Running on Ran: Nuclear Transport and the Mitotic Spindle

One attractive feature of the model discussed above is that it offers a unified picture of Ran's biochemistry in spindle assembly and nuclear transport. In both cases, spatial cues are given through changes in the concentration of Ran-GTP. For transport, high Ran-GTP promotes import receptor unloading and export receptor loading in the nucleus. For spindle assembly, high Ran-GTP promotes the specific activation of APA in the vicinity of chromatin through dissociation of complexes containing the same receptor molecules in their mitotic guise.It will be interesting to determine how well this model is conserved evolutionarily, particularly in organisms that do not undergo an open mitosis. The original experiments demonstrating a role for Ran in spindle assembly were conducted in Xenopus egg extracts. However, recent experiments from Guarguaglini et al. (2000)xGuarguaglini, G, Renzi, L, D'Ottavio, F, Di Fiore, B, Casenghi, M, Cundari, E, and Lavia, P. Cell Growth Differ. 2000; 11: 455–465PubMedSee all ReferencesGuarguaglini et al. (2000) show that Ran has a role in spindle assembly in mammalian tissue culture cells and indicate that Ran's role is probably conserved among all metazoans. This conclusion is further supported by spindle disruption in cells injected with mutant importin β proteins that lack the capacity to bind Ran (Nachury et al., 2001xNachury, M.V, Maresca, T.J, Salmon, W.C, Waterman-Storer, C.M, Heald, R, and Weis, K. Cell. 2001; 104: 95–106Abstract | Full Text | Full Text PDF | PubMed | Scopus (273)See all References(Nachury et al., 2001). It is clear that Ran's role in nuclear transport is well conserved between budding yeast and metazoans (Gorlich and Kutay, 1999xGorlich, D and Kutay, U. Annu. Rev. Cell Dev. Biol. 1999; 15: 607–660Crossref | PubMed | Scopus (1398)See all References(Gorlich and Kutay, 1999). Recent evidence also suggests that Ran has a role in spindle MT integrity in fission yeast that can be distinguished from its role in nuclear transport (Fleig et al., 2000xFleig, U, Salus, S.S, Karig, I, and Sazer, S. J. Cell Biol. 2000; 151: 1101–1112Crossref | PubMed | Scopus (46)See all References(Fleig et al., 2000). However, there is no evidence of dissipation of the Ran-GTP gradient at mitosis in either budding or fission yeast, and RanGAP1 and RanBP1 remain cytosolic throughout the yeast cell cycle (reviewed in Sazer and Dasso, 2000xSazer, S and Dasso, M. J. Cell Sci. 2000; 113: 1111–1118PubMedSee all ReferencesSazer and Dasso, 2000). Spindle assembly occurs within the yeast nucleus, where the concentration of GTP-Ran should be uniformly high throughout the cell cycle, so it is difficult to imagine how a gradient of GTP-Ran might be established. Although Ran may promote spindle assembly in both yeast and metazoan, it therefore appears likely that there will be important differences in how divergent organisms utilize Ran in this process.The theme established for Ran's biochemistry in nuclear transport and spindle assembly could certainly be expanded with a number of further variations. Ran has well-established roles in postmitotic nuclear envelope assembly (Hetzer et al. 2000xHetzer, M, Bilbao-Cortes, D, Walther, T.C, Gruss, O.J, and Mattaj, I.W. Mol. Cell. 2000; 5: 1013–1024Abstract | Full Text | Full Text PDF | PubMedSee all References, Zhang and Clarke 2000xZhang, C and Clarke, P.R. Science. 2000; 288: 1429–1432Crossref | PubMed | Scopus (142)See all References) and in regulating the onset of mitosis (reviewed in Sazer and Dasso, 2000xSazer, S and Dasso, M. J. Cell Sci. 2000; 113: 1111–1118PubMedSee all ReferencesSazer and Dasso, 2000) (Table 1(Table 1). However, the effectors for these processes are uncharacterized at present. Additionally, Nachury et al. (2001)xNachury, M.V, Maresca, T.J, Salmon, W.C, Waterman-Storer, C.M, Heald, R, and Weis, K. Cell. 2001; 104: 95–106Abstract | Full Text | Full Text PDF | PubMed | Scopus (273)See all ReferencesNachury et al. (2001) noted that chromosomal morphology is disrupted in PtK1 cells injected with mutant forms of importin β that are deficient in Ran binding, leading them to speculate that chromatin condensation may also