A molecular basis for the two locus model of human complement component C4

Abstract

The major histocompatibility complex(MHC)-linked fourth component of complement (C4) shows a high degree of polymorphism in several animal species. In man C4 polymorphism was detected by distinct charge differences of the variants. O'Neill et al. showed that this C4 polymorphism was controlled by two closely linked genetic loci, F (C4A) and S (C4B) and these results were extended by Awdeh et al. with an improved typing method. Biochemical analysis of human C4 has revealed that it consists of three polypeptide chains, alpha, beta and gamma. In all reports so far on the molecular analysis of human C4, no molecular weight differences between the A and B locus-encoded molecules have been noticed. Here we demonstrate that the C4A and C4B locus-encoded alpha-chains have a molecular weight (MW) of 96,000 and 94,000, respectively, presenting for the first time a molecular basis for the difference between all C4A and C4B variants tested. Even rare variants that are difficult to allocate to the A or B locus on the basis of charge differences could be identified as C4A or C4B variants in this way, thereby providing new insights into the relationships between the C4A and C4B loci.

Genetic regulation of a structural polymorphism of human C3b receptor.

“…Two forms of human C4 differing in their M, have been described, but they represent products of two separate loci, C4A and C4B (34). Similar to the findings relating to the C3bR are the allotypic a-chain variants of murine C4 that can be distinguished by SDS-PAGE; these have been shown to differ in the extent of their glycosylation (35)(36)(37).…”

“…The number of C3bR on human erythrocytes is regulated by two autosomal codominant alleles, tentatively designated C3BRQ H and C3BRQ0L according to a recommended standard nomenclature (17,18), that (15). These three phenotypes were present in 34,54, and 12%, respectively, of normal individuals. A markedly different distribution was found for patients with systemic lupus erythematosus among whom 5, 42, and 53% had the HH, HL, and LL phenotypes, respectively (15).…”

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

“…This is further proof that a' (p98) is an a-chain molecule with an additional peptide with an M, of -5,000 at the COOH-terminus (8 (34,35). By modifying the bis/ acrylamide ratio in SDS-polyacrylamide gels, the achain of C4 encoded by C4A (Rodgers) has a slightly slower relative mobility than the a-chain of C4 encoded by C4B (Chido) (15). To evaluate the possibility that one peptide (,B-a or a-y) was derived from C4A (Rodgers) and the other peptide derived from C4B (Chido), labeled hepatocyte medium was immunoadsorbed with anti-Chido or anti-Rodgers alloantisera.…”

“…Awdeh and Alpers (41), using agarose gel electrophoresis, further separated six structural variants and a deletion allele at the C4A locus and two structural variants and a deletion allele at the C4B locus. Furthermore, Roos et al (15) have visualized the Chido and Rodgers C4 a-chain variants by electrophoresis on SDS-polyacrylamide gels with a modified bis/acrylamide ratio (15). We considered the hypothesis that if one of the two fragments (13-a or a-) was derived from one of the two C4 genes and the other fragment derived primarily from the other gene, different ,B-a-and a--y-cleavage sites may then give rise to inefficient cleavages in the respective molecules.…”

Hypomorphic C4B∗ 15 variant of the fourth component of complement

“…Human C4 was precipitated from EDTA plasma with goat antihuman C4 antibody [4]. The precipitated gel was washed in 0.25 M EDTA (pH 7.5) and disintegrated in a sample buffer containing 10% 2-mercaptoethanol.…”