Importin-beta mediates Cdc7 nuclear import by binding to the kinase insert II domain, which can be antagonized by importin-alpha

Nuclear import of IRF11 via the importin α/β pathway is essential for its antiviral activity

Interferon regulatory factor 11 (IRF11), an intriguing IRF member found only in fish species, has recently been shown to have antiviral properties that are dependent on its nuclear entry and DNA binding affinity. However, the mechanisms by which IRF11 enters the nucleus are unknown. In the present study, we found orthologs of IRF11 in lamprey and lancelet species by combining positional, phylogenetic and structural comparison data, showing that this gene has an ancient origin. The IRF11 gene (AjIRF11) from the Japanese eel, Anguilla japonica, was subsequently characterized, and it was found that AjIRF11 has antiviral activities against spring viremia of carp virus (SVCV), which are accomplished by regulating the production of type I IFN and IFN-stimulated genes. In addition to its known DNA binding residues in the α3 helix, two residues in Loop 1, His40 and Trp46, are also involved in DNA binding and activation of the IFN promoter. Using immunofluorescence microscopy and site-directed mutagenesis analysis, we confirmed that full nuclear localization of AjIRF11 requires the bipartite nuclear localization sequence (NLS) spanning residues 75 to 101, as well as the monopartite NLS situated between residues 119 and 122. Coimmunoprecipitation assays confirmed that AjIRF11 interacts with importin α via its NLSs and can also bind to importin β directly, implying that IRF11 can be imported to the nucleus by one or more transport receptors.

Characterization of the Interaction Between SARS-CoV-2 Membrane Protein (M) and Proliferating Cell Nuclear Antigen (PCNA) as a Potential Therapeutic Target

SARS-CoV-2 is an emerging virus from the Coronaviridae family and is responsible for the ongoing COVID-19 pandemic. In this work, we explored the previously reported SARS-CoV-2 structural membrane protein (M) interaction with human Proliferating Cell Nuclear Antigen (PCNA). The M protein is responsible for maintaining virion shape, and PCNA is a marker of DNA damage which is essential for DNA replication and repair. We validated the M-PCNA interaction through immunoprecipitation, immunofluorescence co-localization, and PLA (Proximity Ligation Assay). In cells infected with SARS-CoV-2 or transfected with M protein, using immunofluorescence and cell fractioning, we documented a reallocation of PCNA from the nucleus to the cytoplasm and the increase of PCNA and γH2AX (another DNA damage marker) expression. We also observed an increase in PCNA and γH2AX expression in the lung of a COVID-19 patient by immunohistochemistry. In addition, the inhibition of PCNA translocation by PCNA I1 and Verdinexor led to a reduction of plaque formation in an in vitro assay. We, therefore, propose that the transport of PCNA to the cytoplasm and its association with M could be a virus strategy to manipulate cell functions and may be considered a target for COVID-19 therapy.

Role of karyopherin nuclear transport receptors in nuclear transport by nuclear trafficking peptide

Nuclear trafficking peptide (NTP), a cell-penetrating peptide (CPP) composed of 10 amino acids (aa) (RIFIHFRIGC), has potent nuclear trafficking activity. Recently, we established a protein-based cell engineering system by using NTP, but it remained elusive how NTP functions as a CPP with nuclear orientation. In the present study, we identified importin subunit β1 (IMB1) and transportin 1 (TNPO1) as cellular proteins underlying the activity of NTP. These karyopherin nuclear transport receptors were identified as candidate molecules by liquid chromatography/mass spectrometry analysis, and downregulation of each protein by small interfering RNA significantly reduced NTP activity (P < 0.01). Biochemical analyses revealed that NTP bound directly to both molecules, and the forced expression of an IMB1 fragment (296-516 aa) or TNPO1 fragment (1-297 aa), which both contain binding sites to NTP, reduced nuclear NTP-green fluorescent protein (GFP) levels when it was added to cell culture medium. NTP is derived from viral protein R (Vpr) of human immunodeficiency virus-1, and Vpr enters the nucleus and exerts pleiotropic functions. Notably, Vpr bound directly to IMB1 and TNPO1, and its function was significantly impaired by the forced expression of the 296-516-aa fragment of IMB1 and 1-297-aa fragment of TNPO1. Interestingly, NTP completely blocked the physical association of Vpr with IMB1 and TNPO1. Although the nuclear localization mechanism of Vpr remains unknown, our data suggest that NTP functions as a novel nuclear localization signal of Vpr.

