Characterization of an inhibitor of apoptosis protein in Crassostrea gigas clarifies its role in apoptosis and immune defense

The inhibitor of apoptosis (IAP) proteins maintain a balance between cell proliferation and cell death by inhibiting caspase activity and facilitating immune responses. In this study, phylogenetic analysis revealed lineage-specific expansion and tandem duplication of IAPs in the Pacific oyster Crassostrea gigas. We then investigated a representative oyster-specific XIAP-like gene (CgIAP2) to understand how it regulates initiator caspase. Cloning of full-length CgIAP2 from oyster cDNA uncovered a deduced protein containing two BIR domains and a RING domain. Homolog comparison demonstrated that CgIAP2 clustered into the invertebrate branch. We found that CgIAP2 was likely involved in apoptosis inhibition and immune defense, based on high mRNA expression in the gills and labial palps, as well as increased mRNA expression after bacterial challenge. A yeast two-hybrid assay revealed that the BIR2 domain was necessary and sufficient to mediate interaction between CgIAP2 and Cgcaspase-2, providing direct evidence that CgIAP2 participates in apoptosis inhibition.