Export of Importin α from the Nucleus Is Mediated by a Specific Nuclear Transport Factor

1996) and the KNS shuttling domain of the hnRNP K * Zentrum fü r Molekulare Biologie der protein, which allows both import and export (Michael Universitä t Heidelberg et al., 1997). 69120 Heidelberg A fundamental question is how it is possible for the Federal Republic of Germany shuttling transport receptors to carry their cargo in only † Abteilung Molekulare Biologie der Mitose one direction. An import receptor, for example, has to Deutsches Krebsforschungszentrum bind its cargo initially in the cytoplasm, carry the import 69120 Heidelberg substrate into the nucleus, and release the cargo there. Federal Republic of Germany To accomplish multiple rounds of transport, the import ‡ Zentrum fü r Molekulare Medizin receptor has to return to the cytoplasm, leaving the 13122 Berlin-Buch cargo behind. Conversely, a nuclear export receptor has Federal Republic of Germany to bind its substrate in the nucleus, carry it out, and release it in the cytoplasm. This model predicts asym-metric import/re-export cycles and implies that the bind-Summary ing of cargoes to their transport receptors is regulated by the different environments of nucleus and cytoplasm. NLS proteins are transported into the nucleus by the The small GTP binding protein Ran (Drivas et al., 1990; importin ␣/␤ heterodimer. Importin ␣ binds the NLS, Bischoff and Ponstingl, 1991a) is a key component in while importin ␤ mediates translocation through the and low in the cytoplasm, binds importin ␤ and dis-and probably the major source of asymmetry across the places importin ␣. Importin ␣ must then be returned nuclear envelope (discussed in Gö rlich et al., 1996c). to the cytoplasm, leaving the NLS protein behind. Here, Like all G proteins, Ran switches between a GDP-and we report that the previously identified CAS protein a GTP-bound state by nucleotide exchange and GTP mediates importin ␣ re-export. CAS binds strongly to hydrolysis. The intrinsic rates for these reactions are importin ␣ only in the presence of RanGTP, forming very low and are stimulated by specific factors. RCC1 an importin ␣/CAS/RanGTP complex. Importin ␣ is is Ran's major nucleotide exchange factor (Bischoff and released from this complex in the cytoplasm by the Ponstingl, 1991b). It is chromatin-bound (Ohtsubo et al., combined action of RanBP1 and RanGAP1. CAS binds 1989) and thus generates RanGTP inside the nucleus. preferentially to NLS-free importin ␣, explaining why So far, RanGAP1 (Rna1p) is the only known RanGTPase-import substrates stay in the nucleus. Bidirectional transport of macromolecules between …