p95vav associates with the nuclear protein Ku-70

Ezh2 harnesses the intranuclear actin cytoskeleton to remodel chromatin in differentiating Th cells

Following their first interaction with the antigen, quiescent naive T-helper (Th; CD4⁺) cells enlarge, differentiate, and proliferate; these processes are accompanied by substantial epigenetic alterations. We showed previously that the epigenetic regulators the polycomb-group (PcG) proteins have a dual function as both positive and negative transcriptional regulators; however, the underlying mechanisms remain poorly understood. Here, we demonstrate that during Th cell differentiation the methyltransferase activity of the PcG protein Ezh2 regulates post-transcriptionally inducible assembly of intranuclear actin filaments. These filaments are colocalized with the actin regulators Vav1 and WASp, vertically oriented to the T cell receptor, and intermingle with the chromatin fibers. Ezh2 and Vav1 are observed together at chromatin-actin intersections. Furthermore, the inducible assembly of nuclear actin filaments is required for chromatin spreading and nuclear growth. Altogether these findings delineate a model in which the epigenetic machinery orchestrates the dynamic mechanical force of the intranuclear cytoskeleton to reorganize chromatin during differentiation.

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Ezh2 regulates post-transcriptionally the inducible assembly of intranuclear F-actin

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F-actin is oriented toward the TCR and intermingled with the chromatin fibers

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F-actin is required for chromatin spreading and nuclear growth

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The epigenetic machinery harnesses intranuclear cytoskeleton to reorganize chromatin

VAV2 is required for DNA repair and implicated in cancer radiotherapy resistance

Radiotherapy remains the mainstay for treatment of various types of human cancer; however, the clinical efficacy is often limited by radioresistance, in which the underlying mechanism is largely unknown. Here, using esophageal squamous cell carcinoma (ESCC) as a model, we demonstrate that guanine nucleotide exchange factor 2 (VAV2), which is overexpressed in most human cancers, plays an important role in primary and secondary radioresistance. We have discovered for the first time that VAV2 is required for the Ku70/Ku80 complex formation and participates in non-homologous end joining repair of DNA damages caused by ionizing radiation. We show that VAV2 overexpression substantially upregulates signal transducer and activator of transcription 1 (STAT1) and the STAT1 inhibitor Fludarabine can significantly promote the sensitivity of radioresistant patient-derived ESCC xenografts in vivo in mice to radiotherapy. These results shed new light on the mechanism of cancer radioresistance, which may be important for improving clinical radiotherapy.

Identification of Ku70 Domain-Specific Interactors Using BioID2

Since its inception, proximity-dependent biotin identification (BioID), an in vivo biochemical screening method to identify proximal protein interactors, has seen extensive developments. Improvements and variants of the original BioID technique are being reported regularly, each expanding upon the existing potential of the original technique. While this is advancing our capabilities to study protein interactions under different contexts, we have yet to explore the full potential of the existing BioID variants already at our disposal. Here, we used BioID2 in an innovative manner to identify and map domain-specific protein interactions for the human Ku70 protein. Four HEK293 cell lines were created, each stably expressing various BioID2-tagged Ku70 segments designed to collectively identify factors that interact with different regions of Ku70. Historically, although many interactions have been mapped to the C-terminus of the Ku70 protein, few have been mapped to the N-terminal von Willebrand A-like domain, a canonical protein-binding domain ideally situated as a site for protein interaction. Using this segmented approach, we were able to identify domain-specific interactors as well as evaluate advantages and drawbacks of the BioID2 technique. Our study identifies several potential new Ku70 interactors and validates RNF113A and Spindly as proteins that contact or co-localize with Ku in a Ku70 vWA domain-specific manner.

SCF(FBXW7)-mediated degradation of p53 promotes cell recovery after UV-induced DNA damage

Eukaryotic cells have developed sophisticated mechanisms to ensure the integrity of the genome and prevent the transmission of altered genetic information to daughter cells. If this control system fails, accumulation of mutations would increase risk of diseases such as cancer. Ubiquitylation, an essential process for protein degradation and signal transduction, is critical for ensuring genome integrity as well as almost all cellular functions. Here, we investigated the role of the SKP1-Cullin-1-F-box protein (SCF)-[F-box and tryptophan-aspartic acid (WD) repeat domain containing 7 (FBXW7)] ubiquitin ligase in cell proliferation by searching for targets implicated in this process. We identified a hitherto-unknown FBXW7-interacting protein, p53, which is phosphorylated by glycogen synthase kinase 3 at serine 33 and then ubiquitylated by SCF(FBXW7) and degraded. This ubiquitylation is carried out in normally growing cells but primarily after DNA damage. Specifically, we found that SCF(FBXW7)-specific targeting of p53 is crucial for the recovery of cell proliferation after UV-induced DNA damage. Furthermore, we observed that amplification of FBXW7 in wild-type p53 tumors reduced the survival of patients with breast cancer. These results provide a rationale for using SCF(FBXW7) inhibitors in the treatment of this subset of tumors.-Galindo-Moreno, M., Giráldez, S., Limón-Mortés, M. C., Belmonte-Fernández, A., Reed, S. I., Sáez, C., Japón, M. Á., Tortolero, M., Romero, F. SCF(FBXW7)-mediated degradation of p53 promotes cell recovery after UV-induced DNA damage.

The interaction between Vav1 and EBNA1 promotes survival of Burkitt's lymphoma cells by down-regulating the expression of Bim

Vav1 is a guanine nucleotide exchange factor (GEF) predominantly expressed in hematopoietic cells, and functions in the development and antigen-stimulated response of lymphocytes. Burkitt's lymphoma (BL) is characterized as transformed B cell lymphoma, and is highly associated with Epstein-Barr virus (EBV). EBV nuclear antigen 1 (EBNA1) is the only viral protein expressed across all three types of latency and essential for the persistence of EBV genome. It is not clear yet how EBNA1 contributes to the growth advantage of latently infected cells such as in EBV⁺ lymphoma B cells. Here, we reported that Vav1 interacts with EBNA1 via its C-terminal SH3 domain. This interaction suppresses the expression of a pro-apoptotic Bcl-2 family member, Bim, resulting in the resistance of the BL cells to apoptotic inductions. Our data uncovered Vav1 as a novel target for EBNA1, and suggested a pro-survival role of Vav1 in the pathogenesis of EBV associated BLs.

Ku80-deleted cells are defective at base excision repair

Ku80 forms a heterodimer with Ku70, called Ku, that repairs DNA double-strand breaks (DSBs) via the nonhomologous end joining (NHEJ) pathway. As a consequence of deleting NHEJ, Ku80-mutant cells are hypersensitive to agents that cause DNA DSBs like ionizing radiation. Here we show that Ku80 deletion also decreased resistance to ROS and alkylating agents that typically cause base lesions and single-strand breaks (SSBs). This is unusual since base excision repair (BER), not NHEJ, typically repairs these types of lesions. However, we show that deletion of another NHEJ protein, DNA ligase IV (Lig4), did not cause hypersensitivity to these agents. In addition, the ROS and alkylating agents did not induce γ-H2AX foci that are diagnostic of DSBs. Furthermore, deletion of Ku80, but not Lig4 or Ku70, reduced BER capacity. Ku80 deletion also impaired BER at the initial lesion recognition/strand scission step; thus, involvement of a DSB is unlikely. Therefore, our data suggests that Ku80 deletion impairs BER via a mechanism that does not repair DSBs.

