Nuclear Localization Signal and Protein Context both Mediate Importin Specificity of Nuclear Import Substrates (original) (raw)
Related papers
Evidence for distinct substrate specificities of importin α family members in nuclear protein import
1999
Importin ␣ plays a pivotal role in the classical nuclear protein import pathway. Importin ␣ shuttles between nucleus and cytoplasm, binds nuclear localization signal-bearing proteins, and functions as an adapter to access the importin -dependent import pathway. In contrast to what is found for importin , several isoforms of importin ␣, which can be grouped into three subfamilies, exist in higher eucaryotes. We describe here a novel member of the human family, importin ␣7. To analyze specific functions of the distinct importin ␣ proteins, we recombinantly expressed and purified five human importin ␣'s along with importin ␣ from Xenopus and Saccharomyces cerevisiae. Binding affinity studies showed that all importin ␣ proteins from humans or Xenopus bind their import receptor (importin ) and their export receptor (CAS) with only marginal differences. Using an in vitro import assay based on permeabilized HeLa cells, we compared the import substrate specificities of the various importin ␣ proteins. When the substrates were tested singly, only the import of RCC1 showed a strong preference for one family member, importin ␣3, whereas most of the other substrates were imported by all importin ␣ proteins with similar efficiencies. However, strikingly different substrate preferences of the various importin ␣ proteins were revealed when two substrates were offered simultaneously.
Molecular and cellular biology, 1999
Importin alpha plays a pivotal role in the classical nuclear protein import pathway. Importin alpha shuttles between nucleus and cytoplasm, binds nuclear localization signal-bearing proteins, and functions as an adapter to access the importin beta-dependent import pathway. In contrast to what is found for importin beta, several isoforms of importin alpha, which can be grouped into three subfamilies, exist in higher eucaryotes. We describe here a novel member of the human family, importin alpha7. To analyze specific functions of the distinct importin alpha proteins, we recombinantly expressed and purified five human importin alpha's along with importin alpha from Xenopus and Saccharomyces cerevisiae. Binding affinity studies showed that all importin alpha proteins from humans or Xenopus bind their import receptor (importin beta) and their export receptor (CAS) with only marginal differences. Using an in vitro import assay based on permeabilized HeLa cells, we compared the import ...
Classical nuclear localization signals: definition, function, and interaction with importin alpha
The Journal of biological chemistry, 2007
The best understood system for the transport of macromolecules between the cytoplasm and the nucleus is the classical nuclear import pathway. In this pathway, a protein containing a classical basic nuclear localization signal (NLS) is imported by a heterodimeric import receptor consisting of the beta-karyopherin importin beta, which mediates interactions with the nuclear pore complex, and the adaptor protein importin alpha, which directly binds the classical NLS. Here we review recent studies that have advanced our understanding of this pathway and also take a bioinformatics approach to analyze the likely prevalence of this system in vivo. Finally, we describe how a predicted NLS within a protein of interest can be confirmed experimentally to be functionally important.
Classical nuclear localization signals: definition, function, and interaction with importin α
Journal of Biological …, 2007
The best understood system for the transport of macromolecules between the cytoplasm and the nucleus is the classical nuclear import pathway. In this pathway, a protein containing a classical basic nuclear localization signal (NLS) is imported by a heterodimeric import receptor consisting of the -karyopherin importin , which mediates interactions with the nuclear pore complex, and the adaptor protein importin ␣, which directly binds the classical NLS. Here we review recent studies that have advanced our understanding of this pathway and also take a bioinformatics approach to analyze the likely prevalence of this system in vivo. Finally, we describe how a predicted NLS within a protein of interest can be confirmed experimentally to be functionally important.
