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Ku, a DNA repair protein with multiple cellular functions?

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Abstract

The Ku protein binds to DNA ends and other types of discontinuity in double-stranded DNA. It is a tightly associated heterodimer of ∼70 kDa and ∼80 kDa subunits that together with the ∼470 kDa catalytic subunit, DNA-PKcs, form the DNA-dependent protein kinase. This enzyme is involved in repairing DNA double-strand breaks (DSBs) caused, for example, by physiological oxidation reactions, V(D)J recombination, ionizing radiation and certain chemotherapeutic drugs. The Ku-dependent repair process, called illegitimate recombination or nonhomologous end joining (NHEJ), appears to be the main DNA DSB repair mechanism in mammalian cells. Ku itself is probably involved in stabilizing broken DNA ends, bringing them together and preparing them for ligation. Ku also recruits DNA-PKcs to the DSB, activating its kinase function. Targeted disruption of the genes encoding Ku70 and Ku80 has identified significant differences between Ku-deficient mice and DNA-PKcs-deficient mice. Although all three gene products are clearly involved in repairing ionizing radiation-induced damage and in V(D)J recombination, Ku-knockout mice are small, and their cells fail to proliferate in culture and show signs of premature senescence. Recent findings have implicated yeast Ku in telomeric structure in addition to NHEJ. Some of the phenotypes of the Ku-knockout mice may indicate a similar role for Ku at mammalian telomeres.

Section snippets

Establishing the link between Ku and DNA DSB repair

Until 1994, DNA-PK was studied primarily as a possible modulator of transcription because it was first identified as an enzyme that phosphorylated transcription factors such as Sp1, p53 and Oct-1 (for a review, see Ref. [10]). This perspective was largely abandoned when it was discovered that two complementation groups of X-ray-sensitive mammalian cell lines, termed IR5 and IR7, are deficient in DNA-PK activity and can be complemented by the genes encoding human Ku80 (the XRCC5 gene) and

The functions of Ku in DNA DSB repair

Many of the reported biochemical activities of Ku can be envisaged to play important roles in DNA DSB repair. Most obviously, since Ku binds very rapidly and with high affinity to DNA ends in vitro [38], it seems likely that Ku recognizes various types of broken DNA structures that occur in the cell. Once bound, Ku might then prevent digestion of the broken ends by DNA exonucleases. The observation that V(D)J recombination intermediates are relatively stable in the absence of Ku [39], however,

Potential roles for Ku in transcription

DNA-PK phosphorylates a host of transcription factors in vitro (reviewed in Ref. [10]), as well as the regulatory C-terminal domain of RNA polymerase II [69]. In addition to this possible influence on transcription, there are numerous reports of sequence-specific DNA binding by Ku, especially to sequences in transcriptional regulatory elements (for example, Ref. [70]). Despite these long-standing inferences of a role for Ku and DNA-PK in transcription, however, there is still no compelling

Knock-out mice as a route to determine the physiological functions of Ku

One way to examine the physiological roles of Ku is by targeted disruption of the genes for Ku70 and Ku80 in mice. Both gene disruptions cause fundamentally similar phenotypes to those of SCID and DNA-PKcs-knockout mice—V(D)J recombination and the development of lymphocytes is severely impaired—but there are also some interesting differences, discussed below 18, 25, 26, 27, 78, 79(Table 1).

Both Ku70- and Ku80-knockout mice are severely depleted of T and B lymphocytes caused by defects in V(D)J

Functions of yeast Ku in telomere maintenance

Several papers published in recent years have linked Ku and certain other DNA repair proteins to telomere maintenance in S. cerevisiae 35, 81, 82, 83, 84, 85, 86. When the genes that encode either Yku70p or Yku80p are disrupted, not only NHEJ but also telomeric silencing and telomere length maintenance are significantly perturbed. Telomeric silencing, also known as telomere position effect, is observed when a gene is engineered into the telomeric region of a yeast chromosome. In these

Acknowledgements

We thank Penny Jeggo and Guillermo Taccioli for their critical comments on the manuscript. Also, we are grateful to members of our group, especially to Andrew McAinsh and Graeme Smith, for their helpful insights. Many thanks also to Andrew McAinsh who drew the figures. Our research is funded primarily by the Cancer Research Campaign.

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