Characterization of the Mr difference between secreted murine fourth component of complement and the major plasma form: evidence for carboxyl-terminal cleavage of the alpha chain

Dancing with complement C4 and the RP-C4-CYP21-TNX (RCCX) modules of the major histocompatibility complex

The number of the complement component C4 genes varies from 2 to 8 in a diploid genome among different human individuals. Three quarters of the C4 genes in Caucasian populations have the endogenous retrovirus, HERV-K(C4), in the ninth intron. The remainder does not. The C4 serum proteins are highly polymorphic and their concentrations vary from 100 to approximately 1000 microg/ml. There are two distinct classes of C4 protein, C4A and C4B, which have diversified to fulfill (a) the opsonization/immunoclearance purposes and (b) the well-known complement function in the killing of microbes by lysis and neutralization, respectively. Many infectious and autoimmune diseases are associated with complete or partial deficiency of C4A and/or C4B. The adverse effects of high C4 gene dosages, however, are just emerging, as the concepts of human C4 genetics are revised and accurate techniques are applied to distinguish partial deficiencies from differential expression caused by unequal C4A and C4B gene dosages and gene sizes. This review attempts to dissect the sophisticated genetics of complement C4A and C4B. The emphases are on the qualitative and quantitative diversities of C4 genotypes and phenotypes. The many allotypic variants and the processed products of human and mouse C4 proteins are described. The modular variation of C4 genes together with the serine/threonine nuclear kinase gene RP, the steroid 21-hydroxylase CYP21, and extracellular matrix protein TNX (RCCX modules) are investigated for the effects on homogenization of C4 protein polymorphisms, and on the unequal genetic crossovers that knocked out the functions of CYP21 and/or TNX. Furthermore, the influence of the endogenous retrovirus HERV-K(C4) on C4 gene expression and the dispersal of HERV-K(C4) family members in the human genome are discussed.

Thioester function is conserved in SpC3, the sea urchin homologue of the complement component C3

The amino acid sequence of the thioester site in the alpha chain of SpC3, the sea urchin homologue of C3, is conserved. This implies a conserved function of covalent bond formation with amine or hydroxyl groups on target molecules. When coelomic fluid (CF) was incubated with 14C-methylamine, a classic assay for thioester binding function, the alpha chain became labeled. When CF was treated to induce autolysis, peptide bond cleavage occurred at the thioester site. Autolysis could be blocked or reduced by pre-treating CF with either methylamine or yeast, both of which are known to bind to thioester sites C3 proteins from other organisms. The data suggest that SpC3 can bind to target cell surfaces, constituting indirect evidence that it can covalently bind to pathogen surfaces and function as an opsonin in vivo. This activity may be an important aspect of host defense in the sea urchin.

Genetic, structural and functional diversities of human complement components C4A and C4B and their mouse homologues, Slp and C4

