Figure 2 Mre11 controls CtIP phosphorylation in normally dividing cells. Western blot analyses with primary antibodies indicated at left. Tubulin used as protein loading control. Endogenous and cDNA Mre11 alleles (top) are as described in the text and Figure 1 legend. (a) Proteasome inhibition restores CtIP levels in the absence of MRN. MEFs were untreated (−) or treated (+) with MG-132 for 8 h. (b) CtIP phosphorylation depends on the Mre11 C terminus. MEFs of the indicated genotypes (top) were untreated (−) or treated (+) with MG-132 for 8 h. Extracts were subsequently untreated (−)
or treated (+) with lambda phosphatase. Phosphorylated (CtIPP)
and hypophosphorylated (CtIP) forms of CtIP on the western blot are indicated (left). (c) BRCA1–CtIP complex formation depends on the Mre11 C terminus. Western blot of coimmunoprecipitates using anti-BRCA1 antibody (IP) or beads only (M). Three percent of the lysate is shown for comparison (I). Phosphorylated (CtIPP) and hypophosphorylated (CtIP) forms of CtIP are indicated (left). (d) CtIP levels are influenced by CDK2. Wild-type (+/+) or CDK2 knockout (−/−) MEFs were untreated (−) or treated with 10 μM of the CDK inhibitor roscovitine (+) for 48 h.