Nuclear entry of DNA viruses

Abstract

DNA viruses undertake their replication within the cell nucleus, and therefore they must first deliver their genome into the nucleus of their host cells. Thus, trafficking across the nuclear envelope is at the basis of DNA virus infections. Nuclear transport of molecules with diameters up to 39 nm is a tightly regulated process that occurs through the nuclear pore complex (NPC). Due to the enormous diversity of virus size and structure, each virus has developed its own strategy for entering the nucleus of their host cells, with no two strategies alike. For example, baculoviruses target their DNA-containing capsid to the NPC and subsequently enter the nucleus intact, while the hepatitis B virus capsid crosses the NPC but disassembles at the nuclear side of the NPC. For other viruses such as herpes simplex virus and adenovirus, although both dock at the NPC, they have each developed a distinct mechanism for the subsequent delivery of their genome into the nucleus. Remarkably, other DNA viruses, such as parvoviruses and human papillomaviruses, access the nucleus through an NPC-independent mechanism. This review discusses our current understanding of the mechanisms used by DNA viruses to deliver their genome into the nucleus, and further presents the experimental evidence for such mechanisms.

Keywords: DNA virus; Nuclear import; nuclear envelope; nuclear pore complex; nucleoporins; virus nuclear entry; virus nuclear import.

FIGURE 1

Strategies used by enveloped viruses for nuclear entry of their genomes. (A) HBV capsids bind and cross the NPC in an importin-α and -β dependent manner. The capsid then binds directly to Nup153 at the nuclear basket, which somehow triggers disassembly of the capsid and nuclear entry of the viral genome. (B) The HSV1 capsid docks with a distinct orientation to the NPC cytoplasmic filament by binding to Nup358 in an importin-β and Ran dependent manner. The genome is delivered into the nucleus through the NPC by a unique mechanism of genome release through the capsid portal, and the empty capsid remains at the cytoplasmic side of the NPC. (C) The intact nucleocapsid of the baculovirus AcMNPV enters the nucleus through the NPC, but the roles of any viral or cellular components remain to be determined.

FIGURE 2

Hepatitis B virus capsids cross the NPC intact and get arrested within the NPC nuclear basket, depending on genome maturation. Electron micrographs of NPC cross-sections from Xenopus oocytes microinjected with phosphorylated recombinant HBV capsids (EcPC), HBV capsids containing the mature viral genome (MatC), or HBV capsids that did not contain the mature viral genome (ImmatC). While all three types of capsids cross the NPC intact, recombinant capsids and immature capsids remain arrested within the NPC nuclear basket, while mature capsids are released into the nucleus. Scale bar, 100 nm; n, nucleus; c, cytoplasm. Arrowheads point to capsids associated with the nuclear face of the NPC. Some of the micrographs have been reproduced with permission from Rabe et al. (2003).

FIGURE 3

Herpes simplex virus 1 capsids dock at the cytoplasmic side of the NPC. Electron micrographs of NE cross-sections from Vero cells infected with HSV1. Both DNA-containing capsids (A) and empty capsids (B) are located in close proximity to the NPC. HSV1 capsids bind to the NPC with a distinct orientation with one of the vertices facing the NPC. Arrows point to HSV1 capsids and arrowheads point to NPC cytoplasmic filaments. Reproduced with permission from Ojala et al. (2000).

FIGURE 4

Baculovirus AcMNPV capsids cross the NPC intact. Electron micrographs of NE cross-sections from Xenopus oocytes microinjected with baculovirus AcMNPV capsids. Intact capsids (arrows) are detected docked at the NPC prior to nuclear import (A) and midway through the NPC (B). Scale bars, 100 nm; n, nucleus; c, cytoplasm. Arrows point to capsids. Reproduced with permission from Au et al. (2013).

FIGURE 5

Strategies used by non-enveloped viruses for nuclear entry of their genomes. (A) The adenovirus capsid docks at the NPC via binding of hexon protein to Nup214, and uses Nup358-bound kinesin-1 via its heavy chain to completely disassemble the capsid that is also bound to kinesin-1, via its light chain, and deliver the uncoated genomes into the nucleus through the NPC. Kinase-1 binds to the capsid via its light chains and to Nup358 via its heavy chains. Movement of kinase-1 along microtubules exerts a pulling action that disassembles the capsid. (B) Parvoviruses enter the nucleus by transiently disrupting nuclear membranes, by a yet unknown mechanism, and disassembling the nuclear lamina. The latter involves phosphorylation of lamin A/C by PKC and cdk2, and cleavage of lamin-B by caspase-3. (C) SV40 partially disassembles inside the endoplasmic reticulum (ER) and the subviral particles could follow two different pathways to deliver the genome into the nucleus. The first involves exit of the subviral particle into the cytoplasm using the cellular ERAD pathway and cellular chaperones, disassembly at the NPC, and import of the uncoated genome through the NPC. The second is directly from the ER to the nucleus by direct disruption of the inner nuclear membrane and underlying nuclear lamina using caspase-6. (D) HPV partially disassembles in endosomes, and a possible route through the Golgi apparatus, and gains access to the nucleus during mitosis when the NE breaks down.

FIGURE 6

Adenovirus capsids dock at the NPC cytoplasmic face before disassembly. Electron micrographs of NE cross-sections from digitonin-permeabilized cells incubated with adenovirus capsids and cytosol. Arrows point to NPCs. Reproduced with permission from Saphire et al. (2000).

FIGURE 7

Parvoviruses cause disruption of the NE for nuclear import. Electron micrographs of NE cross-sections from Xenopus oocytes mock-injected (A) or injected with the parvovirus MVM (B–E). Arrowheads point to NPCs. Brackets indicate breaks in the NE caused by MVM, which are often found close to NPCs (indicated by ^∗). Virions are found close to the breaks (B, indicated by ^∗∗), associated with the outer nuclear membrane (D, arrows), and in the perinuclear space (C, arrows). Small vesicles are found close to the breaks (E, arrow). Scale bars, 100 nm; n, nucleus; c, cytoplasm. Reproduced with permission from Cohen and Pante (2005).

FIGURE 8

Simian virus 40 is depicted at the NPC after microinjection. Electron micrographs of a cross-section through the NE of a cell injected with SV40. Scale bars, 100 nm; Nuc, nucleus; Cyt, cytoplasm. Arrows point to virions, arrowhead points to a virion at the NPC. Reproduced with permission from Yamada and Kasamatsu (1993).