The complement protein C4 is a non-enzymatic component of the C3 and C5 convertases and thus essential for the propagation of the classical complement pathway. The covalent binding of C4 to immunoglobulins and immune complexes (IC) also enhances the solubilization of immune aggregates, and the clearance of IC through complement receptor one (CR1) on erythrocytes. Human C4 is the most polymorphic protein of the complement system. In this review, we summarize the current concepts on the 1-2-3 loci model of C4A and C4B genes in the population, factors affecting the expression levels of C4 transcripts and proteins, and the structural, functional and serological diversities of the C4A and C4B proteins. The diversities and polymorphisms of the mouse homologues Slp and C4 proteins are described and contrasted with their human homologues. The human C4 genes are located in the MHC class III region on chromosome 6. Each human C4 gene consists of 41 exons coding for a 5.4-kb transcript. The long gene is 20.6 kb and the short gene is 14.2 kb. In the Caucasian population 55% of the MHC haplotypes have the 2-locus, C4A-C4B configurations and 45% have an unequal number of C4A and C4B genes. Moreover, three-quarters of C4 genes harbor the 6.4 kb endogenous retrovirus HERV-K(C4) in the intron 9 of the long genes. Duplication of a C4 gene always concurs with its adjacent genes RP, CYP21 and TNX, which together form a genetic unit termed an RCCX module. Monomodular, bimodular and trimodular RCCX structures with 1, 2 and 3 complement C4 genes have frequencies of 17%, 69% and 14%, respectively. Partial deficiencies of C4A and C4B, primarily due to the presence of monomodular haplotypes and homo-expression of C4A proteins from bimodular structures, have a combined frequency of 31.6%. Multiple structural isoforms of each C4A and C4B allotype exist in the circulation because of the imperfect and incomplete proteolytic processing of the precursor protein to form the beta-alpha-gamma structures. Immunofixation experiments of C4A and C4B demonstrate > 41 allotypes in the two classes of proteins. A compilation of polymorphic sites from limited C4 sequences revealed the presence of 24 polymophic residues, mostly clustered C-terminal to the thioester bond within the C4d region of the alpha-chain. The covalent binding affinities of the thioester carbonyl group of C4A and C4B appear to be modulated by four isotypic residues at positions 1101, 1102, 1105 and 1106. Site directed mutagenesis experiments revealed that D1106 is responsible for the effective binding of C4A to form amide bonds with immune aggregates or protein antigens, and H1106 of C4B catalyzes the transacylation of the thioester carbonyl group to form ester bonds with carbohydrate antigens. The expression of C4 is inducible or enhanced by gamma-interferon. The liver is the main organ that synthesizes and secretes C4A and C4B to the circulation but there are many extra-hepatic sites producing moderate quantities of C4 for local defense. The plasma protein levels of C4A and C4B are mainly determined by the corresponding gene dosage. However, C4B proteins encoded by monomodular short genes may have relatively higher concentrations than those from long C4A genes. The 5' regulatory sequence of a C4 gene contains a Spl site, three E-boxes but no TATA box. The sequences beyond--1524 nt may be completely different as the C4 genes at RCCX module I have RPI-specific sequences, while those at Modules II, III and IV have TNXA-specific sequences. The remarkable genetic diversity of human C4A and C4B probably promotes the exchange of genetic information to create and maintain the quantitative and qualitative variations of C4A and C4B proteins in the population, as driven by the selection pressure against a great variety of microbes. An undesirable accompanying byproduct of this phenomenon is the inherent deleterious recombinations among the RCCX constituents leading to autoimmune and genetic disorders.

Sulfation of tyrosine residues increases activity of the fourth component of complement

Sulfation of tyrosine residues recently has been recognized as a biosynthetic modification of many plasma proteins and other secretory proteins. Effects of this site-specific modification on protein function are not known, but the activity of several peptides such as cholecystokinin is greatly augmented by sulfation. Here, we examine the role of sulfation in the processing and activity of C4 (the fourth component of complement), one of the few proteins in which sites and stoichiometry of tyrosine sulfation have been characterized. Our results, with C4 as a paradigm, suggest that sulfation of tyrosine residues can have major effects on the activity of proteins participating in protein-protein interactions. Sulfation of C4 synthesized by Hep G2 cells was blocked by incubating the cells with NaClO3 and guaiacol. These sulfation inhibitors did not alter secretion or other steps in the processing of C4. However, hemolytic activity of C4 was decreased more than 50%. The inhibitors' effect on C4 activity was prevented by adding Na2SO4 to restore sulfation of C4. Activity of C3, a complement component homologous to C4 but lacking tyrosine sulfate residues, was minimally reduced (19%) by the inhibitors. Decreased hemolytic activity of nonsulfated C4 apparently resulted from impaired interaction with complement subcomponent C1s (EC 3.4.21.42), the protease that physiologically activates C4. Purified C1s was able to cleave nonsulfated C4, but approximately 10-fold higher concentrations of C1s were required for that cleavage than to yield equivalent cleavage of sulfated C4. Our results suggest that activation of C4, a central component in the classical pathway of complement activation, is influenced by the level of sulfation of the protein. Thus, sulfation of C4 provides a potential locus for physiological or pharmacological modulation of complement-mediated opsonization and inflammation.

