AcMNPV PKIP is associated with nucleocapsid of budded virions and involved in nucleocapsid assembly

Highlights

• pkip is required for optimal budded virus production during AcMNPV replication.

• The deletion of pkip impairs nucleocapsid assembly.

• PKIP is distributed in both the cytoplasm and nuclei of viruses-infected cells.

• PKIP is associated with the nucleocapsid of BV.

Abstract

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) orf24 (pkip) is a unique Alphabaculovirus gene. A previous study showed that a temperature-sensitive mutant of AcMNPV with a mutation in pkip displayed severe defects in progeny budded virion (BV) production and very late gene transcription, however, the underlying mechanism has not been determined. To investigate the function of pkip in the baculovirus replication cycle, we constructed a pkip-knockout AcMNPV bacmid in this study. Our results showed that deletion of pkip led to significant reduction of BV production, while the synthesis of viral DNA and the transcription of early and late genes were not affected. Further examination by transmission electron microscopy analysis showed that deletion of pkip resulted in the formation of massive electron-lucent tubular structures in the nucleus of the infected cells, along with some normal electron‐dense nucleocapsids. The pkip-encoded protein PKIP could be detected at late phase during infection and was distributed in both the cytoplasm and nuclei of viruses-infected cells, with a ring pattern near the inner nuclear membrane and punctate distribution in the virogenic stroma area. Biochemical fractionation of virions into nucleocapsid and envelop components showed that PKIP was associated with the nucleocapsid fraction of BV. Taken together, our results indicated that PKIP is associated with nucleocapsids of BV and involved in nucleocapsid assembly, which contributes to the optimal production of BV.

Introduction

Baculoviridae is a family of large, insect-specific viruses with circular double-stranded DNA genomes ranging from 80 to 180 kb that are packaged within rod-shaped nucleocapsids enclosed by lipid envelopes (Rohrmann, 2013). Members of this family exclusively infect larvae of the insect orders Lepidoptera, Hymenoptera and Diptera. The family Baculoviridae is divided into four genera: the lepidopteran-specific Alphabaculovirus and Betabaculovirus, the hymenopteran-specific Gammabaculovirus, and the dipteran-specific Deltabaculovirus. These subdivisions likely reflect differences in gene content, genetic evolution, their permissive host species, and variations in the manner in which virions are occluded or embedded to form occlusion bodies (OBs) (Herniou and Jehle, 2007; Hu et al., 1998; Jehle et al., 2006). The Alphabaculovirus genus falls into two phylogenetic clades representing the group I and group II NPVs based on phylogenetic analysis of polyhedrin (polh) (Zanotto et al., 1993), and the major difference is which envelope fusion protein is used, GP64 or F protein, respectively.

A striking feature of the baculovirus infection cycle is the production of two phenotypically different but genetically identical types of virions: budded virion (BV) and occlusion-derived virion (ODV) (Blissard and Rohrmann, 1990; Volkman et al., 1976). It was previously thought that the nucleocapsids of these two virions were identical; however, the source and composition of the viral envelope were quite distinct (Braunagel and Summers, 1994; Hou et al., 2013; Slack and Arif, 2007). BVs are formed when synthesized nucleocapsids are transported out of the nucleus and eventually bud from the plasma membrane, while ODVs are formed when nucleocapsids retained in the nucleus are enveloped by intranuclear microvesicles. ODV infects midgut epithelial cells to initiate primary infection in insect larvae, while BV is responsible for cell-to-cell spread of infection within the larval body, which is referred to as systemic infection (Summers, 1971; Volkman and Summers, 1977). In addition, BV production of betabaculoviruses in cultured cells is very low; in contrast, the BV yields of alphabaculoviruses can be very high in cultured cells (Winstanley and Crook, 1993).

The gene expression of baculoviruses is mainly regulated by the transcription cascade as characterized into three phases: immediate-early/early, late, and very late. Immediate-early/early genes are transcribed by host RNA polymerase II after viral DNA is released in the nuclei of the infected cells, and the products regulate viral DNA replication and/or late gene expression; while late and very late genes are transcribed by a virus-encoded RNA polymerase and their expressions are dependent of viral DNA replication (Passarelli and Guarino, 2007). When late gene products are produced, nucleocapsid assembly is triggered, BVs and ODVs are formed subsequently. During very late state of infection, the very late viral genes polh and p10 are expressed and the products are to form OBs that embedded with ODVs. The incorporation of virions into the OBs provides stability for the virions and allows them to remain viable for long periods in the environment (Rohrmann, 2013).

Autographa californica multiple nucleopolyhedrovirus (AcMNPV), which is one of the members of Alphabaculovirus, is the first baculovirus to have its genome completely sequenced and is currently the most extensively studied baculovirus. ac24, a unique gene in the genus Alphabaculovirus (Miele et al., 2011; Rohrmann, 2013), encodes a 19.2-kDa protein kinase-interacting protein (PKIP) that stimulates the activity of PK1, a virus-encoded serine/threonine kinase, in vitro (Fan et al., 1998). A previous study showed that an AcMNPV temperature-sensitive (ts) mutant with a mutation in pkip displayed several properties at nonpermissive temperature, including a significant decrease in progeny BV production and a defect in very late gene transcription (McLachlin et al., 1998), however, the underlying mechanism has not been determined.

In the present study, a pkip-knockout AcMNPV bacmid was constructed and the effects of pkip deletion on infectious BV production and virion morphogenesis were monitored. The results showed that pkip was not an essential gene for virus replication, but deletion of this gene significantly decreased infectious BV production, which are consistent with the study of the pkip-ts mutant (McLachlin et al., 1998). Viral DNA replication and transcription of early and late viral genes were not affected by deletion of pkip, but transmission electron microscopy analysis showed that the nucleocapsid assembly within the virus-infected cells was impaired by producing massive electron-lucent tubular structures along with normal nucleocapsids upon pkip deletion. Further experiments revealed that PKIP was a late viral protein and localized in both the nucleus and the cytoplasm of the viruses-infected cells, with a ring pattern near the inner nuclear membrane and punctate distribution in the virogenic stroma (VS) area, and is associated with the nucleocapsid fraction of BV in a fractionation experiment.