Characterization of the mRNA and cloned cDNA specifying the third component of mouse complement

Eighteen cDNA clones containing inserts specific for the third component of complement (C3) have been derived from high molecular weight mouse liver mRNA. The inserts span 4,600 nucleotides of the C3 coding sequence, including the 3' end of C3 mRNA. The length of C3 mRNA was determined to be 5,100 +/- 200 nucleotides, including a poly(A)-containing tail of mean length 170 nucleotides. From cDNA sequence analysis of the 5'-proximal region of C3 mRNA, the NH2-terminal amino acid sequence of the mature C3 beta chain was predicted to be Ile-Pro-Met-Tyr-Ser-Ile-Ile-Thr-Pro-Asn-Val-Leu-Arg-Leu-Glu. This sequence is in good agreement with the reported amino acid sequences of human and guinea pig C3 beta chains. These data position the C3 beta subunit to the NH2-terminal portion of the precursor C3 molecule (pro-C3) and establish the order of subunits in pro-C3 to be NH2-beta-alpha-COOH. In addition, the cDNA sequence indicates that an NH2-terminal extension peptide precedes the beta chain in pro-C3. The amino acid sequence of the mouse C3a fragment and its flanking regions was determined. The data indicate the presence of four arginine residues located between the COOH terminus of the C3 beta and the NH2 terminus of the C3 alpha subunits in pro-C3. The coding sequences of the amino acids that constitute the internal thioester domain in C3 were determined. Unexpectedly, the glutamyl residue that has been shown to participate in the thioester bond in native C3 was found to be encoded as a glutamine.