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PubMed1997

Regulation of the human IgE receptor (Fc epsilonRII/CD23) by Epstein-Barr virus (EBV): Ku autoantigen binds specifically to an EBV-responsive enhancer of CD23.

An early and critical event in immortalization of human B cells by Epstein-Barr virus (EBV) is induction of CD23 expression. CD23 is constitutively expressed in all EBV-immortalized B cells and its expression is tightly linked with immortalization. We have previously shown that activation of CD23 by EBV occurs at the transcriptional level and is mediated, in part, by EBV-responsive enhancer elements in the region of the type a promoter. We have localized one EBV-responsive enhancer (designated EBVRE) to a 37 bp sequence in intron 1 of type a CD23 that contains a GC-rich sequence that binds nuclear protein(s) from EBV-positive but not EBV-negative cells with sequence specificity. This EBVRE-binding activity was enhanced by protein phosphorylation and did not react with antibodies to the ubiquitous GC box transcription factor, Sp1. We have now shown by protein purification with peptide sequencing and immunological reactivity that p70/p80 Ku autoantigen [the DNA-binding component of DNA-dependent protein kinase (DNA-PK)] binds to this EBVRE with high affinity and sequence specificity. Although Ku autoantigen is ubiquitously expressed, an EBV-specific DNA-protein complex that contains Ku was elicited from EBV-positive but not EBV-negative nuclear extracts. Furthermore, the formation of this EBV-specific DNA-Ku complex was dramatically enhanced by protein phosphorylation. Thus, we have identified EBVRE-binding activity that contains the Ku autoantigen, is DNA sequence specific and is present in EBV-positive but not EBV-negative nuclear extracts. The possible functional significance of the Ku autoantigen-EBVRE interaction is discussed in light of the role of DNA-PK in phosphorylation and activation of several transcription factors. We suggest that phosphorylation of the EBV-specific EBVRE-binding activity by DNA-PK may modulate its activity as a transcription factor.

Amino Acid Sequence↗
PubMed1997

Autoantigen Ku and its role in multiple cellular processes.

Ku is a DNA binding protein composed of 70 and 80 kDa subunits which was discovered as autoantigen in a patient with scleroderma-polymyositis overlap syndrome. Ku can bind to the end of DNA and also to some internal sequences. Ku-autoantigen acts as a potential transcription factor for several RNA polymerase II genes and RNA polymerase I gene. Ku is also associated with DNA-dependent protein kinase and involved in V(D)J recombination and DNA break repair mechanisms. Ku may be involved in replication, helicase activity and cell signaling. Therefore, Ku-autoantigen is a very important cellular factor which plays important role in the multiple cellular processes.

Animals↗
PubMed1992

Ku autoantigen is the regulatory component of a template-associated protein kinase that phosphorylates RNA polymerase II.

The carboxyl-terminal domain of RNA polymerase II contains a tandemly repeated heptapeptide sequence. Previous work has shown that this sequence is phosphorylated at multiple sites by a template-associated protein kinase, in a reaction that is closely associated with the initiation of RNA synthesis. We have purified this kinase to apparent homogeneity from human (HeLa) cells. The purified kinase phosphorylates native RNA polymerase II only in the presence of DNA and the general transcription factors TFIID (TBP), TFIIB, and TFIIF. Two kinase components are required for full activity: a catalytic component and a DNA-binding regulatory component. The regulatory component has been identified as Ku autoantigen, based on the molecular weights of its component polypeptides, its DNA-binding properties, and its reactivity with anti-Ku monoclonal antibodies. The Ku autoantigen recruits the catalytic component of the kinase to the template. Ku autoantigen has been previously proposed to interact with DNA by a characteristic bind-and-slide mechanism. This mode of interaction may provide a mechanism for targeting the kinase to the transcription complex and other DNA-bound substrates.

Antigens, Nuclear↗
PubMed2000

Ku autoantigen (DNA helicase) is required for interleukins-13/-4-induction of 15-lipoxygenase-1 gene expression in human epithelial cells.