be directly or indirectly regulated by Ran. Linking several aspects of chromosomal metabolism through a single molecule could have foreseeable advantages for coordinating their regulation through the cell cycle. In principle, importin β or other transport receptors could act in any of these processes, and this paradigm will clearly be extensively tested in the near future.Table 1Mechanisms of Ran Function and Targets of RanProcessRole of RanEffectorTargetReferencesNuclear transportCargo loading and unloading from transport receptor complexesImportin α/β and Importin β-related Ran-GTP binding proteinsMultiple nuclear proteins and shuttling proteinsSee Gorlich and Kutay, 1999xGorlich, D and Kutay, U. Annu. Rev. Cell Dev. Biol. 1999; 15: 607–660Crossref | PubMed | Scopus (1398)See all ReferencesGorlich and Kutay, 1999 for reviewSpindle assemblyPositive regulation of tubulin polymerization, Regulation of MT motor proteinsImportin α/βTPX2 NuMA (Eg5-indirect target?)Gruss et al. 2001xGruss, O.J, Carazo-Salas, R.E, Schatz, C.A, Guarguaglini, G, Kast, K, Wilm, M, Le Bot, N, Vernos, I, Karsenti, E, and Mattaj, I.W. Cell. 2001; 104: 83–93Abstract | Full Text | Full Text PDF | PubMed | Scopus (386)See all References, Nachury et al. 2001xNachury, M.V, Maresca, T.J, Salmon, W.C, Waterman-Storer, C.M, Heald, R, and Weis, K. Cell. 2001; 104: 95–106Abstract | Full Text | Full Text PDF | PubMed | Scopus (273)See all References, Wiese et al. 2001xWiese, C, Wilde, A, Moore, M.S, Adam, S.A, Merdes, A, and Zheng, Y. Science. 2001; 291: 653–656Crossref | PubMed | Scopus (237)See all References, Wilde et al. 2001xWilde, A, Lizarraga, S.B, Zhang, L, Wiese, C, Gliksman, N.R, Walczak, C.E, and Zheng, Y. Nat. Cell Biol. 2001; 3: 221–227Crossref | PubMed | Scopus (162)See all References, Carazo-Salas et al. 2001xCarazo-Salas, R.E, Gruss, O.J, Iain, W, Mattaj, I.W, and Karsenti, E. Nat. Cell Biol. 2001; 3: 228–234Crossref | PubMed | Scopus (153)See all ReferencesPostmitotic nuclear assemblyFusion of vesicles to form nuclear envelopeUnknown (Note: both nucleotide exchange and hydrolysis on Ran are required)UnknownZhang and Clarke 2000xZhang, C and Clarke, P.R. Science. 2000; 288: 1429–1432Crossref | PubMed | Scopus (142)See all References, Hetzer et al. 2000xHetzer, M, Bilbao-Cortes, D, Walther, T.C, Gruss, O.J, and Mattaj, I.W. Mol. Cell. 2000; 5: 1013–1024Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesCell cycle controlPrevention of premature activation of cdc2/cyclin B when unreplicated DNA is presentUnknownUnknownSee Sazer and Dasso, 2000xSazer, S and Dasso, M. J. Cell Sci. 2000; 113: 1111–1118PubMedSee all ReferencesSazer and Dasso, 2000 for reviewHowever, there is one key aspect of this model has been poorly demonstrated experimentally—namely, the structure of the Ran-GTP gradient. During interphase, a wealth of circumstantial evidence supports the notion that Ran-GTP is primarily distributed to the nucleus (Gorlich and Kutay 1999xGorlich, D and Kutay, U. Annu. Rev. Cell Dev. Biol. 1999; 15: 607–660Crossref | PubMed | Scopus (1398)See all References, Sazer and Dasso 2000xSazer, S and Dasso, M. J. Cell Sci. 2000; 113: 1111–1118PubMedSee all References), but direct evidence of this distribution has been lacking, largely due to technical considerations. The existence of a Ran-GTP gradient in mitosis is entirely inferred from the localization of RCC1, and experimental limitations will make the mitotic Ran-GTP distribution even more difficult to document. It will be important not only to verify the existence and nature of the Ran gradient, but also to show how it is remodeled during the cell cycle as the nuclear envelope breaks down and reforms and as chromosomes condense and decondense. Given the central role now proposed for Ran-GTP gradients, it is reasonable to assume that Ran-GTP distribution will be closely regulated. Such regulation may help to account for earlier data implicating phosphorylation in chromatin-induced MT stabilization (Andersen, 1999xAndersen, S.S. Bioessays. 1999; 21: 53–60Crossref | PubMedSee all References(Andersen, 1999). In the end, understanding Ran-GTP distribution may indeed provide the keys to the engine that is driving many machines.