Accelerated Evolution of Venom Gland Isozymes

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Snake venom contains a variety of physiological active enzymes which act on mammalian target molecules with high selectivety. Snakes know everything about mammalian (human) homeostasis. One of the most intriguing questions is how snake toxins have evolved to acquire such functions specific to mammalian constitutes. Recently, we discovered that crotalidae snake phospholipase A_2 (PLA_2) isozyme genes have evolved via accelerated evolution only in the mature protein-coding region to gain diverse physiological functions.A major theme in this project is to prove the universality of this hypothesis for other snake toxins and species to elucidate the adaptive evolutionary mechanism.First, we prepared cDNA libraries from Naja naja kaouthia and Dendroaspis angnsticeps (green mamba) venom gland. We obtained 50 positive clones encoding group I PLA_2 isozyme cDNAs from Naja naja kaouthia venom gland. Two cDNA clones were seqenced. Their nucleotide sequences encoded new PLA_2 isozymes with four amino acid differences to each other.From D.angnsticeps venom gland cDNA library, four new cDNAs encoding "theree finger" shaped isotoxins, which also exhibit various diverse fanctions, were cloned and sequenced. The comparison of the nucleotide sequences of the "three finger" shaped isotoxin cDANs revealed that the mature protein-coding region is much more variable than the noncoding region and that the numbers of nucleotide substitutions per nosnsynonymous site (K_A) are close to or larger than the numbers of nucleotide substitutions per synonymous site (K_S) in the mature protein-coding region. These observations suggest that these isotoxins have also evolved via Darwinian-type accelerated evolution. It is apparent that accelerated eolution is universal in snake venom isozyme or isoprotein families.