The Autographa californica Multiple Nucleopolyhedrovirus ac54 Gene Is Crucial for Localization of the Major Capsid Protein VP39 at the Site of Nucleocapsid Assembly

Baculovirus DNAs are synthesized and inserted into preformed capsids to form nucleocapsids at a site in the infected cell nucleus, termed the virogenic stroma. Nucleocapsid assembly of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) requires the major capsid protein VP39 and nine minor capsid proteins, including VP1054. However, how VP1054 participates in nucleocapsid assembly remains elusive. In this study, the VP1054-encoding gene (ac54) was deleted to generate the ac54-knockout AcMNPV (vAc54KO). In vAc54KO-transfected cells, nucleocapsid assembly was disrupted, leading to the formation of abnormally elongated capsid structures. Interestingly, unlike cells transfected with AcMNPV mutants lacking other minor capsid proteins, in which capsid structures were distributed within the virogenic stroma, ac54 ablation resulted in a distinctive location of capsid structures and VP39 at the periphery of the nucleus. The altered distribution pattern of capsid structures was also observed in cells transfected with AcMNPV lacking BV/ODV-C42 or in cytochalasin D-treated AcMNPV-infected cells. BV/ODV-C42, along with PP78/83, has been shown to promote nuclear filamentous actin (F-actin) formation, which is another requisite for nucleocapsid assembly. Immunofluorescence using phalloidin indicated that the formation and distribution of nuclear F-actin were not affected by ac54 deletion. However, immunoelectron microscopy revealed that BV/ODV-C42, PP78/83, and 38K failed to integrate into capsid structures in the absence of VP1054, and immunoprecipitation further demonstrated that in transient expression assays, VP1054 interacted with BV/ODV-C42 and VP80 but not VP39. Our findings suggest that VP1054 plays an important role in the transport of capsid proteins to the nucleocapsid assembly site prior to the process of nucleocapsid assembly. IMPORTANCE Baculoviruses are large DNA viruses whose replication occurs within the host nucleus. The localization of capsids into the capsid assembly site requires virus-induced nuclear F-actin; the inhibition of nuclear F-actin formation results in the retention of capsid structures at the periphery of the nucleus. In this paper, we note that the minor capsid protein VP1054 is essential for the localization of capsid structures, the major capsid protein VP39, and the minor capsid protein 38K into the capsid assembly site. Moreover, VP1054 is crucial for correct targeting of the nuclear F-actin factors BV/ODV-C42 and PP78/83 for capsid maturation. However, the formation and distribution of nuclear F-actin are not affected by the lack of VP1054. We further reveal that VP1054 interacts with BV/ODV-C42 and a capsid transport-related protein, VP80. Taken together, our findings suggest that VP1054 plays a unique role in the pathway(s) for transport of capsid proteins.