Transport of Proteins and RNAs in and out of the Nucleus

Abstract

Eukaryotic cells are equipped with a machinery charged with the responsibility of transporting a vast number of molecules in and out of the nucleus in a rapid, accurate, and often regulated manner. The cargos for this machinery are diverse, comprising proteins and more elaborate RNA–protein complexes (RNPs). Although the extent and complexity of trafficking between the nucleus and the cytoplasm have long been appreciated, it is only relatively recently that some of the molecular mechanisms involved have been uncovered. Proteins and some RNPs that need to move between the nucleus and cytoplasm in order to perform their normal cell functions are most often recognized by soluble proteins, the nuclear transport receptors. These receptors mediate translocation of cargos through the nuclear pore complex (NPC), the route for all macromolecular traffic across the nuclear membrane. After depositing their cargos in the appropriate compartment, the unloaded receptors shuttle back through the NPC to pick up more cargo. Like many vectorial cellular processes, directionality is imposed on nucleocytoplasmic traffic (at least in part) by a nucleotidase, the small GTPase Ran. Several experimental approaches, including in vitro import and export assays in higher eukaryotic cells, in vivo transport studies in frog oocytes and yeast genetics, have been instrumental in the discovery of molecules and mechanisms of nucleocytoplasmic transport. In this review, we attempt to bring together the large body of information in a manner accessible to the general reader interested in basic cell function. We cover cargos, transporters, route, direction, fuel and translocation mechanisms, and regulation, emphasizing general principles along the way. Cargos identify themselves to the nucleocytoplasmic transport machinery by signals (nuclear localization signals, NLSs, and nuclear export signals, NESs) that can be protein- or RNA-based, or a composite of the two. The signals in many protein cargos, and in some RNP cargos, are recognized by one or more members of the nuclear transport receptor family (Table 1, Table 2, Table 3). Members of this protein family have been given various names, including karyopherins (87Radu A Blobel G Moore M.S Identification of a protein complex that is required for nuclear protein import and mediates docking of import substrate to distinct nucleoporins.Proc. Natl. Acad. Sci. USA. 1995; 92: 1769-1773Crossref PubMed Scopus (323) Google Scholar), p97 (8Chi N.C Adam E.J Adam S.A Sequence and characterization of cytoplasmic nuclear protein import factor p97.J. Cell Biol. 1995; 130: 265-274Crossref PubMed Scopus (223) Google Scholar), PTACs (29Imamoto N Shimamoto T Kose S Takao T Tachibana T Matsubae M Sekimoto T Shimonishi Y Yoneda Y The nuclear pore-targeting complex binds to nuclear pores after association with a karyophile.FEBS Lett. 1995; 368: 415-419Abstract Full Text PDF PubMed Scopus (137) Google Scholar), importins (21Gorlich D Kostka S Kraft R Dingwall C Laskey R.A Hartmann E Prehn S Two different subunits of importin cooperate to recognize nuclear localization signals and bind them to the nuclear envelope Curr.Biol. 1995; 5: 383-392Google Scholar), transportins (84Pollard V.W Michael W.M Nakielny S Siomi M.C Wang F Dreyfuss G A novel receptor-mediated nuclear protein import pathway.Cell. 1996; 86: 985-994Abstract Full Text Full Text PDF PubMed Scopus (502) Google Scholar), exportins (106Stade K Ford C.S Guthrie C Weis K Exportin 1 (Crm1p) is an essential nuclear export factor.Cell. 1997; 90: 1041-1050Abstract Full Text Full Text PDF PubMed Scopus (742) Google Scholar), and Ran-binding proteins (reviewed in 9Corbett A.H Silver P.A Nucleocytoplasmic transport of macromolecules.Microbiol. Mol. Biol. Rev. 1997; 61: 193-211Crossref PubMed Scopus (168) Google Scholar, 14Dingwall C Laskey R.A Nuclear import a tale of two sites.Curr. Biol. 1998; 8: R922-R924Abstract Full Text Full Text PDF PubMed Google Scholar, 20Gorlich D Transport into and out of the cell nucleus.EMBO J. 1998; 17: 2721-2727Crossref PubMed Scopus (253) Google Scholar, 30Izaurralde E Adam S Transport of macromolecules between the nucleus and the cytoplasm.RNA. 1998; 4: 351-364PubMed Google Scholar, 55Mattaj I.W Englmeier L Nucleocytoplasmic transport the soluble phase.Annu. Rev. Biochem. 1998; 67: 265-306Crossref PubMed Scopus (898) Google Scholar, 76Pemberton L.F Blobel G Rosenblum J.S Transport routes through the nuclear pore complex.Curr. Opin. Cell Biol. 1998; 10: 392-399Crossref PubMed Scopus (187) Google Scholar). Here, we refer to the individual nuclear transport receptors by the specific names used in their original descriptions. These receptors are generally large (90–130 kDa) acidic proteins sharing 15%–25% sequence identity within a given organism. They all have an N-terminal RanGTP-binding domain, a C-terminal cargo-binding domain, and the capacity to bind components of the NPC (Figure 1A).Table 1Import Signals and ReceptorsImport Carg