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

Abstract

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. IMPORTANCEBaculoviruses 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. The family Baculoviridae includes a diverse group of insect-specific viruses that contain circular double-stranded DNA within a rod-shaped protein capsid enclosed by a lipid envelope (1, 2). Two forms of virions (bu...

Baculovirus AC102 Is a Nucleocapsid Protein That Is Crucial for Nuclear Actin Polymerization and Nucleocapsid Morphogenesis

“…AC102 is also important for nucleocapsid morphogenesis, as aberrant capsid-like tubular structures accumulated in the RZ of AC102-K66A-infected cells as visualized by TEM. Similar tubular structures have been observed in cells treated with cytochalasin D (10,30), as well as in cells transfected/infected with mutant bacmids/ viruses carrying deletions of ac101/c42 and ac144/ec27 (20), and other genes important for nucleocapsid assembly, including vlf-1/ac77 (31), 49K/ac142 (20), 38K/ac98 (32), ac54 (33,34), ac53 (35), and pk-1/ac10 (36). These aberrant tubular structures contain the major capsid protein VP39 (20,30,32) and are thought to result from perturbed nucleocapsid assembly.…”

Baculovirus AC102 is a nucleocapsid protein that is crucial for nuclear actin polymerization and nucleocapsid morphogenesis

“…AC102 is also important for nucleocapsid morphogenesis, as aberrant capsid-like tubular structures accumulated in the RZ of AC102-K66A-infected cells as visualized by TEM. Similar tubular structures have been observed in cells treated with cytochalasin D (10, 29), as well as in cells transfected/infected with mutant bacmids/viruses carrying deletions of ac101/c42 and ac144/ec27 (19), and other genes important for nucleocapsid assembly including vlf-1/ac77 (30), 49K/ac142 (19), 38K/ac98 (31), ac54 (32, 33), ac53 (34), and pk-1/ac10 (35). These aberrant tubular structures contain the major capsid protein VP39 (19, 29, 31) and are thought to result from perturbed nucleocapsid assembly.…”

Ac102 Participates in Nuclear Actin Polymerization by Modulating BV/ODV-C42 Ubiquitination during Autographa californica Multiple Nucleopolyhedrovirus Infection

“…Taken together, our results suggested that Ac102 not only participates in nucleocapsid morphogenesis but also modulates the distribution of capsid structures in the nucleus. In fact, the morphogenesis and spatial distribution of capsid structures in vAc ac102ko-mc -and vAc ac102D61A-mc -bearing cells strongly resembled the ultrastructure previously seen in cells bearing vAc c42ko or vAc54KO (ac54 encodes VP1054, which interacts with C42) (16,17). This resemblance prompted us to hypothesize that Ac102 is functionally connected to C42.…”