Evidence of the Coevolution of a Snake Toxin and Its Endogenous Antitoxin. Cloning, Sequence and Expression of a Serum Albumin cDNA of the Chinese Cobra

A full-length cDNA of the serum albumin (CSA) of the cobra (Naja naja kaouthia) was cloned from a lambda gt 11 library. It encodes a mature protein of 614 amino-acid residues homologous to the precursor of mammalian serum albumins. The 1 degree and 2 degrees structures of the CSA resemble those of the human variety. The putative toxin binding sites are mainly located in the subdomains IIA and IIIA. The relation between structural homology and function of the serum albumins (SA) is discussed. An analysis of their evolutionary tree revealed that anti-toxicity arose by < 90 amino-acid exchanges. The rate of substitution is much higher in the SA than in cytochrome C, which probably reflects the difference in evolutionary driving forces. The evolutionary period of the SA (6.7 +/- 0.1 M.Y.) significantly exceeds that of hemoglobin (5.8 M.Y.). Eight tripeptides in the nicotinic acetylcholine receptor (ACR), all flanking the putative toxin binding site, are also found in the CSA where they join to form 1 octa-, 1 penta- and 4 tripeptides, thus indicating the concerted evolution of two functionally linked proteins: toxin and antitoxin (CSA).