Structural Basis for the Activation and Target Site Specificity of CDC7 Kinase

CDC7 is an essential Ser/Thr kinase that acts upon the replicative helicase throughout the S phase of the cell cycle and is activated by DBF4. Here, we present crystal structures of a highly active human CDC7-DBF4 construct. The structures reveal a zinc-finger domain at the end of the kinase insert 2 that pins the CDC7 activation loop to motif M of DBF4 and the C lobe of CDC7. These interactions lead to ordering of the substrate-binding platform and full opening of the kinase active site. In a co-crystal structure with a mimic of MCM2 Ser40 phosphorylation target, the invariant CDC7 residues Arg373 and Arg380 engage phospho-Ser41 at substrate P+1 position, explaining the selectivity of the S-phase kinase for Ser/Thr residues followed by a pre-phosphorylated or an acidic residue. Our results clarify the role of DBF4 in activation of CDC7 and elucidate the structural basis for recognition of its preferred substrates.

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DBF4 activates CDC7 kinase via a two-step mechanism

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Zinc-finger domain in CDC7 KI2 interacts with DBF4 motif M

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Invariant CDC7 residues Arg373 and Arg380 engage P+1 substrate site

Complementary functions for the Ran gradient during division

The Ran pathway has a well-described function in nucleocytoplasmic transport, where active Ran dissociates importin/karyopherin-bound cargo containing a nuclear localization signal (NLS) in the nucleus. As cells enter mitosis, the nuclear envelope breaks down and a gradient of active Ran forms where levels are highest near chromatin. This gradient plays a crucial role in regulating mitotic spindle assembly, where active Ran binds to and releases importins from NLS-containing spindle assembly factors. An emerging theme is that the Ran gradient also regulates the actomyosin cortex for processes including polar body extrusion during meiosis, and cytokinesis. For these events, active Ran could play an inhibitory role, where importin-binding may help promote or stabilize a conformation or interaction that favours the recruitment and function of cortical regulators. For either spindle assembly or cortical polarity, the gradient of active Ran determines the extent of importin-binding, the effects of which could vary for different proteins.

Cdc7 kinase stimulates Aurora B kinase in M-phase

The conserved serine-threonine kinase, Cdc7, plays a crucial role in initiation of DNA replication by facilitating the assembly of an initiation complex. Cdc7 is expressed at a high level and exhibits significant kinase activity not only during S-phase but also during G2/M-phases. A conserved mitotic kinase, Aurora B, is activated during M-phase by association with INCENP, forming the chromosome passenger complex with Borealin and Survivin. We show that Cdc7 phosphorylates and stimulates Aurora B kinase activity in vitro. We identified threonine-236 as a critical phosphorylation site on Aurora B that could be a target of Cdc7 or could be an autophosphorylation site stimulated by Cdc7-mediated phosphorylation elsewhere. We found that threonines at both 232 (that has been identified as an autophosphorylation site) and 236 are essential for the kinase activity of Aurora B. Cdc7 down regulation or inhibition reduced Aurora B activity in vivo and led to retarded M-phase progression. SAC imposed by paclitaxel was dramatically reversed by Cdc7 inhibition, similar to the effect of Aurora B inhibition under the similar situation. Our data show that Cdc7 contributes to M-phase progression and to spindle assembly checkpoint most likely through Aurora B activation.

