cDNA cloning and sequence analysis of a lysine-49 phospholipase A2 myotoxin from Agkistrodon contortrix laticinctus snake venom.

A cDNA clone (ACLPREMT1) for a K49 phospholipase A2 (PLA2) myotoxin from Agkistrodon contortrix laticinctus snake venom was isolated from a venom gland library and sequenced. The ACLPREMT1 cDNA is 734 bp in length and has an open reading frame of 414 bp. It codes for a K49 phospholipase A2 with 121 amino acid residues. The sequence of the first 20 amino acid residues of the predicted mature protein matches exactly with the N-terminal sequence of the purified myotoxin. Comparison of the ACLPREMT1 cDNA sequence with PLA2 cDNAs from Viperidae snakes shows that it has a similar organization: highly conserved 5' and 3' untranslated regions, a sequence encoding a 16-amino acid signal peptide, and the mature protein coding region. Comparison of the predicted sequence of ACL myotoxin and other K49 and D49 PLA2 myotoxins shows that, despite the homology (85-97%) at the nucleotide level, K49 PLA2 myotoxins are distinct from the D49 PLA2s and form a highly conserved protein family. In addition to the substitution of D49K, K49 myotoxins have several invariant residues not found in the D49 group, including K7, K78, K80, K115, and K116. There are also some conserved residues (E12, T13, K16, and N17) in all myotoxic proteins, including some neurotoxic and myotoxic PLA2s. Molecular modeling of ACL myotoxin shows that these residues are close together on the surface of one side of the molecule which suggests a potential site for binding to membranes and/or induction of toxicity.