The chemical structure of the C4d fragment of the human complement component C4

The complete amino acid sequence of the C4d fragment (380 residues long) of the human complement component C4 is presented. Most of the sequence was determined by analysis of CNBr peptides and tryptic peptides obtained from S-carboxymethylated protein. The sequence of the amino terminal 88 residues [Campbell R. D., Gagnon J. and Porter R. R. (1981) Biochem. J. 199, 359-370] and a 106 residue polymorphic segment of C4d [Chakravarti D. N., Campbell R. D. and Gagon J. (1983) FEBS Lett. 154, 387-390] was extended. Some overlaps not provided by the protein sequence analysis were obtained from the amino acid sequence predicted by the nucleotide sequence [Belt K. T., Carroll M. C. and Porter R. R. (1984) Cell 36, 907-914]. The present protein sequence data provide information for the isolation of all the CNBr and succinylated tryptic peptides of C4d. In addition to the polymorphism previously described, two other sets of polymorphic amino acid residues at positions 153 (Ile/Ser) and 154 (Gln/Ala) have been identified. The major site of glycosylation has been shown to be an asparagine residue located in the sequence -Asn-Val-Thr- in the carboxy terminal end of C4d. A remarkable difference in the predicted secondary structure of C4d arising from one set of four polymorphic residues in a stretch of six residues and another single polymorphic residue suggests a structural basis for the origin of the different chemical reactivities of the C4 isotypes (C4A and C4B) and their serological difference in the expression of Rodgers or Chido blood group antigens. Possible non-covalent membrane attachment sites have been suggested from the hydropathy profile. Comparison of the C4d sequence with human C3, C5 and alpha 2-macroglobulin revealed extended stretches of sequence similarity (between 19 and 38% homology) with the corresponding regions of these proteins.