Structural Views of the Ran GTPase Cycle
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Keywords:
Ran
Importin
Nuclear pore
Nucleoporin
The trafficking of macromolecules between cytoplasm and nucleus through nuclear pore complexes is mediated by specific carrier molecules such as members of the importin‐β family. Nuclear pore proteins (nucleoporins) frequently contain sequence repeats based on FG cores and carriers appear to move their cargo through the pores by hopping between successive FG cores. A major question is why some macromolecules are transported while others are not. This selectivity may be generated by the ability to bind FG repeats, a local concentration of carrier–cargo complexes near the entrance to the pore channel, and steric hindrance produced by high concentrations of nucleoporins in the channel.
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Nuclear pore
Importin
Ran
Transport protein
Nuclear export signal
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Importin
Nuclear pore
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Nucleoporin
Transport protein
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Author(s): Tang, Jeffrey Hsin Nien | Advisor(s): Liphardt, Jan | Abstract: The nuclear pore complex (NPC) is one of the largest known protein structures in the cell. Evolutionarily conserved in eukaryotes ranging from fungi to plants and animals, the NPC is the main transporter of molecules between the cell cytoplasm and nucleus. Maintaining the proper compartment-specific localization of proteins and RNA is crucial for normal cell function, and the nuclear pore accomplishes this task both robustly and efficiently. Over the past several decades, insight into the composition, organization, structure, and mechanism of the NPC has been gradually teased out through careful experimentation. However, many questions about the pore's function remain unanswered. In this dissertation, I describe efforts aimed at elucidating several aspects of the NPC. First, I investigate the transport properties of the pore, specifically looking at how the nuclear transport receptor importin-β and the Ran GTPase interact not only with each other but also how they may affect the pore itself. The nucleoporin Nup153 is identified as an important player in the nuclear transport process which binds strongly to importin-β in a Ran-sensitive manner. Using multiple experimental techniques, the properties of importin-β, and Nup153's interactions are characterized and shown to be capable of modulating the selective permeability barrier of the NPC.Next, I examine how members of a major class of nuclear pore proteins, the scaffold nucleoporins, are both structurally and functionally similar to the karyopherin family of soluble nuclear transport receptors. Structures of the proteins Nup188 and Nup192 are analyzed and shown to resemble those of karyopherins. Furthermore, in vitro assays indicate that at least a subset of the scaffold nucleoporins behave functionally as transport receptors, hinting at an evolutionary relationship between these two important classes of proteins.Finally, a calcium-mediated phenomenon affecting the permeability of the NPC is explored. I show that certain cytosolic proteases are activated by millimolar concentrations of calcium ion which leads irreversibly to an increase in the nuclear pore's permeability to large molecules. A model for physiological pathways implicated in this effect is proposed.
Nucleoporin
Nuclear pore
Importin
Karyopherin
Ran
Transport protein
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Ran
Nucleoporin
Nuclear pore
Karyopherin
Nuclear export signal
Importin
Small GTPase
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Proteins containing a classical NLS are transported into the nucleus by the import receptor importin β, which binds to cargoes via the adaptor importin α. The import complex is translocated through the nuclear pore complex by interactions of importin β with a series of nucleoporins. Previous studies have defined a nucleoporin binding region in the NH2-terminal half of importin β. Here we report the identification of a second nucleoporin binding region in its COOH-terminal half. Although the affinity of the COOH-terminal region for nucleoporins is dramatically weaker than that of the NH2-terminal region, sets of mutations that perturb the nucleoporin binding of either region reduce the nuclear import activity of importin β to a similar extent (∼50%). An importin β mutant with a combination of mutations in the NH2- and COOH-terminal regions is completely inactive for nuclear import. Thus, importin β possesses two nucleoporin binding sites, both of which are important for its nuclear import function.