Vav1 regulates MHCII expression in murine resting and activated B cells

Vav1 is a guanine nucleotide exchange factor (GEF) for Rho GTPases, which is exclusively expressed in cells of the hematopoietic system. In addition to its well-documented GEF activity, it was suggested to have other functions due to the presence of multiple domains and nuclear localization signals in its protein structure. Although GEF-dependent and GEF-independent functions of vav have been implicated in T-cell development and T-cell receptor signaling, the role of vav1 in antigen-presenting cells is poorly understood. We found that vav1 is an important regulator of MHCII expression and transport. Microarray analysis of unstimulated bone marrow-derived macrophages revealed a novel role of vav1 in transcriptional regulation of the MHCII locus, possibly by indirect means. Primary immune cells from vav1-deficient mice had a significantly lower constitutive surface expression of MHCII with the strongest impact observed on splenic and peritoneal B cells. Impaired MHCII expression resulted in a diminished capacity for T-cell activation. Using 6-thio-GTP, a specific inhibitor of the GEF function of vav1, we were able to show that the GEF activity is required for MHCII upregulation in B cells after stimulation with LPS. Furthermore, our data show that vav1 not only affects transcription of the MHCII locus but also is an important regulator of MHCII protein transport to the cell surface.

Mechanism and function of Vav1 localisation in TCR signalling

The antigen-specific binding of T cells to antigen presenting cells results in recruitment of signalling proteins to microclusters at the cell-cell interface known as the immunological synapse (IS). The Vav1 guanine nucleotide exchange factor plays a critical role in T cell antigen receptor (TCR) signalling, leading to the activation of multiple pathways. We now show that it is recruited to microclusters and to the IS in primary CD4⁺ and CD8⁺ T cells. Furthermore, we show that this recruitment depends on the SH2 and C-terminal SH3 (SH3_B) domains of Vav1, and on phosphotyrosines 112 and 128 of the SLP76 adaptor protein. Biophysical measurements show that Vav1 binds directly to these residues on SLP76 and that efficient binding depends on the SH2 and SH3_B domains of Vav1. Finally, we show that the same two domains are critical for the phosphorylation of Vav1 and its signalling function in TCR-induced calcium flux. We propose that Vav1 is recruited to the IS by binding to SLP76 and that this interaction is critical for the transduction of signals leading to calcium flux.

Vav1 in differentiation of tumoral promyelocytes

The multidomain protein Vav1, in addition to promote the acquisition of maturation related properties by normal hematopoietic cells, is a key player in the ATRA- and PMA-induced completion of the differentiation program of tumoral myeloid precursors derived from APL. This review is focussed on the role of Vav1 in differentiating promyelocytes, as part of interconnected networks of functionally related proteins ended to regulate different aspects of myeloid maturation. The role of Vav1 in determining actin cytoskeleton reorganization alternative to the best known function as a GEF for small G proteins is discussed, as well as the binding of Vav1 with cytoplasmic and nuclear signaling molecules which provides a new perspective in the modulation of nuclear architecture and activity. In particular, new hints are provided on the ability of Vav1 to determine the nuclear amount of proteins implicated in modulating mRNA production and stability and in regulating the ATRA-dependent protein expression also by direct interaction with transcription factors known to drive the ATRA-induced maturation of myeloid cells. The reviewed findings summarize the major advances in the understanding of additional, non conventional functions connected with the vast interactive potential of Vav1.

The distinct role of guanine nucleotide exchange factor Vav1 in Bcl-2 transcription and apoptosis inhibition in Jurkat leukemia T cells

AIM

To investigate a novel function of proto-oncogene Vav1 in the apoptosis of human leukemia Jurkat cells.

METHODS

Jurkat cells, Jurkat-derived vav1-null cells (J.Vav1) and Vav1-reconstituted J.WT cells were treated with a Fas agonist antibody, IgM clone CH11. Apoptosis was determined using propidium iodide (PI) staining, Annexin-V staining, DNA fragmentation, cleavage of caspase 3/caspase 8, and poly (ADP-ribose) polymerase (PARP). Mitochondria transmembrane potential (ΔΨ(m)) was measured using DiOC(6)(3) staining. Transcription and expression of the Bcl-2 family of proteins were evaluated using semi-quantitative RT-PCR and Western blot, respectively. Bcl-2 promoter activity was analyzed using luciferase reporter assays.

RESULTS

Cells lacking Vav1 were more sensitive to Fas-mediated apoptosis than Jurkat and J.WT cells. J.Vav1 cells lost mitochondria transmembrane potential (ΔΨ(m)) more rapidly upon Fas induction. These phenotypes could be rescued by re-expression of Vav1 in J.Vav1 cells. The expression of Vav1 increased the transcription of pro-survival Bcl-2. The guanine nucleotide exchange activity of Vav1 was required for enhancing Bcl-2 promoter activity, and the Vav1 downstream substrate, small GTPase Rac2, was likely involved in the control of Bcl-2 expression.

CONCLUSION

Vav1 protects Jurkat cells from Fas-mediated apoptosis by promoting Bcl-2 transcription through its GEF activity.

Deletion of Ku70, Ku80, or both causes early aging without substantially increased cancer

Ku70 forms a heterodimer with Ku80, called Ku, that is critical for repairing DNA double-stand breaks by nonhomologous end joining and for maintaining telomeres. Mice with either gene mutated exhibit similar phenotypes that include increased sensitivity to ionizing radiation and severe combined immunodeficiency. However, there are also differences in the reported phenotypes. For example, only Ku70 mutants are reported to exhibit a high incidence of thymic lymphomas while only Ku80 mutants are reported to exhibit early aging with very low cancer levels. There are two explanations for these differences. First, either Ku70 or Ku80 functions outside the Ku heterodimer such that deletion of one is not identical to deletion of the other. Second, divergent genetic backgrounds or environments influence the phenotype. To distinguish between these possibilities, the Ku70 and Ku80 mutations were crossed together to generate Ku70, Ku80, and double-mutant mice in the same genetic background raised in the same environment. We show that these three cohorts have similar phenotypes that most resemble the previous report for Ku80 mutant mice, i.e., early aging without substantially increased cancer levels. Thus, our observations suggest that the Ku heterodimer is important for longevity assurance in mice since divergent genetic backgrounds and/or environments likely account for these previously reported differences.

The association of Sam68 with Vav1 contributes to tumorigenesis

Vav1 functions in the hematopoietic system as a specific GDP/GTP nucleotide exchange factor regulated by tyrosine phosphorylation. An intact C-terminal SH3 domain of Vav1 (Vav1SH3C) was shown to be necessary for Vav1-induced transformation, yet the associating protein(s) necessary for this activity have not yet been identified. Using a proteomics approach, we identified Sam68 as a Vav1SH3C-associating protein. Sam68 (Src-associated in mitosis of 68 kD) belongs to the heteronuclear ribonucleoprotein particle K (hnRNP-K) homology (KH) domain family of RNA-binding proteins. The Vav1/Sam68 interaction was observed in vitro and in vivo. Mutants of Vav1SH3C previously shown to lose their transforming potential did not associate with Sam68. Co-expression of Vav1 and Sam68 in Jurkat T cells led to increased localization of Vav1 in the nucleus and changes in cell morphology. We then tested the contribution of Sam68 to known functions of Vav1, such as focus-forming in NIH3T3 fibroblasts and NFAT stimulation in T cells. Co-expression of oncogenic Vav1 with Sam68 in NIH3T3 fibroblasts resulted in a dose-dependent increase in foci, yet no further enhancement of NFAT activity was observed in Jurkat T cells, as compared to cells overexpressing only Vav1 or Sam68. Our results strongly suggest that Sam68 contributes to transformation by oncogenic Vav1.

