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J. Biol. Chem., Vol. 282, Issue 8, 5101-5105, February 23, 2007
Minireview
Classical Nuclear Localization Signals: Definition, Function, and Interaction with Importin
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| ABSTRACT |
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-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. | INTRODUCTION |
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| Overview of Nucleocytoplasmic Transport |
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The active transport of macromolecular cargo between the cytoplasmic and nuclear compartments is facilitated by specific soluble carrier proteins. These carriers are collectively referred to as "karyopherins" (15), with those involved in import and export termed "importins" (16) and "exportins" (17), respectively. This distinction may be artificial though, because in an ideal system, a transport factor would import one molecule, switch to export mode, and return to the cytoplasm carrying a different cargo. For example, both import and export cargos for the S. cerevisiae Msn5 receptor have been identified (1822). Many transport receptors are members of the importin
superfamily and are called collectively "
-karyopherins." Cargo proteins can bind directly to
-karyopherins; however, in classical nuclear import, the interaction between the
-karyopherin and the cargo is mediated by the adaptor molecule importin
. Recent modeling studies have shown that addition of an adaptor to the system results in lower transport efficiency, but this feature also may facilitate the formation of a higher nuclear cytoplasmic gradient (23).
The energy for nuclear transport is provided by the small Ras family GTPase, Ran (24). Like other GTPases (25), Ran cycles between a GTP- and a GDP-bound state. The nucleotide state of Ran is modulated by regulatory proteins, primarily the Ran guanine nucleotide exchange factor (RanGEF) in the nucleus and the Ran GTPase-activating protein (RanGAP) in the cytoplasm (2629). Because these key regulatory factors are compartmentalized, the different forms of Ran are asymmetrically distributed in the cell, with RanGTP enriched in the nucleus and RanGDP enriched in the cytoplasm (30, 31). This compartmentalization allows Ran to impart directionality to nuclear transport by acting as a molecular switch that controls the binding and release of cargo. Therefore, import receptors bind cargo in the cytoplasm in the absence of RanGTP and release cargo in the nucleus upon RanGTP binding to the complex. In contrast, export receptors bind cargo in the nucleus in complex with RanGTP with hydrolysis to GDP in the cytoplasm triggering dissociation.
| Classical Nuclear Import Cycle |
|---|
recognizes and binds cargo in the cytoplasm, linking it to the
-karyopherin, importin
(32). Importin
then mediates interaction of the trimeric complex with the nuclear pore as it translocates into the nucleus. Once the import complex reaches the nucleus, it is dissociated by RanGTP. Binding of RanGTP to importin
causes a conformational change that results in the release of the importin
-cargo complex (33). An autoinhibitory region on the importin
-binding (IBB) domain of importin
(34, 35), the nucleoporin Nup2 (Nup50 or Npap60 in vertebrates) (3638), and the export receptor for importin
, Cse1/RanGTP (CAS/RanGTP in vertebrates) (39) then work together to deliver the cargo into the nucleus. Finally, Cse1/RanGTP recycles importin
back to the cytoplasm in preparation for another round of import (40, 41). | Classical Nuclear Localization Signal |
|---|
measured in vitro correlates with the steady state nuclear accumulation and import rate of the corresponding cNLS cargo in vivo (46, 47). These data establish boundaries for the binding capacity of a cNLS, where a functional cNLS has a binding constant of
10 nM for importin
. Non-functional near-cNLS sequences either bind importin
too weakly to be effectively imported or too tightly to be efficiently released from the receptor in the nucleus, although karyopherin release factors such as Nup2 and Cse1 can aid in this cargo release step (3639, 48). Recent computer simulations also indicate that the import rate of cNLS cargo depends strongly on the rate of formation of the import complex (23). Therefore, the import and accumulation of cNLS cargo in the nucleus is affected by both the affinity of the cNLS cargo for importin
and by the concentration of the importin
receptor itself (23, 47). This relationship is also characteristic of other NLS cargo/receptor interactions (49).
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Importin /cNLS Interaction
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has been defined using x-ray crystallography techniques (44, 45, 50, 51). These structural studies reveal that importin
is composed of a large curved domain consisting of ten armadillo (ARM) motifs, each of which is constructed from three
-helices (Fig. 2A), and a flexible N-terminal domain (the IBB domain) required for both binding to importin
(32) and cargo dissociation (34, 52, 53). The regular sequence of ARM repeats generates a gently curving, elongated molecule where the major and minor cNLS-binding sites are located within a shallow groove on the concave face. Both pockets are formed by solvent-exposed, conserved tryptophans together with a set of invariant asparagines four residues downstream (34, 50). The major pocket, which lies nearer the N terminus, binds monopartite cNLSs and the larger stretch of basic residues in bipartite cNLSs. The minor pocket is more C-terminal and binds the smaller stretch of basic residues in bipartite cNLSs. cNLS motifs bind to the NLS-binding pockets in an extended conformation with their main chains running anti-parallel to the direction of the importin
chain. The key cNLS lysine side chains then lie between the stacked hydrophobic indole side chains of the conserved tryptophans and form salt bridges with negatively charged residues lining the binding groove, whereas the asparagines make key main chain contacts (34, 50).
