Acting Out of Character: Regulatory Roles of Nuclear Pore Complex Proteins
2009; Elsevier BV; Volume: 17; Issue: 5 Linguagem: Inglês
10.1016/j.devcel.2009.10.015
ISSN1878-1551
AutoresNikos Xylourgidis, Maarten Fornerod,
Tópico(s)Genomics and Chromatin Dynamics
ResumoNuclear pore complexes (NPCs) mediate all selective bidirectional transport between the nucleus and the cytoplasm. Additional functions for NPCs and their constituent proteins (nucleoporins) are emerging, some independent of classical transport. Specifically, enzymatic activities at the NPC regulate nucleocytoplasmic transport and use the NPC as a regulatory scaffold. Also, nucleoporins may regulate gene expression by contacting chromatin. Discriminating between effects on transport, scaffolding, and gene expression is a major challenge in understanding the role of the NPC in signaling and development. Nuclear pore complexes (NPCs) mediate all selective bidirectional transport between the nucleus and the cytoplasm. Additional functions for NPCs and their constituent proteins (nucleoporins) are emerging, some independent of classical transport. Specifically, enzymatic activities at the NPC regulate nucleocytoplasmic transport and use the NPC as a regulatory scaffold. Also, nucleoporins may regulate gene expression by contacting chromatin. Discriminating between effects on transport, scaffolding, and gene expression is a major challenge in understanding the role of the NPC in signaling and development. The eukaryotic interphase nucleus is a gene expression factory where signaling molecules and ribonuclear protein complexes enter and leave. These molecules transit across the nuclear envelope (NE) at NPCs, large protein structures that permeate the NE. The core function of the NPCs is mediating selective bidirectional nucleocytoplasmic transport (Görlich and Kutay, 1999Görlich D. Kutay U. Transport between the cell nucleus and the cytoplasm.Annu. Rev. Cell Dev. Biol. 1999; 15: 607-660Crossref PubMed Scopus (1267) Google Scholar, Tran and Wente, 2006Tran E.J. Wente S.R. Dynamic nuclear pore complexes: life on the edge.Cell. 2006; 125: 1041-1053Abstract Full Text Full Text PDF PubMed Scopus (299) Google Scholar). Three layers of selectivity for nucleocytoplasmic transport can be distinguished: size, signal, and directionality. Small molecules and proteins below a molecular weight (MW) of roughly 20 kD have unrestricted access to and from the nucleus. Spherical proteins larger than that are more restricted in their diffusion across the NPC, and such an example is BSA (60 kD), which is essentially incapable of entering the nucleus by itself (e.g., Mohr et al., 2009Mohr D. Frey S. Fischer T. Guttler T. Görlich D. Characterisation of the passive permeability barrier of nuclear pore complexes.EMBO J. 2009; 28: 2541-2553Crossref PubMed Scopus (80) Google Scholar). However, some much larger proteins, if they are elongated, can still slowly diffuse into the nucleus where they can accumulate in the nucleus by binding to chromatin or other nuclear structures (Mohr et al., 2009Mohr D. Frey S. Fischer T. Guttler T. Görlich D. Characterisation of the passive permeability barrier of nuclear pore complexes.EMBO J. 2009; 28: 2541-2553Crossref PubMed Scopus (80) Google Scholar, Tolwinski and Wieschaus, 2001Tolwinski N.S. Wieschaus E. Armadillo nuclear import is regulated by cytoplasmic anchor Axin and nuclear anchor dTCF/Pan.Development. 