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As a step toward understanding the structure and function relationships of phospholipases A2 (PLAp), we isolated and sequenced five cDNAs encoding Trimeresurus flavoviridis (Habu snake) venom PLA2 isozymes. Comparison of the nucleotide sequences of these cDNAs revealed that the homologies of the 5' and 3' untranslated regions were much higher than that of the protein-coding regions and that the base substitution rates at the first, second, and third position of codons are similar in the coding region. To gain further insight into this novel findings, six T . flavoviridis venom gland PLA2 isozyme genes were isolated and sequenced. They were found to consist of four exons and three introns. The numbers of nucleotide substitutions per site (KN) for introns are approximately one-fourth of the numbers of nucleotide substituions per synonymous site (Ks) for the protein-coding regions, indicating that the introns are unusually conserved. The fact that the numbers of nucleotide substitutions per nonsynonymous site ( K A ) are close to or larger than KS values for relevant pairs of genes revealed that Darwinian-type accelerated substitutions have occurred in the protein-coding regions of exons. This is compatible with the presence of PLA2 species with diverse physiological activities in the venom. Phospholipase A2 (PLA2, EC 3.1.1.4) catalyzes the hydrolysis of the 2-acyl ester linkage of 3-snphosphoglycerides with the requirement of Ca2+. To date, the primary structures of more than 90 PLA2s have been determined (1). Aspartate-49 (numbered accoding to the aligned numbering of PLA2 enzymes from various sources) constitutes a part of the Ca2+ binding site (2). The PLA2s are classified into two groups, Asp-49-PLAz with high activity and Lys-49-PLA2 with extremely low activity (3). We isolated and sequenced Asp-49-PLA2 (4, 5 ) and two Lys-49-PLA2s called basic proteins I and I1 (6, 7) from Trimeresurus flavoviridis (Habu snake, crotalidae) venom. These PLAp consist of 122 amino acid residues and are structurally homologous to one another. Especially, basic protein I is identical to basic protein I1 except that aspartate at position 58 of the former is replaced by asparagine in the latter. These T. flavoviridis PLA2 isozymes provide a useful system for understanding the structure-function relationships of PLA2s. In addition, a question arises as to why PLA2 isozymes with low lipolitic activity were produced in the venom. Here we report the cloning and the sequence analysis of cDNAs and genomic DNAs encoding venom gland T . flavoviridis PLA2 isozymes. Comparison and analysis of the nucleotide sequences of five cDNAs and six genes indicated that T.flavoviridis PLA2 isozyme genes have evolved so as to bring about accelerated amino acid substitutions in the protein-coding regions except for the signal peptide-coding domains. Such accelerated substitutions appear to be adaptive and consistent with the fact that T.flavoviridis venom contains PLA2 species with diverse physiological activities. Novel Structures of cDNAs Encoding T. fhvov ir id is PLA2 Isozymes. From T . flavoviridis venom gland cDNA library, five cDNAs encoding Asp-49-PLA2, basic protein I and 11, Thr-37-PLA2, and PLXI-PLA;! were isolated and sequenced. The nucleotide substitutions among five PLA2 isozyme cDNAs are schematically shown in Fig. 1. The 5' and 3' untranslated regions (UTRs)

Darwinian evolution of Trimeresurus flavoviridis venom gland phospholipase A2 isozymes