Ran and Nuclear Transport

nuclear pore complex nuclear localization sequence guanine nucleotide exchange factor GTPase-activating protein. The last several years have witnessed an explosion in our understanding of how proteins and RNAs traffic into and out of the nucleus. Although an increasing number of proteins have been implicated in different nuclear transport pathways, the small GTPase Ran appears to play a central role in coordinating and driving much of this nuclear traffic. Recently there have been several excellent reviews describing the multiple pathways of nuclear transport (1Nigg E.A. Nature. 1997; 386: 779-787Crossref PubMed Scopus (909) Google Scholar, 2Pennisi E. Science. 1998; 279: 1129-1131Crossref PubMed Scopus (26) Google Scholar, 3Ohno M. Mattaj I.W. Fornerod M. Cell. 1998; 92: 327-336Abstract Full Text Full Text PDF PubMed Scopus (316) Google Scholar, 4Mattaj, I. W., and Englmeier, L. (1998) Annu. Rev. Biochem., in pressGoogle Scholar, 5Pemberton L.F. Blobel G. Rosenblum J. Curr. Opin. Cell Biol. 1998; 10: 392-399Crossref PubMed Scopus (211) Google Scholar); consequently this review will focus on what is currently known about Ran and its biochemical properties and what is presently understood of the role of the Ran GTPase cycle during nuclear transport. The 25-kDa protein Ran (Ras-relatednuclear protein) was first cloned because of its homology to Ras (6Drivas G.T. Shih A. Coutavas E. Rush M.G. D'Eustachio P. Mol. Cell. Biol. 1990; 10: 1793-1798Crossref PubMed Scopus (248) Google Scholar). However, most of this homology is found within the conserved domains involved in guanine nucleotide binding, and Ran clearly constitutes its own family of small GTP-binding proteins (reviewed in Ref. 7Rush M.G. Drivas G. D'Eustachio P. Bioessays. 1996; 18: 103-112Crossref PubMed Scopus (91) Google Scholar). Ran is a typical G-protein in that it cycles between a GDP-bound and GTP-bound state; however, one feature that makes Ran unique is the location in the cell where these cycles are thought to occur. Ran, unlike the rest of the known G-proteins, appears to localize in two distinct cellular compartments in all known cell types and probably cycles back and forth between the two (see below). Ran is primarily nuclear during interphase with approximately 80% of it being measured inside the nuclear interior in a baby hamster kidney cell despite the fact it does not appear to contain a nuclear targeting signal in its sequence (Fig. 1) (8Ren M.G. Drivas P. D'Eustachio P. Rush M.G. J. Cell Biol. 1994; 14: 4216-4224Google Scholar). At mitosis when the nuclear envelope breaks down, Ran appears dispersed throughout the cell (Fig. 1). Also unlike many G-proteins, Ran does not appear to undergo any post-translational lipid modifications and probably as a result does not bind membranes inside the cell or require lipids for its activity (reviewed in Ref. 7Rush M.G. Drivas G. D'Eustachio P. Bioessays. 1996; 18: 103-112Crossref PubMed Scopus (91) Google Scholar). Instead of a consensus prenylation domain, human Ran possesses an acidic -DEDDDL at its C terminus that is conserved across species (9Richards S.A. Lounsbury K.M. Macara I.G. J. Biol. Chem. 1995; 270: 14405-14411Abstract Full Text Full Text PDF PubMed Scopus (100) Google Scholar). Ran is a very abundant cellular protein and thus probably constitutes one of the most abundant G-proteins in the cell. HeLa cells, for example, contain an estimated 107 copies of Ran per cell (10Bischoff F.R. Ponstingl H. Proc. Natl. Acad. Sci. U. S. A. 1991; 88: 10830-10834Crossref PubMed Scopus (220) Google Scholar). Additionally, Ran has been very conserved throughout evolution and has been found in every eukaryotic cell examined, although to date no Ran homologs have been identified in prokaryotes (which lack a nucleus) (7Rush M.G. Drivas G. D'Eustachio P. Bioessays. 1996; 18: 103-112Crossref PubMed Scopus (91) Google Scholar). There is only one ran gene found in many cell types (such as HeLa and Schizosaccharomyces pombe), whereas cells of some other species (such asSaccharomyces cerevisiae and tomato) contain two or more very similar ran genes. Though certain cell types do contain multiple ran genes there is no evidence yet that these different ran gene products carry out different functions. Macromolecules journey back and forth between the cytoplasm and nucleus through nuclear pore complexes (NPCs),1 of which there are several thousand studding the vertebrate nuclear envelope (1Nigg E.A. Nature. 