Protein synthesis in pupae of the silkworm Bombyx mori after infection with nuclear polyhedrosis virus: resistance to viral infection acquired during pupal period.

Protein synthesis has been studied in pupae of the silkworm Bombyx mori (Bm) infected with nuclear polyhedrosis virus (BmNPV) at various stages of the pupal period. Nascent proteins were labelled by injection of [35S]methionine into pupae and then analysed by SDS-PAGE. Temporal regulation of synthesis of infected cell-specific proteins (ICSPs) in pupae was demonstrated by electrophoretic analysis of the proteins labelled at different times post-infection (p.i.). The rate of ICSP synthesis reached a maximum at 4 to 5 days p.i., exceeding the rate of synthesis of cellular proteins in uninfected pupae by about twofold. The viral proteins p10 and polyhedrin were the most abundant products synthesized late in the infection. Both proteins were found to be associated with the nuclear matrix after fractionation of nuclei from infected pupae. Two virus-induced phosphoproteins, pp35 and ppB, were found to be the major acceptors of labelled phosphate from [gamma-32P]ATP during in vitro phosphorylation of proteins in pupal homogenates, nuclei and nuclear extracts. These proteins had electrophoretic mobilities comparable to those of structural phosphoproteins of BmNPV virions with M(r)s of 35K and 11K to 16K, respectively. The latter polypeptide was identified as the major DNA-binding protein of the virus. The susceptibility of silkworms to BmNPV decreased markedly during the pupal period. Following injection of BmNPV all young pupae acquired polyhedrosis and finally died whereas most of the older pupae did not exhibit disease and completed metamorphosis normally. Moreover, the later in the pupal period the silkworms were infected, the lower the production of polyhedrin in diseased pupae.