DNA viruses that replicate in the nuclei of infected cells must transport to and deliver their viral genomes into the nucleus to cause infection. How polyomavirus (PyV), a tumor virus that causes debilitating disease in humans, accomplishes this essential step in infection is not well-understood. During entry, PyV sorts from the cell surface to the endoplasmic reticulum (ER) where it penetrates the ER membrane to reach the cytosol. From there, the virus is transported to the nucleus for entry through the narrow nuclear pore complex (NPC). The incoming viral genome is harbored within an icosahedral capsid that is approximately 45 nm in diameter and larger than the NPC cargo limit. Using the archetype PyV, simian virus 40 (SV40), which shares both structural and genetic organization with human PyVs as well as the same infectious life cycle, we recently reported that the virus is disassembled in the cytosol by the bicaudal D2 (BICD2) dynein cargo adaptor to generate a subviral particle competent for transport through the NPC. How this disassembled particle is subsequently targeted to the nuclear membrane as well as the exact mechanism of nuclear import is unclear. In mammalian cells, intracellular transport to the nucleus is facilitated largely by the cytoplasmic dynein motor, which moves cargo along microtubules towards the center of the cell. Interestingly, BICD2 was shown to mediate the recruitment of dynein to the nuclear membrane through its association with either the linker of nucleoskeleton and cytoskeleton (LINC) complex component Nesprin-2 or the NPC cytoplasmic filament RanBP2. Our preliminary experiments reveal that loss of Nesprin-2 (and not RanBP2) blocks nuclear entry, suggesting that SV40 is recruited to the nucleus by a dynein-BICD2-Nesprin-2 nexus. Moreover, SV40 associates with Nesprin-2 and this interaction appears to mediate a second, previously unreported, disassembly step at the nuclear membrane. We are currently investigating the role of BICD2/Nesprin-2 in nuclear arrival of SV40 as well as the functional relationship between Nesprin-2 and the NPC during nuclear import. Our studies will clarify the mechanism responsible for driving these final steps of viral entry, and more broadly, further our understanding of host cellular nucleocytoplasmic transport.
