Complete complementary DNA sequence of the third component of complement of lamprey. Implication for the evolution of thioester containing proteins

Abstract

Lamprey liver mRNA sequences were amplified by reverse transcriptase-polymerase chain reaction using primers synthesized according to the amino acid sequences at the thioester region common to the mammalian C3, C4, and α 2-macroglobulin (α 2M). Two different cDNA species were identified that showed a close similarity to the mammalian C3 or α 2M sequences, respectively. Using the C3-like sequence as a probe, two overlapping cDNA clones were isolated from the lambda ZAP library, which together covered the entire region encoding the putative lamprey pro-C3. The deduced amino acid sequence of the putative lamprey pro-C3 contained 1660 amino acids and showed 31%, 22%, 23%, and 16% amino acid sequence identity with mouse C3, C4, C5, and human α 2M, respectively. The distributions of cysteine residues were completely identical between the mouse C3 and the putative lamprey C3 except that the lamprey sequence had two additional cysteine residues in the α-chain. The possible β-α and α-γ processing sites were found at exactly the same positions as in mammalian C4. These results suggest that the putative lamprey C3 retains a close similarity to the common ancestor of the mammalian C3 and C4, which appeared to have had a three-subunit chain structure.