As reported previously in human monocytes, a human lung epithelial cell line, A549, showed de novo induction of 15-Lipoxygenase-1 (15-LO-1) in response to interleukins-13 (IL-13) and -4 (IL-4). In this cell line, 15-LO-1 expression, by RT-PCR and western blotting, was observed following 6 and 24 h of exposure to human IL-13 (ED50 5 ng/ml) and IL-4 (ED50 0.2 ng/ml). We have previously shown that no cis-acting regulatory elements exist within the 15-LO-1 promoter region. To define IL-13 and IL-4 responsive trans-acting elements, we identified a region (DP2: -353 to -304 bp site) within the 15-LO-1 promoter (by footprinting experiments) to which IL-13-responsive elements (or factors) bind specifically (Kelavkar et al, 1998, Mol Biol Rep 25, 173-182). To further delineate this region, we constructed (by site-directed mutagenesis) several deletion mutants in the 'LOPB5' region containing the 29 bp within the -353 to -304 bp of the DP2 core element. These were: DP3 (site totally deleted), DP4 (5 bp deleted at the center of the site), DP5 (8 bp at the 5'-end of the site) and DP6 (13 bp at the 3'-end of the site). Cotransfection of these deletion constructs (driving luciferase reporter genes) was associated with 90% (DP4, DP5 and DP6) or 100% (DP3) abrogation of promoter activity at 24 h. Purification of nuclear protein extracts from IL-13 and IL-4-stimulated A549 cells, using a DP2 core containing affinity column, identified a 150 kDa protein under non-denaturing conditions, and two, 70 and 85 kDa proteins under denaturing conditions. These were not detectable by Coomassie blue staining in control nuclear protein extracts. Matrix assisted laser desorption ionization mass spectrometry (MALDI-MS) of the tryptic digests of these proteins, identified one as the 86 kDA Lupus KU autoantigen protein P86 and the second as the 70 kDa Lupus KU autoantigen protein P70. Gel shift and supershift experiments using monoclonal antibodies toward Ku antigen and its individual subunits, and utilizing DP2 and other mutant oligonucleotides with purified nuclear protein extracts from control and cytokine-treated A549 cells, confirmed our findings. Furthermore, electroporation of neutralizing anti-Ku70, Ku 80 and Ku70/80 antibodies into A549 cells totally suppressed IL-13 and IL-4-stimulated 15-LO-1 induction in these cells. Further, immunoprecipitation experiments data suggests that IL-4 and IL-13 activate Ku antigens and 15-LO-1 expression through distinct signaling events. In summary, in A549 cells, Ku antigen is induced in response to the cytokines, IL-13 and -4, and a 29 bp region within the -353 to -304 bp region of the 15-LO-1 promoter is required for its binding and subsequent induction of 15-LO-1 gene expression. The findings may provide an important link between the established dysregulated function of Ku antigen in auto-immune diseases, such as systemic lupus erythematosus and thyroiditis, and the increasingly recognized 'anti-inflammatory' role of 15-LO-1.

Antibodies↗
PubMed1995

A constitutive heat shock element-binding factor is immunologically identical to the Ku autoantigen.

Analysis of the heat shock element (HSE)-binding proteins in extracts of rodent cells, during heat shock and their post-heat shock recovery, indicates that the regulation of heat shock response involves a constitutive HSE-binding factor (CHBF), in addition to the heat-inducible heat shock factor HSF1. We purified the CHBF to apparent homogeneity from HeLa cells using column chromatographic techniques including an HSE oligonucleotide affinity column. The purified CHBF consists of two polypeptides with apparent molecular masses of 70 and 86 kDa. Immunoblot and gel mobility shift analysis verify that CHBF is identical or closely related to the Ku autoantigen. The DNA binding characteristics of CHBF to double-stranded or single-stranded DNA are similar to that of Ku autoantigen. In gel mobility shift analysis using purified CHBF and recombinant human HSF1, CHBF competes with HSF1 for the binding of DNA sequences containing HSEs in vitro. Furthermore, when Rat-1 cells were co-transfected with human Ku expression vectors and the hsp70-promoter-driven luciferase reporter gene, thermal induction of luciferase is significantly suppressed relative to cells transfected with only the hsp70-luciferase construct. These data suggest a role of CHBF (or Ku protein) in the regulation of heat response in vivo.

Animals↗
PubMed1994

Isolation of cDNA clones encoding the 80-kd subunit protein of the human autoantigen Ku (p70/p80) by antisera raised against ciliary processes of human eye donors.