2001; 128: 2107-2117Crossref PubMed Google Scholar). In contrast, signal-mediated translocation of protein or protein/RNA complexes of over 1000 kD is extremely rapid (Ribbeck and Görlich, 2001Ribbeck K. Görlich D. Kinetic analysis of translocation through nuclear pore complexes.EMBO J. 2001; 20: 1320-1330Crossref PubMed Scopus (379) Google Scholar). These signals mainly work in one direction, and so a significant accumulation can be achieved either in the nucleus using a nuclear localization signal (NLS) or in the cytoplasm using a nuclear export signal (NES). In contrast to diffusion, signal-mediated accumulation is an active process, i.e., requires energy (reviewed in Mattaj and Englmeier, 1998Mattaj I.W. Englmeier L. Nucleocytoplasmic transport: the soluble phase.Annu. Rev. Biochem. 1998; 67: 265-306Crossref PubMed Scopus (898) Google Scholar). The mechanism of how these three layers of selectivity are imposed by the NPC, at least for proteins and small RNAs, has only recently become clearer, but is still the topic of some controversy (Weis, 2007Weis K. The nuclear pore complex: oily spaghetti or gummy bear?.Cell. 2007; 130: 405-407Abstract Full Text Full Text PDF PubMed Scopus (38) Google Scholar). In essence the system needs four basic ingredients and several accessory factors (Figure 1). These basic ingredients are (1) a signal on the cargo, (2) a nuclear transport receptor (NTR)/karyopherin/importin-exportin that recognizes the signal, (3) nucleoporin phenylalanine-glycine (Nup FG) repeat regions that form a meshwork/brushwork barrier inside the NPC, which essentially can only be accessed by (cargo-loaded) karyopherins/NTRs, and (4) nuclear RanGTP. Nuclear RanGTP binds to nuclear import receptors (importins) to release import cargo, and helps to assemble export cargo onto nuclear export receptors (exportins). Nuclear RanGTP thus imposes directionality on the nuclear transport system without the NPC being required to do this (Figure 1). The functional core of the NPC consists of Nup FG repeat regions, which are unstructured domains (Denning et al., 2003Denning D.P. Patel S.S. Uversky V. Fink A.L. Rexach M. Disorder in the nuclear pore complex: the FG repeat regions of nucleoporins are natively unfolded.Proc. Natl. Acad. Sci. USA. 2003; 100: 2450-2455Crossref PubMed Scopus (227) Google Scholar). They contain interspersed FG dipeptides that can self-associate (Patel et al., 2007Patel S.S. Belmont B.J. Sante J.M. Rexach M.F. Natively unfolded nucleoporins gate protein diffusion across the nuclear pore complex.Cell. 2007; 129: 83-96Abstract Full Text Full Text PDF PubMed Scopus (153) Google Scholar, Ribbeck and Görlich, 2002Ribbeck K. Görlich D. The permeability barrier of nuclear pore complexes appears to operate via hydrophobic exclusion.EMBO J. 2002; 21: 2664-2671Crossref PubMed Scopus (220) Google Scholar), possibly forming a molecular sieve. Concentrated Nup FG repeats in vitro form a hydrogel that is impermeable to noncargo proteins, but permeable to transport receptors and transport receptor/cargo complexes (Frey and Gorlich, 2007Frey S. Gorlich D. A saturated FG-repeat hydrogel can reproduce the permeability properties of nuclear pore complexes.Cell. 2007; 130: 512-523Abstract Full Text Full Text PDF PubMed Scopus (154) Google Scholar, Frey and Gorlich, 2009Frey S. Gorlich D. FG/FxFG as well as GLFG repeats form a selective permeability barrier with self-healing properties.EMBO J. 2009; 28: 2554-2567Crossref PubMed Scopus (38) Google Scholar). Alternatively, FG repeat regions can be seen as nanofilaments, forming a brush that blocks the pore by kinetic movement but attracting (cargo-loaded) transport receptors (Rout et al., 2000Rout M.P. Aitchison J.D. Suprapto A. Hjertaas K. Zhao Y. Chait B.T. The yeast nuclear pore complex: composition, architecture, and transport mechanism.J. Cell Biol. 2000; 148: 635-651Crossref PubMed Scopus (842) Google Scholar). Aspects of the latter model have been recapitulated in vitro (Jovanovic-Talisman et al., 2009Jovanovic-Talisman T. Tetenbaum-Novatt J. McKenney A.S. Zilman A. Peters R. Rout M.P. Chait B.T. Artificial nanopores that mimic the transport selectivity of the nuclear pore complex.Nature. 