Flesh and blood: The story of Vav1, a gene that signals in hematopoietic cells but can be transforming in human malignancies

Cancer results from the interaction of multiple aberrations including activation of dominant oncogenes and upregulation of signal transduction pathways. Identification of the genes involved in malignant transformation is a pre-requisite for understanding cancer and improving its diagnosis and treatment. Quite a few of the genes that have been implicated in cancer are mutant or aberrantly expressed versions of genes that are important mediators of the normal growth that occurs during development. An important example of this is Vav1, a cytoplasmic signal transducer protein initially identified as an oncogene. Physiological expression of Vav1 is restricted to the hematopoietic system, where its best-known function is as a GDP/GTP nucleotide exchange factor for Rho/Rac GTPases, an activity strictly controlled by tyrosine phosphorylation. Vav1 was shown to regulate cytoskeletal rearrangement during activation of hematopoietic cells. Vav1 can also mediate other cellular functions including activation of the JNK, ERK, Ras, NF-kB, and NFAT pathways, in addition to association with numerous adapter proteins such as Shc, NCK, SLP-76, GRB2, and Crk. Although the oncogenic form of Vav1 has not been detected in clinical human tumors, its wild-type form has recently been implicated in mammalian malignancies such as neuroblastoma, melanoma, pancreatic tumors and B-cell chronic lymphocytic leukemia. This review addresses the physiological function of wild-type Vav1, its mode of activation as an oncogene, and its emerging role as a transforming protein in human cancer.

An active form of Vav1 induces migration of mammary epithelial cells by stimulating secretion of an epidermal growth factor receptor ligand

Background

Vav proteins are guanine nucleotide exchange factors (GEF) for Rho family GTPases and are activated following engagement of membrane receptors. Overexpression of Vav proteins enhances lamellipodium and ruffle formation, migration, and cell spreading, and augments activation of many downstream signaling proteins like Rac, ERK and Akt. Vav proteins are composed of multiple structural domains that mediate their GEF function and binding interactions with many cellular proteins. In this report we examine the mechanisms responsible for stimulation of cell migration by an activated variant of Vav1 and identify the domains of Vav1 required for this activity.

Results

We found that expression of an active form of Vav1, Vav1Y3F, in MCF-10A mammary epithelial cells increases cell migration in the absence or presence of EGF. Vav1Y3F was also able to drive Rac1 activation and PAK and ERK phosphorylation in MCF-10A cells in the absence of EGF stimulation. Mutations in the Dbl homology, pleckstrin homology, or cysteine-rich domains of Vav1Y3F abolished Rac1 or ERK activation in the absence of EGF and blocked the migration-promoting activity of Vav1Y3F. In contrast, mutations in the SH2 and C-SH3 domains did not affect Rac activation by Vav1Y3F, but reduced the ability of Vav1Y3F to induce EGF-independent migration and constitutive ERK phosphorylation. EGF-independent migration of MCF-10A cells expressing Vav1Y3F was abolished by treatment of cells with an antibody that prevents ligand binding to the EGF receptor. In addition, conditioned media collected from Vav1Y3F expressing cells stimulated migration of parental MCF-10A cells. Lastly, treatment of cells with the EGF receptor inhibitory antibody blocked the Vav1Y3F-induced, EGF-independent stimulation of ERK phosphorylation, but had no effect on Rac1 activation or PAK phosphorylation.

Conclusion

Our results indicate that increased migration of active Vav1 expressing cells is dependent on Vav1 GEF activity and secretion of an EGF receptor ligand. In addition, activation of ERK downstream of Vav1 is dependent on autocrine EGF receptor stimulation while active Vav1 can stimulate Rac1 and PAK activation independent of ligand binding to the EGF receptor. Thus, stimulation of migration by activated Vav1 involves both EGF receptor-dependent and independent activities induced through the Rho GEF domain of Vav1.

A proteome-wide CDK/CRK-specific kinase inhibitor promotes tumor cell death in the absence of cell cycle progression

The identification of molecular determinants of tumor cell survival is an important objective in cancer research. Here, we describe a small-molecule kinase inhibitor (RGB-286147), which, besides inhibiting tumor cell cycle progression, exhibits potent cytotoxic activity toward noncycling tumor cells, but not nontransformed quiescent fibroblasts. Extensive yeast three-hybrid (Y3H)-based proteome/kinome scanning with chemical dimerizers revealed CDK1/2/3/5/7/9 and the less well-characterized CDK-related kinases (CRKs) p42/CCRK, PCTK1/3, and PFTK1 as its predominant targets. Thus, RGB-286147 is a proteome-wide CDK/CRK-specific kinase inhibitor whose further study could yield new insight into molecular determinants of tumor cell survival. Our results also suggest that the [1, 3, 6]-tri-substituted-pyrazolo[3,4-d]-pyrimidine-4-one kinase inhibitor scaffold is a promising template for the rational design of kinase inhibitors with potential applications to disease indications other than cancer, such as neurodegeneration, cardiac hypertrophic growth, and AIDS.

The development and applications of nonradioactive plate-formatted DNA-binding assay for Ku70/80, a multifunctional DNA-binding protein complex

Ku is a heterodimer composed of p70 and p80, and is the regulatory subunit of DNA-dependent protein kinase. As a multifunctional DNA-binding protein complex, Ku plays important roles in DNA damage repair through non-homologous end joining and in V(D)J recombination. In addition, Ku has also been implicated in various biological functions including growth control, cell proliferation, cell cycle, chromosome maintenance, transcriptional regulation, apoptosis, and viral infection. In particular, using our Inverse Genomics (Immusol, Inc., San Diego, CA) platform technology, we recently identified Ku80 as an essential co-factor for human immunodeficiency virus replication. Although Ku has been studied extensively in the past years, its in-depth study as well as development as a drug target has been limited by conventional DNA-binding activity assay. Here we describe the development and applications of a nonradioactive DNA binding assay in the 96-well format. We show that this plate-formatted assay is more sensitive and allows for direct quantification when compared with an electrophoretic mobility shift assay. The establishment of this assay will not only facilitate structure and function studies on Ku, but also help the development of Ku protein or its DNA repair enzyme complex as a drug target.

Characterization of VIK-1: a new Vav-interacting Kruppel-like protein

Binding partners of the Src homology domains of Vav-1 were characterized by a two-hybrid screening of a Jurkat cell cDNA library. One of the isolated clones encoded a new protein named VIK that belongs to the Kruppel-like zinc-finger gene family. Genome mapping showed that a single gene positioned at chromosome 7q22.1 generated three possible isoforms containing alternative domains such as proline-rich and Kruppel-associated box A or B repressor domains. The isolated isoform, VIK-1, did not contain such motifs but presented six tandemly arranged zinc-fingers and consensus Kruppel H-C links. VIK-1 interacted both with Vav-1 and cyclin-dependent kinase 4 through two independent domains and corresponded to a Vav C-Src homology domain (SH)3 partner able to shuttle between the nucleus and the cytoplasm exhibiting functional nuclear addressing and export sequences. The results indicated a restricted expression of the protein during the G1 phase and its overexpression resulted in an inhibition of the cell-cycle progression that was reversed in the presence of Vav 1. Thus, this ubiquitous factor provides a first link between Vav-1 and the cell-cycle machinery.

