The proteins that bind phospholipase A(2) (PLA(2)) isozymes of Trimeresurus flavoviridis (habu snake, crotalinae) venom were fractionated from sera on four columns, each conjugated with one of four PLA, isozymes, Five proteins, termed PLA(2) inhibitors (PLI) I-V, were obtained as the binding components. The combinations of the binding components differed depending on the PLA(2) isozymes. PLI-IV and PLI-V correspond to PLI-A and PLI-B, respectively, which were known to bind to a major [Asp49]PLA(2), PLA2, and contained a segment similar to the carbohydrate-recognition domain of C-type lectins. PLI-I, which is a major component of inhibitory proteins against three basic PLA2 isozymes, PLA-B (a basic [Asp49]PLA(2)) and basic proteins I and II (both [Lys49]PLA(2)s), has been isolated, and its partial amino acid sequence has been determined. A cDNA encoding PLI-I was isolated from a T. flaviridis liver cDNA library and sequenced. PLI-I cDNA encoded 200 amino acid residues, including a signal peptide of 19 amino acid residues. One sugar chain was predicted to occur at position 157. A gene coding for PLI-I was isolated. It is 9.6-kb long and consists of five exons and four introns. Comparison of the exonintron structure of the PLI-I gene with those of genes encoding urokinase-type-plasminogen-activator receptor (uPAR), Ly-6, CD59 and neurotoxins showed that they have characteristic unit encoding approximately 90 amino acid residues, which is divided over two exons. This strongly suggests that the PLI-I gene belongs to the uPAR, Ly-6, CD59 and neurotoxin gene family. There are two types of structurally different inhibitors against PLA, isozymes in T. flavoviridis serum with different evolutionary origins.