Importin α: functions as a nuclear transport factor and beyond

Nucleocytoplasmic transport is an essential process in eukaryotes. The molecular mechanisms underlying nuclear transport that involve the nuclear transport receptor, small GTPase Ran, and the nuclear pore complex are highly conserved from yeast to humans. On the other hand, it has become clear that the nuclear transport system diverged during evolution to achieve various physiological functions in multicellular eukaryotes. In this review, we first summarize the molecular mechanisms of nuclear transport and how these were elucidated. Then, we focus on the diverse functions of importin α, which acts not merely an import factor but also as a multi-functional protein contributing to a variety of cellular functions in higher eukaryotes.

Identification of karyopherins involved in the nuclear import of RNA exosome subunit Rrp6 in Saccharomyces cerevisiae

The exosome is a conserved multiprotein complex essential for RNA processing and degradation. The nuclear exosome is a key factor for pre-rRNA processing through the activity of its catalytic subunits, Rrp6 and Rrp44. In Saccharomyces cerevisiae, Rrp6 is exclusively nuclear and has been shown to interact with exosome cofactors. With the aim of analyzing proteins associated with the nuclear exosome, in this work, we purified the complex with Rrp6-TAP, identified the co-purified proteins by mass spectrometry, and found karyopherins to be one of the major groups of proteins enriched in the samples. By investigating the biological importance of these protein interactions, we identified Srp1, Kap95, and Sxm1 as the most important karyopherins for Rrp6 nuclear import and the nuclear localization signals recognized by them. Based on the results shown here, we propose a model of multiple pathways for the transport of Rrp6 to the nucleus.

Cdk1-mediated phosphorylation of Cdc7 suppresses DNA re-replication

To maintain genetic stability, the entire mammalian genome must replicate only once per cell cycle. This is largely achieved by strictly regulating the stepwise formation of the pre-replication complex (pre-RC), followed by the activation of individual origins of DNA replication by Cdc7/Dbf4 kinase. However, the mechanism how Cdc7 itself is regulated in the context of cell cycle progression is poorly understood. Here we report that Cdc7 is phosphorylated by a Cdk1-dependent manner during prometaphase on multiple sites, resulting in its dissociation from origins. In contrast, Dbf4 is not removed from origins in prometaphase, nor is it degraded as cells exit mitosis. Our data thus demonstrates that constitutive phosphorylation of Cdc7 at Cdk1 recognition sites, but not the regulation of Dbf4, prevents the initiation of DNA replication in normally cycling cells and under conditions that promote re-replication in G2/M. As cells exit mitosis, PP1α associates with and dephosphorylates Cdc7. Together, our data support a model where Cdc7 (de)phosphorylation is the molecular switch for the activation and inactivation of DNA replication in mitosis, directly connecting Cdc7 and PP1α/Cdk1 to the regulation of once-per-cell cycle DNA replication in mammalian cells.

Identification of cargo proteins specific for importin-β with importin-α applying a stable isotope labeling by amino acids in cell culture (SILAC)-based in vitro transport system

The human importin (Imp)-β family consists of 21 nucleocytoplasmic transport carrier proteins, which transport thousands of proteins (cargoes) across the nuclear envelope through nuclear pores in specific directions. To understand the nucleocytoplasmic transport in a physiological context, the specificity of cargoes for their cognate carriers should be determined; however, only a limited number of nuclear proteins have been linked to specific carriers. To address this biological question, we recently developed a novel method to identify carrier-specific cargoes. This method includes the following three steps: (i) the cells are labeled by stable isotope labeling by amino acids in cell culture (SILAC); (ii) the labeled cells are permeabilized, and proteins in the unlabeled cell extracts are transported into the nuclei of the permeabilized cells by a particular carrier; and (iii) the proteins in the nuclei are quantitatively identified by LC-MS/MS. The effectiveness of this method was demonstrated by the identification of transportin (Trn)-specific cargoes. Here, we applied this method to identify cargo proteins specific for Imp-β, which is a predominant carrier that exclusively utilizes Imp-α as an adapter for cargo binding. We identified candidate cargoes, which included previously reported and potentially novel Imp-β cargoes. In in vitro binding assays, most of the candidate cargoes bound to Imp-β in one of three binding modes: directly, via Imp-α, or via other cargoes. Thus, our method is effective for identifying a variety of Imp-β cargoes. The identified Imp-β and Trn cargoes were compared, ensuring the carrier specificity of the method and illustrating the complexity of these transport pathways.