The molecular genetics of components of complement

Rapid progress has been made in establishing linkages and in chromosome allocation of the genes of some 9 complement components. In the MHC, C2, Factor B, and two C4 or C4 related genes have been placed in some detail in both man and mouse. The gene coding for the cytochrome P-450 21-hydroxylase has been shown to be duplicated and immediately 3' to the two C4 genes, though it appears to be functionally and structurally unrelated to the complement components. Thus six genes have been mapped to this region where particular haplotypes are associated with increased susceptibility to a number of diseases, some of which are autoimmune in character. The complete gene structure of Factor B has been solved in man and rapid progress is being made with the C2 and C4 genes. The structural basis of the polymorphisms of these genes is being established. In C4, the polymorphism is exceptionally complex with varying numbers of loci and probably more than 50 allotypes occurring in man. A structural basis has also been found for the big differences in the biological activity of some of the C4 allotypes in man. Apart from the genes in the MHC, linkage has been found between the genes coding for C4bp, CR1, and Factor H. Remarkably there are sequence homologies between these proteins and C2 and Factor B, probably related to the ability to bind to one or other of the structurally similar proteins C3b and C4b. The complete cDNA sequences of C3 and C4 in mouse and man have given much information on the many posttranslational modifications of these proteins. A partial structure has been obtained for the C3 gene and the homology shown between C3, C4, C5, alpha 2-macroglobulin, and pregnancy zone protein. Although the amount of detailed information in the molecular genetics of complement components is accumulating rapidly, there appears to be a reasonable prospect that linkages and homologies will classify the data into a comprehensible form.

Molecular biology of the human and mouse MHC class III genes: phylogenetic conservation, genetics and regulation of expression

The generation of complementary and genomic DNA clones for the human and mouse MHC class III genes has advanced the study of the organization, structure, genetics and expression of these loci. These clones have been useful in defining new polymorphic markers in each species and therefore permit a more complete genetic analysis of the complement cluster and the MHC as a whole. The coding sequences of the factor B and C4 genes are extensively conserved both within and between species, in contrast to the coding sequences of other MHC products. In human and mouse, the organization of the class III genes is similar with respect to order and position between the class II and class I regions of the MHC. However, these inter-species similarities in the organization and products of the class III genes does not extend to their regulation. In addition to complement gene expression being regulated differently between tissue sites within a species, expression is apparently regulated differently in analogous tissues between species. The considerable progress which has been made in the molecular analysis of C2, factor B and C4 using DNA clones forms the basis for the future study of the biology of the class III genes and the role of complement in inflammatory processes and in the immune system.

Biosynthesis of the third (C3), eighth (C8), and ninth (C9) complement components by guinea pig hepatocyte primary cultures

In the present report guinea pig hepatocyte primary cultures were established in order to study the synthesis of the eighth (C8) and ninth (C9) complement component. As reference-protein, the third complement component (C3) was measured antigenetically and hemolytically. Synthesis of C8 and C9 was determined by means of the hemolytic activity of the culture supernatant harvested every 24 h during a 6-day incubation period in vitro. The data to be reported demonstrated that the hepatocytes are able to synthesize spontaneously and secrete C8 and C9; in their culture medium a hemolytic activity of about 15-25 X 10(8) em/10(6) cells/24 h for C8 and of 25-90 x 10(8) em/10(6) cells/24 h for C9 were found. The same hepatocyte cultures produced 2500-6000 micrograms/10(6) cells/24 h of C3. Hemolytic C3 activity was also found in the culture media. The synthesis of C8 and C9 could be reversibly inhibited by addition of 30-50 micrograms cycloheximide per ml of culture medium. The kinetics of synthesis show a slight decrease after the first day of culture and a recovery in the following days up to a rate that is two- to threefold higher than that of the first day. The data suggest that hepatocytes could contribute to the production of C8 and C9 present in the plasma.

