NH2-terminal sequence analysis of pro-C4, the precursor of the fourth component of guinea pig complement

The amino acid sequence of the NH2-terminal regions of the intracellular precursor of the fourth component of guinea pig complement (pro-C4) and isolated alpha, beta, and gamma chains of native C4, synthesized by peritoneal macrophages in culture, were determined with a microradiosequencing technique. Radiolabeled pro-C4 was immunoprecipitated from cell lysates and native C4 from culture media which contained one of six 3H-amino-acids or [35S]methionine. The purity of these proteins was established and molecular weights were estimated by electrophoresis in sodium dodecyl sulfate. Seventeen of the first 22 residues of pro-C4 were identified. Fifteen of these 17 were nonpolar. The eight amino acids ascertained within the first 9 NH2-terminal residues of guinea pig pro-C4 were identical with those reported for human C4 beta chain, and identity with residues determined for guinea pig C4 beta chain was established. These data indicate that beta chain is the NH2-terminal segment of pro-C4, that no residues are cleaved from the NH2 terminus during the conversion to native C4, and that this segment of the pro-C4 molecule is conserved phylogenetically.