1997; 386: 779-787Crossref PubMed Scopus (909) Google Scholar). The NPC is a quite large and complex organelle and is thought to be composed of at least 50 different proteins although the actual number remains unknown (1Nigg E.A. Nature. 1997; 386: 779-787Crossref PubMed Scopus (909) Google Scholar, 2Pennisi E. Science. 1998; 279: 1129-1131Crossref PubMed Scopus (26) Google Scholar, 3Ohno M. Mattaj I.W. Fornerod M. Cell. 1998; 92: 327-336Abstract Full Text Full Text PDF PubMed Scopus (316) Google Scholar, 4Mattaj, I. W., and Englmeier, L. (1998) Annu. Rev. Biochem., in pressGoogle Scholar, 5Pemberton L.F. Blobel G. Rosenblum J. Curr. Opin. Cell Biol. 1998; 10: 392-399Crossref PubMed Scopus (211) Google Scholar). Movement of macromolecules through the aqueous tunnels formed by these organelles has been observed to occur by at least two distinct mechanisms, passive diffusion and active transport (reviewed in Ref. 11Feldherr C. Akin D. Int. Rev. Cytol. 1994; 151: 183-227Crossref PubMed Scopus (35) Google Scholar). Small molecules diffuse very quickly through the NPC (in either direction) whereas those larger than 50–60 kDa are too large to visibly diffuse in any reasonable time frame and must contain a targeting signal to be actively transported. Active nuclear transport is by necessity highly selective if the distinct environments found in the cytoplasm and nucleus are to be maintained, and the cell shows great discrimination both in its choice of substrates to be transported and in the direction in which those substrates are moved. The types of transported molecules include proteins, mRNAs, tRNAs, ribosomal subunits, and some Usn ribonucleoproteins, and to handle such diverse cargo there appear to be a number of distinct but in many cases uncharacterized transport pathways (1Nigg E.A. Nature. 1997; 386: 779-787Crossref PubMed Scopus (909) Google Scholar, 2Pennisi E. Science. 1998; 279: 1129-1131Crossref PubMed Scopus (26) Google Scholar, 3Ohno M. Mattaj I.W. Fornerod M. Cell. 1998; 92: 327-336Abstract Full Text Full Text PDF PubMed Scopus (316) Google Scholar, 4Mattaj, I. W., and Englmeier, L. (1998) Annu. Rev. Biochem., in pressGoogle Scholar, 5Pemberton L.F. Blobel G. Rosenblum J. Curr. Opin. Cell Biol. 1998; 10: 392-399Crossref PubMed Scopus (211) Google Scholar). Much of the excitement in the nuclear transport field the last several years has resulted from the characterization of new transport pathways, each apparently using different but structurally related proteins as receptors and carriers for the different transport signals. The first evidence for the involvement of Ran in nuclear transport came when Ran was shown to be essential for the nuclear import in permeabilized cells of a reporter construct containing the nuclear localization sequence (NLS) of the SV40 T antigen (PKKKRKV) (12Moore M.S. Blobel G. Nature. 1993; 365: 661-663Crossref PubMed Scopus (638) Google Scholar, 13Melchior F. Paschal B. Evans J. Gerace L. J. Cell Biol. 1993; 123: 1649-1659Crossref PubMed Scopus (472) Google Scholar). This nuclear transport pathway remains the best characterized because for many years this type of NLS, consisting primarily of a single or bipartite stretch of basic amino acids, was the only nuclear transport signal that had been defined for some proteins at the sequence level (reviewed in Ref. 14Jans D.A. Hübner S. Physiol. Rev. 1996; 76: 651-685Crossref PubMed Scopus (383) Google Scholar). This type of basic NLS is recognized in the cytosol by its receptor karyopherin α (importin α, KAP60p in yeast) (15Adam S.A. Gerace L. Cell. 1991; 66: 837-847Abstract Full Text PDF PubMed Scopus (318) Google Scholar, 16Görlich D. Prehn S. Laskey R.A. Hartmann E. Cell. 1994; 79: 767-778Abstract Full Text PDF PubMed Scopus (597) Google Scholar, 17Moroianu J. Blobel G. Radu A. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 2008-2011Crossref PubMed Scopus (249) Google Scholar, 18Weis K. Mattaj I.W. Lamond A.L. Science. 