The factor B and C2 genes

The echinoid complement system inferred from genome sequence searches

The vertebrate complement cascade is an essential host protection system that functions at the intersection of adaptive and innate immunity. However, it was originally assumed that complement was present only in vertebrates because it was activated by antibodies and functioned with adaptive immunity. Subsequently, the identification of the key component, SpC3, in sea urchins plus a wide range of other invertebrates significantly expanded the concepts of how complement functions. Because there are few reports on the echinoid complement system, an alternative approach to identify complement components in echinoderms is to search the deduced proteins encoded in the genomes. This approach identified known and putative members of the lectin and alternative activation pathways, but members of the terminal pathway are absent. Several types of complement receptors are encoded in the genomes. Complement regulatory proteins composed of complement control protein (CCP) modules are identified that may control the activation pathways and the convertases. Other regulatory proteins without CCP modules are also identified, however regulators of the terminal pathway are absent. The expansion of genes encoding proteins with Macpf domains is noteworthy because this domain is a signature of perforin and proteins in the terminal pathway. The results suggest that the major functions of the echinoid complement system are detection of foreign targets by the proteins that initiate the activation pathways resulting in opsonization by SpC3b fragments to augment phagocytosis and destruction of the foreign targets by the immune cells.

Targeting the Complement Pathway in Malignant Glioma Microenvironments

Malignant glioma is a highly fatal type of brain tumor, and its reoccurrence is largely due to the ordered interactions among the components present in the complex microenvironment. Besides its role in immune surveillance and clearance under physiological conditions, the complement system is expressed in a variety of tumor types and mediates the interactions within the tumor microenvironments. Recent studies have uncovered the broad expression spectrum of complement signaling molecules in the tumor microenvironment and various tumor cells, in particular, malignant glioma cells. Involvement of the complement system in tumor growth, immunosuppression and phenotype transition have also been elucidated. In this review, we enumerate the expression and function of complement molecules in multiple tumor types reported. Moreover, we elaborate the complement pathways in glioma cells and various components of malignant glioma microenvironments. Finally, we summarize the possibility of the complement molecules as prognostic factors and therapeutic targets in the treatment of malignant glioma. Specific targeting of the complement system maybe of great significance and value in the future treatment of multi-type tumors including malignant glioma.

Regulation of complement protein biosynthesis in mononuclear phagocytes

Proteins of the complement system (with the exception of the terminal components C6-9) are synthesized in mononuclear phagocytes. The extrahepatic macrophage is therefore an important local source of the complement proteins which may serve as a first-line host defence mechanism. Net synthesis and secretion of complement by these cells is a function of maturation of the mononuclear phagocytic series, the tissue from which the cells are isolated, and the state of macrophage activation. To define some of the mechanisms for regulation of complement gene expression in mononuclear phagocytes, the major histocompatibility complex class III genes and C3 have been investigated. These genes are expressed constitutively in hepatocytes and monocytes/macrophages. In the mononuclear phagocyte, interferon-gamma, at physiological concentrations, effects a dose- and time-dependent increase in factor B and C2 mRNA and a corresponding increase in factor B and C2 biosynthesis. This effect is specific inasmuch as the expression of other genes (e.g. C3) is decreased by interferon-gamma, and interferon-alpha and beta at concentrations one to two logs greater have only a minimal effect on C2 and factor B gene expression. Endotoxin acting directly on monocytes has qualitatively different effects on expression of the complement genes. These complex regulatory mechanisms are being investigated with the use of murine fibroblasts transfected with human DNA bearing the relevent complement genes.

Human GABA(B)R genomic structure: evidence for splice variants in GABA(B)R1 but not GABA(B)R2

The type B gamma-aminobutryic acid receptor (GABA(B)R) is a G protein coupled receptor that mediates slow pre- and post-synaptic inhibition in the nervous system. We find that the human GABA(B)R2 gene spans greater than 350 kb and contains 2.8 kb of coding region in 19 exons. The overall similarity in genomic structure with regard to conservation of intron position and exon size between human or Drosophila GABA(B)R1 and GABA(B)R2 genes suggests a common ancestral origin. Multiple transcripts GABA(B)R1a-c and GABA(B)R2a-c have been described and alternative splicing has been proposed to result in GABA(B)R1c, GABA(B)R2b and GABA(B)R2c. The results described here provide support for the existence of GABA(B)R1c but not for GABA(B)R2b and GABA(B)R2c. Splice junctions present in the GABA(B)R1 gene sequence are consistent with the formation of GABA(B)R1c by exon skipping of one sushi domain module. The GABA(B)R2 gene lacks canonical splice junctions for the reported variants. Consistent with this, RNA analysis demonstrates the presence of GABA(B)R1c and GABA(B)R2 transcripts in fetal and adult human brain RNA but GABA(B)R2b and GABA(B)R2c transcripts are not detected. These results provide insight into the evolution and transcript diversity of the mammalian GABA(B)R genes.

