MdBVe46 is an envelope protein that is required for virion formation by Microplitis demolitor bracovirus

Abstract

Bracoviruses (BVs) are endogenized nudiviruses that braconid parasitoid wasps have coopted for functions in parasitizing hosts. Microplitis demolitor is a braconid wasp that produces Microplitis demolitor bracovirus (MdBV) and parasitizes the larval stage of the moth Chrysodeixis includens. Some BV core genes are homologs of genes also present in baculoviruses while others are only known from nudiviruses or other BVs. In this study, we had two main goals. The first was to separate MdBV virions into envelope and nucleocapsid fractions before proteomic analysis to identify core gene products that were preferentially associated with one fraction or the other. Results indicated that nearly all MdBV baculovirus-like gene products that were detected by our proteomic analysis had similar distributions to homologs in the occlusion-derived form of baculoviruses. Several core gene products unknown from baculoviruses were also identified as envelope or nucleocapsid components. Our second goal was to functionally characterize a core gene unknown from baculoviruses that was originally named HzNVorf64-like. Immunoblotting assays supported our proteomic data that identified HzNVorf64-like as an envelope protein. We thus renamed HzNVorf64-like as MdBVe46, which we further hypothesized was important for infection of C. includens. Knockdown of MdBVe46 by RNA interference (RNAi) greatly reduced transcript and protein abundance. Knockdown of MdBVe46 also altered virion morphogenesis, near-fully inhibited infection of C. includens, and significantly reduced the proportion of hosts that were successfully parasitized by M. demolitor.

Identification of a viral gene essential for the genome replication of a domesticated endogenous virus in ichneumonid parasitoid wasps

“…Functional studies, guided in part by these similarities, have provided insights into several key processes underlying BV virion production. Identification of BV core genes that regulate the expression of other BV core genes encoding structural proteins [25], are involved in BV virion formation [25, 26, 35], or are required for processing proviral segments into circular DNA molecules packaged into capsids [25, 26] have been documented. In contrast, identifying the components of IV genomes and functions of IV genes regulating replication is more challenging because the hypothesized NCLDV ancestor is unknown.…”

Envelope-Fusion-Syncytium Formation in Microplitis bicoloratus bracovirus Maturation

“…We identified EFCs, in which the fusion stage produces three types of multi-cored viral particles. The enveloped and matured virions are released into the calyx lumen by lysing calyx cells that cross the epithelial layer [ 5 , 14 , 16 ]. We redesignated two types of epithelial cells on the border of the ovary lumen, i.e., PCECs and NPCECs.…”

“…Recently, an envelope protein, MdBVe46, from Microplitis demolitor bracovirus was shown to be necessary but not alone sufficient for virus maturation [ 16 ]. This raised the question whether the fused envelope is regulated by calyx cells.…”