Expression of huCdc7 in colorectal cancer

AIM: To detect the expression of huCdc7 in colorectal cancer.

METHODS: The mRNA and protein expression of huCdc7 in 39 colorectal cancer tissue specimens and matched tumor-adjacent normal colorectal tissue specimens was detected by reverse transcription-polymerase chain reaction and immunohistochemistry, respectively.

RESULTS: The relative expression level of huCdc7 mRNA in colorectal cancer was significantly higher than that in tumor-adjacent normal colorectal tissues (0.03675 ± 1.00 vs 0.01199 ± 0.44, P < 0.05). huCdc7-positive cells displayed brown granules in the nucleus. Tumor tissues contained many huCdc7-positive cells, whereas normal colorectal tissues contained very few positive cells.

CONCLUSION: huCdc7 may play an important role in the development and progression of colorectal cancer.

Dbf4: the whole is greater than the sum of its parts

Together with cyclin-dependent kinases, the Dbf4-dependent kinase (DDK) is essential to activate the Mcm2-7 helicase and, hence, initiate DNA replication in eukaryotes. Beyond its role as the regulatory subunit of the DDK complex, the Dbf4 protein also regulates the activity of other cell cycle kinases to mediate the checkpoint response and prevent premature mitotic exit under stress. Two features that are unusual in DNA replication proteins characterize Dbf4. The first is its evolutionary divergence; the second is how its conserved motifs are combined to form distinct functional units. This structural plasticity appears to be at odds with the conserved functions of Dbf4. In this review, we summarize recent genetic, biochemical and structural work delineating the multiple interactions mediated by Dbf4 and its various functions during the cell cycle. We also discuss how the limited sequence conservation of Dbf4 may be an advantage to regulate the activities of multiple cell cycle kinases.

Subcellular proteomics reveals a role for nucleo-cytoplasmic trafficking at the DNA replication origin activation checkpoint

Depletion of DNA replication initiation factors such as CDC7 kinase triggers the origin activation checkpoint in healthy cells and leads to a protective cell cycle arrest at the G1 phase of the mitotic cell division cycle. This protective mechanism is thought to be defective in cancer cells. To investigate how this checkpoint is activated and maintained in healthy cells, we conducted a quantitative SILAC analysis of the nuclear- and cytoplasmic-enriched compartments of CDC7-depleted fibroblasts and compared them to a total cell lysate preparation. Substantial changes in total abundance and/or subcellular location were detected for 124 proteins, including many essential proteins associated with DNA replication/cell cycle. Similar changes in protein abundance and subcellular distribution were observed for various metabolic processes, including oxidative stress, iron metabolism, protein translation and the tricarboxylic acid cycle. This is accompanied by reduced abundance of two karyopherin proteins, suggestive of reduced nuclear import. We propose that altered nucleo-cytoplasmic trafficking plays a key role in the regulation of cell cycle arrest. The results increase understanding of the mechanisms underlying maintenance of the DNA replication origin activation checkpoint and are consistent with our proposal that cell cycle arrest is an actively maintained process that appears to be distributed over various subcellular locations.

