The baculovirus is a classic example of a parasite that alters the behavior or physiology of its host so that progeny transmission is maximized. Baculoviruses do this by inducing enhanced locomotory activity (ELA) that causes the host caterpillars to climb to the upper foliage of plants. We previously reported that this behavior is not induced in silkworms that are infected with a mutant baculovirus lacking its protein tyrosine phosphatase ( ptp ) gene, a gene likely captured from an ancestral host. Here we show that the product of the ptp gene, PTP, associates with baculovirus ORF1629 as a virion structural protein, but surprisingly phosphatase activity associated with PTP was not required for the induction of ELA. Interestingly, the ptp knockout baculovirus showed significantly reduced infectivity of larval brain tissues. Collectively, we show that the modern baculovirus uses the host-derived phosphatase to establish adequate infection for ELA as a virion-associated structural protein rather than as an enzyme.

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The Bombyx mori nucleopolyhedrovirus (BmNPV) genome contains five related members of the bro gene family, all of which are actively expressed in infected BmN cells. Although their functions are unknown, their amino acid sequences contain a motif found in all known viral and prokaryotic single-stranded DNA binding proteins. To determine if they bind to nucleic acids, we fractionated the nuclei of BmNPV-infected BmN cells using a histone extraction protocol. We detected BRO-A, BRO-C, and BRO-D in the histone H1 fraction using anti-BRO antibodies. Micrococcal nuclease treatment released these BRO proteins from the chromatin fraction, suggesting their involvement in nucleosome structures. Chromatographic fractionation showed that BRO-A and/or BRO-C interacted with core histones. Expression of partial sequences of BRO-A proved that the N-terminal 80 amino acid residues were required for DNA binding activity. We also demonstrated that BmNPV BRO proteins underwent phosphorylation and ubiquitination followed by proteasome degradation, which may explain their distribution in the cytoplasm as well as the nucleus. We propose that BRO-A and BRO-C may function as DNA binding proteins that influence host DNA replication and/or transcription.Bombyx mori nucleopolyhedrovirus (BmNPV) belongs to the Baculoviridae, a large family of viruses with double-stranded (ds) DNA genomes that are pathogenic mainly for lepidopteran insects. The BmNPV genome is about 128 kb in length and is predicted to contain 136 open reading frames (ORFs) (9). Among these ORFs, five genes (bro-a, bro-b, bro-c, bro-d, and bro-e) were found to belong to a unique baculovirus multigene family, since they demonstrated high homology to each other (13). Multiple members of this gene family have been reported in the genomes of the Orgyia pseudotsugata NPV, Lymantria dispar NPV (LdNPV), and Xestia c-nigrum granulovirus (1,11,16). However, the well-characterized Autographa californica MNPV (AcMNPV) genome contains only a single member (ORF2), which is related to BmNPV bro-d (2, 13) with 80% amino acid sequence identity. This is much lower than the average identity of predicted proteins from these two viruses, which is over 93% (9). In addition, NPV pathogenic for Spodoptera exigua lacks a bro homolog (12).Most bro genes share a related core sequence and demonstrate differing degrees of similarity in other regions (16). Although BmNPV BRO proteins show high homology within the family and with other baculovirus BROs, they have no strong similarity with any known proteins. Thus, it has been difficult to predict their function during the viral infection cycle.Recently, we reported that all bro genes of BmNPV are actively transcribed as delayed-early genes and that BRO proteins are produced at high levels between 8 and 14 h postinfection (p.i.) (13). We also reported that one BmNPV bro gene (bro-d) is essential for viral infection and that bro-a and bro-c may functionally complement each other (13). Since our immunohistochemical analysis using confocal microscopy showe...