摘要：

Almost all RNAs are synthesized in the nucleus, and must be exported to the cytoplasm before they can function in cell metabolism. Specific proteins, export receptors, carry the RNA cargoes out of the nucleus, mediating the interaction between the cargo and components of the nuclear pore complex (NPC). To date, two RNA export receptors that bind the export cargo in a Ran·GTP-dependent manner have been identified: Xpo-t (exportin-t), which binds directly to its tRNA export cargo, and CRM1 (Xpo-1 or exportin-1), which uses specific protein adapters to bind export cargos like snRNAs, certain cellular mRNAs, unspliced HIV-1 retroviral mRNA and rRNAs. Like other proteins that are exported by CRM1, these RNA export adapters have specific (often leucine-rich) nuclear export signals (NESs) (Chapter 2). Both Xpo-t and CRM1 are members of the large family of importin -like transport receptors. A different type of RNA export receptor, TAP (or NXF1), which does not directly require Ran·GTP for association with its cargo, is responsible for export of most mRNAs.Many proteins that associate with RNAs during nuclear processing exit the nucleus with the RNAs and must be imported back into the nucleus, for use in continued rounds of RNA export. Import of these shuttling transport factors generally requires importin -like import receptors and therefore is dependent on nuclear Ran-GTP (Chapter 1). Thus, Ran-GTP plays both a direct and an indirect role in RNA export, by promoting formation of export complexes and by recycling limited export factors, respectively.

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