Nuclear-to-cytoplasmic relocalization of the proliferating cell nuclear antigen (PCNA) during differentiation involves a chromosome region maintenance 1 (CRM1)-dependent export and is a prerequisite for PCNA antiapoptotic activity in mature neutrophils

Neutrophils are deprived of proliferative capacity and have a tightly controlled lifespan to avoid their persistence at the site of injury. We have recently described that the proliferating cell nuclear antigen (PCNA), a nuclear factor involved in DNA replication and repair of proliferating cells, is a key regulator of neutrophil survival. In neutrophils, PCNA was localized exclusively in the cytoplasm due to its nuclear-to-cytoplasmic relocalization during granulocytic differentiation. We showed here that leptomycin B, an inhibitor of the chromosome region maintenance 1 (CRM1) exportin, inhibited PCNA relocalization during granulocytic differentiation of HL-60 and NB4 promyelocytic cell lines and of human CD34(+) primary cells. Using enhanced green fluorescent protein fusion constructs, we have demonstrated that PCNA relocalization involved a nuclear export signal (NES) located from Ile-11 to Ile-23 in the PCNA sequence. However, this NES, located at the inner face of the PCNA trimer, was not functional in wild-type PCNA, but instead, was fully active and leptomycin B-sensitive in the monomeric PCNAY114A mutant. To test whether a defect in PCNA cytoplasmic relocalization would affect its antiapoptotic activity in mature neutrophils, a chimeric PCNA fused with the SV40 nuclear localization sequence (NLS) was generated to preclude its cytoplasmic localization. As expected, neutrophil-differentiated PLB985 cells expressing ectopic SV40NLS-PCNA had an increased nuclear PCNA as compared with cells expressing wild-type PCNA. Accordingly, the nuclear PCNA mutant did not show any antiapoptotic activity as compared with wild-type PCNA. Nuclear-to-cytoplasmic relocalization that occurred during myeloid differentiation is essential for PCNA antiapoptotic activity in mature neutrophils and is dependent on the newly identified monomerization-dependent PCNA NES.

Molecular mechanism of activation of human Cdc7 kinase: bipartite interaction with Dbf4/activator of S phase kinase (ASK) activation subunit stimulates ATP binding and substrate recognition

Cdc7 is a serine/threonine kinase conserved from yeasts to human and is known to play a key role in the regulation of initiation at each replication origin. Its catalytic function is activated via association with the activation subunit Dbf4/activator of S phase kinase (ASK). It is known that two conserved motifs of Dbf4/ASK are involved in binding to Cdc7, and both are required for maximum activation of Cdc7 kinase. Cdc7 kinases possess unique kinase insert sequences (kinase insert I-III) that are inserted at defined locations among the conserved kinase domains. However, precise mechanisms of Cdc7 kinase activation are largely unknown. We have identified two segments on Cdc7, DAM-1 (Dbf4/ASK interacting motif-1; amino acids 448-457 near the N terminus of kinase insert III) and DAM-2 (C-terminal 10-amino acid segment), that interact with motif-M and motif-C of ASK, respectively, and are essential for kinase activation by ASK. The C-terminal 143-amino acid polypeptide (432-574) containing DAM-1 and DAM-2 can interact with Dbf4/ASK. Characterization of the purified ASK-free Cdc7 and Cdc7-ASK complex shows that ATP binding of the Cdc7 catalytic subunit requires Dbf4/ASK. However, the "minimum" Cdc7, lacking the entire kinase insert II and half of kinase insert III, binds to ATP and shows autophosphorylation activity in the absence of ASK. However, ASK is still required for phosphorylation of exogenous substrates by the minimum Cdc7. These results indicate bipartite interaction between Cdc7 and Dbf4/ASK subunits facilitates ATP binding and substrate recognition by the Cdc7 kinase.

Importin beta plays an essential role in the regulation of the LysRS-Ap(4)A pathway in immunologically activated mast cells

We recently reported that diadenosine tetraphosphate hydrolase (Ap(4)A hydrolase) plays a critical role in gene expression via regulation of intracellular Ap(4)A levels. This enzyme serves as a component of our newly described lysyl tRNA synthetase (LysRS)-Ap(4)A biochemical pathway that is triggered upon immunological challenge. Here we explored the mechanism of this enzyme's translocation into the nucleus and found its immunologically dependent association with importin beta. Silencing of importin beta prevented Ap(4)A hydrolase nuclear translocation and affected the local concentration of Ap(4)A, which led to an increase in microphthalmia transcription factor (MITF) transcriptional activity. Furthermore, immunological activation of mast cells resulted in dephosphorylation of Ap(4)A hydrolase, which changed the hydrolytic activity of the enzyme.

