ReviewThe Ku heterodimer: Function in DNA repair and beyond
Introduction
Ku was first identified in the early 1980s as an autoantigen targeted by autoantibodies in the serum of patients diagnosed with an autoimmune disease known as scleroderma polymyositis overlap syndrome [1]. The name Ku comes from the first two letters of the name of the original patient in whose serum it was identified. Autoantibodies directed against Ku were subsequently found in several other autoimmune diseases, including systemic lupus erythematosus, Sjorgren's syndrome, polymyositis and scleroderma [2], [3], [4]. Early studies using serum from Ku-positive patients identified Ku as an abundant, mostly nuclear protein [1], [5]. Subsequent reports showed that Ku had unusual DNA binding properties, binding avidly to the ends of double-stranded DNA molecules in a sequence-independent manner, and to a lesser extent, to other forms of DNA discontinuities such as hairpins gaps and nicks [5], [6], [7]. These unusual end-binding properties made Ku an appealing candidate for a role in DSB repair and V(D)J recombination, which was confirmed when the two Ku subunits (XRCC5, Ku80 and XRCC6, Ku70) were found to complement the DNA repair defect of several IR-sensitive cell lines [8], [9], [10], [11], [12], [13], [14]. It is now well established that while not essential to individual life in the short term, Ku function is critical to the maintenance of genomic integrity and to the proper cellular and organismal development. A better understanding of Ku's diverse roles at the cellular and organismal level have implications for the study and the treatment of other human diseases, such as immune system disorders, cancer and aging.
Section snippets
Ku structure
Ku is a highly abundant protein found in vivo as a stable heterodimer consisting of two subunits, Ku70 and Ku80 (70 and 80 kDa, respectively). Both Ku70 and Ku80 eukaryotic Ku subunits contain three domains (Fig. 1A): an N-terminal alpha helix/beta barrel von Willebrand A (vWA) domain; a central core domain required for DNA binding and dimerization; and a helical C-terminal domain.
The Ku70/80 crystal structure (Fig. 1B) shows that the two subunits dimerize through the central domain to form a
Non-homologous end joining
There are three main DSB repair pathways in eukaryotes (Fig. 2): the classical NHEJ (C-NHEJ), alternative NHEJ or microhomology-mediated end joining (A-NHEJ or MMEJ) and homologous recombination (HR)[42], [43], [44]. C-NHEJ and A-NHEJ have the potential to be active in all stages of the cell cycle, however HR, is only active in the S and G2 phases because it utilizes the complementarity of the sister chromatid to repair the DSB with high accuracy [43], [44]. C-NHEJ is capable of ligating any
Ku as a DNA damage signaling molecule
The DNA damage response (DDR) pathway (Fig. 3) is initiated by sensor proteins that form large complexes that accumulate in foci at the site of damage (reviewed in [131]). This complex formation promotes the activation of a phosphorylation cascade, and the modification of surrounding chromatin to allow DNA repair factors to access the break. These initial sensors include the MRN complex, 53BP1, BRCA1, as well as the serine/threonine (S/T) PIKK family members Ataxia Telangiectasia Mutated (ATM),
Ku at telomeres
Telomeres are the linear ends of chromosomes and therefore have the potential to be recognized as DSBs and processed by DSB repair pathways. Specific protein complexes, such as the shelterin complex in mammals and the Rap1 and the CST (Cdc13–Stn1–Ten1) complexes in yeast have evolved to bind and form protective caps on the DNA ends to prevent access by the DNA repair complexes (Fig. 4) [146], [147]. A role for Ku in the maintenance of telomeres in yeast has been long established. Foundational
Immune system disorders
The vertebrate immune system utilizes Ku and C-NHEJ to repair physiological DSBs generated to create immune system genetic diversity. Recombination of the V, D, and J segments of immunoglobulins in lymphoid cells and class switch recombination of the T-cell receptor genes in mature B cells allows for the recognition of a wide variety antigens, which results in an adaptive immune response [193]. Ku70 and Ku80 knockout mice display many immune system abnormalities including B cell developmental
Conclusion
Ku is a versatile, multifunctional protein that has integral roles in DNA repair, telomere maintenance and DNA damage signaling. The central importance of Ku to cellular homeostasis is best demonstrated by the knockout of Ku in mouse models, which results in a complex phenotype, highlighted by extreme immunodeficiency, susceptibility to cancer and accelerated aging. Interestingly, despite the identification of human diseases stemming from mutation in the majority of other important DSB and DDR
Acknowledgement
This work was supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant to C.S.P.
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