Baculovirus nuclear import: open, nuclear pore complex (NPC) sesame

Abstract

Baculoviruses are one of the largest viruses that replicate in the nucleus of their host cells. During infection, the rod-shape, 250-nm long nucleocapsid delivers its genome into the nucleus. Electron microscopy evidence suggests that baculoviruses, specifically the Alphabaculoviruses (nucleopolyhedroviruses) and the Betabaculoviruses (granuloviruses), have evolved two very distinct modes for doing this. Here we review historical and current experimental results of baculovirus nuclear import studies, with an emphasis on electron microscopy studies employing the prototypical baculovirus Autographa californica multiple nucleopolyhedrovirus infecting cultured cells. We also discuss the implications of recent studies towards theories of nuclear transport mechanisms.

Figure 1

Classification of the Baculoviridae family. Only a small subset of characterized species within each group is listed. AcMNPV, the most studied baculovirus and most commonly used viral vector for baculovirus expression vector systems, belongs to the type I Alphabaculovirus genus.

Figure 2

Schematic diagrams of the structure of baculovirus occlusion bodies (OB), occlusion-derived virion (ODV) and budded virion (BV). ODVs are embedded in a crystalline matrix of protein to form OBs. Shown here is the OB of nucleopolyhedroviruses (NPVs) (the vesicle containing three round structures is a cross section of an ODV). The ODV and BV envelope and their nucleocapsid(s) contain numerous proteins; only the proteins discussed in this review are outlined in the diagram. The major nucleocapsid protein VP39 constitutes the barrel of the nucleocapsid and is present in the whole nucleocapsid. VP78/83 is located at one end of the nucleocapsid, presumably at the blunt end. The fusion protein GP64 (NPV group I) or F protein (NPV group II) is found throughout the BV envelope but forms peplomers on one end, presumably the conical end.

Figure 3

Budded virus infection cycle in cultured cells. See text for details.

Figure 4

Nuclear import of AcMNPV Nucleocapsids. Electron micrographs of nuclear envelope cross-sections from Xenopus oocytes that have been microinjected with AcMNPV nucleocapsids and processed for embedding and thin-section electron microscopy. Intact nucleocapsids (arrows) were detected docked at the nuclear pore complex (NPC) prior to nuclear import (A), midway through the NPC (B), and inside the nucleus (E). (C, D) Schematic diagrams of NPCs illustrating “close” and “open” states respectively. In (C) NPC cytoplasmic filaments and nucleoporins within the central channel prevent the passage of the nucleocapsid. In (D) NPC cytoplasmic filaments straighten out, and nucleoporins in the central channel retract towards the body of the NPC to free up the passageway for the nucleocapsid to cross the NPC.