Four ubiquitously expressed genes, RD (D6S45)-SKI2W (SKIV2L)-DOM3Z-RP1 (D6S60E), are present between complement component genes factor B and C4 in the class III region of the HLA

The association of the HLA class III region with many diseases motivates the investigation of unidentified genes in the 30-kb segment between complement component genes Bf and C4. RD, which codes for a putative RNA binding protein, is 205 bp downstream of Bf. SKI2W (HGMW-approved symbol SKIV2L), a DEVH-box gene probably involved in RNA turnover, is 171 bp downstream of RD (HGMW-approved symbol D6S45). RP1 (HGMW-approved symbol D6S60E) is located 611 bp upstream of C4. The DNA sequence between human RD and RP1 was determined and the exon-intron structure of SKI2W elucidated. SKI2W consists of 28 exons. The putative RNA helicase domain of Ski2w is encoded by 9 exons. Further analysis of the 2.5-kb intergenic sequence between SKI2W and RP1 led to the discovery of DOM3Z. The full-length cDNA sequence of DOM3Z encodes 396 amino acids with a leucine zipper motif. Dom3z-related proteins are present in simple and complex eukaryotes. In Caenorhabditis elegans, Dom3z-related protein could be involved in the development of germ cells. Human RD-SKI2W and DOM3Z-RP1 are arranged as two head-to-head oriented gene pairs with unmethylated CpG sequences at the common 5' regulatory region of each gene pair. The ubiquitous expression pattern suggests that these four genes are probably housekeeping genes.

Mice deficient for the complement factor B develop and reproduce normally

Factor B is an essential component of the complement cascade which forms the C3 and C5 convertase of the alternative pathway. Factor B cleavage products also function as cofactors in antibody-independent monocyte-mediated cytotoxicity, macrophage spreading, plasminogen activation and proliferation of B lymphocytes. Several healthy kindreds heterozygous for the factor B null or non-functional allele have been reported but the absence of homozygous factor B deficiency in humans or in animals has been speculated to be caused by the lethality of the phenotype. Here we report the generation of factor B-deficient mice by gene targeting in vivo. These mice were born at the expected Mendelian ratio and they both develop and breed normally in a conventional animal facility. These mice represent a model of complete alternative pathway deficiency. This model enables the dissection of the complement cascade in vivo and the elucidation of the relative contribution of this complement pathway in the various physiological and pathological phenomena ascribed to the complement system.

Human complement factor B: cDNA cloning, nucleotide sequencing, phenotypic conversion by site-directed mutagenesis and expression

A full-length cDNA clone, BHL4-1, encoding factor B was isolated from a human liver cDNA library and sequenced in its entirety. It consists of 2388 bp which include a 5'-untranslated region of 40 bp, a single open reading frame, 2292 bp in length, and a 3'-untranslated region of 56 bp followed by a poly-A tail. The deduced amino acid sequence comprises 25 residues of a putative leader peptide and 739 residues of the mature polypeptide chain of the F allele of factor B. We constructed an S allele-like Q7R mutant of BHL4-1 by site-directed mutagenesis. Both the wild-type and mutant factor B cDNA were expressed transiently in a eukaryotic system. The specific hemolytic activities of the two recombinant factor B alleles and of native B were not significantly different from each other.

Type A modules: interacting domains found in several non-fibrillar collagens and in other extracellular matrix proteins

A 200-amino acid long motif first recognized in von Willebrand Factor (type A module) has been found in components of the extracellular matrix, hemostasis, cellular adhesion, and immune defense mechanisms. At present the extracellular matrix is the predominant site of expression of type A modules since at least four non-fibrillar collagens and two non-collagenous proteins contain a variable number of modules ranging from one to twelve. The modules conform to a consensus motif made of short conserved subregions separated by stretches of variable length. The proteins that incorporate type A modules participate in numerous biological events such as cell adhesion, migration, homing, pattern formation, and signal transduction after interaction with a large array of ligands.

