Ku is a dimeric protein complex that binds to
DNA double-strand break
ends and is required for the
non-homologous end joining (NHEJ) pathway of
DNA repair. Ku is evolutionarily conserved from bacteria to humans. The ancestral bacterial Ku is a
homodimer (two copies of the same protein bound to each other).[2] Eukaryotic Ku is a
heterodimer of two
polypeptides,
Ku70 (XRCC6) and
Ku80 (XRCC5), so named because the
molecular weight of the human Ku proteins is around 70 kDa and 80 kDa. The two Ku subunits form a basket-shaped structure that threads onto the
DNA end.[1] Once bound, Ku can slide down the DNA strand, allowing more Ku molecules to thread onto the end. In higher eukaryotes, Ku forms a complex with the
DNA-dependent protein kinase catalytic subunit (DNA-PKcs) to form the full DNA-dependent
protein kinase, DNA-PK.[3] Ku is thought to function as a molecular scaffold to which other proteins involved in NHEJ can bind, orienting the double-strand break for ligation.
The Ku70 and Ku80 proteins consist of three structural
domains. The
N-terminal domain is an
alpha/beta domain. This domain only makes a small contribution to the dimer interface. The domain comprises a six-stranded
beta sheet of the
Rossmann fold.[4] The central domain of Ku70 and Ku80 is a
DNA-binding
beta-barrel domain. Ku makes only a few contacts with the sugar-phosphate backbone, and none with the
DNA bases, but it fits
sterically to
major and
minor groove contours forming a ring that encircles duplex DNA, cradling two full turns of the DNA molecule. By forming a bridge between the broken DNA ends, Ku acts to
structurally support and align the DNA ends, to protect them from degradation, and to prevent promiscuous binding to unbroken DNA. Ku effectively aligns the DNA, while still allowing access of
polymerases,
nucleases and
ligases to the broken DNA ends to promote end joining.[5] The
C-terminal arm is an
alpha helical region which embraces the central
beta-barrel domain of the opposite
subunit.[1] In some cases a fourth domain is present at the C-terminus, which binds to DNA-dependent
protein kinase catalytic subunit.[6]
In many organisms, Ku has additional functions at
telomeres in addition to its role in DNA repair.[9]
Abundance of Ku80 seems to be related to species longevity.[10]
Aging
Mutant mice defective in Ku70, or Ku80, or double mutant mice deficient in both Ku70 and Ku80 exhibit early aging.[11] The mean lifespans of the three mutant mouse strains were similar to each other, at about 37 weeks, compared to 108 weeks for the wild-type control. Six specific signs of aging were examined, and the three mutant mice were found to display the same aging signs as the control mice, but at a much earlier age. Cancer incidence was not increased in the mutant mice. These results suggest that Ku function is important for longevity assurance and that the NHEJ pathway of DNA repair (mediated by Ku) has a key role in repairing DNA double-strand breaks that would otherwise cause early aging.[12] (Also see
DNA damage theory of aging.)
Plants
Ku70 and Ku80 have also been experimentally characterized in plants, where they appear to play a similar role to that in other eukaryotes. In rice, suppression of either protein has been shown to promote
homologous recombination (HR)[13] This effect was exploited to improve
gene targeting (GT) efficiency in Arabidopsis thaliana. In the study, the frequency of HR-based GT using a
zinc-finger nuclease (ZFN) was increased up to sixteen times in ku70 mutants[14] This result has promising implications for genome editing across eukaryotes as DSB repair mechanisms are highly conserved. A substantial difference is that in plants, Ku is also involved in maintaining an
alternate telomere morphology characterized by blunt-ends or short (≤ 3-nt) 3’ overhangs.[15] This function is independent of the role of Ku in DSB repair, as removing the ability of the Ku complex to translocate along DNA has been shown to preserve blunt-ended telomeres while impeding DNA repair.[16]
Bacteria and archaea
Bacteria usually have only one Ku gene (if they have one at all). Unusually, Mesorhizobium loti has two, mlr9624 and mlr9623.[17]
Archaea usually also only have one Ku gene (for the ~4% of species that have one at all). The evolutionary history is blurred by extensive horizontal gene transfer with bacteria.[18]
Bacterial and archaeal Ku proteins are unlike their eukaryotic counterparts in that they only have the central beta-barrel domain.
Name
The name 'Ku' is derived from the surname of the Japanese patient in which it was discovered.[19]
^Sugihara T, Wadhwa R, Kaul SC, Mitsui Y (April 1999). "A novel testis-specific metallothionein-like protein, tesmin, is an early marker of male germ cell differentiation". Genomics. 57 (1): 130–6.
doi:
10.1006/geno.1999.5756.
PMID10191092.
^Harris R, Esposito D, Sankar A, Maman JD, Hinks JA, Pearl LH, Driscoll PC (January 2004). "The 3D solution structure of the C-terminal region of Ku86 (Ku86CTR)". J. Mol. Biol. 335 (2): 573–82.
doi:
10.1016/j.jmb.2003.10.047.
PMID14672664.