Vav1: an oncogene that regulates specific transcriptional activation of T cells

The nuclear factor of activated T cells (NFAT) proteins are a family of transcription factors whose activation is controlled by calcineurin, a Ca2+-dependent phosphatase. Once dephosphorylated, these proteins move to the nucleus where they interact with cofactors to form transcription factor complexes. Inhibition of NFAT proteins by immunosuppressants, such as cyclosporin A (CsA) and FK506, is used clinically to prevent transplant rejection. Although these drugs have revolutionized organ transplantation, their use is associated with severe side effects in other organs in which NFAT proteins are important. One of the signal transducers that controls NFAT activity is Vav1, which is exclusively expressed in the hematopoietic system. Vav1 contains numerous modular domains that enable its function as a guanine exchange factor (GEF) toward RhoGTPases as well as participate in protein-protein interactions. This review focuses on the mechanisms by which Vav1 regulates NFAT through GEF-dependent and -independent cascades, emphasizing the newly assigned role of Vav1 in the regulation of Ca2+ release. Because of its restriction to hematopoietic cell lineages and its importance in the regulation of NFAT, targeting Vav1 and, in particular, its association with other proteins may offer a highly selective means of modifying T-cell behavior, thus allowing the development of more specific immunosuppressive therapies.

Association of PI 3-K with tyrosine phosphorylated Vav is essential for its activity in neutrophil-like maturation of myeloid cells

The importance of the Vav family of signal transduction molecules in hematopoietic cells has long been acknowledged, even though its role and its regulatory mechanism are not completely understood. We have previously demonstrated that tyrosine-phosphorylated Vav, also located inside the nucleus of myeloid cells, is up-regulated during maturation of promyelocytic precursors induced by all-trans-retinoic acid (ATRA). Here, we report that the tyrosine phosphorylation of Vav during granulocytic maturation is dependent on the tyrosine kinase Syk and is essential for the morphological changes of the cell nucleus. These ATRA-induced events are independent on the guanine nucleotide exchange activity of Vav. We also found that, in differentiating cells, and in both cytoplasmic and nuclear compartments, tyrosine phosphorylated Vav associates with the regulatory subunit of phosphoinositide 3-kinase (PI 3-K). The Vav/p85 interaction is essential for the ATRA-induced PI 3-K activity and for association of PI 3-K with actin, particularly in the nucleus. Our data indicate an unprecedented crucial function for Vav in modulating the morphological maturation process of myeloid cells in a GDP-GTP exchange factor (GEF)-independent manner and suggest a role of Vav as an adaptor protein responsible of targeting PI 3-K to its intranuclear substrates.

A Three-Hybrid Approach to Scanning the Proteome for Targets of Small Molecule Kinase Inhibitors

In this study, we explored the application of a yeast three-hybrid (Y3H)-based compound/protein display system to scanning the proteome for targets of kinase inhibitors. Various known cyclin-dependent kinase (CDK) inhibitors, including purine and indenopyrazole analogs, were displayed in the form of methotrexate-based hybrid ligands and deployed in cDNA library or yeast cell array-based screening formats. For all inhibitors, known cell cycle CDKs as well as novel candidate CDK-like and/or CDK-unrelated kinase targets could be identified, many of which were independently confirmed using secondary enzyme assays and affinity chromatography. The Y3H system described here may prove generally useful in the discovery of candidate drug targets.

HIV-1 Nef triggers Vav-mediated signaling pathway leading to functional and morphological differentiation of dendritic cells

The accessory HIV-1 Nef protein plays a key role in AIDS pathogenesis. We recently demonstrated that exogenous Nef triggers phenotypic and functional differentiation of immature dendritic cells (DCs). Here we investigated whether the Nef-induced DC differentiation occurs with morphological remodeling and have focused on the interference of Nef in the signaling pathways that regulates DC maturation. We found that exogenous Nef enters immature DCs, promoting their functional and morphological differentiation. Specifically, Nef promotes interleukin (IL) -12 release, which closely fits with nuclear factor (NF) -kappaB activation. Nef induces rearrangement of actin microfilaments, leading to uropod and ruffle formation. Moreover, Nef increases the capacity of DCs to form clusters with allogeneic CD4+ T cells, improving immunological synapse formation. Searching for molecules involved in Nef-triggered signaling pathways driving the DC maturation, we found that Nef targets Vav and promotes its tyrosine phosphorylation, associated with its nucleus-to-cytoplasm redistribution. This has a direct effect on Vav guanine nucleotide exchange factor activity for the small GTPase Rac1. We hypothesize that targeting Vav, Nef modulates both early signaling events (such as cytoskeletal rearrangement) and delayed responses (such as transcriptional regulation), promoting DC differentiation. Our results highlight how Nef may enhance T lymphocyte activation, thus fostering virus dissemination, manipulating the DC arm of the immune response.

Vav1: a key signal transducer downstream of the TCR

Vav1 is a 95-kDa protein expressed in all hemopoietic cells that becomes rapidly tyrosine phosphorylated following T cell antigen receptor (TCR) stimulation. Vav1 contains multiple domains characteristic of signal transducing proteins, including a Dbl homology domain, a hallmark of a guanine nucleotide exchange factor (GEF) for Rho-family GTPases. Indeed Vav1 is a GEF for Rac1, Rac2 and RhoG, and it is activated following tyrosine phosphorylation. Generation of mice deficient in Vav1 has shown that it plays an important role in selection events within the thymus, including both positive and negative selection, consistent with Vav1 transducing TCR signals required to drive these processes. Furthermore, Vav1-deficient T cells are defective in TCR-induced proliferation and cytokine synthesis. Analysis of TCR signaling pathways in Vav1-deficient T cells and thymocytes has shown that Vav1 is required to transduce signals to the activation of a calcium flux, extracellular signal-regulated kinase (ERK) and the nuclear factor kappaB (NF-kappaB) transcription factor. Vav1 has also been shown to control the activation of phospholipase Cgamma1 (PLCgamma1) via both phosphoinositide-3-kinase (PI3K)-dependent and -independent pathways. Finally, Vav1 has been shown to transduce TCR signals to some but not all cytoskeleton-dependent pathways. In particular, Vav1 is required for efficient TCR-induced conjugate formation with antigen presenting cells (APCs), activation of the integrin leukocyte function-associated antigen-1 (LFA-1) and cell polarization.

Vav1 is a component of transcriptionally active complexes

The importance of the hematopoietic protooncogene Vav1 in immune cell function is widely recognized, although its regulatory mechanisms are not completely understood. Here, we examined whether Vav1 has a nuclear function, as past studies have reported its nuclear localization. Our findings provide a definitive demonstration of Vav1 nuclear localization in a receptor stimulation-dependent manner and reveal a critical role for the COOH-terminal Src homology 3 (SH3) domain and a nuclear localization sequence within the pleckstrin homology domain. Analysis of DNA-bound transcription factor complexes revealed nuclear Vav1 as an integral component of transcriptionally active nuclear factor of activated T cells (NFAT)- and nuclear factor (NF)kappaB-like complexes, and the COOH-terminal SH3 domain as being critical in their formation. Thus, we describe a novel nuclear role for Vav1 as a component and facilitator of NFAT and NFkappaB-like transcriptional activity.

