NUCLEAR IMPORT AND EXPORT OF VIRUSES AND VIRUS GENOMES

Many viruses replicate in the nucleus of their animal and plant host c ells. Nuclear import, export, and nucleo-cytoplasmic shuttling play a central role in their replication cycle. Although the trafficking of i ndividual virus proteins into and out of the nucleus has been well stu died for some virus systems, the nuclear transport of larger entities such as viral genomes and capsids has only recently become a subject o f molecular analysis. In this review, the general concepts emerging ar e discussed and a survey is provided of current information on both pl ant and animal viruses. Summarizing the main findings in this emerging field, it is evident that most viruses that enter or exit the nucleus take advantage of the cell's nuclear import and export machinery. Wit h a few exceptions, viruses seem to cross the nuclear envelope through the nuclear pore complexes, making use of cellular nuclear import and export signals, receptors, and transport factors. In many cases, they capitalize on subtle control systems such as phosphorylation that reg ulate traffic of cellular components into and out of the nucleus. The large size of viral capsids and their composition (they contain large RNA and DNA molecules for which there are few precedents in normal nuc lear transport) make the processes unique and complicated. Prior capsi d disassembly (or deformation) is required before entry of viral genom es and accessory proteins can occur through nuclear pores. Capsids of different virus families display diverse uncoating programs which culm inate in genome transfer through the nuclear pores. (C) 1998 Academic Press.