The C4 and Slp genes of the complement region of the murine H-2 major histocompatibility complex

Recent analyses, at the protein and DNA levels of structure, of the murine complement components C4 and the closely related sex-limited protein, Slp have led to new insights into the H-2/S region-linked C4 and Slp genes and their products. The primary products are 200 000 Da precursors which are cleaved, intracellularly and extracellularly, into the the mature alpha-beta-gamma-subunit molecules of plasma. Precursor order of subunits is beta-alpha-gamma; a complementary DNA clone spanning the alpha-gamma junction has been extensively analysed. The C-terminal of the alpha-chain is of particular interest because of post-secretion processing which differentiates 'secreted' and 'plasma' forms of C4, both apparently functional, and because allelic variants of C4 and the Slp protein, which differ substantially in molecular masses, owe their differences principally to different levels of glycosylation of the alpha-chain. Allelic variations in rate of C4 synthesis (C4-high compared with C4-low) have been analysed in cultures of hepatocytes and macrophages. Three distinct modes of genetic regulation of the expression of the Slp protein have been identified.

Sequence heterogeneity of murine complementary DNA clones related to the C4 and C4-Slp isoforms of the fourth complement component

Two classes of mRNA encoding the murine C4 protein were identified by sequence analysis of clones isolated from a liver complementary DNA library. The divergence found within a 357 base pair sequence available for comparison is limited to five nucleotide replacements located in the region corresponding to the carboxy-terminal end of the C4d peptide fragment. One of the nucleotide substitutions influences the presence of a site for the Hind III restriction endonuclease. That this restriction site indeed discriminates the two non-allelic genes encoding the mouse C4 and C4-Slp isoforms has been demonstrated by Southern blot analysis and nucleotide sequencing at the genomic level. Circumstantial evidence supports the identification of the gene lacking the Hind III site in the region corresponding to the carboxy-terminal end of the C4d fragment as the one encoding the C4-Slp isotype.

Expression of the MHC class III genes

The second (C2) and fourth (C4) components of complement and factor B (B) are coded for by genes within the major histocompatibility complex (MHC). These proteins are synthesized in liver and in extrahepatic mononuclear phagocytes. The isolation of complementary DNA probes corresponding to each of these proteins now permits analysis of the molecular mechanisms controlling expression of the class III MHC genes. Genetic control of C4 gene expression has been examined in two model systems. A defect in post transcriptional processing of C4-specific RNA accounts for a failure to generate mature C4 mRNA in homozygous deficients of a C4 deficient guinea-pig strain. On the other hand, a quantitative difference in the amounts of mature C4 liver mRNA accounts for the genetic variation in C4 levels observed among several mouse strains. The maturation of monocytes to macrophages results in changes in biosynthesis of the MHC class III products; for example, a significant increase in rate of secretion of C2 and B is noted in human monocytes during the first 3 d in culture and the proportion of C2-producing cells is greater in freshly isolated macrophages than in monocytes. Macrophages demonstrate selective increases in factor B and C2 mRNA characteristic of specific tissues. In the guinea-pig macrophage, C4 gene expression is regulated by a selective feedback mechanism induced by extracellular native C4. The C4 binds to the macrophage cell surface mediating a change in transcription or, less likely, a change in stability of C4 mRNA. Regulation of C4 synthesis in the mouse macrophage is accomplished by mechanisms that are independent of this feedback control but the murine cells also display separate mechanisms for regulation of C4 and factor B-specific mRNA levels. Resident and elicited macrophages from either mouse or guinea-pig differ with respect to expression of the class III MHC gene products. These studies form the basis for evaluating the molecular regulation of inflammation, maturation of mononuclear phagocytes and the genetic variants and deficiencies of complement proteins.

