XRCC1 phosphorylation affects aprataxin recruitment and DNA deadenylation activity

Aprataxin (APTX) is a DNA-adenylate hydrolase that removes 5'-AMP blocking groups from abortive ligation repair intermediates. XRCC1, a multi-domain protein without catalytic activity, interacts with a number of known repair proteins including APTX, modulating and coordinating the various steps of DNA repair. CK2-phosphorylation of XRCC1 is thought to be crucial for its interaction with the FHA domain of APTX. In light of conflicting reports, the importance of XRCC1 phosphorylation and APTX function is not clear. In this study, a phosphorylation mutant of XRCC1 designed to eliminate APTX binding was stably expressed in Xrcc1^-/- cells. Analysis of APTX-GFP accumulation at micro-irradiation damage confirmed that phosphorylated XRCC1 is required for APTX recruitment. APTX-mediated DNA deadenylation activity (i.e., 5'-AMP removal) was measured in extracts of cells expressing wild-type XRCC1 or the XRCC1 phosphorylation mutant, and compared with activity in APTX-deficient and APTX-complemented human cells. APTX activity was lower in extracts from Xrcc1^-/- and XRCC1 phosphorylation mutant cells compared to the robust activity in extract from wild-type XRCC1 expressing cells. Taken together, results verify that interaction with phosphorylated XRCC1 is a requirement for significant APTX recruitment to cellular DNA damage and enzymatic activity in cell extracts.