Solution structure of N-terminal SH3 domain of Vav and the recognition site for Grb2 C-terminal SH3 domain

The three-dimensional structure of the N-terminal SH3 domain (residues 583-660) of murine Vav, which contains a tetra-proline sequence (Pro 607-Pro 610), was determined by NMR. The solution structure of the SH3 domain shows a typical SH3 fold, but it exists in two conformations due to cis-trans isomerization at the Gly614-Pro615 bond. The NMR structure of the P615G mutant, where Pro615 is replaced by glycine, reveals that the tetra-proline region is inserted into the RT-loop and binds to its own SH3 structure. The C-terminal SH3 domain of Grb2 specifically binds to the trans form of the N-terminal SH3 domain of Vav. The surface of Vav N-terminal SH3 which binds to Grb2 C-terminal SH3 was elucidated by chemical shift mapping experiments using NMR. The surface does not involve the tetra-proline region but involves the region comprising the n-src loop, the N-terminal and the C-terminal regions. This surface is located opposite to the tetra-proline containing region, consistent with that of our previous mutagenesis studies.

Hyaluronan promotes CD44v3-Vav2 interaction with Grb2-p185(HER2) and induces Rac1 and Ras signaling during ovarian tumor cell migration and growth

In this study we initially examined the interaction between CD44v3 (a hyaluronan (HA) receptor) and Vav2 (a guanine nucleotide exchange factor) in human ovarian tumor cells (SK-OV-3.ipl cell line). Immunological data indicate that both CD44v3 and Vav2 are expressed in SK-OV-3.ipl cells and that these two proteins are physically linked as a complex in vivo. By using recombinant fragments of Vav2 and in vitro binding assays, we have detected a specific binding interaction between the SH3-SH2-SH3 domain of Vav2 and the cytoplasmic domain of CD44. In addition, we have observed that the binding of HA to CD44v3 activates Vav2-mediated Rac1 signaling leading to ovarian tumor cell migration. Further analyses indicate that the adaptor molecule, growth factor receptor-bound protein 2 (Grb2) that is bound to p185(HER2) (an oncogene product), is also associated with the CD44v3-Vav2 complex. HA binding to SK-OV-3.ipl cells promotes recruitment of both Grb2 and p185(HER2) to the CD44v3-Vav2 complex leading to Ras activation and ovarian tumor cell growth. In order to determine the role of Grb2 in CD44v3 signaling, we have transfected SK-OV-3.ipl cells with Grb2 mutant cDNAs (e.g. Delta N-Grb2 that has a deletion in the amino-terminal SH3 domain or Delta C-Grb2 that has a deletion in the carboxyl-terminal SH3 domain). Our results clearly indicate that the SH3 domain deletion mutants of Grb2 (i.e. the Delta N-Grb2 (and to a lesser extent the Delta C-Grb2) mutant) not only block their association with p185(HER2) but also significantly impair their binding to the CD44v3-Vav2 complex and inhibit HA/CD44v3-induced ovarian tumor cell behaviors. Taken together, these findings strongly suggest that the interaction of CD44v3-Vav2 with Grb2-p185(HER2) plays an important role in the co-activation of both Rac1 and Ras signaling that is required for HA-mediated human ovarian tumor progression.

Inhibition of Ku heterodimer DNA end binding activity during granulocytic differentiation of human promyelocytic cell lines

The heterodimeric Ku protein (composed of the Ku 86 and Ku 70 sub-units) is a nuclear protein which binds to DNA termini without sequence specificity. Ku is the DNA-targeting component of the large catalytic sub-unit of the DNA-dependent protein kinase complex that is required for the repair of DNA double-strand breaks in mammalian cells. We studied the expression and function of Ku/DNA-PK during granulocytic differentiation of two human promyelocytic cell lines, HL60 and NB4, a process associated to decreased radiation resistance. After 3 days exposure to differentiating agents (either all-trans-retinoic acid or DMSO), Ku binding to double stranded (ds)-DNA ends declined dramatically whereas Ku protein levels remain unchanged. The nuclear, but not cytoplasmic, fraction of differentiated HL60 cells extracts exhibited a heat-sensitive inhibitory activity towards DNA binding of recombinant Ku heterodimer. We further demonstrate that immunoprecipitation of Ku is impaired in extracts from differentiated cells by using two antibodies that recognize epitopes within the C-terminus DNA binding domains of Ku 70 and Ku 86 proteins. These results favor the hypothesis of a protein interacting with Ku that would prevent DNA binding of heterodimerized Ku protein by steric hindrance.

The binding of Ku antigen to homeodomain proteins promotes their phosphorylation by DNA-dependent protein kinase

The Ku antigen (70- and 80-kDa subunits) is a regulatory subunit of DNA-dependent protein kinase (DNA-PK) that promotes the recruitment of the catalytic subunit of DNA-PK (DNA-PKcs) to DNA ends and to specific DNA sequences from which the kinase is activated. Ku and DNA-PKcs plays essential roles in double-stranded DNA break repair and V(D)J recombination and have been implicated in the regulation of specific gene transcription. In a yeast two-hybrid screen of a Jurkat T cell cDNA library, we have identified a specific interaction between the 70-kDa subunit of Ku heterodimer and the homeodomain of HOXC4, a homeodomain protein expressed in the hematopoietic system. Unexpectedly, a similar interaction with Ku was observed for several additional homeodomain proteins including octamer transcription factors 1 and 2 and Dlx2, suggesting that specific binding to Ku may be a property shared by many homeodomain proteins. Ku-homeodomain binding was mediated through the extreme C terminus of Ku70 and was abrogated by amino acid substitutions at Lys595/Lys596. Ku binding allowed the recruitment of the homeodomain to DNA ends and dramatically enhanced the phosphorylation of homeodomain-containing proteins by DNA-PK. These results suggest that Ku functions as a substrate docking protein for signaling by DNA-PK to homeodomain proteins from DNA ends.

Human Ku70 interacts with heterochromatin protein 1alpha

Ku is involved in the metabolism of DNA ends, DNA repair, and the maintenance of telomeres. It consists of a heterodimer of 70- and 80-kDa subunits. Recently we have demonstrated that Ku70 interacted with TRF2, a mammalian telomere-binding protein. Using the same yeast two-hybrid screening system, we now show that Ku70 also interacts with heterochromatin protein 1alpha (HP1alpha), a protein known to be associated with telomeres as well as heterochromatin. HP1 is a suppressor of the position effect variegation in Drosophila and acts as a transcriptional suppressor in mammalian cells. The interaction with Ku70 in the two-hybrid system was confirmed by a glutathione S-transferase pull-down study using bacterial recombinant proteins in vitro. The interaction was also reproduced in vivo in HeLa cells, where endogenous Ku70 coimmunoprecipitated with HP1alpha. This interaction was more effective in acidic pH and weakened considerably as the pH of the reaction buffer was elevated up to 7.5. Ku80 did not interact with HP1alpha directly. The interaction domains of Ku70 and HP1alpha included the Leu-Ser repeat (amino acids 200-385) and the chromo shadow domain, respectively. Ku70 was largely colocalized with transfected HP1alpha but not with a C-terminal deletion mutant, HP1alpha(Delta)C. In contrast to HP1alpha, Ku70 did not repress transcriptional activity of the reporter gene when tethered to DNA after transfection to mammalian cells. The implication of this interaction is discussed.

