A SLiM-dependent conformational change in baculovirus IE1 controls its focus formation ability

The baculovirus IE1 gene encodes a multifunctional protein that is essential for both DNA replication and RNA transcription of the virus. Prior to viral DNA replication, IE1 promotes early gene transcription when localized in hr-dependent foci. During viral DNA replication, the IE1 foci expand and fuse to generate the virogenic stroma (VS) with IE1 found in the VS reticulum. To explore the IE1 structural features essential for this coordinated localization, we constructed various IE1 mutants based on three putative domains (N, I, and C). We determined that a BDI motif located in the intrinsic disorder region (IDR) between the N and I domains acts as a nuclear localization signal, whereas BDII and HLH in the C domain are required for VS localization in infected cells or for chromosomal association in uninfected mitotic cells. Deletion of the SLiM (short linear motif) located in the I domain restrains both nuclear- and VS localization. Intra-molecular fluorescence resonance energy transfer (FRET) probes of IE1 mutants revealed a conformational change of the I-C two-domain fragment during infection, which was inhibited by aphidicolin, suggesting that IE1 undergoes a stage-dependent conformational change. Further, homo-dimerization of the I domain and stage-dependent conformational changes require an intact SLiM. Mutational analysis of SLiM revealed that VS localization and chromosomal association were retained following S291A and S291E substitutions, but hr-dependent focus formation differed between the two mutations. These results suggest that coordinated IE1 localization is controlled by SLiM-dependent conformational changes that are potentially switched by the phosphorylation state of the SLiM.

Identification and characterization of BmNPV Bm5 protein required for the formation of nuclear vesicle structures

BmNPV infection induces nuclear vesicle-like structures and its Bm5 protein mediates the intranuclear lipid accumulation, which is thought to participate in the formation of nuclear vesicles. However, the relationship between viral-induced nuclear vesicles and Bm5 protein is still unclear. Here, our results indicated that BmNPV Bm5 protein participated in the baculovirus infection-induced nuclear vesicle-like structures' invagination thereby influencing the production of occlusion-derived virion (ODV) and occlusion body (OB). The process of nuclear vesicle-like structures' formation was dispensable for the transport or recruitment of ODV major envelope proteins, such as P74 and Bm14. Furthermore, baculovirus-induced nuclear F-actin might provide a direct mechanical force to mediate the scission of large vesicle-like structures from the nuclear membrane. Collectively, these findings illustrated a BmNPV Bm5 protein-induced nuclear membrane invagination pathway and revealed the function of nuclear vesicle-like structures in ODV production.

Identification of the Key Functional Domains of Bombyx mori Nucleopolyhedrovirus IE1 Protein

The immediate early protein 1 (IE1) acts as a transcriptional activator and is essential for viral gene transcription and viral DNA replication. However, the key regulatory domains of IE1 remain poorly understood. Here, we analyzed the sequence characteristics of Bombyx mori nucleopolyhedrovirus (BmNPV) IE1 and identified the key functional domains of BmNPV IE1 by stepwise truncation. Our results showed that BmNPV IE1 was highly similar to Autographa californica nucleopolyhedrovirus (AcMNPV) IE1, but was less conserved with IE1 of other baculoviruses, the C-terminus of IE1 was more conserved than the N-terminus, and BmNPV IE1 was also necessary for BmNPV proliferation. Moreover, we found that IE1_158–208 was a major nuclear localization element, and IE1_1–157 and IE1_539–559 were minor nuclear localization elements, but the combination of these two minor elements was equally sufficient to fully mediate the nuclear entry of IE1. Meanwhile, IE1_1–258, IE1_560–584, and the association of amino acids 258 and 259 were indispensable for the transactivation activity of BmNPV IE1. These results systematically resolve the functional domains of BmNPV IE1, which contribute to the understanding of the mechanism of baculovirus infection and provide a possibility to synthesize a small molecule IE1-truncated mutant as an agonist or antagonist.

Effects of CRM1-dependent nuclear export inhibition on viral structural protein nuclear accumulation during Autographa californica multiple nucleopolyhedrovirus infection

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) assembles its nucleocapsids and occlusion-derived virions (ODVs) in the nucleus, which requires AcMNPV regulation for viral structural proteins to accumulate in the nucleus during its replication in cells. It is generally accepted that the nuclear import receptor plays a predominant role in this process. CRM1 is a nuclear export receptor that forms an export complex with its cargo protein to exit the nucleus. We previously discovered that AcMNPV inhibited CRM1-dependent nuclear export by the viral protein Ac34. This finding suggested that Ac34 could sequester CRM1-dependent proteins in the nucleus and play a novel role in the nuclear accumulation of viral structural proteins. Using the CRM1 inhibitor leptomycin B (LMB), we demonstrated that CRM1 inhibition promoted AcMNPV replication, as LMB treatment readily increased the virus titer, and even functionally surrogate Ac34 to rescue the infectivity of an ac34-knockout virus. To elucidate whether CRM1 inhibition contributes to the nuclear accumulation of viral structural proteins, we systematically analyzed the impact of CRM1 inhibition on viral protein spatial distribution patterns. We found that the nucleocapsid protein Ac102 and ODV envelope protein E26 were retained in the nucleus in response to CRM1 inhibition by Ac34. This finding indicates that AcMNPV is evolving to simultaneously exploit bidirectional nucleocytoplasmic trafficking to assist in viral replication.