Open reading frame 193R of Chilo iridescent virus encodes a functional inhibitor of apoptosis (IAP)

Programmed cell death or apoptosis is a major defense mechanism in insects in response to viral infections. The genome of Chilo iridescent virus (CIV) has three ORFs with homology to baculovirus inhibitor of apoptosis (iap) genes. The proteins encoded by the 157L, 193R, and 332L ORFs contain 152, 208 and 234 amino acids, respectively. While all three proteins contain C-terminal RING domains, only the protein encoded by ORF 193R contains a baculoviral iap repeat (BIR) domain, indicative of a putative IAP protein. The 193R protein has 28 and 27% similarity in amino acid sequence to the Orgyia pseudotsugata MNPV and Cydia pomonella granulovirus IAP-3 proteins, respectively. ORF 193R from CIV is the only gene known to exist among the Iridoviridae that encodes a BIR domain. 193R is transcribed early during CIV infection, and its transcription is not dependent on the synthesis of early viral proteins. When this putative CIV IAP was transiently expressed in SPC-BM-36 and Sf21 cells under the control of an immediate early baculovirus promoter it significantly reduced apoptosis induced by actinomycin-D. Silencing of the CIV iap gene (193R) in CIV infected SPC-BM-36 cells with 193R-specific dsRNA resulted in apoptosis. Thus, CIV ORF 193R is the first iap gene identified in an iridovirus, which encodes a functional IAP protein.