Human securin, hPTTG, is associated with Ku heterodimer, the regulatory subunit of the DNA-dependent protein kinase

We have previously isolated the hpttg proto-oncogene, which is expressed in normal tissues containing proliferating cells and in several kinds of tumors. In fact, expression of hPTTG correlates with cell proliferation in a cell cycle-dependent manner. Recently it was reported that PTTG is a vertebrate analog of the yeast securins Pds1 and Cut2, which are involved in sister chromatid separation. Here we show that hPTTG binds to Ku, the regulatory subunit of the DNA-dependent protein kinase (DNA-PK). hPTTG and Ku associate both in vitro and in vivo and the DNA-PK catalytic subunit phosphorylates hPTTG in vitro. Furthermore, DNA double-strand breaks prevent hPTTG-Ku association and disrupt the hPTTG-Ku complexes, indicating that genome damaging events, which result in the induction of pathways that activate DNA repair mechanisms and halt cell cycle progression, might inhibit hPTTG-Ku interaction in vivo. We propose that hPTTG might connect DNA damage-response pathways with sister chromatid separation, delaying the onset of mitosis while DNA repair occurs.

Potentiation of chemosensitivity in multidrug-resistant human leukemia CEM cells by inhibition of DNA-dependent protein kinase using wortmannin

DNA-dependent protein kinase (DNA-PK) is activated by DNA strand breaks and participates in DNA repair. Its regulatory subunit, Ku autoantigen, binds to DNA and recruits the catalytic subunit (DNA-PKcs). We show here a new role of DNA-PK in the development of multidrug resistance (MDR). The Ku-DNA binding activity, the levels of Ku70/Ku80 and DNA-PKcs in MDR variants, CEM/VLB(10-2), CEM/VLB(55-8) and CEM/VLB100 were higher than those in their parental drug-sensitive CEM cells in a drug resistance-dependent fashion. Also, CEM/VLB100 cells showed about 3-fold increase of DNA-PK enzyme activity as compared with CEM cells. Similar results were observed in another MDR cell line, FM3A/M mouse mammary carcinoma cells. Moreover, we observed that CEM/VLB100 cells were about 11-fold sensitive to wortmannin, which inhibits DNA-PK, compared with the CEM cells, and sensitized the MDR cells when combined with either bleomycin or vincristine, but have a little effect on CEM cells. Wortmannin was shown to inhibit DNA-PK and Ku-DNA binding activity in CEM/VLB100 cells dose dependently but had a little or no effect on their parental cells. Our results suggested that enhanced expression of DNA-PK participates in the development of MDR, and the use of DNA-PK inhibitors such as wortmannin is likely to improve the effectiveness of anticancer drugs and thus could partially overcome drug resistance in MDR cells, through its ability to inhibit Ku/DNA-PK activity.

Interaction of human Ku70 with TRF2

Ku, a heterodimer of 70- and 80-kDa subunits, plays a general role in the metabolism of DNA ends in eukaryotic cells, including double-strand DNA break repair, V(D)J recombination, and maintenance of telomeres. We have utilized the yeast two-hybrid system to identify Ku70-interacting proteins other than Ku80. Two reactive clones were found to encode the dimerization domain of TRF2, a mammalian telomeric protein that binds to duplex TTAGGG repeats at chromosome ends. This interaction was confirmed using bacterial fusion proteins and co-immunoprecipitations from eukaryotic cells overexpressing TRF2. The transfected TFR2 colocalized with Ku70.

Ku autoantigen: a multifunctional DNA-binding protein

Ku is a heterodimeric protein composed of approximately 70- and approximately 80-kDa subunits (Ku70 and Ku80) originally identified as an autoantigen recognized by the sera of patients with autoimmune diseases. Ku has high binding affinity for DNA ends and that is why originally it was known as a DNA end binding protein, but now it is known to also bind the DNA structure at nicks, gaps, hairpins, as well as the ends of telomeres. It has been reported also to bind with sequence specificity to DNA and with weak affinity to RNA. Ku is an abundant nuclear protein and is present in vertebrates, insects, yeast, and worms. Ku contains ssDNA-dependent ATPase and ATP-dependent DNA helicase activities. It is the regulatory subunit of the DNA-dependent protein kinase that phosphorylates many proteins, including SV-40 large T antigen, p53, RNA-polymerase II, RP-A, topoisomerases, hsp90, and many transcription factors such as c-Jun, c-Fos, oct-1, sp-1, c-Myc, TFIID, and many more. It seems to be a multifunctional protein that has been implicated to be involved directly or indirectly in many important cellular metabolic processes such as DNA double-strand break repair, V(D)J recombination of immunoglobulins and T-cell receptor genes, immunoglobulin isotype switching, DNA replication, transcription regulation, regulation of heat shock-induced responses, regulation of the precise structure of telomeric termini, and it also plays a novel role in G2 and M phases of the cell cycle. The mechanism underlying the regulation of all the diverse functions of Ku is still obscure.

Major transcript variants of VAV3, a new member of the VAV family of guanine nucleotide exchange factors

VAV3 is a new member of the VAV oncogene family with a strong homology to VAV and VAV2. A conceptual translation of the cDNA indicates that VAV3 is between 40 and 77% identical to VAV and VAV2 at the amino acid level in all identified functional motifs. This homology suggests that VAV3 occupies a similar position in signal transduction as the other family members. A major variant transcript, VAV3.1, found in both humans and mice, appears to encode only the 3' SH3-SH2-SH3 region, which suggests that it may substitute for the full-length isoform in functions mediated by this domain, or compete with the full-length isoform in functions mediated by more N-terminal motifs. VAV3.1 either is a partly unspliced mRNA or originates from a different promoter. VAV3 transcripts are found in cells of hematopoietic origin, where VAV is primarily expressed. However, unlike, VAV, the VAV3 and VAV3.1 transcripts are also found at varying levels in a wide variety of other tissues and cell lines. TGF-beta and EGF reversibly down-regulate the abundance of the VAV3. 1 transcript in HaCaT keratinocytes, representing the first observation of transcript regulation of a member of the VAV family by a growth factor.

Interferon-alpha signaling promotes nucleus-to-cytoplasmic redistribution of p95Vav, and formation of a multisubunit complex involving Vav, Ku80, and Tyk2

Interferons (IFNs) are a family of hormone-like secretory proteins with multiple phenotypical changes, including gene expression and morphological alterations. Earlier studies have shown that IFN-activated Tyk2 kinase physical associates with p95Vav (Vav), a proto-oncogene gene product expressed in hematopoietic cells. Since Tyk2 is a cytoplasmic kinase and Vav is believed to be localized in the nuclear compartment, here we explored the possibility of Vav redistribution in IFN-alpha-activated cells, using the U266 human myeloma cell line as a model system. Using biochemical assays and in situ confocal microscopy, we demonstrate that IFN-alpha treatment triggers a rapid (10 min) translocation of Vav from the nuclear compartment to the cytoplasm. In addition, we also show the existence of IFN-alpha-induced physical interaction between Vav and Ku80, Ku80, and Tyk2, and among Vav, Ku80, and Tyk2 in the cytoplasmic compartment of IFN-stimulated cells. The observed IFN-alpha-induced association among Vav, Ku80, and Tyk2 was dependent on cellular tyrosine kinase activity. Since recently Vav has been shown to promote the GDP/GTP exchange activity of the cytoskeleton signaling molecule small GTPase Rac1 and activates its downstream signaling, our present findings raise the possibility of involvement of the small GTPase in IFN signaling leading to its biological effects, including cytoskeleton reorganization.

