Functional identification of multiple nucleocytoplasmic trafficking signals in the broad-spectrum resistance protein RPW8.2

Nuclear localization signals (NLSs) and nuclear export signals (NESs) are important intramolecular regulatory elements for protein nucleocytoplasmic trafficking. This regulation confers spatial specificity to signal initiation and transduction in eukaryotic cells and thus is fundamental to the viability of all eukaryotic organisms. Here, we developed a simple and rapid method in which activity of putative NLSs or NESs was reported by subcellular localization of two tandem fluorescent proteins in fusion with the respective NLSs or NESs after agroinfiltration-mediated transient expression in leaves of Nicotiana benthamiana (Nb). We further demonstrated that the predicted NES from amino acid residue (aa) 9 to 22 and the NLS from aa91 to 101 in the broad-spectrum disease resistance protein RPW8.2 possess nuclear export and import activity, respectively. Additionally, by testing overlapping fragments covering the full length of RPW8.2, we identified another NLS from aa65 to 74 with strong nuclear import activity and two tandem non-canonical NESs in the C-terminus with strong nuclear export activity. Taken together, our results demonstrated the utility of a simple method to evaluate potential NLSs and NESs in plant cells and suggested that RPW8.2 may be subject to opposing nucleocytoplasmic trafficking forces for its subcellular localization and functional execution.     

Identification and characterization of nuclear localization signals within the nucleocapsid protein VP15 of white spot syndrome virus

The nucleocapsid protein VP15 of white spot syndrome virus (WSSV) is a basic DNA-binding protein. Three canonical bipartite nuclear localization signals (NLSs), called NLS1 (aa 11-27), NLS2 (aa 33-49) and NLS3 (44-60), have been detected in this protein, using the ScanProsite computer program. To determine the nuclear localization sequence of VP15, the full-length open reading frame, or the sequence of one of the three NLSs, was fused to the green fluorescent protein (GFP) gene, and transiently expressed in insect Sf9 cells. Transfection with full-length VP15 resulted in GFP fluorescence being distributed exclusively in the nucleus. NLS 1 alone could also direct GFP to the nucleus, but less efficiently. Neither of the other two NLSs (NLS2 and 3) was functional when expressed alone, but exhibited similar activity to NLS1 when they were expressed as a fusion peptide. Furthermore, a mutated VP15, in which the two basic amino acids (11RR12) of NLSI were changed to two alanines (11AA12), caused GFP to be localized only in the cytoplasm of Sf9 cells. These results demonstrated that VP15, as a nuclear localization protein, needs cooperation between its three NLSs, and that the two residues (11RR12) of NLS1 play a key role in transporting the protein to the nucleus.     

BARD1 induces BRCA1 intranuclear foci formation by increasing RING-dependent BRCA1 nuclear import and inhibiting BRCA1 nuclear export

BRCA1 is a tumor suppressor with several important nuclear functions. BRCA1 has no known cytoplasmic functions. We show here that the two previously identified nuclear localization signals (NLSs) are insufficient for nuclear localization of BRCA1 due to the opposing action of an NH2-terminal nuclear export signal. In transfected breast cancer cells, BRCA1 nuclear localization requires both the NLSs and NH2-terminal RING domain region; mutating either of these sequences shifts BRCA1 to the cytoplasm. The BRCA1 RING element mediates nuclear import via association with BARD1, and this is not affected by cancer-associated RING mutations. Moreover, BARD1 directly masks the BRCA1 nuclear export signal, and the resulting block to nuclear export is requisite for efficient import and nuclear localization of ectopic and endogenous BRCA1. Our results explain why BRCA1 exon 11 splice variants, which lack the NLSs but retain the RING domain, are frequently detected in the nucleus and in nuclear foci in vivo. In fact, co-expression of BARD1 promoted formation of DNA damage-induced nuclear foci comprising ectopic wild-type or NLS-deficient BRCA1, implicating BARD1 in nuclear targeting of BRCA1 for DNA repair. Our identification of BARD1 as a BRCA1 nuclear chaperone has regulatory implications for its reported effects on BRCA1 protein stability, ubiquitin ligase activity, and DNA repair.     

Cooperative nuclear localization sequences lend a novel role to the N-terminal region of MSH6

Human mismatch repair proteins MSH2-MSH6 play an essential role in maintaining genetic stability and preventing disease. While protein functions have been extensively studied, the substantial amino-terminal region (NTR*) of MSH6 that is unique to eukaryotic proteins, has mostly evaded functional characterization. We demonstrate that a cluster of three nuclear localization signals (NLS) in the NTR direct nuclear import. Individual NLSs are capable of partially directing cytoplasmic protein into the nucleus; however only cooperative effects between all three NLSs efficiently transport MSH6 into the nucleus. In striking contrast to yeast and previous assumptions on required heterodimerization, human MSH6 does not determine localization of its heterodimeric partner, MSH2. A cancer-derived mutation localized between two of the three NLS significantly decreases nuclear localization of MSH6, suggesting altered protein localization can contribute to carcinogenesis. These results clarify the pending speculations on the functional role of the NTR in human MSH6 and identify a novel, cooperative nuclear localization signal.     

