Human complement component C3: cDNA coding sequence and derived primary structure.

The complete cDNA coding sequence and derived amino acid sequence of human complement component C3 are presented. The encoded precursor molecule contains a signal peptide of 22 amino acid residues, the beta chain (645 residues), and the alpha chain (992 residues). The two chains are joined by four arginine residues not present in the mature protein. Several functionally important sites have been localized, such as the thiolester site, the cleavage site liberating the anaphylatoxin, and two sites of cleavage by the serine protease factor I, as well as a peptide fragment with leukocyte mobilizing activity. At least two carbohydrate attachment sites, one on each chain, have been identified. Human C3 has 79% identity to mouse C3 at the nucleotide level and 77% identity at the amino acid level. The protease alpha 2-macroglobulin and complement component C4 show considerable homology to C3, suggesting that the three proteins have evolved from a common ancestor.