PURPOSE: To isolate and characterize human nonpigmented ocular ciliary epithelial cDNA clones by screening a cDNA library constructed from the established human nonpigmented ciliary epithelial cell line (ODM-2) with sera raised against ciliary processes of human eye donors (cadavers). METHODS: An ODM-2 cDNA expression library in lambda Uni-ZAP-XR vector was screened with sera of mouse anti-human ciliary processes. The cDNA inserts from plaque-purified clones were sequenced by the dideoxy-chain termination method with T3 and T7 polymerase. Sera were assayed by Western blot analysis and indirect immunofluorescence and were then compared with sera, enriched in anti-Ku antibodies, collected from patients with scleroderma polymyositis overlap syndrome. RESULTS: Immunoscreening of 2.5 x 10(5) independent clones with sera induced experimentally in total tissue homogenates of human ciliary processes led to the isolation of four cDNA-positive clones. Three clones, designated A1, D1, and D3 and containing inserts measuring 1.4 kb to 1.8 kb, were found to be identical in DNA sequence analysis to that of the 80-kd subunit protein of the human autoantigen Ku (p70/p80). The fourth clone, designated D2, failed to show any significantly similarity with previously known genes. Cell extracts from human ciliary processes and ODM-2 cells were analyzed by immunoblotting with anti-human ciliary process sera. The pattern of proteins immunodetected was compared with the profile of proteins immunodetected with human anti-Ku sera from a patient with scleroderma polymyositis overlap syndrome. These results demonstrated that sera from anti-human ciliary processes recognize a variety of antigens in ODM-2 cells and are strikingly rich in antibodies to the 80-kd subunit protein of Ku (p70/p80). Furthermore, indirect immunofluorescence analysis supported the observation that anti-human ciliary processes sera labeled nuclear antigens in a human ciliary epithelium cell line and cross species in bovine ciliary epithelial cell, thus demonstrating the nuclear localization of Ku antigen. The mRNA in the human eye was found to be widely expressed in all tested ocular tissues and in ODM-2 cells. CONCLUSIONS: The ocular ciliary epithelium induces a species-specific immune response in xenogenic animals. In particular, the human ciliary epithelium elicits a strong immune recognition of the 80-kd subunit protein of the human autoantigen Ku (p70/p80) in ODM-2 cells.

Animals↗
PubMed1994

Involvement of the Ku autoantigen in the cellular response to DNA double-strand breaks.

The Ku autoantigen is a well-characterized heterodimer of 70 and 86 kDa that binds to DNA ends, but its cellular function has been obscure. An electrophoretic mobility-shift assay and Ku antisera were used to show that Ku or a closely related protein was deficient in three mutant hamster cell lines from x-ray-sensitive complementation group 5, which is characterized by defects in DNA double-strand break repair and V(D)J recombination. Furthermore, Ku protein expression was restored when the cells reverted to x-ray resistance. The Ku p86 gene maps to human chromosome 2q33-35, and group 5 cells are rescued by almost precisely the same region, 2q34-36. Thus, biochemical and genetic evidence suggests that Ku is involved in pathways for DNA recombination and repair. By its association with a DNA-dependent protein kinase activated by DNA ends, Ku may also initiate a signaling pathway induced by DNA damage, perhaps for cell cycle arrest.

Animals↗
PubMed1990

Characterization of the 70KDA component of the human Ku autoantigen expressed in insect cell nuclei using a recombinant baculovirus vector.

The Ku autoantigen is a human nuclear, DNA-binding heterodimer of 70kDa and 86kDa proteins. It is the target of autoantibodies in several autoimmune diseases. We now report the expression of a cDNA encoding the 70kDa Ku protein. Large amounts of protein were obtained using a recombinant baculovirus vector, in contrast with earlier unsuccessful attempts using other expression systems. We demonstrate that the 70kDa Ku protein is targeted to the nucleus and is associated with the nuclear matrix when expressed in the absence of the 86kDa Ku component. No post-translational modifications were observed. The 70kDa protein binds double and single-stranded DNA with very high affinity. Our results suggest that the baculovirus expression system may be of widespread use in the production and characterization of human autoantigens.

