Complete intron/exon organization of DNA encoding the alpha' chain of human C3

The third component of human complement (C3), a central molecule in both the classical and alternative pathways of complement, is comprised of two polypeptides, termed the alpha and beta chains. Activation of C3 cleaves the alpha chain into two fragments, C3a, an inflammatory peptide, and the alpha' chain which remains covalently linked to the beta chain. Proteolytic fragments derived from the alpha' chain during activation and regulation of complement play a significant role in host defense and regulation of the immune response. Two cosmid clones covering the alpha' chain region were used to characterize the structure of this portion of the C3 gene. The alpha' chain is encoded by 24 exons, which range in size from 52 to 213 base pairs (bp) with an average size of 115 bp. The splice donor sequence at the beginning of intron 12 has a rare sequence variant of GC instead of the usual GT sequence. Ten introns have been completely sequenced and were surprisingly short, ranging in size from 85 to 242 bp with an average of 140 bp. Other introns range in size from 250 bp to over 4 kilobases in length. The gene size for this portion of C3 is estimated to be 23-24 kilobases. Comparison of exon structure with protein domains and with peptide mapping studies demonstrates that several binding sites on C3 are encoded by single exons. These data support the hypothesis that individual exons can code for functional protein domains.