2009; 457: 1023-1027Crossref PubMed Scopus (103) Google Scholar, Lim et al., 2007Lim R.Y. Fahrenkrog B. Koser J. Schwarz-Herion K. Deng J. Aebi U. Nanomechanical basis of selective gating by the nuclear pore complex.Science. 2007; 318: 640-643Crossref PubMed Scopus (113) Google Scholar). Together with the directionality imposed by RanGTP, the interplay between FG Nups and NTRs can explain the three layers of selectivity of nucleocytoplasmic transport of proteins and small RNAs. For mRNA export, the signal requirement is not so clear-cut. In fact, RNAs above a certain size can be exported though a set of transport factors unrelated to karyopherins (Masuyama et al., 2004Masuyama K. Taniguchi I. Kataoka N. Ohno M. RNA length defines RNA export pathway.Genes Dev. 2004; 18: 2074-2085Crossref PubMed Scopus (40) Google Scholar). Therefore RNAs can be thought to contain a "constitutive signal" for export, which is the ability to associate with proteins of the TAP/NXF1 and pp15/NTF2 families (Iglesias and Stutz, 2008Iglesias N. Stutz F. Regulation of mRNP dynamics along the export pathway.FEBS Lett. 2008; 582: 1987-1996Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar, Vinciguerra and Stutz, 2004Vinciguerra P. Stutz F. mRNA export: an assembly line from genes to nuclear pores.Curr. Opin. Cell Biol. 2004; 16: 285-292Crossref PubMed Scopus (118) Google Scholar). One important insight obtained from karyopherin/NTR-dependent nuclear transport is that the translocation event itself is "passive," i.e., nondirectional and not requiring energy. The energy requirement lies in the maintenance of the RanGTP gradient across the NE. The same may be true for messenger ribonucleoprotein (mRNP) translocation. Conformational changes imposed by RNA helicases in the nucleus and cytoplasm could account for both the directionality and energy requirement of mRNP export. Clearly the core is not the whole story. Otherwise the NPC would only require one type of Nup and not 30–35 (Cronshaw et al., 2002Cronshaw J.M. Krutchinsky A.N. Zhang W. Chait B.T. Matunis M.J. Proteomic analysis of the mammalian nuclear pore complex.J. Cell Biol. 2002; 158: 915-927Crossref PubMed Scopus (523) Google Scholar, Rout et al., 2000Rout M.P. Aitchison J.D. Suprapto A. Hjertaas K. Zhao Y. Chait B.T. The yeast nuclear pore complex: composition, architecture, and transport mechanism.J. Cell Biol. 2000; 148: 635-651Crossref PubMed Scopus (842) Google Scholar). To what extent is this complexity due to regulatory functions of the NPC? It is likely that much of the diversity in Nup is still directly related to bulk transport processes. There are more than 20 different karyopherins that serve different import and/or export pathways (reviewed in Mosammaparast and Pemberton, 2004Mosammaparast N. Pemberton L.F. Karyopherins: from nuclear-transport mediators to nuclear-function regulators.Trends Cell Biol. 2004; 14: 547-556Abstract Full Text Full Text PDF PubMed Scopus (180) Google Scholar). Each karyopherin needs to pass the NPC in at least two different conformations: cargo-loaded and empty, and these are structurally quite distinct (Cook et al., 2007Cook A. Bono F. Jinek M. Conti E. Structural biology of nucleocytoplasmic transport.Annu. Rev. Biochem. 