Genetic deficiencies of the complement system and association with disease--early components

Genetic deficiency of one of the early components of the classical pathway of complement (C1q, C1r, C1s, C4 and C2) is often associated with clinical symptoms and immunochemical abnormalities common in idiopathic autoimmune diseases, such as lupus erythematosus, but also with an increased incidence of various, local and generalized infections. These observations are consistent with the current view of the complement system's role in handling immune complexes and combating microbial invasion. However, the absence of absolute correlations in these experiments of nature suggests that genetic defects of the classical pathway act only epistatically to other host factors and the primary etiologies of the associated diseases. In contrast, the strong association of properdin and factor D deficiency with serious infections caused by encapsulated Gram-negative bacteria suggests a more immediate involvement of the alternative pathway in a specific segment of immunity and its pathology. This concept is also supported by the primordial role of the alternative pathway in the evolution of the complement system and the apparent lethality of factor B deficiency. The gene structures of most of these early components have now been elucidated providing the basis for detailed analyses of the defective alleles, the determination of carrier status, and prenatal diagnosis.

HLA and disease

Many human diseases are associated with HLA class I, class II and class III antigens. It appears that the class III antigen disease associations can be explained by a direct defect operating at the level of either the class III gene or its gene product. The mechanism underlying class I and class II antigen disease associations is at present unknown. In this review we have considered thirty diseases which have been ranked according to their relative risk as defined by the frequency of a given HLA antigen in patient and control populations. The chronic inflammatory disorder, ankylosing spondylitis and its association with HLA B27 has been used as a model to study the HLA linked diseases. We have suggested that the disease may be caused by the Gram-negative microorganism Klebsiella which has antigenic similarity to HLA B27. It is proposed that some antibodies made against Klebsiella bind to HLA B27, thereby acting as autoantibodies leading to the pathological sequelae of chronic inflammatory arthritis. This is the crosstolerance hypothesis or molecular mimicry model and it has been compared to the receptor model. It is further suggested that the crosstolerance hypothesis can be utilised as a general theory to explain the association of other diseases with the class I and class II antigens, and offer a possible explanation for the polymorphism of HLA.

Complete structure of the chicken alpha 2(VI) collagen gene

Type VI collagen is a hybrid molecule consisting of a short triple helix flanked by two large globular domains. These globular domains are composed of several homologous repeats which show a striking similarity to the collagen-binding motifs found in von Willebrand factor. The alpha 2(VI) subunit contains three of these homologous repeats termed D1, D2 and D3. We have isolated and characterized the entire gene for chicken alpha 2(VI) collagen. This gene, which is present as a single copy in the chicken genome, is 26 kbp long and comprises 28 exons. All exons can be classified in three groups. (a) The triple-helical domain is encoded by 19 short exons (27-90 bp) separated by introns of phase class 0. These exons are multiples of 9 bp and encode an integral number of collagenous Gly-Xaa-Yaa triplets. (b) The homologous repeats D1-D3 are encoded by one or two very long exons each (153-1578 bp). These exons are separated by introns of phase class 1. (c) The homologous repeats and the collagen sequence are linked to each other by three short adapter segments which are each encoded by a single exon (21-46 bp). The modular nature of the polypeptide is thus clearly reflected by the mosaic structure of its gene. The size of the exons and the phase class of the introns suggest that the alpha 2(VI) gene evolved by duplication and shuffling of two different primordial exons, one of 9 bp encoding a collagen Gly-Xaa-Yaa triplet and one of 600 bp encoding the precursor of the homologous repeats.

Differences in gene copy number carried by different MHC ancestral haplotypes. Quantitation after physical separation of haplotypes by pulsed field gel electrophoresis

We have examined the hypothesis that MHC ancestral haplotypes have a specific content of genes regulating the extent of autoimmune reactions. Gene copy number was quantitated by objective densitometry after PFGE was used to separate heterozygous AHs of different lengths. Initially we analyzed examples of known gene copy number at the C4 and 21 hydroxylase loci and showed that the approach provides predictable results. We then studied heterozygotes containing one characterized and one uncharacterized AH with particular attention to the gene copy number at the C4, Cyp21, and DRB loci. Each AH studied has a characteristic gene copy number at each locus studied. The same may be true of TNF, but other possibilities must be considered. AHs are markers for extensive chromosomal segments including particular numbers of several functional genes. Since AHs mark susceptibility to autoimmune disease, differences in gene copy number may be implicated.

