Identification of novel serum proteins in a Japanese viper: Homologs of mammalian PSP94

Three small serum proteins (SSP-1, -2, and -3), with molecular masses of 6.5–10 kDa, were isolated from Habu (Trimeresurus flavoviridis) serum, and the amino acid sequences were determined by protein and cDNA analysis. Despite only limited sequence identity to any mammalian prostatic secretory protein of 94 amino acids (PSP94), all of the Cys residues in these SSPs were well conserved. SSPs are the first PSP94 family proteins to be identified in reptiles. SSP-1 and -3 weakly inhibited the proteolytic activity of a snake venom metalloproteinase. On the other hand, SSP-2 formed a tight complex with triflin, a snake venom-derived Ca2+ channel blocker that suppresses the smooth muscle contraction. This suggests a role for SSP-2 in the self defense system of venomous snakes.