A GTPase controlling nuclear trafficking: running the right way or walking RANdomly?

The entry of transport factors into the nucleus during nuclear protein import provokes two questions: how do the transport factors exit the nucleus, and do import factors play a role in RNA export as well? Gorlich et al. (1996 [this issue of Cell]) now present data implicating importin α directly in export of snRNAs. They find that a significant fraction of yeast importin α associates with the cap binding complex (CBC), a multicomponent structure which shuttles between the nucleus and the cytoplasm while directing the export of capped U snRNAs. While it is not surprising that importin α should bind to a protein targeted to the nucleus, it is unusual that importin α does not dissociate from CBC upon nuclear entry. Even more intriguing is the observation that importin β can dissociate RNA from the importin α–CBC complex. This result leads Gorlich et al. 1996xGorlich, D, Kraft, R, Kostka, S, Vogel, F, Hartmann, E, Laskey, R.A, Mattaj, I.W, and Izaurralde, E. Cell. 1996; 87Abstract | Full Text | Full Text PDF | PubMed | Scopus (150)See all ReferencesGorlich et al. 1996 to suggest a model (shown in Figure 8 of their paper) in which a cytoplasmic complex of importin α and β function in the import of CBC and a nuclear complex of importin α and CBC function in the export of snRNA. An alternative interpretation is that importin α “hitches a ride” with CBC to exit the nucleus and plays no direct role in snRNA export. Furthermore, export of importin α in complex with CBC cannot be the only means of exiting the nucleus for importin α, as the absence of CBC is not lethal, at least for the yeast cell. In addition, an untested prediction of this model is that mutant alleles of importin α should exhibit defects in the export of capped U snRNAs.The export of mRNA from the nucleus may proceed by a similar pathway that is, in principle, the reciprocal of the protein import pathway. Numerous RNA binding proteins, such as some hnRNPs, have been shown to shuttle between the nucleus and the cytoplasm (seeIzaurralde and Mattaj 1995xIzaurralde, E and Mattaj, I.W. Cell. 1995; 81: 153–159Abstract | Full Text PDF | PubMed | Scopus (201)See all ReferencesIzaurralde and Mattaj 1995, for review). As such, these shuttling RNA-binding proteins have been proposed to act as carriers of mRNA out of the nucleus. Once in the cytoplasm, the mRNA dissociates from its carrier, perhaps by replacement with cytoplasmic RNA-binding proteins or recruitment into ribosomes. The carrier proteins would then reenter the nucleus for additional rounds of export. Support for such a mechanism comes from experiments with the yeast shuttling protein Npl3p; mutations in Npl3p block both mRNA export and export of Npl3p from the nucleus. Similarly, export of Npl3p depends on ongoing RNA synthesis (Lee et al. 1996xLee, M.S, Henry, M, and Silver, P.A. Genes Dev. 1996; 10: 1233–1246Crossref | PubMedSee all ReferencesLee et al. 1996). Taken together, Npl3p and similar proteins, such as hnRNPA1 and the HIV Rev protein, exhibit characteristics expected for RNA transporters. It remains to be shown whether or not these proteins, like CBC, are coupled to importins for export. However, the observation that the nucleotide bound state of Ran causes defects in both mRNA and importin α export provides an important link.Implicit in these proposed models is the notion that some or all of the “import” factors must cycle between the nucleus and the cytoplasm. The molecular details of the export of transport factors themselves are not clear. Nevertheless, we can speculate about the export process using the principles already discussed (Figure 3Figure 3). Once inside the nucleus, importin α must dissociate from the NLS-bearing substrate, which may be accomplished by competition with RNA-binding proteins. Ran may move out of the nucleus as a complex of Ran-GTP–importin β. Dissociation of these two proteins could be a result of the GAP activity of Rna1p, either inside the NPC or on the cytoplasmic face of the NPC. There is evidence that Rna1p can interact with importin β (Koepp et al. 1996xKoepp, D.M, Wong, D.H, Corbett, A.H, and Silver, P.A. J. Cell Biol. 1996; 133: 1163–1176Crossref | PubMed | Scopus (108)See all ReferencesKoepp et al. 1996). The precise signal for an irreversible step of export is unclear, but it is possible that free importin β could dissociate importin α from RNA-binding proteins. Thus, the key players in nuclear protein import would be regenerated for another round of transport.Figure 3Model for Transport Factor CyclingNuclear transport factors cycle between the nucleus and the cytoplasm in order to promote multiple rounds of transport. See text for details.View Large Image | View Hi-Res Image | Download PowerPoint Slide