Periodic Ubiquitination of Ku70 subunit regulates the cell cycle related activity of Ku protein

Abstract

Ku, a heterodimer of Ku70 and Ku80 subunits, is well-known for its role in DNA repair as the DNA-binding component of DNA-PKcs. In addition, it is involved in other important cellular processes including replication initiation. Periodic pphosphorylation of Ku70 by cyclin dependent kinases (Cdk) inhibits its interaction with replication origin, resulting in prevention of re-replication during S, G2 and M-phases. Interestingly, Ku70 is also ubiquitinated at K299 and K605 residues that are located on its dimerization and DNA-binding interfaces, respectively, suggesting crucial effect on its function by the posttranslational modification. Significantly, when the target lysine residues are mutated, Ku70 is ubiquitinated on alternative sites which are also located in the DNA-binding domain. Moreover, the identification of Cdc20 as the responsible ubiquitin ligase implicates periodicity in Ku70 ubiquitination. Expectedly, Ku70 ubiquitination occurs once at G1-phase and again at early S-phase, affecting its dimerization with Ku80. Overall, the earlier observation of Ku binding to replication origin during early G1 and S-phases along with detailed time-point experiments in the present study establish that, in the absence of Cdk activity, the windows of reversible Ku70 ubiquitination during G1 and early S-phases remove Ku from replication origin after its replication initiation related function.