Participation of C3 and its ligands in complement activation

C3, the most abundant complement protein in blood, plays a central role in the activation sequence of the complement system as well as in host defense. Expression of the multiple functions of C3 requires its cleavage by highly specific enzymes termed C3 convertases. C3 in a conformationally altered form, C3H2O, resulting from the slow spontaneous hydrolysis of the internal thioester bond of native C3, initiates the assembly of a C3 convertase which continuously cleaves C3 in the blood at slow rates generating a constant supply of small amounts of C3b. When an activator of the alternative complement pathway is present, C3b becomes covalently attached to its surface via an ester or amide bond. Activator surface-bound C3b initiates the assembly of an "amplification" C3 convertase, C3bBb(P), which can efficiently activate C3 and generate additional convertase complexes on the surface of the activator. C3b generated by an amplification or classical pathway C3 convertase can also bind covalently to the noncatalytic subunit, C3b or C4b, respectively, resulting in the generation of a C5 convertase, an enzyme catalyzing the cleavage/activation of C5. In terms of participation in host defense, several fragments of C3, including C3a, C3b, iC3b, and C3dg, mediate a number of important functions such as increased vascular permeability, enhancement of phagocytosis, elimination of immune complexes, and perhaps also proliferative responses and/or differentiation of B cells.

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

The exon-intron structure of the human complement C1s gene displays a striking similarity with that of the gene encoding haptoglobin, a peculiar transport protein distantly related to the serine proteases. While the protease regions of the serine zymogens are typically encoded by multiple exons, the protease domains of C1s and of its genetically linked and functionally interacting homolog C1r are encoded as intronless domains, not unlike a region of haptoglobin, which in fact is devoid of proteolytic activity. The close similarity of the C1s gene with haptoglobin includes the precise conservation of exon-intron junctions and it extends to upstream exons encoding the short repeats typical of several complement components, but found also in other functionally unrelated proteins. Additional evidence of the common ancestry of C1r, C1s and haptoglobin is the presence, within the protease domain, of a set of sequence markers that distinguish these three proteins from all known serine proteases. The finding of vertebrate serine protease genes with an uninterrupted protease-encoding exon supports the definition of a novel evolutionary branch of this gene family and rules out the hypothesis that regards this unusual exon as an irrelevant byproduct of the extravagant functional divergence of haptoglobin.

Disulfide bonds are localized within the short consensus repeat units of complement regulatory proteins: C4b-binding protein

Several plasma and membrane proteins belong to a superfamily of structurally related proteins that contain internal homology of a variable number (2-30) of repeating units. Each SCR (short consensus repeat) unit is approximately 60 amino acid residues in length, with the positions of 1 Trp, 2 Pro, and 4 Cys residues being conserved. The aim of this study was to provide experimental evidence that each SCR may exist as an independent structural domain maintained by disulfide bonds. The well-characterized C4b-binding protein (C4BP) with eight SCR units in each of its seven identical chains was chosen for this study. Analysis of the disulfide-bonding pattern indicated that intrachain disulfide bonds may be localized within each SCR unit, with the first and third and the second and fourth half-cystines in each unit being linked. This pattern of disulfides may confer to C4BP (and to other structurally related proteins) a conformation which apparently allows the assembly of the SCR units (4-30) in a tandem fashion. Such an arrangement of the polypeptide chain(s) may explain, in part, the elongated shape of these protein molecules. The structural motif of the SCR units of C4BP is discussed in relation to those previously described for the type II domain of fibronectin and the kringle structure present in various proteins of the coagulation system.