1995; 268: 1049-1053Crossref PubMed Scopus (306) Google Scholar). In addition to binding the import substrate, karyopherin α forms a complex with a second protein called karyopherin β (importin β, KAP95p in yeast), and it is this interaction that targets the import complex to the cytoplasmic face of the NPC (19Radu A. Blobel G. Moore M.S. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 1769-1773Crossref PubMed Scopus (382) Google Scholar, 20Görlich D. Kostka S. C. Laskey R.A. Hartmann E. Prehn S. Curr. Biol. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar, J. Cell Biol. 1995; PubMed Scopus Google Scholar, and M. Scholar, S.A. J. Biol. Chem. 1994; Scholar). The karyopherin α apparently the nuclear interior in with the transport to whereas the karyopherin β appears to only as as the nuclear of the NPC D. F. Hartmann E. Laskey R.A. Nature. 1995; PubMed Scopus Google Scholar, J. M. Blobel G. Radu A. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: PubMed Scopus (248) Google Scholar). be a that the receptors for nuclear transport through the NPC in with In Ran is not for the of the import at the NPC but for the of the import in the nuclear interior (12Moore M.S. Blobel G. Nature. 1993; 365: 661-663Crossref PubMed Scopus (638) Google Scholar). of during transport has been the of much was that karyopherin β not with M. Blobel G. Cell. 1995; Full Text PDF PubMed Scopus Google Scholar). of the came from the and sequence of karyopherin β with the and of transport homology of proteins in the a number of proteins that of homology with karyopherin β, and at least some of this homology is thought to to Ran binding M. J. S. A. D. J. G. J. 1997; PubMed Scopus Google Scholar, D. M. F.R. U. P. Hartmann E. Prehn S. E. J. Cell Biol. 1997; PubMed Scopus Google Scholar). great of excitement has resulted from the that a number of these proteins as receptors and carriers in other nuclear transport it is that all do At the protein of this have been implicated in the nuclear transport of binding proteins S. F. G. Cell. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar, Blobel G. Science. 1996; PubMed Scopus Google Scholar, J. Radu A. Blobel G. Proc. Natl. Acad. Sci. U. S. A. 1997; PubMed Scopus Google Scholar, L.F. Rosenblum Blobel G. J. Cell Biol. 1997; PubMed Scopus Google ribosomal proteins Blobel G. Cell. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar, G. E. J. U. D. Ponstingl H. F.R. J. 1997; PubMed Scopus Google proteins L.F. Blobel G. J. Cell Biol. 1997; PubMed Scopus Google U. G. E. F.R. P. Hartmann E. D. Mol. Cell. 1998; Full Text Full Text PDF PubMed Scopus Google Usn RNAs I. W., and Englmeier, L. (1998) Annu. Rev. Biochem., in pressGoogle substrates containing the nuclear sequence K. C. K. Cell. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar, M. M. M. Mattaj I.W. Cell. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar, B. F. C. Science. 1997; PubMed Scopus Google Scholar, M. S. M. M. M. E. Nature. 1997; PubMed Scopus Google and in the of the karyopherin α back to the cytoplasm U. F.R. Kostka S. D. Cell. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar). to the transport substrates recognized by the uncharacterized of this and in a number of cases to the recognized on the transport substrates by the characterized at a In most of the pathways that have been to the receptor does in that it to bind the targeting sequence an α and with the NPC and of the of this family that have been for to bind appear to do one D. M. F.R. U. P. Hartmann E. Prehn S. E. J. Cell Biol. 1997; PubMed Scopus Google Scholar). is that the rest although the amino sequence Ran binding on each has not yet been This to bind Ran be to the of these related proteins in nuclear and this binding in is on the Ran GTPase The of all G-proteins is on which guanine nucleotide or currently have and it is this in that to as in many diverse cellular pathways (reviewed in M.S. F. Nature. 1993; Scopus Google Scholar). Ran has been in the GDP-bound K. C. K. A. Nature. 