Differences in primary structure among five phospholipases A2 from Heloderma suspectum

Abstract

Five increasingly anionic phospholipases A2 (Pa1-Pa5) exist in the venom of the lizard Heloderma suspectum. We recently elucidated the sequence of Pa5, the most abundant and most active variant, towards emulsified phosphatidylcholines. Here we present the primary structures of Pa2, Pa3 (subvariants a and b) and Pa4, based on Edman degradation of tryptic, endoproteinase Arg-C and chymotryptic fragments of the reduced and S-carboxymethylated proteins. Pa1-Pa5, considered collectively, belong to an original class of secretory phospholipases A2 with 141-143 residues, a short hydrophobic N-terminus, 10 half-cystine residues and an extended C-terminus. The only known phospholipase A2 with characteristics close enough to be a member of the same class is that present in the venom from the insect Apis mellifera. More specifically, the sequences of Pa3 and Pa5 are almost identical, and those of Pa2 and Pa4 are also quite similar. Both groups diverge enough to indicate the translation of two mRNA species in the venom gland. The primary structure of Pa3 reveals the existence of subvariants a and b, the sequence of which is identical to that previously defined for Pa5, except that the C-terminal tripeptide GEG in Pa5 is replaced by the dipeptide GE in Pa3a and the tetrapeptide GEGR in Pa3b, Pa4, when compared to Pa5, shows 21 substitutions with a cluster of five modified amino acids in positions 40-44, immediately after the catalytic segment amino acids 30-39, and added changes scattered before the C-terminus. Pa2 differs from Pa4 only by the absence of the Gly142 C-terminal residue. The 15% difference in primary structure observed between the Pa3-Pa5 and Pa2-Pa4 subgroups might be largely responsible for their distinct biological properties.