The molecular genetics of components of the complement system

Rapid progress has been made recently on the elucidation of the structural components of the complement system by the application of recombinant DNA techniques. The derived amino acid sequences of most of the complement proteins are now available through cDNA cloning, and significant progress has been made in the discovery of the genetic organization of the corresponding genes. The linkage of some of the complement component genes has been established through the study of phenotypic genetics. Of particular interest has been the mapping of two clusters of genes which encode proteins involved in the activation of C3. C2, C4 and factor B, three of the structural components of the classical and alternative pathway C3 convertases, are encoded by genes which map to the MHC on human chromosome 6. The linkage of the genes with each other in a 100 kb segment of DNA has been established through the isolation of overlapping cosmid clones of genomic DNA, and PFGE has defined the molecular map position of these genes within the class III region of the MHC. The regulatory proteins factor H, C4BP, CR1 and DAF, which are involved in the control of C3 convertase activity, are encoded by closely-linked genes (termed the regulators of complement activation or RCA linkage group) that have been mapped to human chromosome 1. PFGE has defined the linkage of the CR1, C4BP and DAF genes, together with the CR2 gene in an 800 kb segment of DNA, and it is clear that this technique will eventually be applied to the molecular mapping of other complement genes in relation to their flanking loci. Polymorphism is a feature of many of the complement proteins, especially those encoded by genes in the MHC class III region. Of these, C4 is by far the most polymorphic, and differences in gene size and gene number, in addition to the functional and antigenic differences in the gene products, have been recognized. Null alleles at either of the C4 loci are rather common and may be important susceptibility factors in some HLA-associated diseases, particularly SLE. The molecular basis of complement deficiency states has begun to be elucidated. In many cases, the deficiency is not caused by a major gene deletion or rearrangement, and techniques which detect single point mutations in DNA (Cotton et al, 1988) will have to be applied to fully characterize the nature of the defect.

A putative functional domain of human placental alkaline phosphatase predicted from sequence comparisons

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Normal polymorphic variations and transcription of the decay accelerating factor gene in paroxysmal nocturnal hemoglobinuria cells

In paroxysmal nocturnal hemoglobinuria (PNH), an acquired hemolytic anemia, deficiency of decay accelerating factor (DAF) renders blood cells susceptible to increased deposition of autologous complement activation fragments (C3b) and complemented-mediated injury. To investigate the mechanism of the DAF defect, DNA and mRNA from normal and PNH leukocytes were compared in blot hybridization assays by using DAF cDNA and oligonucleotide probes. Southern analyses of DNA from normal cells revealed a single gene spanning approximately equal to 35 kilobases of DNA. Six HindIII banding patterns were distinguishable among normal individuals. In family studies, the patterns segregated as three homozygous and three heterozygous genotypes deriving from three haplotypes: A, B, and C with frequencies of 0.47, 0.36, and 0.17, respectively. Oligonucleotide mapping localized the polymorphic HindIII sites to two noncoding regions in the vicinity of exons encoding (i) the protein oligosaccharide-rich domain and (ii) the mRNA 3'-untranslated region. Analyses of DNA from DAF-negative leukocytes of eight PNH patients demonstrated restriction fragment profiles identical to those of normal individuals for all enzymes studied. Three patients had the BC (normals = 3/32), three patients had the AA (normals = 6/32), and two patients had the AC (normals = 8/32) HindIII genotype. Of the three PNH patients exhibiting the BC genotype, family studies of two demonstrated the expected inheritance patterns, and RNA gel blot analyses of two showed mRNA transcripts indistinguishable from those in normal cells. The absence of DAF gene or mRNA alterations in affected PNH cells that lack other glycolipid-anchored proteins as well as DAF argues that the lesion underlying PNH cells resides in the glycolipid-anchor pathway.

Primary structure of human coagulation factor XIII

The complete primary structures of the a and b subunits of human factor XIII were determined by a combination of cDNA cloning and amino acid sequencing. The a subunit is composed of 731 amino acids including an activation peptide (37 amino acids), an active site (-Tyr-Gly-Gln-Cys-Glu-), a putative calcium binding site(s), and a thrombin-inactivation site. The functional regions of the a subunit appear to be located in separate exons of its gene. The b subunit consists of 641 amino acids including ten tandem repeats that are homologous with those in at least 13 other proteins. Each GP-I structure in the b subunit is probably encoded by a separate exon.

Gene deletions correlate with the development of alloantibodies in von Willebrand disease

Among all patients with von Willebrand disease (vWD), alloantibodies to von Willebrand factor (vWF) have been described only in severe vWD (type III). The relationship between the development of alloantibodies and the nature of the genetic lesion in vWD is not known. In hemophilia B, large deletions within the factor IX gene appear to correlate with the occurrence of alloantibodies, whereas in hemophilia A no such correlation is apparent. We have studied 19 patients with severe recessive vWD (type III) and 19 with autosomal dominant vWD (type I) by Southern blotting with probes encompassing the full 9 kilobases (kb) of the vWF cDNA. Two apparently unrelated patients were shown to have large deletions within the vWF gene. Both patients had severe vWD (type III) and were the only patients among those studied that had inhibitory alloantibodies to vWF. The extent of deletion was similar in both patients, corresponding to at least the 3'-7.4 kb of the vWF cDNA. The deletion in each patient was estimated to exceed 110 kb. In addition, the localization of the vWF gene to chromosome 12 was confirmed, and a homologous sequence on chromosome 22 was identified.