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Importin
Nuclear pore
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Nuclear pore
Nucleoporin
Importin
Transport protein
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Nucleocytoplasmic transport is sustained by karyopherins (Kaps) and a Ran guanosine triphosphate (RanGTP) gradient that imports nuclear localization signal (NLS)–specific cargoes (NLS-cargoes) into the nucleus. However, how nuclear pore complex (NPC) barrier selectivity, Kap traffic, and NLS-cargo release are systematically linked and simultaneously regulated remains incoherent. In this study, we show that Kapα facilitates Kapβ1 turnover and occupancy at the NPC in a RanGTP-dependent manner that is directly coupled to NLS-cargo release and NPC barrier function. This is underpinned by the binding affinity of Kapβ1 to phenylalanine–glycine nucleoporins (FG Nups), which is comparable with RanGTP·Kapβ1, but stronger for Kapα·Kapβ1. On this basis, RanGTP is ineffective at releasing standalone Kapβ1 from NPCs. Depleting Kapα·Kapβ1 by RanGTP further abrogates NPC barrier function, whereas adding back Kapβ1 rescues it while Kapβ1 turnover softens it. Therefore, the FG Nups are necessary but insufficient for NPC barrier function. We conclude that Kaps constitute integral constituents of the NPC whose barrier, transport, and cargo release functionalities establish a continuum under a mechanism of Kap-centric control.
Nucleoporin
Ran
Nuclear pore
Importin
NLS
Karyopherin
Barrier function
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Nucleoporin
Nuclear pore
Importin
Ran
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Importin
Nuclear pore
Ran
Nucleoporin
NLS
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A major question in nuclear import concerns the identity of the nucleoporin(s) that interact with the nuclear localization sequences (NLS) receptor and its cargo as they traverse the nuclear pore. Ligand blotting and solution binding studies of isolated proteins have attempted to gain clues to the identities of these nucleoporins, but the studies have from necessity probed binding events far from an in vivo context. Here we have asked what binding events occur in the more physiological context of a Xenopus egg extract, which contains nuclear pore subcomplexes in an assembly competent state. We have then assessed our conclusions in the context of assembled nuclear pores themselves. We have used immunoprecipitation to identify physiologically relevant complexes of nucleoporins and importin subunits. In parallel, we have demonstrated that it is possible to obtain immunofluorescence localization of nucleoporins to subregions of the nuclear pore and its associated structures. By immunoprecipitation, we find the nucleoporin Nup153 and the pore-associated filament protein Tpr, previously shown to reside at distinct sites on the intranuclear side of assembled pores, are each in stable subcomplexes with importin alpha and beta in Xenopus egg extracts. Importin subunits are not in stable complexes with nucleoporins Nup62, Nup93, Nup98, or Nup214/CAN, either in egg extracts or in extracts of assembled nuclear pores. In characterizing the Nup153 complex, we find that Nup153 can bind to a complete import complex containing importin alpha, beta, and an NLS substrate, consistent with an involvement of this nucleoporin in a terminal step of nuclear import. Importin beta binds directly to Nup153 and in vitro can do so at multiple sites in the Nup153 FXFG repeat region. Tpr, which has no FXFG repeats, binds to importin beta and to importin alpha/beta heterodimers, but only to those that do not carry an NLS substrate. That the complex of Tpr with importin beta is fundamentally different from that of Nup153 is additionally demonstrated by the finding that recombinant beta or beta45-462 fragment freely exchanges with the endogenous importin beta/Nup153 complex, but cannot displace endogenous importin beta from a Tpr complex. However, the GTP analogue GMP-PNP is able to disassemble both Nup153- and Tpr-importin beta complexes. Importantly, analysis of extracts of isolated nuclei indicates that Nup153- and Tpr-importin beta complexes exist in assembled nuclear pores. Thus, Nup153 and Tpr are major physiological binding sites for importin beta. Models for the roles of these interactions are discussed.
Nucleoporin
Nuclear pore
Importin
Karyopherin
Immunoprecipitation
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