1995; PubMed Scopus Google and although its in the GTP-bound is not yet its binding for different of proteins on it is in the or GTP-bound other of the Ras Ran both guanine and very at the cellular of approximately but these be by the of several protein that has been shown to as a guanine nucleotide exchange factor on Ran by increasing its of guanine nucleotide exchange approximately is a nuclear protein called in yeast) (reviewed in Ref. J. 1996; PubMed Scopus Google Scholar). of is a protein that a basic NLS and is found inside the nucleus to Much of of amino and its has that these are in a L. I. J. C. M. A. Nature. 1998; PubMed Scopus Google Scholar). In a HeLa is about abundant than Ran, being at about J. 1996; PubMed Scopus Google Scholar). has been to bind to and this protein play in the of (in addition to its role as the of is the of an J. 1996; PubMed Scopus Google Scholar). appears to play a role in the eukaryotic cell by the of nuclear to There is a hamster cell called that a in which this protein to be the cells to the of J. 1996; PubMed Scopus Google Scholar). several at this Ran its primarily nuclear localization and out into the that Ran to be in the GTP-bound to in the nucleus (8Ren M.G. Drivas P. D'Eustachio P. Rush M.G. J. Cell Biol. 1994; 14: 4216-4224Google Scholar). Although the for the nuclear of Ran at remains the binding of to other nuclear proteins the of Ran and its diffusion out of the nucleus. cells in nuclear and in a of nuclear transport pathways at the as do a number of in the in Ran and in several other proteins that the Ran GTPase cycle (see (reviewed in Ref. Mol. Biol. Rev. 1997; PubMed Scopus Google Scholar). Ran has been shown to with many different proteins that it is not in that its nucleotide result in such in the and of the nucleus. be that the interaction of and Ran in a does not result in as a guanine nucleotide exchange as is found for some other Ran has a for than whereas will bind either of Ran with C. F.R. Ponstingl H. A. 1995; PubMed Scopus Google Scholar). a when with of the two will Ran to exchange any for the of what is thought to inside the nucleus. is thought to Ran being with than inside the cell is the estimated of which in the of Ran with because of the of this guanine This be an unknown by other nuclear proteins inside the cell. Although the of to inside the the Ran GTPase-activating protein out the in the in yeast) is a protein has been shown to the of by Ran approximately F.R. H. I. Ponstingl H. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: PubMed Scopus (220) Google Scholar). in to the of is in the cytoplasm by although there has been a that small of this protein to the nucleus S. Proc. Natl. Acad. Sci. U. S. A. 1996; PubMed Scopus Google Scholar). This of the of the in the nucleus and the of the in the cytoplasm is thought to play a role in the of nuclear transport by a of in the cytoplasm and in the nucleus (see below). There is abundant protein called binding protein 1) that and when to Ran its for by about F.R. H. E. Ponstingl H. J. 1995; 14: PubMed Scopus Google Scholar). However, has been to a complex with and karyopherin β, whereas will bind protein S.A. J. Cell Biol. 1996; PubMed Scopus Google Scholar). of such a complex be in the of nuclear import when the of Ran in the cytoplasm is thought to be has been shown to the binding of Ran to other of the karyopherin β and it be these in addition to its to that such observed nuclear transport pathways in S.A. J. Cell Biol. 1996; PubMed Scopus Google Scholar, F.R. D. 1997; PubMed Scopus Google Scholar). The on the cytoplasmic of the NPC domains thus and and karyopherin β, each by a different (see I. W., and Englmeier, L. (1998) Annu. Rev. Biochem., in pressGoogle Scholar). In vertebrate cells, a of the post-translational in the of a addition C. Gerace L. Cell. 1997; 88: Full Text Full Text PDF PubMed Scopus Google Scholar, Coutavas E. Blobel G. J. Cell Biol. 1996; PubMed Scopus Google Scholar). this does not appear to this protein for but targets it to the on the cytoplasmic face of the NPC the does not appear to undergo a similar and the NPC protein to which the vertebrate is does not appear to have a In NPC proteins a small of across and the role of this and targeting in vertebrate nuclear transport will be an this of the at the cytoplasmic of the NPC it in an to on Ran molecules either the of proteins that have been shown to with small G-proteins are the guanine nucleotide M.S. F. Nature. 