2007; 76: 647-671Crossref PubMed Scopus (214) Google Scholar). Also, the conformation of the karyopherin may be influenced by its specific cargo (Wohlwend et al., 2007Wohlwend D. Strasser A. Dickmanns A. Ficner R. Structural basis for RanGTP independent entry of spliceosomal U snRNPs into the nucleus.J. Mol. Biol. 2007; 374: 1129-1138Crossref PubMed Scopus (18) Google Scholar). At the same time, the window for interaction strengths between FG Nups and karyopherins is limited; translocation is impeded by overly strong interactions, as is the case for an N-terminally truncated form of importin β (Kutay et al., 1997Kutay U. Izaurralde E. Bischoff F.R. Mattaj I.W. Görlich D. Dominant-negative mutants of importin-beta block multiple pathways of import and export through the nuclear pore complex.EMBO J. 1997; 16: 1153-1163Crossref PubMed Scopus (268) Google Scholar, Mohr et al., 2009Mohr D. Frey S. Fischer T. Guttler T. Görlich D. Characterisation of the passive permeability barrier of nuclear pore complexes.EMBO J. 2009; 28: 2541-2553Crossref PubMed Scopus (80) Google Scholar). Adding to the complexity, transport receptors involved in mRNP export belong to a different class than karyopherins do and likely have their own requirements for Nup FG interactions. Furthermore, very large cargoes, such as preribosomal complexes and possibly the proteasome, most likely require the NPC to undergo significant conformational changes, requiring a flexible structure. In conclusion, a "one-Nup-fits-all" design for NTRs seems implausible, as the need to precisely balance the affinity of the Nup for the transport receptor is likely to limit the number of receptors a single Nup can handle. Given the number of different transport receptors, and the additional conformational complexity conferred on them by their cargoes, it is logical that diverse receptor-specific Nups would be required for efficient movement across the pore. The complexity of the NPC in terms of the type and number of its constituent proteins therefore is not necessarily indicative of a regulatory capacity. A large body of evidence indicates that nucleocytoplasmic transport of individual cargoes is regulated at the level of the transport signal (Kaffman and O'Shea, 1999Kaffman A. O'Shea E.K. Regulation of Nuclear Localization: A Key to a Door.Annu. Rev. Cell Dev. Biol. 1999; 15: 291-339Crossref PubMed Scopus (229) Google Scholar, Terry et al., 2007Terry L.J. Shows E.B. Wente S.R. Crossing the nuclear envelope: hierarchical regulation of nucleocytoplasmic transport.Science. 2007; 318: 1412-1416Crossref PubMed Scopus (263) Google Scholar) and rarely at the level of the transport receptor (Bohnsack et al., 2006Bohnsack M.T. Stuven T. Kuhn C. Cordes V.C. Gorlich D. A selective block of nuclear actin export stabilizes the giant nuclei of Xenopus oocytes.Nat. Cell Biol. 2006; 8: 257-263Crossref PubMed Scopus (72) Google Scholar). Evidence that nuclear transport of single cargoes is specifically regulated at the level of the NPC is essentially absent, although certain nuclear transport pathways are more dependent on specific Nups than others (Bernad et al., 2004Bernad R. van der Velde H. Fornerod M. Pickersgill H. Nup358/RanBP2 attaches to the nuclear pore complex via association with Nup88 and Nup214/CAN and plays a supporting role in CRM1-mediated nuclear protein export.Mol. Cell. Biol. 2004; 24: 2373-2384Crossref PubMed Scopus (80) Google Scholar, Hutten and Kehlenbach, 2006Hutten S. Kehlenbach R.H. Nup214 is required for CRM1-dependent nuclear protein export in vivo.Mol. Cell. Biol. 2006; 26: 6772-6785Crossref PubMed Scopus (61) Google Scholar, Sabri et al., 2007Sabri N. Roth P. Xylourgidis N. Sadeghifar F. Adler J. Samakovlis C. Distinct functions of the Drosophila Nup153 and Nup214 FG domains in nuclear protein transport.J. Cell Biol. 2007; 178: 557-565Crossref PubMed Scopus (31) Google Scholar, Terry and Wente, 2007Terry L.J. Wente S.R. Nuclear mRNA export requires specific FG nucleoporins for translocation through the nuclear pore complex.J. Cell Biol. 2007; 178: 1121-1132Crossref PubMed Scopus (51) Google Scholar, Walther et al., 2001Walther T.C. Fornerod M. Pickersgill H. Goldberg M. Allen T.D. Mattaj I.W. The