Isolation of an Apoptosis Suppressor Gene of the Spodoptera littoralis Nucleopolyhedrovirus

Abstract

Spodoptera frugiperda SF9 cells infected with mutants of the Autographa californica nucleopolyhedrovirus (Ac M NPV) which lack a functional p35 gene undergo apoptosis, aborting the viral infection. The Spodoptera littoralis nucleopolyhedrovirus (SlNPV) was able to suppress apoptosis triggered by vΔP35K/pol + , an Ac M NPV p35 null mutant. To identify the putative apoptotic suppressor gene of SlNPV, overlapping cosmid clones representing the entire SlNPV genome were individually cotransfected along with genomic DNA of vΔP35K/pol + . Using this complementation assay, we isolated a SlNPV DNA fragment that was able to rescue the vΔP35K/pol + infection in SF9 cells. By further subcloning and rescue, we identified a novel SlNPV gene, Slp49 . The Slp49 sequence predicted a 49-kDa polypeptide with about 48.8% identity to the Ac M NPV apoptotic suppressor P35. SLP49 displays a potential recognition site, TVTDG, for cleavage by death caspases. Recombinant Ac M NPVs deficient in p35 bearing the Slp49 gene did not induce apoptosis and showed successful productive infections in SF9 cells, indicating that Slp49 is a functional homologue of p35 . A 1.5-kbp Slp49 -specific transcript was identified in SF9 cells infected with SlNPV or with vAc496, a vΔP35K/pol + -recombinant bearing Slp49 . The discovery of Slp49 contributes to the identification of important functional motifs conserved in p35 -like apoptotic suppressors and to the future isolation of p35 -like genes from other baculoviruses.