A GTPase Controlling Nuclear Trafficking: Running the Right Way or Walking RANdomly?

In addition, certain GTP analogs inhibit nuclear import, indicating that GTP hydrolysis by Ran is neces-Macromolecular movement across the nuclear envelope sary for proper nuclear protein import. Failure by Ran is distinctly bidirectional. Proteins are targeted to and to hydrolyze and/or exchange nucleotide also results in enter the nucleus at nuclear pore complexes (NPCs) in a block in RNA export, providing a link between protein the nuclear envelope. The same NPCs promote passage and RNA transit with the nucleotide bound state of Ran of RNAs and proteins out of the nucleus. The precise (Amberg et al., 1993; Koepp et al., 1996). orchestration of these movements is essential for proper In a normal cell, Ran can be found in both the nucleus gene expression, DNA replication, RNA processing, and and the cytoplasm. Although the relative levels vary in viral maturation. different systems, in most cells the steady state distribu-Several key factors have been identified that are im-tion of Ran is primarily nuclear. Ran's GTP exchange portant for trafficking of proteins and RNAs through the factor, termed RCC1 in mammalian cells and Prp20p nuclear pore. These include importin ␣ and ␤ (also in yeast, is located in the nucleus. Conversely, Ran's termed karyopherins), which recognize proteins with a GTPase-activating protein (Ran-GAP in higher organ-nuclear localization sequences (NLS); the small GTP-isms, Rna1p in yeast) is localized to the cytoplasm. The binding protein Ran and its associated regulators; RNA compartmentalization of its regulators suggests that binding proteins for escorting mRNAs and snRNAs out Ran moves in and out of the nucleus (see Figure 1). In of the nucleus; and numerous components of the nu-fact, exogenously added Ran has been shown to move clear pore complex, generically termed nucleoporins. into the nucleus in import assays in vitro (Melchior et The focus of this review is how the GTPase cycle, cata-al., 1995). Furthermore, since the GAP is cytoplasmic, lyzed by Ran, drives the directional movement of certain Ran is predicted to be in the GDP-bound state in the proteins into the nucleus and RNAs out of the nucleus. cytoplasm and converted to the GTP-bound state in the The nuclear pore is an immense protein complex, ap-nucleus by the nuclear exchange factor. These observa-proximately thirty times the size of the ribosome. Like tions suggest that the nucleotide-bound state of Ran is the ribosome, it has a large number of protein compo-important in conferring direction of Ran movement. …