nucleoporin Nup153 is required for nuclear pore basket formation, nuclear pore complex anchoring and import of a subset of nuclear proteins.EMBO J. 2001; 20: 5703-5714Crossref PubMed Scopus (110) Google Scholar). These specificities so far have not been implicated in signaling developmental regulation, most likely because control of nucleocytoplasmic transport on the level of the single substrate is more easily achieved by modifying the cargo. In contrast to this, the NPC has been implicated in the modulation of higher-order processes. For example, changes in the metabolic activity of vertebrate cells can lead to an increase in the number of NPCs during interphase (Maul et al., 1980Maul G.G. Deaven L.L. Freed J.J. Campbell G.L. Becak W. Investigation of the determinants of nuclear pore number.Cytogenet. Cell Genet. 1980; 26: 175-190Crossref PubMed Google Scholar, Oberleithner et al., 1994Oberleithner H. Brinckmann E. Schwab A. Krohne G. Imaging nuclear pores of aldosterone-sensitive kidney cells by atomic force microscopy.Proc. Natl. Acad. Sci. USA. 1994; 91: 9784-9788Crossref PubMed Scopus (58) Google Scholar). In addition, the transport channel of NPCs is larger in proliferating than in quiescent fibroblasts, and cell-cycle-dependent variation has also been found (Feldherr and Akin, 1993Feldherr C.M. Akin D. Regulation of nuclear transport in proliferating and quiescent cells.Exp. Cell Res. 1993; 205: 179-186Crossref PubMed Scopus (66) Google Scholar, Feldherr and Akin, 1994Feldherr C.M. Akin D. Variations in signal-mediated nuclear transport during the cell cycle in BALB/c 3T3 cells.Exp. Cell Res. 1994; 215: 206-210Crossref PubMed Scopus (21) Google Scholar). Whether these changes in the number of NPCs or the transport channel size actively influence global cellular activity, or rather are adaptations to the requirements of the cell, is currently unclear, although there is some evidence to suggest it is an active mechanism. For example, atomic force microscopy using isolated Xenopus NEs showed that the distal ring of the nuclear basket opens and closes in a calcium-dependent way (Stoffler et al., 1999Stoffler D. Goldie K.N. Feja B. Aebi U. Calcium-mediated structural changes of native nuclear pore complexes monitored by time-lapse atomic force microscopy.J. Mol. Biol. 1999; 287: 741-752Crossref PubMed Scopus (110) Google Scholar), possibly linking NPC gate size to the cells' metabolic state. In yeast, mutations in several Nups led to mRNA export defects (Dimaano and Ullman, 2004Dimaano C. Ullman K.S. Nucleocytoplasmic transport: integrating mRNA production and turnover with export through the nuclear pore.Mol. Cell. Biol. 2004; 24: 3069-3076Crossref PubMed Scopus (48) Google Scholar), and as such could be used to modulate overall gene expression and cellular activity. Such a mechanism may be exploited by the vesicular stomatitis virus (VSV) (Enninga et al., 2002Enninga J. Levy D.E. Blobel G. Fontoura B.M.A. Role of nucleoporin induction in releasing an mRNA nuclear export block.Science. 2002; 295: 1523-1525Crossref PubMed Scopus (95) Google Scholar) during cellular infection. The VSV matrix (M) protein targets the nucleoporin Nup98 and its binding partner Gle2/Rae1, thereby inhibiting host cell mRNA export (Faria et al., 2005Faria P.A. Chakraborty P. Levay A. Barber G.N. Ezelle H.J. Enninga J. Arana C. van Deursen J. Fontoura B.M. VSV disrupts the Rae1/mrnp41 mRNA nuclear export pathway.Mol. Cell. 2005; 17: 93-102Abstract Full Text Full Text PDF PubMed Scopus (106) Google Scholar, von Kobbe et al., 2000von Kobbe C. van Deursen J.M. Rodrigues J.P. Sitterlin D. Bachi A. Wu X. Wilm M. Carmo-Fonseca M. Izaurralde E. Vesicular stomatitis virus matrix protein inhibits host cell gene expression by targeting the nucleoporin