A protein homologous to human Ku p70-protein is required for reconstitution of Xenopus sperm pronuclei

Characterization of DNA end-binding activities in higher plants

DNA double-strand-breaks (DSB) are the most severe lesion in cells exposing to ionizing radiation and many other stress environments. Repair of DNA DSB is therefore critical to cellular survival. In this work, we observed the double-stranded DNA end-binding (DEB) like activities in rice (Oryza sativa L. cv. TN5) suspension cells and hypocotyls from etiolated mung bean (Vigna radiata L. TN5) seedlings. Higher plant DEB-like protein binds primarily to linearized double-stranded DNA ends. Competition of unlabeled probe was examined in double-stranded DEB assay of cell extracts from rice and mung bean. DEB-like activities of higher plants did not depend on sequence and types of double-stranded DNA ends. Distinct electrophoretic mobility shift patterns and binding features further indicate that DEB-like factors from various sources might not share identical structure and function, and probably belong to different types of DEB proteins from higher plants. Our evidence suggests that DEB proteins are certainly ubiquitous in all organisms probably for repairing and processing double-stranded DNA breaks from formidable lethal lesion.

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.

Telomerase: a therapeutic target for the third millennium?

Telomerase offers the potential opportunity to control cell proliferation by interfering with a totally new and unique biological process which is cell senescence. The aim of this review is to impartially present the state of the art in telomerase with the pros and the cons of the current scientific situation of this fast-growing and fascinating topic for answering the key question asked by experimental and medical oncologists: Will telomerase be a therapeutic target for the third millenium? The most convincing argument (which is a scientifically documented one) for going ahead with this target is obviously the strong correlation existing between the level and frequency of telomerase expression and the malignant properties of tumors. This has been now largely documented in established tumor cell lines and fresh tumor samples obtained from patients. Noteworthy is the very important difference of telomerase expression between malignant and normal tissues. This difference is much higher than those observed for classical enzymatic targets of chemotherapy such as thymidylate synthetase, dihydrofolate reductase and topoisomerases. If this translates to the clinical situation, telomerase inhibitors might display a good selectivity for tumor cells with a minimal toxicity for normal tissues. The most appealing criticism (which is still purely speculative) is obviously the clinical relevance of inhibiting telomerase in cancer patients. According to the paradigm currently proposed for telomeres and telomerases, it can be predicted that telomerase inhibition will not affect a tumor until its telomeres reach the critical size for entering senescence. This means that during anti-telomerase therapy, the tumor cells will continue grow undergoing 20-30 divisions until the telomeres reach a critical size leading to tumor senescence. Does this make sense, especially in patients with advanced tumors at the beginning of the therapy? Ultimately, the definitive answer to the question will not come from intellectual speculation but from the properties of telomerase inhibitors, first in tumor bearing animals, then finally in cancer patients! Several institutions are very active in the development of telomerase inhibitors. Different stategies are used: direct inhibition of telomerase, interference with telomeres (G quartets), interaction with other proteins involved in the regulation of telomerase and telomeres.

Molecular cloning and sequencing of cDNAs encoding homologues of human Ku70 and Ku80 autoantigen from Xenopus and their expression in various Xenopus tissues

We isolated cDNA clones encoding Ku70 and Ku80 homologues of Xenopus laevis from a cDNA library prepared from Xenopus oocytes. The nucleotide sequences of these Ku70 and Ku80 homologues have coding sequences of 1833 bp and a 611 aa protein, and 2178 bp and a 726 aa protein, respectively. The amino acid sequences deduced from the open reading frame of the Ku70 and Ku80 cDNA clones were highly homologous to those from Ku genes previously isolated, such as human (ca. 65% and ca. 62% identity, respectively) and mouse (ca. 65% and ca. 60%), and show a certain degree of homology to Drosophila (ca. 27% with Ku70), Caenorhabditis elegans (ca. 20% with Ku80) and Saccharomyces cerevisiae (ca. 23% and ca. 19%). Our detailed comparison of the predicted amino acid sequences among these species revealed the highly conserved octa-peptide LPFXXDIR common to both Xenopus Ku70 and Ku80 homologues in the region showing the high homology throughout the species tested. A Northern analysis using specific cDNA probes showed that Ku poly(A)+ mRNAs are expressed at high levels in Xenopus adult oocyte and testis.