Primary structure of human complement component C2. Homology to two unrelated protein families

The primary structure of the second component of human complement (C2) was determined by cDNA cloning and sequence analysis. C2 has 39% identity with the functionally analogous protein Factor B. The C-terminal half of C2a is homologous to the catalytic domains of other serine proteinases. C2b contains three direct repeats of approx. 60 amino acid residues. They are homologous to repeats in Factor B, C4b-binding protein and Factor H, suggesting a functional significance of the repeat in C4b and C3b binding. The repeats are also found in the non-complement proteins beta 2-glycoprotein I and interleukin-2 receptor, and this repeat family may be widespread.

Studies on the structure of the human C4b-binding protein gene

Protein and cDNA sequence data have shown human C4b-binding protein to contain eight internally homologous repeat units, each approx. 61 amino acids in length. Repeat units conforming to the same consensus sequence have been found in other complement and non-complement proteins. Southern blot analysis together with isolation, characterisation and sequencing of genomic clones has allowed the study of intron/exon organisation in the human C4b-binding protein gene and the identification of a Bg/II restriction fragment length polymorphism.

Murine protein H is comprised of 20 repeating units, 61 amino acids in length

A cDNA library constructed from size-selected (greater than 28 S) poly(A)+ RNA isolated from the livers of C57B10. WR mice was screened by using a 249-base-pair (bp) cDNA fragment encoding 83 amino acid residues of human protein H as a probe. Of 120,000 transformants screened, 30 hybridized with this cDNA probe. Ten positives were colony-purified, and the largest plasmid cDNA insert, MH8 (4.4 kb), was sequenced by the dideoxy chain termination method. MH8 contained the complete coding sequence for the precursor of murine complement protein factor H (3702 bp), 100 bp of 5'-untranslated sequence, 448 bp of 3'-untranslated sequence, and a polyadenylylated tail of undetermined length. Murine pre-protein H was deduced to consist of an 18-amino acid signal peptide and 1216 residues of H-protein sequence. Murine H was composed of 20 repetitive units, each about 61 amino acid residues in length. Similar repetitive units are present in the C4b binding protein, the C3b-receptor (CR1), complement factor B and C2, and in beta 2-glycoprotein I and the interleukin 2 receptor. This finding suggests a common evolutionary origin for regions of these proteins.

Partial characterization of human complement factor H by protein and cDNA sequencing: homology with other complement and non-complement proteins

Factor H, a control protein of the human complement system, is closely related in functional activity to two other complement control proteins, C4b-binding protein (C4bp) and complement receptor type 1 (CR1). C4bp is known to have an unusual primary structure consisting of eight homologous units each about 60 amino acids long. Such units also occur in the N-terminal regions of the complement proteins C2 and factor B, and in the non-complement serum glycoprotein beta 2I. Amino acid sequencing, and sequencing of a factor H cDNA clone, show that factor H also contains internal repeating units, and is homologous to the proteins listed above.

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.

DNA polymorphism of the C2 locus

The extent of the C2 locus in the HLA class III region has been determined by Southern blotting techniques and by DNA sequence analysis. The gene is 18 kb in length and therefore provides a marked contrast to the adjacent factor B gene of 6 kb. A novel restriction fragment length polymorphism (RFLP) has been identified using the endonuclease Sst I and a genomic probe derived from the 5' region of the C2 gene. Four variants have been detected in a sample of unrelated individuals with haplotypes carrying the C2C allele. Further analysis using C2 and factor B cDNA probes has determined the relationship between this and the other RFLPs previously identified in this region of the genome. Together, the three polymorphisms identified so far make the subdivision of previously indistinguishable haplotypes possible. They therefore constitute a series of markers which increase the resolution of genetic variation in the C2 locus and they may be important in studies of diseases associated with this region of the major histocompatibility complex.