Importin α: a multipurpose nuclear-transport receptor
Trends in Cell Biology, 2004
The importin a/b heterodimer targets hundreds of proteins to the nuclear-pore complex (NPC) and facilitates their translocation across the nuclear envelope. Importin a binds to classical nuclear localization signal (cNLS)-containing proteins and links them to importin b, the karyopherin that ferries the ternary complex through the NPC. A second karyopherin, the exportin CAS, recycles importin a back to the cytoplasm. In this article, we discuss control mechanisms that importin a exerts over the assembly and disassembly of the ternary complex and we describe how new groups of importin a genes arose during the evolution of metazoan animals to function in development and differentiation. We also describe activities of importin a that seem to be distinct from its housekeeping functions in nuclear transport.
Stimulated nuclear import by β-like importins
F1000Prime Reports, 2013
Classic nuclear shuttling is mediated by an importin-a • b heterodimer that binds to cargoes containing a nuclear localization signal, and shuttles most nuclear proteins immediately after their translation. Aside from this canonical mechanism, kariopheryn-bs or b-like importins operate by binding to non-canonical nuclear localization signals to mediate translocation without the assistance of importin-a. The mechanism by which these components operate is much less understood and is currently under investigation. Recently, several b-like importins have been implicated in the stimulated nuclear translocation of signaling proteins. Here, we propose that this group of importins might be responsible for the swift nuclear shuttling of many proteins following various stimuli.
Dynamic localization of the nuclear import receptor and its interactions with transport factors
The Journal of Cell Biology, 1996
Characterization of the interactions between soluble factors required for nuclear transport is key to understanding the process of nuclear trafficking. Using a synthetic lethal screen with the rnal-1 strain, we have identified a genetic interaction between Rnalp, a GTPase activating protein required for nuclear transport, and yeast importin-[3, a component of the nuclear localization signal receptor. By the use of fusion proteins, we demonstrate that Rnalp physically interacts with importin-~. Mutants in importin-~ exhibit in vivo nuclear protein import defects, and importin-[3 localizes to the nuclear envelope along with other proteins associated with the nuclear pore complex. In addition, we present evidence that importin-ot, but not importin-13, mislocalizes to the nucleus in cells where the GTPase Ran is likely to be in the GDP-bound state. We suggest a model of nuclear transport in which Ran-mediated hydrolysis of GTP is necessary for the import of importin-a and the nuclear localization signal-bearing substrate into the nucleus, while exchange of GDP for GTP on Ran is required for the export of both mRNA and importin-a from the nucleus.
Structural biology and regulation of protein import into the nucleus
Journal of molecular biology, 2015
Proteins are translated in the cytoplasm, but many need to access the nucleus to perform their functions. Understanding how these nuclear proteins are transported through the nuclear envelope and how the import processes are regulated is therefore an important aspect of understanding cell function. Structural biology has played a key role in understanding the molecular events during the transport processes and their regulation, including the recognition of nuclear targeting signals by the corresponding receptors. Here, we review the structural basis of the principal nuclear import pathways and the molecular basis of their regulation. The pathways involve transport factors that are members of the β-karyopherin family, which can bind cargo directly (e.g. importin-β, transportin-1, transportin-3, importin-13) or through adaptor proteins (e.g. importin-α, snurportin-1, symportin-1), as well as unrelated transport factors such as Hikeshi, involved in the transport of heat-shock proteins,...
Importin-beta-like nuclear transport receptors
Genome biology, 2001
In recent years, our understanding of macromolecular transport processes across the nuclear envelope has grown dramatically, and a large number of soluble transport receptors mediating either nuclear import or nuclear export have been identified. Most of these receptors belong to one large family of proteins, all of which share homology with the protein import receptor importin beta (also named karyopherin beta). Members of this family have been classified as importins or exportins on the basis of the direction they carry their cargo. To date, the family includes 14 members in the yeast Saccharomyces cerevisiae and at least 22 members in humans. Importins and exportins are regulated by the small GTPase Ran, which is thought to be highly enriched in the nucleus in its GTP-bound form. Importins recognize their substrates in the cytoplasm and transport them through nuclear pores into the nucleus. In the nucleoplasm, RanGTP binds to importins, inducing the release of import cargoes. In ...