This characteristic binding mechanism, which is found for importin
with each of its cNLS-bearing cargos (34, 50), allows an autoinhibitory region within importin
to specifically compete for binding with all cNLSs, contributing to the delivery of cNLS cargo in the nucleus (Fig. 2B). This autoinhibitory sequence consists of KRR residues within the N-terminal domain of importin
(35, 53) that mimic the basic cNLS. Upon entry of the trimeric import complex into the nucleus, the N-terminal domain of importin
is released by importin
, freeing it to fold over and interact with the major cNLS-binding pocket on the body of importin
(34, 50, 51). Because both monopartite and bipartite cNLSs bind to the major pocket, the autoinhibitory region affects the binding affinity of both types of cNLS, preventing re-binding and, therefore, facilitating the release of cNLS cargo within the nucleus.
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| How Common Is cNLS Import? |
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For this analysis, the search algorithm from PSORT II (54) for monopartite cNLSs and bipartite cNLSs was used to query three sets of data. First, we examined the known yeast proteome represented by the 5850 proteins in the S. cerevisiae GenBankTM (55) to identify the fraction of yeast proteins predicted to contain a cNLS and, hence, which have the potential to enter the nucleus via the classical import pathway. Second, we analyzed the 1515 proteins localized to either the nucleus or the nucleolus in a comprehensive subcellular localization study that was performed with a global yeast GFP-fusion library (56). We reasoned that these proteins were targeted to the nucleus by some nuclear import pathway and that a fraction probably used classical nuclear import. Third, we analyzed the 224 proteins that interact with importin
according to the BioGRID data base (57), because proteins that contain functional cNLSs should interact with importin
. Given that the bipartite consensus sequence consists of the monopartite consensus sequence plus a linker and upstream basic residues, all proteins that contain a putative bipartite sequence also, by definition, contain a monopartite consensus sequence. To eliminate overlap, proteins containing a bipartite cNLS were removed from the monopartite tally.
|
, 49 (21.9%) contain a predicted bipartite cNLS and 61 (27.2%) contain a predicted monopartite cNLS, meaning that about one-half do not contain predicted cNLSs. At first, this number seems surprisingly high because cNLSs are conventionally thought to mediate interactions with importin
; however, many of the interactions in the BioGRID data base are genetic in nature and do not require or even necessarily imply a direct physical interaction. In addition, many proteins that interact with importin
in non-cargo roles are included, such as importin
(the import partner for importin
) and Cse1 (the export factor for importin
), which interact with importin
through non-NLS-mediated interfaces (39, 58). The current definition of a cNLS also may need refining, because some of these proteins may contain cNLSs and utilize traditional importin
-mediated import but escape recognition by the standard algorithm. For example, the primary sequence of the STAT1 (signal transducers and activators of transcription 1) protein does not contain a functional classical targeting sequence; however, upon dimerization, each subunit contributes basic residues that form a cNLS recognized by importin
(59). Although a large fraction of the proteins examined in these three data bases show evidence for import by the classical system, our analysis collectively indicates that alternative nuclear localization mechanisms also are likely to be prevalent. Thus, further studies and novel experimental approaches will be necessary to fully explore these additional systems. | Demonstrating That an NLS Is Functional in Vivo |
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or importin
. Unfortunately, testing the import pathway is much more complicated in non-yeast systems because specific inhibitors of nuclear import have not yet been developed. After a putative sequence has met these requirements, then it may be deemed a functional NLS for the protein of interest. However, it is important to note that a specific protein may have more than one nuclear targeting sequence that may be used by different pathways or respond to different types of signals, highlighting the complexity and intricacy of nuclear transport in vivo. | Conclusions |
|---|
. We have also assessed the prevalence of the classical nuclear import system in the cell by determining the fraction of proteins that contain a predicted cNLS. However, binding cargo is only the first step in an extremely complex and complicated cycle. Several recent studies have shed light on many of the other steps of classical import such as the cargo release step, where the structure of importin
with the karyopherin release factor Nup2 has been solved (37, 38), and importin
recycling, where the structure of importin
with its export receptor Cse1 has been deciphered (39). Future studies will focus on illuminating one of the enduring mysteries of transport, the translocation of cargo through the nuclear pore. Additionally, although a great deal is understood about the classical nuclear import pathway, very little is known about the details of other
-karyopherin-mediated transport pathways. Lee et al. (61) have begun to address this question and have recently proposed rules for NLS recognition by karyopherin
2 (also known as transportin). Thorough analogous structural studies complemented with bioinformatics analyses similar to the one performed here should facilitate the identification of the mechanism of interaction between other non-classical NLSs and their receptors, allowing development of a more comprehensive understanding of nucleocytoplasmic transport in vivo. | FOOTNOTES |
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The on-line version of this article (available at http://www.jbc.org) contains a supplemental table. ![]()
1 To whom correspondence should be addressed. Tel.: 404-727-4546; Fax: 404-727-2738; E-mail: acorbe2{at}emory.edu.
2 The abbreviations used are: SV40, simian virus 40; NLS, nuclear localization signal; cNLS, classical nuclear localization signal; IBB, importin
-binding; GFP, green fluorescent protein. ![]()
| ACKNOWLEDGMENTS |
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| REFERENCES |
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