The structure and genetics of the C2 and factor B genes

This review summarises our current knowledge of the genetic organisation, structure and polymorphism of the loci for the complement proteins, C2 and Factor B--class III gene products of the major histocompatibility complex. cDNA probes specific for C2 and Factor B have been used to screen cosmid libraries of human genomic DNA, and this has allowed isolation and characterisation of the corresponding genes. Southern blot analysis of the cosmid clones and of uncloned genomic DNA has shown that there are single C2 and Factor B loci that are less than 500 bp apart. Molecular mapping has revealed that the C2 gene spans approximately 18 kb of DNA. This is in marked contrast to the Factor B gene which is 6 kb in length. The entire gene structure of the Factor B gene has been determined and the interesting features of this gene which have emerged from an examination of the intron-exon boundaries are discussed. C2 and Factor B are polymorphic and structural variants have been detected by differences in charge. The degree of polymorphism at the C2 and Factor B loci has been examined by Southern blot analysis of restriction digests of genomic DNA. Three DNA polymorphisms have been identified in the C2 gene. These polymorphisms subdivide the common allelic variant of C2 (C2C) and reveal that there is much greater variability at the C2 locus than that detected by protein typing. It is suggested that these DNA polymorphisms may serve as useful markers in the genetic analysis of diseases that are related to the major histocompatibility complex.

Molecular cloning and characterization of the cDNA coding for C4b-binding protein, a regulatory protein of the classical pathway of the human complement system

By using synthetic oligonucleotides as probes, plasmid clones containing portions of cDNA coding for human C4b-binding protein were isolated from a liver cDNA library. The entire amino acid sequence of the C4b-binding protein can be predicted from this study of the cloned cDNA when allied to a previous sequence study at the protein level [Chung, Gagnon & Reid (1985) Mol. Immunol. 22, 427-435], in which over 55% of the amino acid sequence, including the N-terminal 62 residues, was obtained. The plasmid clones isolated allowed the unambiguous determination of 1717 nucleotides of cDNA sequence between the codon for the 32nd amino acid in the sequence of C4b-binding protein and the 164th nucleotide in the 3' non-translated region. The sequence studies show that the secreted form of C4b-binding protein, found in plasma, is composed of chains of apparent Mr 70 000 that contains 549 amino acid residues. Examination of the protein and cDNA sequence results show that there are at least two polymorphic sites in the molecule. One is at position 44, which can be glutamine or threonine, and the other is at position 309, which can be tyrosine or histidine. Northern-blot analysis indicated that the mRNA for C4b-binding protein is approx. 2.5 kilobases long. The N-terminal 491 amino acids of C4b-binding protein can be divided into eight internal homologous regions, each approx. 60 amino acids long, which can be aligned by the presence in each region of four half-cystine, one tryptophan and several other conserved residues. These regions in C4b-binding protein are homologous with the three internal-homology regions that have been reported to be present within the Ba region of the complement enzyme factor B and also to the internal-homology regions found in the non-complement beta 2-glycoprotein I.

The polymorphism of the complement genes in HLA

Genes coding for the complement proteins C2, C4A, C4B and factor B lie between HLA-D and HLA-B in HLA, the major histocompatibility complex in man. All the complement components are polymorphic, particularly C4, which has many alleles at each locus. The genetic complexity of C4 extends to the number of loci each of which may be deleted or duplicated on the chromosome. The different forms of C4 showed markedly different reactivities with small molecules and on haemolytic activity in the complement system. Surprisingly, amino acid sequences of the several allelic forms of C4 appear to be very similar, with less than 1% of residue positions being changed between alleles of C4A and C4B. These results may be relevant to the increased susceptibility to autoimmune disease which is associated with particular haplotypes of the HLA complex.

DNA polymorphism of the C2 and factor B genes. Detection of a restriction fragment length polymorphism which subdivides haplotypes carrying the C2C and factor B F alleles

Factor B and the second component of complement (C2) in man are encoded within the major histocompatibility complex by single loci that are less than 1 kb apart. A 2.3 kb factor B-specific cDNA probe has been used to examine, by Southern blot analysis, the genomic DNA of individuals typed for C2 and factor B by protein electrophoresis. We have identified a restriction fragment length polymorphism using the endonuclease Taq I, which subdivides haplotypes carrying both the common variant of C2 (C2C) and the fast (F) variant of factor B. This DNA polymorphism has been mapped to lie in the C2 gene and represents a new genetic marker not defined by protein electrophoresis. This polymorphism may serve as a useful marker in the genetic analysis of diseases that are related to the major histocompatibility complex.