Ku is a dimeric protein complex that binds to
DNA double-strand break
ends and is required for the
non-homologous end joining (NHEJ) pathway of
DNA repair. Ku is evolutionarily conserved from bacteria to humans. The ancestral bacterial Ku is a
homodimer (two copies of the same protein bound to each other).[2] Eukaryotic Ku is a
heterodimer of two
polypeptides,
Ku70 (XRCC6) and
Ku80 (XRCC5), so named because the
molecular weight of the human Ku proteins is around 70 kDa and 80 kDa. The two Ku subunits form a basket-shaped structure that threads onto the
DNA end.[1] Once bound, Ku can slide down the DNA strand, allowing more Ku molecules to thread onto the end. In higher eukaryotes, Ku forms a complex with the
DNA-dependent protein kinase catalytic subunit (DNA-PKcs) to form the full DNA-dependent
protein kinase, DNA-PK.[3] Ku is thought to function as a molecular scaffold to which other proteins involved in NHEJ can bind, orienting the double-strand break for ligation.
The Ku70 and Ku80 proteins consist of three structural
domains. The
N-terminal domain is an
alpha/beta domain. This domain only makes a small contribution to the dimer interface. The domain comprises a six-stranded
beta sheet of the
Rossmann fold.[4] The central domain of Ku70 and Ku80 is a
DNA-binding
beta-barrel domain. Ku makes only a few contacts with the sugar-phosphate backbone, and none with the
DNA bases, but it fits
sterically to
major and
minor groove contours forming a ring that encircles duplex DNA, cradling two full turns of the DNA molecule. By forming a bridge between the broken DNA ends, Ku acts to
structurally support and align the DNA ends, to protect them from degradation, and to prevent promiscuous binding to unbroken DNA. Ku effectively aligns the DNA, while still allowing access of
polymerases,
nucleases and
ligases to the broken DNA ends to promote end joining.[5] The
C-terminal arm is an
alpha helical region which embraces the central
beta-barrel domain of the opposite
subunit.[1] In some cases a fourth domain is present at the C-terminus, which binds to DNA-dependent
protein kinase catalytic subunit.[6]
In many organisms, Ku has additional functions at
telomeres in addition to its role in DNA repair.[9]
Abundance of Ku80 seems to be related to species longevity.[10]
Aging
Mutant mice defective in Ku70, or Ku80, or double mutant mice deficient in both Ku70 and Ku80 exhibit early aging.[11] The mean lifespans of the three mutant mouse strains were similar to each other, at about 37 weeks, compared to 108 weeks for the wild-type control. Six specific signs of aging were examined, and the three mutant mice were found to display the same aging signs as the control mice, but at a much earlier age. Cancer incidence was not increased in the mutant mice. These results suggest that Ku function is important for longevity assurance and that the NHEJ pathway of DNA repair (mediated by Ku) has a key role in repairing DNA double-strand breaks that would otherwise cause early aging.[12] (Also see
DNA damage theory of aging.)
Plants
Ku70 and Ku80 have also been experimentally characterized in plants, where they appear to play a similar role to that in other eukaryotes. In rice, suppression of either protein has been shown to promote
homologous recombination (HR)[13] This effect was exploited to improve
gene targeting (GT) efficiency in Arabidopsis thaliana. In the study, the frequency of HR-based GT using a
zinc-finger nuclease (ZFN) was increased up to sixteen times in ku70 mutants[14] This result has promising implications for genome editing across eukaryotes as DSB repair mechanisms are highly conserved. A substantial difference is that in plants, Ku is also involved in maintaining an
alternate telomere morphology characterized by blunt-ends or short (≤ 3-nt) 3’ overhangs.[15] This function is independent of the role of Ku in DSB repair, as removing the ability of the Ku complex to translocate along DNA has been shown to preserve blunt-ended telomeres while impeding DNA repair.[16]
Bacteria and archaea
Bacteria usually have only one Ku gene (if they have one at all). Unusually, Mesorhizobium loti has two, mlr9624 and mlr9623.[17]
Archaea usually also only have one Ku gene (for the ~4% of species that have one at all). The evolutionary history is blurred by extensive horizontal gene transfer with bacteria.[18]
Bacterial and archaeal Ku proteins are unlike their eukaryotic counterparts in that they only have the central beta-barrel domain.
Name
The name 'Ku' is derived from the surname of the Japanese patient in which it was discovered.[19]
^Sugihara T, Wadhwa R, Kaul SC, Mitsui Y (April 1999). "A novel testis-specific metallothionein-like protein, tesmin, is an early marker of male germ cell differentiation". Genomics. 57 (1): 130–6.
doi:
10.1006/geno.1999.5756.
PMID10191092.
^Harris R, Esposito D, Sankar A, Maman JD, Hinks JA, Pearl LH, Driscoll PC (January 2004). "The 3D solution structure of the C-terminal region of Ku86 (Ku86CTR)". J. Mol. Biol. 335 (2): 573–82.
doi:
10.1016/j.jmb.2003.10.047.
PMID14672664.