Ku (protein)

Ku is a protein 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 human. The ancestral bacterial Ku is a homodimer (two copies of the same protein bound to each other). 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. 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. Ku is thought to function as a molecular scaffold to which other proteins involved in NHEJ can bind.

Both subunits of Ku have been experimentally knocked out in mice. These mice exhibit chromosomal instability, indicating that NHEJ is important for genome maintenance.

In many organisms, Ku has additional functions at telomeres in addition to its role in DNA repair.

Abundance of Ku80 seems to be related to species longevity.