Cell-free translation of Drosophila C virus RNA: identification of a virus protease activity involved in capsid protein synthesis and further studies on in vitro processing of cricket paralysis virus specified proteins

pelo is required for high efficiency viral replication

Viruses hijack host factors for their high speed protein synthesis, but information about these factors is largely unknown. In searching for genes that are involved in viral replication, we carried out a forward genetic screen for Drosophila mutants that are more resistant or sensitive to Drosophila C virus (DCV) infection-caused death, and found a virus-resistant line in which the expression of pelo gene was deficient. Our mechanistic studies excluded the viral resistance of pelo deficient flies resulting from the known Drosophila anti-viral pathways, and revealed that pelo deficiency limits the high level synthesis of the DCV capsid proteins but has no or very little effect on the expression of some other viral proteins, bulk cellular proteins, and transfected exogenous genes. The restriction of replication of other types of viruses in pelo deficient flies was also observed, suggesting pelo is required for high level production of capsids of all kinds of viruses. We show that both pelo deficiency and high level DCV protein synthesis increase aberrant 80S ribosomes, and propose that the preferential requirement of pelo for high level synthesis of viral capsids is at least partly due to the role of pelo in dissociation of stalled 80S ribosomes and clearance of aberrant viral RNA and proteins. Our data demonstrated that pelo is a host factor that is required for high efficiency translation of viral capsids and targeting pelo could be a strategy for general inhibition of viral infection.

Induction of host defence responses by Drosophila C virus

Insect responses that are specific for virus infection have been investigated using the genetically tractableDrosophila melanogaster. Most studies focus on interactions withDrosophilaC virus (DCV), which is a member of the familyDicistroviridae. DCV is a non-enveloped,T=3 icosahedral virus with a positive-sense RNA genome. It was demonstrated recently that several genes controlled by the Jak-STAT pathway are specifically upregulated upon DCV infection. To investigate the virus factors that induce these responses, we used the Jak-STAT regulated genes as reporter genes. Challenge of flies with non-infectious DCV particles or double-stranded RNA did not stimulate significant upregulation of the antiviral response genes. In addition, there was no difference in reporter gene upregulation betweenDrosophilachallenged with three different strains of DCV. This suggests that upregulation of theseDrosophilagenes may require virus replication and may involve the non-structural proteins of DCV.

Genome-wide RNAi screen reveals a specific sensitivity of IRES-containing RNA viruses to host translation inhibition

The widespread class of RNA viruses that utilize internal ribosome entry sites (IRESs) for translation include poliovirus and Hepatitis C virus. To identify host factors required for IRES-dependent translation and viral replication, we performed a genome-wide RNAi screen in Drosophila cells infected with Drosophila C virus (DCV). We identified 66 ribosomal proteins that, when depleted, specifically inhibit DCV growth, but not a non-IRES-containing RNA virus. Moreover, treatment of flies with a translation inhibitor is protective in vivo. Finally, this increased sensitivity to ribosome levels also holds true for poliovirus infection of human cells, demonstrating the generality of these findings.

A comparative study on the cell-free translation of the genomic RNAs of two aphid picorna-like viruses

The genomic RNAs of aphid lethal paralysis virus (ALPV) and Rhopalosiphum padi virus (RhPV)--two distinct picorna-like viruses found in aphids [Williamson et al. (1988) J Gen Virol 69: 787-795]--were both efficiently translated in rabbit reticulocyte lysates. ALPV RNA was translated into primary products with molecular weights ranging from 92 kDa to 170 kDa. These underwent time-dependent post-translational cleavage to produce smaller polypeptides including some with molecular weights comparable to those of the viral structural proteins. A 92 kDa polypeptide as well as smaller proteins were immunoprecipitated with capsid protein antisera, indicating the presence of at least one large capsid subunit protein precursor. RhPV RNA was translated into products of molecular weights ranging from 45 kDa to 175 kDa. There was no evidence for time-dependent post-translation cleavage of RhPV translation products. However, a 60 kDa polypeptide was precipitated with antiserum to RhPV virions, indicating that at least one capsid protein of RhPV is derived by proteolysis of a precursor protein, like those of ALPV and most other picornaviruses.