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

## TL;DR

The E18 protein is crucial for virus particle formation in Autographa californica multiple nucleopolyhedrovirus, affecting intracellular structures and virus release.

## Contribution

This study identifies the functional domains of E18 and its role in intranuclear envelopment and virion morphogenesis.

## Key findings

- E18 is essential for intranuclear microvesicle formation and nucleocapsid egress.
- The transmembrane domain and two low-complexity domains in E18 are critical for virion morphogenesis.
- An α-helix structure in the transmembrane domain facilitates nuclear trafficking of a fusion protein.

## Abstract

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 e18 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 e18 knockout virus and an e18 repair virus to investigate the effects of e18 deletion on virion morphogenesis. Our data indicated that e18 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 e18 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.

The 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 e18 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.