Autographa californica multiple nucleopolyhedrovirus <i>e18</i> is essential for the formation of normal intranuclear membrane microvesicles and intranuclear envelopment and nuclear egress of nucleocapsids

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) E18 (AC143, ODV-E18) is an envelope protein common to both occlusion-derived virions (ODVs) and budded virions (BVs). The <i>e18</i> gene has been demonstrated to be essential for generating infectious BVs. However, its functional role in virion morphogenesis remains unclear. In this study, we constructed an <i>e18</i> knockout virus and an <i>e18</i> repair virus to investigate the effects of <i>e18</i> deletion on virion morphogenesis. Our data indicated that <i>e18</i> is required for normal intranuclear microvesicle (IMV) formation and accumulation, for intranuclear envelopment and nuclear egress of nucleocapsids, as well as for embedding of ODVs into occlusion bodies (OBs) and BV production. Additionally, we created and characterized a series of recombinant viruses with truncated <i>e18</i> of varying lengths to identify domains involved in nuclear translocation and virion morphogenesis. We identified two low-complexity domains (LCDs) in E18, in addition to a known transmembrane domain (TM). The AA30-34 sequence within the TM was found to be essential, but not sufficient for nuclear translocation. However, an α-helix structure encompassing the TM domain proved adequate to mediate a fusion protein's trafficking into the nucleus in the context of additional viral factors. Furthermore, we discovered that the TM was required for the accumulation of IMVs, while both the TM and LCD 1 were necessary for intranuclear envelopment, nuclear egress of nucleocapsids, and the embedding of ODVs into OBs; LCD 2 influenced the processing of IMVs and ODV formation. Both the TM and the two LCDs were essential for BV production.IMPORTANCEThe envelope protein E18 is a conserved component common to both ODV and BV virion types of baculoviruses, yet its functional role in virion morphogenesis remains unclear. This study investigated the <i>e18</i> gene of Autographa californica multiple nucleopolyhedrovirus, determining that it is essential for normal IMV formation and accumulation, intranuclear envelopment and nuclear egress of nucleocapsids, as well as for the embedding of ODVs into occlusion bodies and BV production. The functional roles of the single TM domain and two LCD domains within E18 during virion morphogenesis were identified. Furthermore, it was found that an α-helix structure encompassing the TM domain is sufficient to facilitate the trafficking of a fusion protein into the nucleus in the context of other viral factors, with AA30-34 being critical for the nuclear import of E18.