Baculovirus VP1054 is an acquired cellular PURα, a nucleic acid-binding protein specific for GGN repeats

Baculovirus VP1054 protein is a structural component of both of the virion types budded virus (BV) and occlusion-derived virus (ODV), but its exact role in virion morphogenesis is poorly defined. In this paper, we reveal sequence and functional similarity between the baculovirus protein VP1054 and the cellular purine-rich element binding protein PUR-alpha (PURα). The data strongly suggest that gene transfer has occurred from a host to an ancestral baculovirus. Deletion of the Autographa californica multiple nucleopolyhedrovirus (AcMNPV) vp1054 gene completely prevented viral cell-to-cell spread. Electron microscopy data showed that assembly of progeny nucleocapsids is dramatically reduced in the absence of VP1054. More precisely, VP1054 is required for proper viral DNA encapsidation, as deduced from the formation of numerous electron-lucent capsid-like tubules. Complementary searching identified the presence of genetic elements composed of repeated GGN trinucleotide motifs in baculovirus genomes, the target sequence for PURα proteins. Interestingly, these GGN-rich sequences are disproportionally distributed in baculoviral genomes and mostly occurred in proximity to the gene for the major occlusion body protein polyhedrin. We further demonstrate that the VP1054 protein specifically recognizes these GGN-rich islands, which at the same time encode crucial proline-rich domains in p78/83, an essential gene adjacent to the polyhedrin gene in the AcMNPV genome. While some viruses, like human immunodeficiency virus type 1 (HIV-1) and human JC virus (JCV), utilize host PURα protein, baculoviruses encode the PURα-like protein VP1054, which is crucial for viral progeny production.