p95(vav) associates with the type I interferon (IFN) receptor and contributes to the antiproliferative effect of IFN-alpha in megakaryocytic cell lines

The vav proto-oncogene product is a 95 kDa protein predominantly expressed in hematopoietic cells. Vav presents a wide range of functional domains, including structural domains known to be involved in signal transduction. Triggering of various cytokine receptors among which type I interferon receptor induces a rapid and transient tyrosine phosphorylation of p95(vav). Nevertheless, the biological functions of p95(vav) are still unclear. This report is the first documentation on the physical association of p95(vav) with both alpha and beta type I interferon receptor chains, as demonstrated by co-immunoprecipitation and Western blot analysis in megakaryocytic cells (Dami and UT7). This interaction is increased by interferon-alpha/beta stimulation. Moreover, p95(vav) phosphorylated subsequently to type I interferon treatment, is translocated in the nucleus; a concomitant increase of its association with the regulatory subunit of the nuclear DNA-dependent protein kinase, KU-70 is observed in the nucleus. To determine whether p95(vav) participates in the biological response to type I interferons, we studied the effects of non modified Vav oligodeoxynucleotides on the antiproliferative effect of interferon-alpha on megakaryocytic cells. By this oligodeoxynucleotide strategy, we show that p95(vav) contributes greatly to the cell proliferation inhibition induced by type I IFN.

The Src homology 2 domain of Vav is required for its compartmentation to the plasma membrane and activation of c-Jun NH(2)-terminal kinase 1

Vav is a hematopoietic cell-specific guanine nucleotide exchange factor (GEF) whose activation is mediated by receptor engagement. The relationship of Vav localization to its function is presently unclear. We found that Vav redistributes to the plasma membrane in response to Fcin receptor I (FcinRI) engagement. The redistribution of Vav was mediated by its Src homology 2 (SH2) domain and required Syk activity. The FcinRI and Vav were found to colocalize and were recruited to glycosphingolipid-enriched microdomains (GEMs). The scaffold protein, linker for activation of T cells (LAT), and Rac1 (a target of Vav activity) were constitutively present in GEMs. Expression of an SH2 domain-containing COOH-terminal fragment of Vav inhibited Vav phosphorylation and movement to the GEMs but had no effect on the tyrosine phosphorylation of the adaptor protein, SLP-76 (SH2 domain-containing leukocyte protein of 76 kD), and LAT. However, assembly of the multiprotein complex containing these proteins was inhibited. In addition, FcinRI-dependent activation of c-Jun NH(2)-terminal kinase 1 (JNK1) was also inhibited. Thus, Vav localization to the plasma membrane is mediated by its SH2 domain and may serve to regulate downstream effectors like JNK1.

Hypermethylation of metallothionein-I promoter and suppression of its induction in cell lines overexpressing the large subunit of Ku protein

We have shown previously that the heavy metal-induced metallothionein-I (MT-I) gene expression is specifically repressed in a rat fibroblast cell line (Ku-80) overexpressing the 80-kDa subunit of Ku autoantigen but not in cell lines overexpressing the 70-kDa subunit or Ku heterodimer. Here, we explored the molecular mechanism of silencing of MT-I gene in Ku-80 cells. Genomic footprinting analysis revealed both basal and heavy metal-inducible binding at specific cis elements in the parental cell line (Rat-1). By contrast, MT-I promoter in Ku-80 cells was refractory to any transactivating factors, implying alteration of chromatin structure. Treatment of two clonal lines of Ku-80 cells with 5-azacytidine, a potent DNA demethylating agent, rendered MT-I gene inducible by heavy metals, suggesting that the gene is methylated in these cells. Bisulfite genomic sequencing revealed that all 21 CpG dinucleotides in MT-I immediate promoter were methylated in Ku-80 cells, whereas only four CpG dinucleotides were methylated in Rat-1 cells. Almost all methylated CpG dinucleotides were demethylated in Ku-80 cells after 5-azacytidine treatment. To our knowledge, this is the first report that describes hypermethylation of a specific gene promoter and its resultant silencing in response to overexpression of a cellular protein.

N-terminally truncated Vav induces the formation of depolymerization-resistant actin filaments in NIH 3T3 cells

The Dbl family proto-oncogene vav is a guanine nucleotide exchange factor (GEF) for Rho family GTPases. Deletion of the N-terminus of Vav, harboring the single calponin homology (CH) domain, activates Vav's transforming potential, suggesting an important role of the CH domain in influencing Vav function. Since calponin binds actin, it has been suggested that the CH domain may mediate association with the actin cytoskeleton. In this study we have analyzed the subcellular localization and investigated the putative actin association of the Vav protein using enhanced green fluorescent protein (EGFP) fusion constructs. Our data show that both EGFP-tagged full length Vav and the CH domain-depleted EGFPvav 143-845 construct localize throughout the cytoplasm but fail to colocalize with F-actin. However, the latter construct of Vav was more strongly retained in the Triton-insoluble cytoskeleton fraction than full length Vav. Whereas removal of the CH domain had no apparent influence on the subcellular localization of Vav, deletion of the SH domains caused nuclear localization, indicating that Vav contains a functional nuclear localization signal. Expression of N-terminally truncated Vav constructs caused depolarization of fibroblasts and triggered the bundling of actin stress fibers into parallel arrays in NIH 3T3 cells. Notably, the parallel actin bundles showed prolonged resistance to the actin polymerization antagonists cytochalasin B and latrunculin B. These data point towards a regulatory role for the CH domain in Vav and suggest an actin cross-linking or bundling protein as a downstream effector molecule of vav-mediated signalling pathways.

Effector recognition by the small GTP-binding proteins Ras and Ral

The Ral effector protein RLIP76 (also called RIP/RalBP1) binds to Ral.GTP via a region that shares no sequence homology with the Ras-binding domains of the Ser/Thr kinase c-Raf-1 and the Ral-specific guanine nucleotide exchange factors. Whereas the Ras-binding domains have a similar ubiquitin-like structure, the Ral-binding domain of RLIP was predicted to comprise a coiled-coil region. In order to obtain more information about the specificity and the structural mode of the interaction between Ral and RLIP, we have performed a sequence space and a mutational analysis. The sequence space analysis of a comprehensive nonredundant assembly of Ras-like proteins strongly indicated that positions 36 and 37 in the core of the effector region are tree-determinant positions for all subfamilies of Ras-like proteins and dictate the specificity of the interaction of these GTPases with their effector proteins. Indeed, we could convert the specific interaction with Ras effectors and RLIP by mutating these residues in Ras and Ral. We therefore conclude that positions 36 and 37 are critical for the discrimination between Ras and Ral effectors and that, despite the absence of sequence homology between the Ral-binding and the Ras-binding domains, their mode of interaction is most probably similar.

Ku antigen-DNA conformation determines the activation of DNA-dependent protein kinase and DNA sequence-directed repression of mouse mammary tumor virus transcription

Mouse mammary tumor virus (MMTV) transcription is repressed by DNA-dependent protein kinase (DNA-PK) through a DNA sequence element, NRE1, in the viral long terminal repeat that is a sequence-specific DNA binding site for the Ku antigen subunit of the kinase. While Ku is an essential component of the active kinase, how the catalytic subunit of DNA-PK (DNA-PKcs) is regulated through its association with Ku is only beginning to be understood. We report that activation of DNA-PKcs and the repression of MMTV transcription from NRE1 are dependent upon Ku conformation, the manipulation of DNA structure by Ku, and the contact of Ku80 with DNA. Truncation of one copy of the overlapping direct repeat that comprises NRE1 abrogated the repression of MMTV transcription by Ku-DNA-PKcs. Remarkably, the truncated element was recognized by Ku-DNA-PKcs with affinity similar to that of the full-length element but was unable to promote the activation of DNA-PKcs. Analysis of Ku-DNA-PKcs interactions with DNA ends, double- and single-stranded forms of NRE1, and the truncated NRE1 element revealed striking differences in Ku conformation that differentially affected the recruitment of DNA-PKcs and the activation of kinase activity.