Quantitative proteomics reveals a "poised quiescence" cellular state after triggering the DNA replication origin activation checkpoint

An origin activation checkpoint has recently been discovered in the G1 phase of the mitotic cell cycle, which can be triggered by loss of DNA replication initiation factors such as the Cdc7 kinase. Insufficient levels of Cdc7 activate cell cycle arrest in normal cells, whereas cancer cells appear to lack this checkpoint response, do not arrest, and proceed with an abortive S phase, leading to cell death. The differential response between normal and tumor cells at this checkpoint has led to widespread interest in the development of pharmacological Cdc7 inhibitors as novel anticancer agents. We have used RNAi against Cdc7 in combination with SILAC-based high resolution MS proteomics to investigate the cellular mechanisms underlying the maintenance of the origin activation checkpoint in normal human diploid fibroblasts. Bioinformatics analysis identified clear changes in wide-ranging biological processes including altered cellular energetic flux, moderate stress response, reduced proliferative capacity, and a spatially distributed response across the mitochondria, lysosomes, and the cell surface. These results provide a quantitative overview of the processes involved in maintenance of the arrested state, show that this phenotype involves active rather than passive cellular adaptation, and highlight a diverse set of proteins responsible for cell cycle arrest and ultimately for promotion of cellular survival. We propose that the Cdc7-depleted proteome maintains cellular arrest by initiating a dynamic quiescence-like response and that the complexities of this phenotype will have important implications for the continued development of promising Cdc7-targeted cancer therapies.

Proliferating cell nuclear antigen in the cytoplasm interacts with components of glycolysis and cancer

Proliferating cell nuclear antigen (PCNA) is involved in a wide range of functions in the nucleus. However, a substantial amount of PCNA is also present in the cytoplasm, although their function is unknown. Here we show, through Far-Western blotting and mass spectrometry, that PCNA is associated with several cytoplasmic oncoproteins, including elongation factor, malate dehydrogenase, and peptidyl-prolyl isomerase. Surprisingly, PCNA is also associated with six glycolytic enzymes that are involved in the regulation of steps 4-9 in the glycolysis pathway.

Lys-110 is essential for targeting PCNA to replication and repair foci, and the K110A mutant activates apoptosis

BACKGROUND INFORMATION

PCNA (proliferating cell nuclear antigen) is required for a wide range of cellular functions, including DNA replication and damage repair. To be functional, PCNA must associate with the replication and repair foci. In addition, PCNA also mediates targeting of certain replication and repair proteins to these foci. However, the mechanism is not yet known by which PCNA is imported into the nucleus, and then localized to the replication and repair foci.

RESULTS

We have found that an NLS (nuclear localization sequence) is present within the amino acid 101-120 segment of PCNA. An NLS-deleted PCNA was localized in the cytoplasm and showed 5-fold lower affinity for importin-beta than wild-type, suggesting that PCNA may be imported into the nucleus by importin-beta via its NLS. We previously reported that the functional unit of PCNA is a double trimer (as opposed to single homotrimer), and Lys-110 is essential for the formation of the double trimer complex [Naryzhny, Zhao and Lee (2005) J. Biol. Chem. 280, 13888-13894]. The present study shows that the substitution of Lys-110 within the NLS to an alanine residue did not affect its nuclear localization. However, the double-trimer-defective PCNA(K110A) was not localized at replication or repair foci. In contrast, the double-trimer-intact PCNA(K117A) mutant was targeted normally to replication and repair foci. Interestingly, in cells transfected with PCNA(K110A), but not PCNA(K117A), caspase-3-mediated chromosome fragmentation was activated.

CONCLUSIONS

The present study suggests that the regulation of PCNA is intimately connected with that of DNA replication, repair and cell death signals, and raises the possibility that defects in the formation of the PCNA double-trimer complex can cause apoptosis.

