1. ¹Key Laboratory of Proteomics and Laboratory of Molecular Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200031, China
2. ²SHARF Laboratory, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200031, China
3. ³Graduate School of the Chinese Academy of Sciences, Shanghai 200031, China
4. ⁴Department of Biochemistry, The Chinese University of Hong Kong, Hong Kong SAR, China
5. ⁵Division of Molecular Radiation Biology, Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, Texas, USA

Correspondence: Zhengjun Chen, Tel/Fax: +86-21-54921081 E-mail: zjchen@sibs.ac.cn

Received 18 December 2006; Revised 19 December 2006; Accepted 20 December 2006.

Abstract

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/Ku80 was enhanced by -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 downregulating 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.