Figure 1. The MRN complex controls CtIP protein levels in mammals.

(a–d) Western blot analyses with primary antibodies indicated at left and genotype of cells at top. GAPDH or tubulin used as protein loading controls.

(a) Comparison of CtIP levels. (left) Murine cells are B lymphocyte lines from two Mre11^ATLD1/ATLD1 and two Mre11^+/+ littermate mice. Human cells are immortalized fibroblasts from an ATLD1 patient, and these cells complemented with human Mre11 cDNA (WT). (Right) ATM knockout (ATM^−/−) and control (ATM^+/+) murine embryonic fibroblasts (MEFs).

(b–d) Analyses of MEFs with endogenous Mre11 alleles as follows; wild–type (Mre11⁺), null (Mre11⁻), and nuclease deficient (Mre11^H129N).

(b) CtIP deficiency in Mre11^−/− cells is observed in the cyclin A positive population (S/G2 phase). MEFs were synchronized at G0/G1 and released from serum starvation for the indicated time (hours).

(c–d) Mre11 cDNA alleles expressed in Mre11^−/− cells are as follows; empty expression vector (−), full length wild–type fused to a C–terminal tag of either 54 (C54) or 5 (C5) amino acids, or the ATLD1 78 amino acid C-terminal deletion (ATLD1).

(c) (left) A 54 amino acid C-terminal tag on Mre11 causes CtIP deficiency. (right) A 78 amino acid C-terminal deletion of Mre11 causes CtIP deficiency.

(d) The 54 amino acid C-terminal tag (left) or 78 amino acid C-terminal deletion (right) does not prevent ATM activation (ser¹⁹⁸⁷ autophosphorylation) induced by 10 Gy ionizing radiation (+).