Caspase-mediated cleavage of importin-alpha increases its affinity for MCM and downregulates DNA synthesis by interrupting the binding of MCM to chromatin

Importin-alpha is essential for classical nucleocytoplasmic transport of nuclear proteins. Here, we report that importin-alpha is cleaved by caspases during apoptosis, generating importin-alpha lacking an IBB domain. This truncated importin-alpha binds tightly to the MCM replication licensing factor and, thus, prevents its binding to chromatin and downregulates DNA synthesis. Together, our data reveal for the first time that a dying cell effectively salvages limited supplies of cellular energy to ensure an orderly process of its own demise by simultaneously downregulating nucleocytoplasmic protein transport and DNA synthesis. Strikingly, cells can achieve this multi-task process by simply cleaving-off a key nuclear import protein.

Identification and characterization of human cdc7 nuclear retention and export sequences in the context of chromatin binding

The Cdc7 serine/threonine kinase activates the initiation of DNA replication by phosphorylating MCM proteins that are bound to the origins of DNA replication. We reported previously that human Cdc7 nuclear import is mediated directly by importin-beta through its binding to the Cdc7 nuclear localization sequence (NLS). Here, we report that human Cdc7 nuclear localization is regulated by two additional elements: nuclear retention (NRS) and export sequences (NES). Cdc7 proteins imported into the nucleus are retained in the nucleus by associating with chromatin, for which NRS-(306-326) is essential. Importantly, this binding appears to be specific to the origin of DNA replication, because the binding of wild-type Cdc7 to origin is 2.4-fold higher than to non-origin DNA. Furthermore, an NRS-defective Cdc7 mutant could not be retained in the nucleus, although it was imported into the nucleus normally. Together, our data suggest that NRS plays an important role in the activation of DNA replication by Cdc7. The Cdc7 proteins unassociated with chromatin are bound by CRM1 via two NES elements: NES1 at 458-467 within kinase insert III, and NES2 at 545-554 within the kinase IX domain. The primary function of the Cdc7-CRM1 association may be to translocate nuclear Cdc7 to the cytoplasm. However, the binding of CRM1 with Cdc7 at NES2 raises an interesting possibility that CRM1 may also down-regulate Cdc7 by masking its kinase domain.

Nuclear import of human sexual regulator DMRT1 is mediated by importin-beta

Human DMRT1 (Doublesex-Mab3-Related Transcription factor 1) encodes a male-specific transcriptional regulator with a conserved zinc-finger-like DNA-binding domain, so called DM domain, which is similar to male sexual regulatory genes doublesex of Drosophila and mab-3 of Caenorhabditis elegans. As a key transcription factor critical to sex determination and differentiation, however, human DMRT1 nuclear import mechanism remains unknown. We have identified a functional nuclear localization signal (NLS) located between the two intertwined zinc-binding sites of the DM domain. Site-directed mutagenesis indicates that K92 and R93 within the DM domain are critical for DMRT1 nuclear localization. Analysis of deletion mutants shows that importin-beta1 binds directly to DMRT1 via the DM domain, mediating its nuclear import. Co-immunoprecipitation analysis confirms the interaction of mouse Dmrt1 in Sertoli cells with importin-beta1 in vivo. In addition, in vitro docking or nuclear transport assay in digitonin-permeabilized cells shows that DMRT1 is docked at the nuclear pore complex (NPC) or accumulated in the nucleus when importin-beta1, but not importin-alpha1 added. Furthermore, transduction of anti-importin-beta1 antibody into live Sertoli cells effectively inhibits DMRT1 nuclear import. These results suggest that zinc finger domain of DMRT1 functions as a nuclear localization signal and DMRT1 is transported into the nucleus in an importinbeta1-mediated manner. Thus, effective nuclear import of DMRT1 and its interaction with importin-beta1 insure the nuclear retention of the DMRT1 and further exertion of its influence on downstream targets in the cascade of sexual development.