Cell polarity protein Par3 complexes with DNA-PK via Ku70 and regulates DNA double-strand break repair

The partitioning-defective 3 (Par3),a key component in the conserved Par3/Par6/aPKC complex,plays fundamental roles in cell polarity.Herein we report the identification of Ku70 and Ku80 as novel Par3-interacting proteins through an in vitro binding assay followed by liquid chromatography-tandem mass spectrometry.Ku70/Ku80 proteins are two key regulatory subunits of the DNA-dependent protein kinase (DNA-PK),which plays an essential role in repairing double-strand DNA breaks (DSBs).We determined that the nuclear association of Par3 with Ku70/KuS0 was enhanced by y-irradiation (IR),a potent DSB inducer.Furthermore,DNA-PKcs,the catalytic subunit of DNA-PK,interacted with the Par3/Ku70/Ku80 complex in response to IR.Par3 over-expression or knockdown was capable of up-or downregulat- ing DNA-PK activity,respectively.Moreover,the Par3 knockdown cells were found to be defective in random plasmid integration,defective in DSB repair following IR,and radiosensitive,phenotypes similar to that of Ku70 knockdown cells.These findings identify Par3 as a novel component of the DNA-PK complex and implicate an unexpected link of cell polarity to DSB repair.