Nup98.Mol. Cell. 2000; 6: 1243-1252Abstract Full Text Full Text PDF PubMed Google Scholar). In response to the cytokine interferon γ, which is released on activation of the host immune response, protein levels of Nup98, Nup96, and Gle2/Rae1 are increased, thereby relieving the mRNA export block. Another Nup that may be involved in higher-order regulation of transport is yeast Nup53 (Makhnevych et al., 2003Makhnevych T. Lusk C.P. Anderson A.M. Aitchison J.D. Wozniak R.W. Cell cycle regulated transport controlled by alterations in the nuclear pore complex.Cell. 2003; 115: 813-823Abstract Full Text Full Text PDF PubMed Scopus (89) Google Scholar), which regulates nuclear import during mitosis via the import receptor Kap121 by alternating physical interactions with neighboring Nups. Specifically, during interphase Nup170 occludes the Kap121-binding domain of Nup53, but during mitosis molecular rearrangements cause Nup53 to bind to another Nup, Nic96, exposing the Kap121 binding site on Nup53. This attracts the import receptor to the NPC and inhibits its translocation to the nucleus. How Nup53 is changed during mitosis remains unclear (Lusk et al., 2007Lusk C.P. Waller D.D. Makhnevych T. Dienemann A. Whiteway M. Thomas D.Y. Wozniak R.W. Nup53p is a target of two mitotic kinases, Cdk1p and Hrr25p.Traffic. 2007; 8: 647-660Crossref PubMed Scopus (17) Google Scholar), but the result is that a class of proteins fails to enter the nucleus, specifically during mitosis. Another example involves vertebrate Nup50, which is phosphorylated by the subfamily of mitogen-activated protein kinases (MAPKs) called Extracellular signal-Regulated Kinases (ERKs; Kosako et al., 2009Kosako H. Yamaguchi N. Aranami C. Ushiyama M. Kose S. Imamoto N. Taniguchi H. Nishida E. Hattori S. Phosphoproteomics reveals new ERK MAP kinase targets and links ERK to nucleoporin-mediated nuclear transport.Nat. Struct. Mol. Biol. 2009; 16: 1026-1035Crossref PubMed Scopus (50) Google Scholar). Phosphorylation decreases Nup50's affinity for the transport receptors importin β and transportin 1, repressing their accumulation inside the nucleus. De-novo-phosphorylated Nup50 is predominantly localized inside the nucleoplasm, raising the possibility that release of Nup50 from the NPC could modulate nuclear import events. However, Nup50 depletion itself had no effect on importin β localization, consistent with earlier reports (Smitherman et al., 2000Smitherman M. Lee K. Swanger J. Kapur R. Clurman B.E. Characterization and targeted disruption of murine Nup50, a p27(Kip1)-interacting component of the nuclear pore complex.Mol. Cell. Biol. 2000; 20: 5631-5642Crossref PubMed Scopus (52) Google Scholar), raising the possibility that alternative nucleoplasmic functions of Nup50 are involved (see below). Prominent features of the NPC are the filamentous appendices that project into the cytoplasm and nucleoplasm (Figure 2A). At the cytoplasmic side these filaments are waving free, while at the nuclear side they are connected to form the so-called nuclear basket structure. Mutational analysis and immunolocalization studies have indicated that the cytoplasmic filaments are mainly composed of the nucleoporin RanBP2/Nup358, whereas the key architectural element of the nuclear basket is the Nup named Tpr (Mlp1/2 in yeast) (Cordes et al., 1997Cordes V.C. Reidenbach S. Rackwitz H.R. Franke W.W. Identification of protein p270/Tpr as a constitutive component of the nuclear pore complex-attached intranuclear filaments.J. Cell Biol. 1997; 136: 515-529Crossref PubMed Scopus (148) Google Scholar, Krull et al., 2004Krull S. Thyberg J. Björkroth B. Rackwitz H.