Active Expression of Genes for Protein Modification Enzymes in Habu Venom Glands

Genes encoding snake venom toxins have been studied extensively. However, genes involved in the modification and functioning of venom proteins are little known. Protobothrops is a genus of pit vipers, which are venomous and inhabit the Nansei (Southwest) islands of Japan, Taiwan China, Vietnam, Thailand, Myanmar, Nepal, Bhutan, and India. Our previous study decoded the genome of Protobothrops flavoviridis, a species endemic to the Nansei Islands, Japan, and revealed unique evolutionary processes of some venom genes. In this study, we analyzed genes that are highly expressed in venom glands to survey genes for candidate enzymes or chaperone proteins involved in toxin folding and modification. We found that, in addition to genes that encode venom proteins and ribosomal proteins, genes that encode protein disulfide isomerase (PDI) family members (orthologs of human P4HB and PDIA3), Selenoprotein M (SELENOM), and Calreticulin (CALR) are highly expressed in venom glands. Since these enzymes or chaperones are involved in protein modification and potentially possess protein folding functions, we propose that P4HB, SELENOM, CALR, and PDIA3 encode candidate enzymes or chaperones to confer toxic functions upon the venom transcriptome.