Animals↗
PubMed1996

Human Ku autoantigen binds cisplatin-damaged DNA but fails to stimulate human DNA-activated protein kinase.

We have identified a series of proteins based on an affinity for cisplatin-damaged DNA. One protein termed DRP-1 has been purified to homogeneity and was isolated as two distinct complexes. The first complex is a heterodimer of 83- and 68-kDa subunits, while the second complex is a heterotrimer of 350-, 83-, and 68-kDa subunits in a 1:1:1 ratio. The 83- and 68-kDa subunits in each complex are identical. The 83-kDa subunit of DRP-1 was identified as the p80 subunit of Ku autoantigen by N-terminal protein sequence analysis and reactivity with a monoclonal antibody directed against human Ku p80 subunit. The 68-kDa subunit of DRP-1 cross-reacted with monoclonal antisera raised against the Ku autoantigen p70 subunit. The 350-kDa subunit was identified as DNA-PKcs, the catalytic subunit of the human DNA-activated protein kinase, DNA-PK. DRP-1/Ku DNA binding was assessed in mobility shift assays and competition binding assays using cisplatin-damaged DNA. Results indicate that DNA binding was essentially unaffected by cisplatin-DNA adducts in the presence or absence of DNA-PKcs. DNA-PK activity was only stimulated with undamaged DNA, despite the ability of Ku to bind to cisplatin-damaged DNA. The lack of DNA-PK stimulation by cisplatin-damaged DNA correlated with the extent of cisplatin-DNA adduct formation. These results demonstrate that Ku can bind cisplatin-damaged DNA but fails to activate DNA-PK. These results are discussed with respect to the repair of cisplatin-DNA adducts and the role of DNA-PK in coordinating DNA repair processes.

Antibodies, Monoclonal↗
PubMed1993

Lupus autoantigen Ku protein binds HIV-1 TAR RNA in vitro.

The control of HIV-1 gene expression depends upon interaction of Tat with the trans-activation responsive (TAR) element present at the 5' end of all HIV-1 transcripts. The TAR sequence forms a stable hair-pin structure that binds Tat and several cellular factors in vitro. In the absence of Tat, TAR acts as a transcription terminator. Tat in conjunction with a cellular factor(s) acts to increase the elongation capacity of the transcription complex. Here we report that Ku protein, the autoantigen in patients with systemic lupus erythematosus, binds TAR RNA in vitro with high affinity and specificity forming a single protein-RNA complex. Ku, a heterodimer of Ku72 and Ku86 that non-specifically binds the ends of DNA fragments, appears to recognize the terminal loop of TAR RNA. UV-crosslinking showed that both subunits of Ku are in proximity to the RNA. Further, Ku shows a 5-fold higher affinity for TAR RNA than for the ends of dsDNA. As Ku is involved in the stimulation of the elongation property of the RNA polymerase II and activation of several transcription factors, the specific interaction of Ku with TAR raises intriguing possibilities for its function in HIV-1 gene expression.

Antigens, Nuclear↗
PubMed1996

Cloning and characterization of rat Ku70: involvement of Ku autoantigen in the heat-shock response.

To study the role of the Ku autoantigen in the heat-shock response, we have cloned the 70-kDa subunit (the DNA-binding component of Ku) from a rat cDNA library. The deduced amino acid sequence of rat Ku70 bears extensive homology with the human and murine counterpart (83 and 97% identity, respectively). Overexpression of Ku70 in rat fibroblasts results in the specific repression of hsp-70 upon heat shock. The inhibition of induction of hsp-70 was greatest in cells expressing the highest level of Ku70. Induction of other heat-shock proteins besides hsp-70 by hyperthermia appears normal, as does the activation of the heat-shock transcription factor, HSF-1. It is likely that other factors besides HSF-1 are involved in controlling heat-shock gene transcription. While the formation of the Ku70 and Ku80 complex is important for repair of DNA double-strand breaks our data suggest that the 70-kDa subunit of Ku plays a role in regulating hsp-70 expression.

Amino Acid Sequence↗
PubMed1999

Poly(ADP-ribose) polymerase and Ku autoantigen form a complex and synergistically bind to matrix attachment sequences.