Genes for murine fourth complement component (C4) and sex-limited protein (Slp) identified by hybridization to C4- and Slp-specific cDNA

Murine fourth component of complement (C4) and sex-limited protein (Slp) are two closely related serum proteins whose structural genes lie in the S region of the murine H-2 complex. We have cloned two very similar (95% nucleotide sequence identity) cDNAs; they encode amino acid sequences that are distinct but that correspond equally well with the limited amino acid sequence available for murine and human C4. We have identified one of these as C4 cDNA and the other as Slp cDNA by comparing, in a RNA blot hybridization experiment, the extent of hybridization of the two cDNAs to liver mRNAs from inbred mouse strains expressing varying amounts of C4 and Slp proteins in their plasma. The C4 and Slp cDNAs were used in a Southern blot experiment to identify the C4 and Slp genes in the molecular map of the S region.

Quantitative determination of complement components produced by purified hepatocytes

In this report we describe, on a quantitative basis, the secretion of complement components by hepatocytes. Primary cultures were established after isolation of the cells from guinea-pig liver and the synthesis of C3, C5, C4 and C2 was measured. The cells were isolated by collagenase perfusion of the liver followed by differential centrifugation. The contamination of the hepatocyte suspension with non-parenchymal cells was less than 1%. At 24 h after plating the cells the kinetics of complement production were measured. C3 and C5 content in the culture medium harvested at different time intervals was determined by a sensitive ELISA. Secretion of C2 and C4 was measured haemolytically using C2 or C4 deficient guinea-pig serum. Under the conditions used hepatocytes secreted C3 at a rate of about 100 ng/10(6) cells/h with a plateau of secretion after 24 h of culture corresponding to about 350,000 molecules/cell/h. C5 secretion was detectable after 3-6 h of culture. The C5 secretion rate was about 15 ng/10(6) cells/24 h. The functional activity of C4 and C2 in the supernatants amounted to about 80 SFU/cell/h if the culture medium was changed every 3 h but dropped significantly if the medium was changed every 12 h. The decrease of the haemolytic activity became stronger if the medium was changed every 24 h. Cycloheximide reversibly inhibited the complement production. Our results show that guinea-pig hepatocytes synthesize considerably more C3 and C5 compared to peritoneal macrophages supporting the hypothesis that hepatocytes provide the major source of plasma complement.

Identification and structural characterization of two incompletely processed forms of the fourth component of human complement

Immunoprecipitates of human C4 from EDTA-plasma were incubated with [14C]methylamine and analyzed by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and fluorography. In addition to finding label in the alpha-chains of the secreted (C4s) and predominant plasma (C4p) forms of C4, two additional molecules with apparent molecular weights of approximately 168,000 (p168) and approximately 125,000 (p125) covalently incorporated methylamine, indicating the presence of an internal thioester bond. These two molecules were present at a concentration of approximately 5% of total plasma C4 and were not immunoprecipitated by antisera to C3 or alpha 2-macroglobulin. A human hepatoma-derived cell line (HepG2), in addition to synthesizing C4s and small quantities of the polypeptide precursor of C4 (pro-C4), was found to secrete p168 and p125 at concentrations of 14 +/- 4.8 and 21 +/- 9.2% (mean +/- SD), respectively, of total secreted C4. These molecules were not found intracellularly. Both molecules were present on reduced, but not nonreduced, SDS-polyacrylamide gels. Chido (C4B) and Rodgers' (C4A) alloantisera precipitated the C4A and C4B variants of pro-C4, p168, p125, and C4s. Both tryptic and Staphylococcus aureus V8 protease peptide analyses showed homology between p168 and the beta- and alpha-chains and between p125 and the alpha- and gamma-chains. Partial NH2-terminal sequencing revealed that the beta-chain was NH2-terminal in p168 and that the alpha-chain was NH2-terminal in p125. Taken together, these data indicate that p168 and p125 represent uncleaved beta-alpha- and alpha-gamma-fragments of pro-C4, respectively. Thus, in most individuals, plasma C4 consists of five structurally distinct molecules, the single polypeptide precursor (pro-C4), the three-subunit secreted (C4s) and predominant plasma (C4p) forms of C4, and two incompletely processed two-subunit molecules with uncleaved beta-alpha- (p168) or uncleaved alpha-gamma (p125)-subunits. In addition, all five molecules are observed for both C4A (Rodgers) and C4B (Chido) structural genes.