DNA-dependent protein kinase protects against heat-induced apoptosis

Purified heat shock transcription factor 1 (HSF1) binds to both the regulatory and catalytic components of the DNA-dependent protein kinase (DNA-PK). This observation suggests that DNA-PK may have a physiological role in the heat shock response. To investigate this possibility, we performed a comparison of cell lines that were deficient in either the Ku protein or the DNA-PK catalytic subunit versus the same cell lines that had been rescued by the introduction of a functional gene. DNA-PK-negative cell lines were up to 10-fold more sensitive to heat-induced apoptosis than matched DNA-PK-positive cell lines. There may be a regulatory interaction between DNA-PK and HSF1 in vivo, because constitutive overexpression of HSF1 sensitized the DNA-PK-positive cells to heat but had no effect in DNA-PK-negative cells. The initial burst of hsp70 mRNA expression was similar in DNA-PK-negative and -positive cell lines, but the DNA-PK-negative cells showed an attenuated rate of mRNA synthesis at later times and, in some cases, lower heat shock protein expression. These findings provide evidence for an antiapoptotic function of DNA-PK that is experimentally separable from its mechanical role in DNA double strand break repair.

hSiah2 is a new Vav binding protein which inhibits Vav-mediated signaling pathways

The hematopoietic proto-oncogene vav has been characterized as a Rac1-GDP/GTP exchanger protein which regulates cytoskeletal reorganization as well as signaling pathways leading to the activation of stress-activated protein kinases (SAPK/JNKs). Furthermore, vav overexpression enhances basal and T-cell receptor (TCR)-mediated stimulation of the nuclear factor of activated T cells (NFAT). We report here the interaction between Vav and hSiah2, a mammalian homolog of Drosophila Seven in absentia (Sina) that has been implicated in R7 photoreceptor cell formation during Drosophila eye development via the proteasome degradation pathway. Vav and hSiah2 interact in vitro and in vivo and colocalize in the cytoplasm of hematopoietic cells. The Src homology domain of Vav and the C-terminal region of hSiah2 are required for this interaction. We provide evidence for a negative regulation by hSiah2 of Vav-induced basal and TCR-mediated NFAT-dependent transcription. Overexpression of hSiah2 also inhibits the onco-Vav-induced JNK activation. Although the Vav-interacting domain is located in the C-terminal portion of hSiah2, the N-terminal region of hSiah2 is necessary for the inhibitory role that seems to be independent of the proteasome degradation.

The Ku70 autoantigen interacts with p40phox in B lymphocytes

Ku70, a regulatory component of the DNA-dependent protein kinase, was identified by a yeast two-hybrid screen of a B lymphocyte cDNA library as a partner of p40phox, a regulatory component of the O2--producing NADPH oxidase. Truncated constructs of p40phox and Ku70 were used to map the interacting sites. The 186 C-terminal amino acids (aa) of Ku70 were found to interact with two distinct regions of p40phox, the central core region (aa 50–260) and the C-terminal extremity (aa 260–339). In complementary experiments, it was observed that Ku70 binds to immobilized recombinant p40phox fusion protein and that p40phox and Ku70 from a B lymphocyte cell extract comigrate in successive chromatographies on Q Separose, Superose 12 and hydroxylapatite columns. Moreover, we report that Ku70 and p40phox colocalize in B lymphocytes and in transfected Cos-7 cells. We also show that the two NADPH oxidase activating factors, p47phox and p67phox are substrates for DNA-PK in vitro and that they are present together with p40phox in the nucleus of B cells. These results may help solve the paradox that the phox protein triad, p40phox, p47phox and p67phox, is expressed equally in B lymphocytes and neutrophils, whereas the redox component of the NADPH oxidase, a flavocytochrome b, which is well expressed in neutrophils, is barely detectable in B lymphocytes.

Regulation of the DNA-dependent protein kinase (DNA-PK) activity in eukaryotic cells

The DNA-dependent protein kinase (DNA-PK) is a trimeric nuclear serine/threonine protein kinase consisting of a large catalytic sub-unit and the Ku heterodimer that regulates kinase activity by its association with DNA. DNA-PK is a major component of the DNA double strand break repair apparatus, and cells deficient in one of its component are hypersensitive to ionizing radiation. DNA-PK is also required to lymphoid V(D)J recombination and its absence confers in mice a severe combined immunodeficiency phenotype. The purpose of this review is to summarize the current knowledge on the mechanisms that contribute to regulate DNA-PK activity in vivo or in vitro and relates them to the role of DNA-PK in cellular functions. Finally, the studies devoted to drug-inhibition of DNA-PK in order to enhance cancer therapy by DNA-damaging agents are presented.

Nuclear association of tyrosine-phosphorylated Vav to phospholipase C-gamma1 and phosphoinositide 3-kinase during granulocytic differentiation of HL-60 cells

The granulocytic differentiation of HL-60 cells induced by all-trans retinoic acid was accompanied by a progressive tyrosine phosphorylation of specific proteins in either cells or isolated nuclei. Among these phosphoproteins, we identified the Vav adaptor in whole cells as well as in the inner nuclear compartment, where the increase in its tyrosine phosphorylation level was more conspicuous. We also demonstrated the differentiation-dependent association of nuclear phosphorylated Vav to phospholipase C-gamma1 and to the p85 regulatory subunit of phosphoinositide 3-kinase. The role of the Vav/phospholipase C-gamma1/phosphoinositide 3-kinase phosphoprotein complexes in the nuclei of HL-60 induced to differentiate along the granulocytic lineage is discussed.

Sequence-specific binding of Ku autoantigen to single-stranded DNA

Glucocorticoid-induced transcription of mouse mammary tumor virus is repressed by Ku antigen/DNA-dependent protein kinase (DNA-PK) through a DNA sequence element (NRE1) in the viral long terminal repeat. Nuclear factors binding to the separated single strands of NRE1 have been identified that may also be important for transcriptional regulation through this element. We report the separation of the upper-stranded NRE1 binding activity in Jurkat T cell nuclear extracts into two components. One component was identified as Ku antigen. The DNA sequence preference for Ku binding to single-stranded DNA closely paralleled the sequence requirements of Ku for double-stranded DNA. Recombinant Ku bound the single, upper strand of NRE1 with an affinity that was 3-4-fold lower than its affinity for double-stranded NRE1. Sequence-specific single-stranded Ku binding occurred rapidly (t1/2 on = 2.0 min) and was exceptionally stable, with an off rate of t1/2= 68 min. While Ku70 cross-linked to the upper strand of NRE1 when Ku was bound to double-stranded and single-stranded DNAs, the Ku80 subunit only cross-linked to single-stranded NRE1. Intriguingly, addition of Mg2+ and ATP, the cofactors required for Ku helicase activity, induced the cross-linking of Ku80 to a double-stranded NRE1-containing oligonucleotide, without completely unwinding the two strands.