Mutation of a BRCT domain selectively disrupts DNA single-strand break repair in noncycling Chinese hamster ovary cells.

The DNA single-strand break repair protein XRCC1 contains a BRCT domain that binds and stabilizes intracellular DNA ligase III protein. We recently demonstrated that this domain is largely dispensable for single-strand break repair and cellular resistance to DNA base damage in cycling cells. Here, we report that the BRCT domain is required for single-strand break repair in noncycling cells. Mutations that disrupt the BRCT domain and prevent DNA ligase III interaction abolished XRCC1-dependent repair in serum-starved Chinese hamster ovary cells, and reentry into cell cycle induced by readdition of serum restored repair. Elevating DNA ligase III levels in XRCC1 mutant cells using proteosome inhibitors or by expressing XRCC1 protein in which the BRCT domain is disrupted but can still bind DNA ligase III failed to restore repair in noncycling cells. The requirement for the BRCT domain for DNA strand break repair is thus for more than simply binding and stabilizing DNA ligase III. These data provide evidence in support of a selective role for a DNA repair protein or protein domain in noncycling cells. We propose that the XRCC1 C-terminal BRCT domain may be important for genetic stability in postmitotic cells in vivo.