Structure and expression of the C3 gene

To map the multiple interactive sites on the C3 polypeptide, it is advantageous to combine the approaches of protein chemistry, nucleic acid technology, and molecular biology. This review summarizes the currently known molecular properties of mouse liver C3 mRNA, cloned C3 cDNA, and genomic DNA. Original data communicated have specified the amino acid sequence of the 215 amino-terminal residues of mature mouse C3 beta. Southern blot analysis of liver DNA indicated that the mouse genome contains only one type of C3 gene, that murine and human C3 sequences strongly cross-hybridize, and that the human C3 gene is not somatically rearranged. Included are descriptions of the first human C3 genomic DNA clones, their preparation, and their use to map the human C3 gene to chromosome 19 in linkage with the myotonic dystrophy (DM) locus. After a brief survey of reports describing inherited human C3 deficiencies, we discuss a Dutch family and their three members with total homozygous C3 deficiency who were the subjects of a recent publication. The restricted synthesis of C3 in major and minor producer tissues is discussed and it is proposed that the C3 gene provides a good model system for studying the molecular basis of tissue-specific gene expression. Data are presented documenting the production of C3 in two established mouse macrophage-like cell lines and two rat hepatoma cell lines in tissue cultures. A short account covers the extensive literature on regulation of C3 serum concentrations in acute and chronic inflammation and the very incomplete picture that presently depicts hormonal regulation of C3 synthesis. The final experiment reported demonstrates that nucleic acid hybridization with cloned cDNA probes is a sensitive assay for quantitative determinations of C3 mRNA. With the help of cloned cDNA and genomic DNA, researchers can address questions concerning the functional topography of the C3 polypeptide, the gene's structure, and the molecular nature of inherited C3 deficiencies in humans.

cDNA clone spanning the alpha-gamma subunit junction in the precursor of the murine fourth complement component (C4)

cDNA clones carrying parts of murine fourth complement component (C4, serum substance protein) mRNA sequences have been identified by differential hybridization to mRNA from a high C4-producing strain, B10.WR, and a congeneic low C4 strain, B10.BR, followed by hybrid-selected translation and DNA sequence analysis. One clone, pMLC4/w7-2, encodes an open amino acid reading frame that includes four tandem arginine residues immediately preceding a sequence 85% homologous with the NH2-terminal sequence of the human C4 gamma-chain. The amino acid composition of the predicted sequence upstream of the tandem arginines matches quite closely with the composition of a similar sized peptide at the COOH terminus of the human C4 alpha chain. The latter result raises questions regarding the nature and extent of plasma-mediated postsynthetic processing of the C4 alpha-chain COOH terminus. The results also demonstrate that strain differences in plasma C4 levels (low C4 vs. high C4) reflect differences in steady-state levels of liver C4 mRNA in these strains.

Identification and partial characterization of the secreted form of the fourth component of human complement: evidence that it is different from major plasma form

Immunoprecipitation of human C4 from plasma followed by NaDodSO4/polyacrylamide gel electrophoresis under reducing conditions revealed the expected alpha, beta, and gamma chains as well as a smaller quantity of a molecule containing an alpha chain (p98) approximately equal to 5,000 daltons heavier than the normal alpha chain. Further studies on p98 indicated that it covalently incorporated methyl-amine, was present at a concentration of approximately equal to 8% of the principal plasma form of the C4 alpha chain, and was found in highly purified C4 preparations. Hep G2, a human hepatoma-derived cell line, was found to secrete a C4 molecule in which the alpha chain had a molecular weight identical to that of the p98 protein found in plasma. The secreted C4 molecule possessed hemolytic activity. The 5,000-dalton difference in the alpha chain was localized to the COOH terminus and was attributed to an additional polypeptide. We propose that p98 is the alpha chain of the secreted form of C4, which is processed extracellularly by proteolytic cleavage to the principal C4 molecule found in plasma.