Journey to the Center of the Cell

This brief overview of the strategies used by viruses to transport their genomes into the nuclei of target cells reveals that there are considerable differences in detail. Nevertheless, several common themes are apparent. Thus, the nuclear import of viral nucleic acids appears to be generally mediated by associated viral proteins that contain NLS sequences that are, in turn, recognized by cellular nuclear import factors. Also, nuclear import of viral genomes invariably occurs via the NPC, with the exception of certain retroviruses, which simply wait for mitosis to enter the nucleus. Most importantly, viral infection generally requires the controlled, step-wise disassembly of virions. This is particularly apparent for virions that contain large genomes. For example, the disassembly of Ad2 virions clearly involves a series of carefully choreographed steps beginning shortly after binding to the cell surface and ending with final dissolution of the Ad2 capsid at the NPC (Greber et al., 1993xGreber, U.F., Willetts, M., Webster, P., and Helenius, A. Cell. 1993; 75: 477–486Abstract | Full Text PDF | PubMed | Scopus (549)See all References(Greber et al., 1993). Nuclear import of the HSV-1 genome is in many respects similar to Ad2, although the HSV-1 capsid is not entirely disassembled after docking at the NPC. HIV-1 virions undergo extensive disassembly in the cytoplasm, in a process that remains only poorly understood, to generate a PIC that may be only just small enough to move through the NPC. Finally, influenza virions undergo complete disassembly at the time of cytoplasmic entry and disgorge eight viral ribonucleoproteins that independently enter the nucleus after recruitment of cellular import factors.While considerable progress has been made in the last decade in deciphering key steps in the process of viral nuclear entry, much remains to be discovered. Progress in this area has clearly been slowed by the technical difficulties of working with partially disassembled virions, as these are generally far more labile than intact virion particles. In addition, for viruses such as HIV-1, the high total particle to infectious particle ratio makes it difficult to be sure that total HIV-1 PICs isolated from the cytoplasm of infected cells are in fact truly representative of infectious PICs. This concern is increasingly being addressed by the development of in vitro assays that can accurately recreate the nuclear import of viral genomes and that can therefore be used to not only confirm the biological activity of isolated subviral particles but also identify viral and cellular factors that play a critical role in the import process (Saphire et al. 2000xSaphire, A.C.S., Guan, T., Schirmer, E.C., Nemerow, G.R., and Gerace, L. J. Biol. Chem. 2000; 275: 4298–4304Crossref | PubMed | Scopus (91)See all References, Ojala et al. 2000xOjala, P.M., Sodeik, B., Ebersold, M.W., Kutay, U., and Helenius, A. Mol. Cell. Biol. 2000; 20: 4922–4931Crossref | PubMed | Scopus (145)See all References).Regardless of how each virus achieves virion disassembly and nuclear import, it appears clear that drugs that could stabilize the virion structure, and hence interfere with nuclear entry of the viral genome, would potentially be effective inhibitors of viral replication. In fact, a family of antiviral agents that selectively interfere with the disassembly of influenza virions, by inhibiting the action of the M2 ion channel (Pinto et al., 1992xPinto, L.H., Holsinger, L.J., and Lamb, R.A. Cell. 1992; 69: 517–528Abstract | Full Text PDF | PubMed | Scopus (726)See all References(Pinto et al., 1992), is already in clinical use. It has also been demonstrated that drugs, such as cyclosporin A, that block the interaction of the cellular chaperonin cyclophilin A with the viral capsid protein, can selectively inhibit the appropriate disassembly of HIV-1 virions and hence prevent HIV-1 infection in culture (Braaten et al., 1996xBraaten, D., Franke, E.K., and Luban, J. J. Virol. 1996; 70: 3551–3560PubMedSee all References(Braaten et al., 1996). These precedents suggest that a more complete understanding of how other pathogenic viruses achieve the nuclear delivery of their genetic information could lead to the development of novel, and potentially highly effective, antiviral drugs.