Genomic sequences with a cluster of ATC sequence stretches where one strand consists exclusively of well mixed As, Ts, and Cs confer high base unpairing propensity under negative superhelical strain. Such base unpairing regions (BURs) are typically found in scaffold or matrix attachment regions (SARs/MARs) that are thought to contribute to the formation of the loop domain structure of chromatin. Several proteins, including cell type-specific proteins, have been identified that bind specifically to double-stranded BURs either in vitro or in vivo. By using BUR-affinity chromatography to isolate BUR-binding proteins from breast cancer SK-BR-3 cells, we almost exclusively obtained a complex of poly(ADP-ribose) polymerase (PARP) and DNA-dependent protein kinase (DNA-PK). Both PARP and DNA-PK are activated by DNA strand breaks and are implicated in DNA repair, recombination, DNA replication, and transcription. In contrast to the previous notion that PARP and Ku autoantigen, the DNA-binding subunit of DNA-PK, mainly bind to free ends of DNA, here we show that both proteins individually bind BURs with high affinity and specificity in an end-independent manner using closed circular BUR-containing DNA substrates. We further demonstrate that PARP and Ku autoantigen form a molecular complex in vivo and in vitro in the absence of DNA, and as a functional consequence, their affinity to the BURs are synergistically enhanced. ADP-ribosylation of the nuclear extract abrogated the BUR binding activity of this complex. These results provide a mechanistic link toward understanding the functional overlap of PARP and DNA-PK and suggest a novel role for these proteins in the regulation of chromatin structure and function.

Antigens, Nuclear↗
PubMed1996

The Ku autoantigen: cast in a new light.

With his associates at the University of Ottawa, Dr. Robert Haché has demonstrated that the Ku autoantigen, a protein that plays a role in DNA repair and in immunoglobulin gene recombination, also modifies the action of steroid hormones. This finding has potential implications for the treatment of tumours whose growth is influenced by steroid hormones. The researchers suggest that Ku could be involved in the control of gene expression through a mechanism involving protein phosphorylation. They hope that a better understanding of the role Ku plays in many cellular processes may improve our understanding of autoimmune diseases, diseases of immune deficiency and cancer.

Antigens, Nuclear↗
PubMed1994

A DNA-binding activity, TRAC, specific for the TRA element of the transferrin receptor gene copurifies with the Ku autoantigen.

We have previously described purification and characterization of a nuclear protein, TREF, which interacts specifically with the transcriptional control element, TRA, of the human transferrin receptor (TR) gene. In this report we show that TREF can be separated into two functionally distinct DNA-binding activities. The first DNA-binding activity (TRAC) is highly specific for the 8-bp element TRA and the related Escherichia coli cAMP receptor binding site. This motif is homologous to the phorbol 12-tetradecanoate 13-acetate- and cAMP-responsive elements of eukaryotic genes and the regulatory proximal sequence elements of the U1 small nuclear RNA gene and is also present in the promoter of the Drosophila melanogaster yolk protein factor 1 gene. In striking contrast, the second activity exhibits high affinity for the ends of double-stranded DNA in a sequence-unspecific manner and is attributable to the heterodimeric Ku autoantigen. Notably, transcription of Ku is induced during mid-late G0/G1 with kinetics similar to the TR gene. Ku is a highly abundant nuclear protein possessing nonspecific affinity for the ends of DNA, whose biological role remains to be elucidated. A transcriptional role for this protein has been proposed, however, on the basis of studies attributing DNA sequence-specific binding activity, notably for TRA-like sequences described above, directly to the Ku heterodimer. The observation that Ku-mediated nonspecific DNA-binding activity copurifies with the TRA-specific activity, TRAC, clearly has implications for these and related studies. The unusual properties of TRAC activity and its relationship, if any, with the enigmatic Ku protein, are discussed.

Antigens, Nuclear↗
PubMed1998

The 86-kD subunit of Ku autoantigen mediates homotypic and heterotypic adhesion of multiple myeloma cells.

