Identification of a novel occlusion derived virus-specific protein in Spodoptera exigua multicapsid nucleopolyhedrovirus

Understanding the molecular basis of the distinct biological properties of Spodoptera exigua multicapsid nucleopolyhedrovirus (SeMNPV), such as its narrow host range and high virulence, requires detailed information on the temporal expression and subcellular localization of SeMNPV gene products. The expression of two unique SeMNPV ORFs, 116 (Se116) and 117 (Se117), which show 45% amino acid similarity, was analyzed. Se116 and Se117 were expressed both in cultured cells and in larvae of S. exigua as polyadenylated transcripts of 0.80 and 0.75 kb, respectively. These transcripts initiated from ATCA(G/T)T promoter motifs, commonly found for baculovirus early genes. Se116 transcripts were detected with increasing abundance from 8 to 48 h p.i., whereas Se117 transcripts were present from 4 h p.i. and most abundantly at 24 h p.i. Western blot analysis of infected Se301 cells revealed 27- and 23-kDa proteins for Se116 and Se117, respectively. C-terminal GFP-fusion proteins of Se116 and Se117 were primarily localized in the nucleus of Se301 cells. When Se301 cells were infected with SeMNPV, both GFP-fusion proteins were localized in the virogenic stroma of the nucleus. While the function of the Se116 protein is still enigmatic, the Se117 protein appeared to be a structural protein associated with nucleocapsids of occlusion-derived SeMNPV virions but not of budded virus.