1993; Scopus Google Scholar). Although of this have been shown to play an role in other protein pathways, no Ran guanine nucleotide have been as such to one protein in the cell has been identified that has a for than and this protein is called is a protein that has been implicated in nuclear import both and M.S. Blobel G. Proc. Natl. Acad. Sci. U. S. A. 1994; PubMed Scopus Google Scholar, Gerace L. J. Cell Biol. 1995; PubMed Scopus Google Scholar, J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar). Ran but unlike any of the or other proteins, this protein does not appear to contain any type of targeting signal and is small to diffuse through the however, to be observed by to to a large all through the NPC when into either the cytoplasm or nucleus of C. Akin D. Moore M.S. J. Cell Sci. 1998; PubMed Google Scholar). This be in to the of to with a of NPC proteins Gerace L. J. Cell Biol. 1995; PubMed Scopus Google Scholar, U. Blobel G. Science. 1996; PubMed Scopus Google (see although its in this import pathway and involvement in nuclear transport pathways for the most substrates from the cytoplasm to the nuclear interior several distinct of the NPC during journey the cytoplasmic the central and the large on the the not through the NPC of substrates must the NPC in the and all of the evidence that the NPC out active transport in both (1Nigg E.A. Nature. 1997; 386: 779-787Crossref PubMed Scopus (909) Google Scholar, 4Mattaj, I. W., and Englmeier, L. (1998) Annu. Rev. Biochem., in pressGoogle Scholar). does this and what is the role of Ran in this the NPC is a very and an unknown number of NPC proteins remains The and sequence of NPC proteins that a transport or its or with during transport is but some NPC proteins appear to be in certain pathways but not in Mol. Biol. Rev. 1997; PubMed Scopus Google Scholar). family of NPC proteins have been implicated in multiple nuclear transport pathways are the NPC proteins contain many copies of a which appears in forms but in the amino sequence number of these proteins have been cloned from both vertebrate cells and is and both biochemical and that these bind certain of the karyopherin β family I. W., and Englmeier, L. (1998) Annu. Rev. Biochem., in pressGoogle Scholar, 5Pemberton L.F. Blobel G. Rosenblum J. Curr. Opin. Cell Biol. 1998; 10: 392-399Crossref PubMed Scopus (211) Google Scholar, A. Blobel G. Moore M.S. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 1769-1773Crossref PubMed Scopus (382) Google Scholar, M. Blobel G. Cell. 1995; Full Text PDF PubMed Scopus Google Scholar). has been to with of this family Gerace L. J. Cell Biol. 1995; PubMed Scopus Google Scholar, U. Blobel G. Science. 1996; PubMed Scopus Google Scholar). these are throughout the of the this of proteins different through the NPC that different transport substrates as through the a transport from any NPC to the and how many biochemical are to a transport from one of the NPC to the other of Ran in nuclear transport in its to the and of transport In nuclear import for example, the addition of not has been shown to the karyopherin complex and to the β from a I. W., and Englmeier, L. (1998) Annu. Rev. Biochem., in pressGoogle Scholar, M. Blobel G. Cell. 1995; Full Text PDF PubMed Scopus Google Scholar, M. Blobel G. M. J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar). with the addition of Ran in the GTP-bound to the cytoplasmic of the nuclear envelope of import substrates and nuclear import C. Akin D. Moore M.S. J. Cell Sci. 1998; PubMed Google Scholar). several substrates have been shown to only bind substrates in the of as occur in the and in the of as occur in the cytoplasm F.R. D. 1997; PubMed Scopus Google Scholar). for in the cytoplasm a for inside the nucleus during nuclear transport has been by the level inside the nucleus by or with GTPase cycles and that this the nuclear of a number of of RNAs I. W., and D. Scholar). that the in the cellular Ran be an that the cell the of nuclear transport. be however, that because of a to activity during the time it to cellular Ran for the of Ran in the GTP-bound inside the cell at any time its unknown K.M. S.A. Macara I.G. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar). Additionally, because of its small Ran is of diffusion through the This is by the of nuclear Ran at of the a that is known to an nuclear envelope (1Nigg E.A. Nature. 1997; 386: 779-787Crossref PubMed Scopus (909) Google Scholar, M.S. Blobel G. Nature. 1993; 365: 661-663Crossref PubMed Scopus (638) Google Scholar). what the of Ran through the is by its with other proteins during active transport and what diffusion remains In either a of Ran throughout the of the NPC in cells on S. is the of the Ran within the NPC with a transport complex that the of that complex through the that the during through the NPC or is not of the found in nuclear transport is that the for the most understood pathway of basic nuclear import example, the addition of either a or Ran to has been to basic import (12Moore M.S. Blobel G. Nature. 1993; 365: 661-663Crossref PubMed Scopus (638) Google Scholar, 13Melchior F. Paschal B. Evans J. Gerace L. J. Cell Biol. 1993; 123: 1649-1659Crossref PubMed Scopus (472) Google Scholar). In has been not to be in nuclear import pathway S. G. Curr. Biol. 1998; Full Text Full Text PDF PubMed Scopus Google Scholar). certain forms of nuclear appear to require Ran in the GTP-bound on but do not appear to require by Ran S.A. Macara I.G. Science. 1997; PubMed Scopus Google Scholar). this that Ran has to in some nuclear transport pathways, but not in is not is not the nucleotide of Ran has to only or more than to of a transport from the cytoplasm to the nuclear interior nuclear on the cytoplasmic of the nuclear envelope a of appear to be for import whereas for nuclear Ran in the GTP-bound inside the nucleus appears to be Although only be during import for the of the import into the nuclear interior of the import D. U. U. J. 1996; PubMed Scopus Google the of Ran during nuclear transport remains remains that Ran have inside the NPC in addition to its to with transport In addition to Ran, there are any other proteins for nuclear transport remains that remains is that of import substrates have been to at the cytoplasmic face of the This an for of an import across the of the NPC Cell. Full Text PDF PubMed Scopus Google Scholar, Laskey R.A. C. Cell. Full Text PDF PubMed Scopus Google Scholar). in nuclear transport remains to be nuclear import does not appear to be by as is K. C. Lamond J. 1996; PubMed Scopus Google but the of some very large is by of the NPC which E. B. J. Mol. Biol. 1996; PubMed Scopus Google Scholar). nuclear transport pathways, with different substrates of different have different have the of being for nuclear import by a into Ran that its nucleotide from to has that when this Ran is to permeabilized cells with no basic nuclear import K. C. Lamond J. 1996; PubMed Scopus Google Scholar). has that nuclear import with the targeting signal similar but not is by Gerace L. J. Cell Biol. 1996; PubMed Scopus Google Scholar). date the of multiple (in this and other nuclear transport remains and other about nuclear transport probably the fact that any that an nuclear envelope and a NPC (which is to and nuclear transport in is by necessity a complex containing many proteins in a number of cases is In an of and a is the that many not nuclear transport pathways are probably example, the transport of a in one direction be on the of a of this pathway in the other U. F.R. Kostka S. D. Cell. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar). of the in the of nuclear transport is to between what is a and of a transport pathway and what be a pathway by or In Ran appears to play a role in many nuclear transport pathways, but its to be nuclear transport pathway that has been not to require Ran is the of in to cellular and there be C. 1996; 10: PubMed Scopus Google Scholar). the of transport pathways that are by Ran, the of the in the cytoplasm and the in the nucleus is thought to be a factor in the of transport by a where is inside the nucleus and very in the in with the of Ran to and transport complexes in to the nucleotide of Ran, be to its in nuclear transport. how transport complexes through the NPC is the to understanding the of Ran in nuclear transport. the of and for many about Ran and a of the