Implication of DNA-dependent protein kinase in an early, essential, local phosphorylation event during end-joining of DNA double-strand breaks in vitro

Previous work with Xenopus egg extracts suggested that a wortmannin-sensitive protein phosphorylation event precedes both the removal of modified termini from DNA double-strand break ends and the joining of unmodified ends. To assess the possible role of DNA-dependent protein kinase in effecting this phosphorylation, both DNA end-joining and DNA-stimulated phosphorylation were examined in the presence of various inhibitors. Linear but not supercoiled DNA stimulated the phosphorylation of several endogenous proteins in the extracts, including species of approximately 48, 87, and 96 kDa. This phosphorylation was selectively suppressed by the kinase inhibitors wortmannin, dimethylaminopurine, and LY294002, with a dose response that in each case paralleled the inhibition of DNA end-joining. If wortmannin was added while the end-joining reaction was in progress, end-joining of DNA already present in the reaction continued for some time, but newly added DNA was not joined or processed at all. Ends with 3'-hydroxyl termini were joined much faster than those with 3'-phosphoglycolate termini, although both were equally effective in stimulating protein phosphorylation. The results support a role for DNA-dependent protein kinase in regulating end-joining in vitro, and suggest that at least one of the necessary phosphorylations involves a protein bound at or near the DNA end to be joined. In contrast, nuclear extracts from human cells joined double-strand breaks with normal but not modified termini, and the joining was unaffected by kinase inhibitors, suggesting that the dominant mechanism of end-joining in these extracts did not involve DNA-PK.

Mutation of yeast Ku genes disrupts the subnuclear organization of telomeres

The mammalian Ku70 and Ku86 proteins form a heterodimer that binds to the ends of double-stranded DNA in vitro and is required for repair of radiation-induced strand breaks and V(D)J recombination [1,2]. Deletion of the Saccharomyces cerevisiae genes HDF1 and HDF2--encoding yKu70p and yKu80p, respectively--enhances radiation sensitivity in a rad52 background [3,4]. In addition to repair defects, the length of the TG-rich repeat on yeast telomere ends shortens dramatically [5,6]. We have shown previously that in yeast interphase nuclei, telomeres are clustered in a limited number of foci near the nuclear periphery [7], but the elements that mediate this localization remained unknown. We report here that deletion of the genes encoding yKu70p or its partner yKu80p altered the positioning of telomeric DNA in the yeast nucleus. These are the first mutants shown to affect the subnuclear localization of telomeres. Strains deficient for either yKu70p or yKu80p lost telomeric silencing, although they maintained repression at the silent mating-type loci. In addition, the telomere-associated silencing factors Sir3p and Sir4p and the TG-repeat-binding protein Rap1p lost their punctate pattern of staining and became dispersed throughout the nucleoplasm. Our results implicate the yeast Ku proteins directly in aspects of telomere organization, which in turn affects the repression of telomere-proximal genes.

Ku selectively transfers between DNA molecules with homologous ends

Double strand break repair and V(D)J recombination in mammalian cells require the function of the Ku protein complex and the DNA-dependent protein kinase. The DNA-dependent protein kinase is targeted to DNA through its interaction with the Ku protein complex, and thus the specificity of template recognition in the repair and recombination reactions depend on Ku. We have studied Ku binding to DNA using competitive gel shift analysis. We find that Ku bound to one DNA molecule can transfer directly to another DNA molecule when the two DNA molecules have homologous ends containing a minimum of four matched bases. This remarkable reaction can give a false impression of sequence specificity of Ku DNA binding under certain assay conditions. A model is proposed for the DNA binding function of Ku on the basis of these results and the discovery of a novel type of DNA-Ku complex formed at high Ku/DNA ratios is discussed.