-R. Cordes V.C. Nucleoporins as components of the nuclear pore complex core structure and Tpr as the architectural element of the nuclear basket.Mol. Biol. Cell. 2004; 15: 4261-4277Crossref PubMed Scopus (92) Google Scholar, Walther et al., 2002Walther T.C. Pickersgill H.S. Cordes V.C. Goldberg M.W. Allen T.D. Mattaj I.W. Fornerod M. The cytoplasmic filaments of the nuclear pore complex are dispensable for selective nuclear protein import.J. Cell Biol. 2002; 158: 63-77Crossref PubMed Scopus (122) Google Scholar). RanBP2 functions in export complex disassembly (Engelsma et al., 2004Engelsma D. Bernad R. Calafat J. Fornerod M. Supraphysiological nuclear export signals bind CRM1 independently of RanGTP and arrest at Nup358.EMBO J. 2004; 23: 3643-3652Crossref PubMed Scopus (54) Google Scholar) and recycling of Ran and karyopherins. Consequently, RanBP2 is required for efficient nuclear transport in vivo (Bernad et al., 2004Bernad R. van der Velde H. Fornerod M. Pickersgill H. Nup358/RanBP2 attaches to the nuclear pore complex via association with Nup88 and Nup214/CAN and plays a supporting role in CRM1-mediated nuclear protein export.Mol. Cell. Biol. 2004; 24: 2373-2384Crossref PubMed Scopus (80) Google Scholar, Forler et al., 2004Forler D. Rabut G. Ciccarelli F.D. Herold A. Köcher T. Niggeweg R. Bork P. Ellenberg J. Izaurralde E. RanBP2/Nup358 provides a major binding site for NXF1-p15 dimers at the nuclear pore complex and functions in nuclear mRNA export.Mol. Cell. Biol. 2004; 24: 1155-1167Crossref PubMed Scopus (41) Google Scholar, Hutten et al., 2008Hutten S. Flotho A. Melchior F. Kehlenbach R.H. The Nup358-RanGAP complex is required for efficient importin alpha/beta-dependent nuclear import.Mol. Biol. Cell. 2008; 19: 2300-2310Crossref PubMed Scopus (45) Google Scholar) but not in vitro (Walther et al., 2002Walther T.C. Pickersgill H.S. Cordes V.C. Goldberg M.W. Allen T.D. Mattaj I.W. Fornerod M. The cytoplasmic filaments of the nuclear pore complex are dispensable for selective nuclear protein import.J. Cell Biol. 2002; 158: 63-77Crossref PubMed Scopus (122) Google Scholar). Tpr and yeast Mlp1/2 have been implicated in mRNA export (Bangs et al., 1998Bangs P. Burke B. Powers C. Craig R. Purohit A. Doxsey S. Functional analysis of Tpr: identification of nuclear pore complex association and nuclear localization domains and a role in mRNA export.J. Cell Biol. 1998; 143: 1801-1812Crossref PubMed Scopus (63) Google Scholar, Green et al., 2003Green D.M. Johnson C.P. Hagan H. Corbett A.H. The C-terminal domain of myosin-like protein 1 (Mlp1p) is a docking site for heterogeneous nuclear ribonucleoproteins that are required for mRNA export.Proc. Natl. Acad. Sci. USA. 2003; 100: 1010-1015Crossref PubMed Scopus (74) Google Scholar, Shibata et al., 2002Shibata S. Matsuoka Y. Yoneda Y. Nucleocytoplasmic transport of proteins and poly(A)+ RNA in reconstituted Tpr-less nuclei in living mammalian cells.Genes Cells. 2002; 7: 421-434Crossref PubMed Scopus (29) Google Scholar, Xu et al., 2007Xu X.M. Rose A. Muthuswamy S. Jeong S.Y. Venkatakrishnan S. Zhao Q. Meier I. NUCLEAR PORE ANCHOR, the Arabidopsis homolog of Tpr/Mlp1/Mlp2/megator, is involved in mRNA export and SUMO homeostasis and affects diverse aspects of plant development.Plant Cell. 2007; 19: 1537-1548Crossref PubMed Scopus (74) Google Scholar), suggesting that the nuclear basket seems ideally positioned for a final quality check of mRNAs before export to the cytoplasm. Indeed, depletion of Mpl1 and Mpl2 in yeast leads to the accumulation of misspliced RNAs in the cytoplasm (Galy et al., 2004Galy V. Gadal O. Fromont-Racine M. Romano A. Jacquier A. Nehrbass U. Nuclear retention of unspliced mRNAs in yeast is mediated by perinuclear Mlp1.Cell. 2004; 116: 63-73Abstract Full Text Full Text PDF PubMed Scopus (164) Google Scholar). For a diagram mapping out some of these architectural elements of the NPC, see Figure 2B. Most