Characterization and evolution of a gene encoding a Trimeresurus flavoviridis serum protein that inhibits basic phospholipase A₂ isozymes in the snake's venom

The proteins that bind phospholipase A₂ (PLA₂) isozymes of Trimeresurus flavoviridis (habu snake, crotalinae) venom were fractionated from sera on four columns, each conjugated with one of four PLA₂ isozymes. Five proteins, termed PLA₂ inhibitors (PLI) I-V, were obtained as the binding components. The combinations of the binding components differed depending on the PLA₂ isozymes. PLI-IV and PLI-V correspond to PLI-A and PLI-B, respectively, which were known to bind to a major [Asp49]PLA₂, 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]PLA₂) and basic proteins I and II (both [Lys49]PLA₂s), has been isolated, and its partial amino acid sequence has been determined. A cDNA encoding PLI-I was isolated from a T. flavoviridis 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 exon-intron 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 PLA₂ isozymes in T. flavoviridis serum with different evolutionary origins.