The basic domain of plant B-ZIP proteins facilitates import of a reporter protein into plant nuclei.

The import of large molecules into the nucleus is an active process that requires the presence in cis of a nuclear localization signal (NLS). Although these signals have been well characterized in mammalian, yeast, and amphibian nuclear proteins, no plant NLS has yet been described. The NLSs identified so far generally contain clusters of basic amino acids. This characteristic feature prompted us to test several basic domains from the plant DNA-binding proteins TGA-1A and TGA-1B and the TATA box-binding protein TFIID for nuclear targeting function. When tested as N-terminal fusions to the beta-glucuronidase protein, only those constructs containing the DNA binding (basic) domain of the basic-zipper (B-ZIP) region of TGA-1A or TGA-1B conferred nuclear import. These results suggest a close association or overlap of the DNA binding and nuclear targeting domains of B-ZIP proteins. We also demonstrated that a wild-type but not a mutant simian virus 40 large T-antigen NLS facilitates import into plant nuclei, indicating a strong conservation between nuclear import mechanisms in animals and plants.     

A simple, solid-phase binding assay for the nuclear import receptor karyopherin alpha. Part 1: direct binding

The nuclear import receptor karyopherin alpha recognizes nuclear localization signals (NLSs), peptides that direct the transport of proteins into the nucleus. A simple, colorimetric assay has been developed to facilitate the identification and comparison of karyopherin ligands by direct and competitive binding using NLSs immobilized on the solid phase (TentaGel resin).     

Dissection of a nuclear localization signal

The regulated process of protein import into the nucleus of a eukaryotic cell is mediated by specific nuclear localization signals (NLSs) that are recognized by protein import receptors. This study seeks to decipher the energetic details of NLS recognition by the receptor importin alpha through quantitative analysis of variant NLSs. The relative importance of each residue in two monopartite NLS sequences was determined using an alanine scanning approach. These measurements yield an energetic definition of a monopartite NLS sequence where a required lysine residue is followed by two other basic residues in the sequence K(K/R)X(K/R). In addition, the energetic contributions of the second basic cluster in a bipartite NLS ( approximately 3 kcal/mol) as well as the energy of inhibition of the importin alpha importin beta-binding domain ( approximately 3 kcal/mol) were also measured. These data allow the generation of an energetic scale of nuclear localization sequences based on a peptide's affinity for the importin alpha-importin beta complex. On this scale, a functional NLS has a binding constant of approximately 10 nm, whereas a nonfunctional NLS has a 100-fold weaker affinity of 1 microm. Further correlation between the current in vitro data and in vivo function will provide the foundation for a comprehensive quantitative model of protein import.     

The l2 minor capsid protein of low-risk human papillomavirus type 11 interacts with host nuclear import receptors and viral DNA

Analysis of the interactions of low-risk human papillomavirus type 11 (HPV11) L2 with karyopherin beta (Kap beta) nuclear import receptors revealed that L2 interacted with Kap beta 1, Kap beta 2, and Kap beta 3 and formed a complex with the Kap alpha 2 beta 1 heterodimer. HPV11 L2 contains two nuclear localization signals (NLSs)-in the N terminus and the C terminus-that could mediate its nuclear import via a classical pathway. Each NLS was functional in vivo, and deletion of both of them abolished L2 nuclear localization. Both NLSs interacted with the viral DNA. Thus, HPV11 L2 can interact with several karyopherins and the viral DNA and may enter the nucleus via multiple pathways.     

NLSdb: database of nuclear localization signals

NLSdb is a database of nuclear localization signals (NLSs) and of nuclear proteins. NLSs are short stretches of residues mediating transport of nuclear proteins into the nucleus. The database contains 114 experimentally determined NLSs that were obtained through an extensive literature search. Using 'in silico mutagenesis' this set was extended to 308 experimental and potential NLSs. This final set matched over 43% of all known nuclear proteins and matches no currently known non-nuclear protein. NLSdb contains over 6000 predicted nuclear proteins and their targeting signals from the PDB and SWISS-PROT/TrEMBL databases. The database also contains over 12 500 predicted nuclear proteins from six entirely sequenced eukaryotic proteomes (Homo sapiens, Mus musculus, Drosophila melanogaster, Caenorhabditis elegans, Arabidopsis thaliana and Saccharomyces cerevisiae). NLS motifs often co-localize with DNA-binding regions. This observation was used to also annotate over 1500 DNA-binding proteins. NLSdb can be accessed via the web site: http://cubic.bioc.columbia.edu/db/NLSdb/.     