interestingly, both the nuclear and cytoplasmic periphery of the NPC are associated with enzymatic activities (Figure 3). The first of these to be discovered was the vertebrate Ran GTPase activating enzyme (RanGAP1) associated with RanBP2 (Saitoh et al., 1996Saitoh H. Cooke C.A. Burgess W.H. Earnshaw W.C. Dasso M. Direct and indirect association of the small GTPase ran with nuclear pore proteins and soluble transport factors: studies in Xenopus laevis egg extracts.Mol. Biol. Cell. 1996; 7: 1319-1334Crossref PubMed Google Scholar). The activity of RanGAP1, together with RanBP1 or RanBP1-like domains in RanBP2, stimulates hydrolysis of RanGTP bound to nuclear export complexes, leading to their disassembly (Figure 1). Thus, this enzymatic activity is inherently linked to the core transport activity of the NPC. RanGAP1 needs to be SUMOylated to bind to RanBP2 (Mahajan et al., 1997Mahajan R. Delphin C. Guan T. Gerace L. Melchior F. A small ubiquitin-related polypeptide involved in targeting RanGAP1 to nuclear pore complex protein RanBP2.Cell. 1997; 88: 97-107Abstract Full Text Full Text PDF PubMed Google Scholar, Matunis et al., 1996Matunis M.J. Coutavas E. Blobel G. A novel ubiquitin-like modification modulates the partitioning of the Ran-GTPase-activating protein RanGAP1 between the cytosol and the nuclear pore complex.J. Cell Biol. 1996; 135: 1457-1470Crossref PubMed Scopus (693) Google Scholar). Surprisingly, this posttranslational modification is dependent on a SUMO E3 ligase domain in RanBP2, which was the second enzymatic activity found at the NPC (Pichler et al., 2002Pichler A. Gast A. Seeler J.S. Dejean A. Melchior F. The nucleoporin RanBP2 has SUMO1 E3 ligase activity.Cell. 2002; 108: 109-120Abstract Full Text Full Text PDF PubMed Scopus (456) Google Scholar). At the other side of the NPC, the opposite enzymatic activity is found; the SUMO protease SENP2 (Ulp1 in yeast) is bound to Nup153 (Panse et al., 2003Panse V.G. Küster B. Gerstberger T. Hurt E. Unconventional tethering of Ulp1 to the transport channel of the nuclear pore complex by karyopherins.Nat. Cell Biol. 2003; 5: 21-27Crossref PubMed Scopus (70) Google Scholar, Zhang et al., 2002Zhang H. Saitoh H. Matunis M.J. Enzymes of the SUMO modification pathway localize to filaments of the nuclear pore complex.Mol. Cell. Biol. 2002; 22: 6498-6508Crossref PubMed Scopus (165) Google Scholar). These findings raised the possibility that these enzymatic activities are linked to specifically modify nuclear transport cargoes, since they sit opposite each other on either side of the NPC. Supporting this is the observation that several SUMOylated proteins accumulate in the nucleus, and SUMOylation of a protein can affect its nucleocytoplasmic distribution (e.g., Hong et al., 2001Hong Y. Rogers R. Matunis M.J. Mayhew C.N. Goodson M.L. Park-Sarge O.K. Sarge K.D. Regulation of heat shock transcription factor 1 by stress-induced SUMO-1 modification.J. Biol. Chem. 2001; 276: 40263-40267Abstract Full Text Full Text PDF PubMed Google Scholar). Indeed it has been found that SUMOylation can inhibit the NES activity of KLF5 (an important transcriptional regulator of cell proliferation; Du et al., 2008Du J.X. Bialkowska A.B. McConnell B.B. Yang V.W. SUMOylation regulates nuclear localization of Krüppel-like factor 5.J. Biol. Chem. 2008; 283: 31991-32002Crossref PubMed Scopus (29) Google Scholar) or enhance the NLS activity of viral proteins (Kindsmüller et al., 2007Kindsmüller K. Groitl P. Härtl B. Blanchette P. Hauber J. Dobner T. Intranuclear targeting and nuclear export of the adenovirus E1B-55K protein are regulated by SUMO1 conjugation.Proc. Natl. Acad. Sci. USA. 2007; 104: 6684-6689Crossref PubMed Scopus (36) Google Sch
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