Previous studies have shown that triggering multiple myeloma (MM) cells via CD40 induces IL-6-mediated autocrine growth as well as increased expression of cell surface adhesion molecules including CD11a, CD11b, CD11c, and CD18. In this study, we generated the 5E2 mAb which targets an antigen that is induced upon CD40 ligand (CD40L) activation of MM cells. Immunofluorescence, immunoprecipitation, and protein sequencing studies identified the target antigen of 5E2 mAb as the 86-kD subunit of the Ku autoantigen. We demonstrate that increased cell surface expression of Ku on CD40L-treated cells is due to migration of Ku from the cytoplasm to the cell surface membrane. Moreover, cell surface Ku on CD40L-treated MM cells mediates homotypic adhesion of tumor cells, as well as heterotypic adhesion of tumor cells to bone marrow stromal cells and to human fibronectin; and 5E2 mAb abrogates IL-6 secretion triggered by tumor cell adherence to bone marrow stromal cells. These data suggest that CD40L treatment induces a shift of Ku from the cytoplasm to the cell surface, and are the first to show that Ku functions as an adhesion molecule. They further suggest that cell surface Ku may play a role in both autocrine and paracrine IL-6-mediated MM cell growth and survival.

3T3 Cells↗
PubMed1995

Complementation of the ionizing radiation sensitivity, DNA end binding, and V(D)J recombination defects of double-strand break repair mutants by the p86 Ku autoantigen.

Two ionizing radiation-sensitive (IRs) and DNA double-strand break (DSB) mutants, sxi-3 and sxi-2, were shown to be severely deficient in a DNA end binding activity, similar to a previously described activity of the Ku autoantigen, correlating with the xrs (XRCC5) mutations. Cell fusions with xrs-6, another IRs, DSB repair-deficient cell line, defined these sxi mutants in the XRCC5 group. sxi-3 cells have low expression levels of the p86Ku mRNA. Introduction of the Ku p86 gene, but not the p70 Ku gene, complemented the IRs, DNA end binding, and variable (diversity) joining [V(D)J] recombination signal and coding junction deficiencies of sxi-3. Thus, the p86 Ku gene product is essential for DSB repair and V(D)J recombination.

Animals↗
PubMed1996

Modulation of thermal induction of hsp70 expression by Ku autoantigen or its individual subunits.

Previously, we proposed a dual control mechanism for the regulation of the heat shock response in mammalian cells: a positive control mediated by the heat shock transcription factor HSF1 and a negative control mediated by the constitutive heat shock element-binding factor (CHBF). To study the physiological role of CHBF in the regulation of heat shock response, we purified CHBF to apparent homogeneity and showed it to be identical to the Ku autoantigen, a heterodimer consisting of 70-kDa (Ku-70) and 86-kDa (Ku-80) polypeptides. To study further the functional significance of Ku/CHBF in the cellular response to heat shock, we established rodent cell lines that stably and constitutively overexpressed one or both subunits of the human Ku protein, and examined the thermal induction of hsp70 and other heat shock proteins in these Ku-overexpressing ing cells. We show that expression of the human Ku-70 and Ku-80 subunits jointly or of the Ku-70 subunit alone specifically inhibits heat-induced hsp70 expression. Conversely, expression of human Ku-80 alone does not have this effect. Thermal induction of other heat shock proteins in all of the Ku-overexpressing cell lines appears not to be significantly affected, nor is the state of phosphorylation or the DNA-binding ability of HSF1 affected. These findings support a model in which hsp70 expression is controlled by a second regulatory factor in addition to the positive activation of HSF1. The Ku protein, specifically the Ku-70 subunit, is involved in the regulation of hsp70 gene expression.

Animals↗
PubMed2002

KAP-2, a protein that binds to the H-box in a bean chalcone synthase promoter, is a novel plant transcription factor with sequence identity to the large subunit of human Ku autoantigen.

The KAP-2 protein that binds to the H-box (CCTACC) element in the bean CHS15 chalcone synthase promoter was purified, and internal peptide sequence used to design primers leading to the cloning of KAP-2 from bean (Phaseolus vulgaris) and barrel medic (Medicago truncatula). KAP-2 shares sequence similarity to the large subunit of mammalian Ku autoantigen, a protein proposed to be involved in control of DNA recombination and transcription. KAP-2 sequences were present in genomic DNA from a range of legumes, and a related protein is found in Arabidopsis thaliana. Recombinant KAP-2 expressed in insect cells showed the same binding specificity for the CHS15 H-box as the protein purified from bean cell extracts. In vitro transcription assays confirmed that KAP-2 stimulates transcription from a promoter harboring the H-box cis element.

Acyltransferases↗