examples of complement and blood coagulation proteases

Several extracellular modular proteins, including proteases of the complement and blood coagulation cascades, are shown here to exhibit conserved sequence patterns specific for a particular module-domain association. This was detected by comparative analysis of sequence variability in different multiple sequence alignments, which provides a new tool to investigate the evolution of modular proteins. A first example deals with the proteins featuring a common complement control protein (CCP) module-serine protease (SP) domain pattern at their C-terminal end, defined here as the CCP-SP sub-family. These proteins include the complement proteases C1r, C1s and MASPs, the Limulus clotting factor C, and the proteins of the haptoglobin family. A second example deals with blood coagulation factors VII, IX and X and protein C, all featuring a common epidermal growth factor (EGF)-SP C-terminal assembly. Highly specific motifs are found at the connection between the CCP or EGF module and the activation peptide of the SP domain: [P/A]-x-C-x-[P/A]-[I/V]-C-G-x-[P/S/K] in the case of the CCP-SP proteins, and C-x-[P/S]-x-x-x-[Y/F]-P-C-G in the case of the EGF-SP proteins. Each motif is strictly conserved in the whole sub-family and it is detected in no more than one other known protein sequence. Strikingly, most of the conserved residues specific to each sub-family appear to be clustered at the interface between the SP domain and the CCP or EGF module. We propose that a rigid module-domain interaction occurs in these proteins and has been conserved through evolution. The functional implications of these assemblies, underlined by such evolutionary constraints, are discussed.

MASP-2, the C3 convertase generating protease of the MBLectin complement activating pathway

Mannan-binding lectin (MBL) activates the complement system through cleavage of C4 and C2. Until recently it was thought that only one serine protease in complex with MBL (MBL-associated serine protease, MASP) mediates complement activation, but with the finding of a second MBL-associated serine protease, MASP-2, the activation process appears more elaborate, possibly resembling that of the C1 complex. The two MASPs share the domain organisation of C1r and C1s and it may be speculated that interaction between the two MASPs is required for complement activation in the same manner as with the C1 proteases. We have demonstrated that MASP-2 is a C4 cleaving component of the MBL/MASP complex. By analogy, one may thus speculate that, upon binding of MBL to carbohydrate, MASP-1 autoactivates and then activates MASP-2, but there is as yet no evidence for this. The components of C1 are present in serum in approximately equimolar amounts, whereas MASP-1 is in large excess over MBL. Pairwise comparison of the four proteases shows the primary structures to be approximately 40% identical. Phylogenetic analysis indicates that MASP-2 is closer to C1r and C1s than is MASP-1, but no particular association between MASP-2 and the C4 cleaving enzyme, C1s, can be deduced from sequence comparison.

Non-coordinated biosynthesis of early complement components in a deficiency of complement proteins C1r and C1s

We report on a 60-year-old woman with systemic lupus erythematosus and a total (95%) C1r and a partial (36%) C1s deficiency. The patient complained about cutaneous lesions on forearms and legs without other systemic involvement. Elevated anti-nuclear, anti-native DNA and anti-SSA antibodies were present. The finding of persistently depressed levels of haemolytic complement activity (CH50) on both serum and plasma, associated with normal levels of C3, C4 and C2 components, and normal alternative pathway haemolytic activity showed a deficiency of an early component of the classical pathway. Indeed C1r component was below the limits of detection whereas C1s component was lowered (36%). The depressed CH50 was only corrected by purified C1r. Biosynthesis of C1r and C1s by patient's monocytes was spontaneously normal but not up-regulated by interferon-gamma for C1r alone, whereas the biosynthesis of C1s, but also of interleukin-6, was increased, indicating a specific disregulation of C1r. The deficiency was associated with a lupus syndrome and a fatal assumed septic shock. This is in agreement with other reported cases.

Identity of the putative serine-proteinase fold in proteins of the complement system with nine relevant crystal structures

The serine-proteinase domain is responsible for the proteolytic events that occur during complement activation. The sequences of nine serine proteinases of known crystal structure were compared with the serine-proteinase sequences in the six complement proteins C1r, C1s, C2, factor B, factor I and factor D to assess the degree of structural homology of the latter with the crystal structures. All sequence insertions and deletions were readily located at the protein surface. The internal location of disulphide bridges and the surface location of putative glycosylation sites are compatible with this structure. Secondary-structure predictions for the sequences were fully consistent with the crystal structures. It is concluded that the double subdomain beta-sheet motif is retained in the complement sequences, but that localized differences are observed for factor I, C2 and factor B.

Evolutionary implications of a new bypass activation pathway of the complement system

The classical pathway of complement activation is a highly specific and amplifiable effector system responding to recognition of foreign antigens by antibody. It comprises a group of well characterized proteins in mammalian plasma. There are many similarities with the alternative pathway of complement activation, which suggests that they have a common evolutionary origin. Both pathways have homologous components, use related activation and regulatory mechanisms, result in the release of the anaphylatoxins C3a and C5a, and deposit C3b onto activating surfaces. This fixed C3b then becomes the focus of further immune reactions, involving either the lytic complement components or C3b receptors on effector cells. Phylogenetic data indicate that the alternative pathway is the older, and that the classical pathway evolved from it. Here Timothy Farries and colleagues review this evolutionary process and present a possible sequence of events that is suggested by recent functional data from their laboratory.