Functional characterization of Bombyx mori nucleopolyhedrovirus CG30 protein

The baculovirus cg30 gene is present in the genomes of most alphabaculoviruses. The gene product, CG30, contains two putative functional domains, a RING finger motif and a leucine zipper motif. A gene-knockout study in Autographa californica nucleopolyhedrovirus (AcMNPV) revealed that a cg30-disrupted virus did not show any striking differences compared with wild-type virus. To determine the roles of cg30 in another alphabaculovirus, we constructed two Bombyx mori NPV (BmNPV) mutants lacking a functional cg30 by lacZ cassette insertion and characterized its infectivity in BmN cells and B. mori larvae. The mutants produced fewer budded viruses (BVs) in BmN cells and B. mori larvae compared with wild-type BmNPV. We also observed a decrease in the release of occlusion bodies (OBs) in the hemolymph of the larvae infected with the cg30 mutants. To investigate the role of a RING finger domain of CG30 during virus growth, we further constructed two mutants; one expressed a mutant CG30 where a RING finger motif is disrupted by a single amino acid substitution, whereas the other possessed a CG30 derivative that completely lacked the RING finger domain. Both produced fewer OBs in the hemolymph of B. mori larvae, demonstrating that a RING finger domain of CG30 is involved in maximum OB production in the hemolymph of B. mori larvae. We also revealed that CG30 is expressed as a nuclear protein with a molecular mass of 30kDa in BmNPV-infected cells.