The C-terminal nuclear localization signal of the sex-determining region Y (SRY) high mobility group domain mediates nuclear import through importin beta 1

The sex-determining factor SRY is a DNA-binding protein that diverts primordial gonads from the ovarian pathway toward male differentiation to form testes. It gains access to the nucleus through two distinct nuclear localization signals (NLSs) that flank the high mobility group (HMG) DNA-binding domain, but the mechanisms through which these NLSs operate have not been studied. In this study, we reconstitute the nuclear import of SRY in vitro, demonstrating a lack of requirement for exogenous factors for nuclear accumulation and a significant reduction in nuclear transport in the presence of antibodies to importin beta but not importin alpha. Using a range of quantitative binding assays including enzyme-linked immunosorbent assay, fluorescence polarization, and native gel mobility electrophoresis, we assess the binding of importins to SRY, demonstrating a high affinity recognition (in the low nm range) by Imp beta independent of Imp alpha. In assessing the contribution of each NLS, we found that the N-terminal NLS was recognized poorly by importins, whereas the C-terminal NLS was bound by importin beta with similar affinity to SRY. We also found that RanGTP, but not RanGDP, could dissociate the SRY-importin beta complex in solution using FP. We describe a novel double-fluorescent label DNA binding assay to demonstrate mutual exclusivity between importin beta recognition and DNA binding on the part of SRY, which may represent an alternative release mechanism upon nuclear entry. This study represents the first characterization of the nuclear import pathway for a HMG domain-containing protein. Importantly, it demonstrates for the first time that recognition of SRY by Imp beta is of comparable affinity to that with which Imp alpha/beta recognizes conventional NLS-containing substrates.     

Characterization of Nuclear Localization Signals (NLSs) and Function of NLSs and Phosphorylation of Serine Residues in Subcellular and Subnuclear Localization of Transformer-2β (Tra2β)

The serine/arginine-rich (SR) proteins are one type of major actors in regulation of pre-mRNA splicing. Their functions are closely related to the intracellular spatial organization. The RS domain and phosphorylation status of SR proteins are two critical factors in determining the subcellular distribution. Mammalian Transformer-2β (Tra2β) protein, a member of SR proteins, is known to play multiple important roles in development and diseases. In the present study, we characterized the subcellular and subnuclear localization of Tra2β protein and its related mechanisms. The results demonstrated that in the brain the nuclear and cytoplasmic localization of Tra2β were correlated with its phosphorylation status. Using deletional mutation analysis, we showed that the nuclear localization of Tra2β was determined by multiple nuclear localization signals (NLSs) in the RS domains. The point-mutation analysis disclosed that phosphorylation of serine residues in the NLSs inhibited the function of NLS in directing Tra2β to the nucleus. In addition, we identified at least two nuclear speckle localization signals within the RS1 domain, but not in the RS2 domain. The nuclear speckle localization signals determined the localization of RS1 domain-contained proteins to the nuclear speckle. The function of the signals did not depend on the presence of serine residues. The results provide new insight into the mechanisms by which the subcellular and subnuclear localization of Tra2β proteins are regulated.     

Identification of a karyopherin alpha 2 recognition site in PLAG1, which functions as a nuclear localization signal

The activation of the pleomorphic adenoma gene 1 (PLAG1) is the most frequent gain-of-function mutation found in pleomorphic adenomas of the salivary glands. To gain more insight into the regulation of PLAG1 function, we searched for PLAG1-interacting proteins. Using the yeast two-hybrid system, we identified karyopherin alpha2 as a PLAG1-interacting protein. Physical interaction between PLAG1 and karyopherin alpha2 was confirmed by an in vitro glutathione S-transferase pull-down assay. Karyopherin alpha2 escorts proteins into the nucleus via interaction with a nuclear localization sequence (NLS) composed of short stretches of basic amino acids. Two putative NLSs were identified in PLAG1. The predicted NLS1 (KRKR) was essential for physical interaction with karyopherin alpha2 in glutathione S-transferase pull-down assay, and its mutation resulted in decreased nuclear import of PLAG1. Moreover, NLS1 was able to drive the nuclear import of the cytoplasmic protein beta-galactosidase. In contrast, predicted NLS2 of PLAG1 (KPRK) was not involved in karyopherin alpha2 binding nor in its nuclear import. The residual nuclear import of PLAG1 after mutation of the NLS1 was assigned to the zinc finger domain of PLAG1. These observations indicate that the nuclear import of PLAG1 is governed by its zinc finger domain and by NLS1, a karyopherin alpha2 recognition site.     

A novel nuclear localization signal in human DNA topoisomerase I

DNA topoisomerase (topo) I is a nuclear enzyme that plays an important role in DNA metabolism. Based on conserved nuclear targeting sequences, four classic nuclear localization signals (NLSs) have been proposed at the N terminus of human topo I, but studies with yeast have suggested that only one of them (amino acids (aa) 150-156) is sufficient to direct the enzyme to the nucleus. In this study, we expressed human topo I fused to enhanced green fluorescent protein (EGFP) in mammalian cells and demonstrated that whereas aa 150-156 are sufficient for nuclear localization, the nucleolar localization requires aa 157-199. More importantly, we identified a novel NLS within aa 117-146. In contrast to the classic NLSs that are rich in basic amino acids, the novel NLS identified in this study is rich in acidic amino acids. Furthermore, this novel NLS alone is sufficient to direct not only EGFP into the nucleus but also topo I; and the EGFP.topo I fusion driven by the novel NLS is as active in vivo as the wild-type topo I in response to the topo I inhibitor topotecan. Together, our results suggest that human topo I carries two independent NLSs that have opposite amino acid compositions.