The MRN complex controls CtIP protein levels in mammals. 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 Mre11ATLD1/ATLD1 and two Mre11+/+ littermate mice (left). Human cells are immortalized fibroblasts from an individual with ATLD1, and these cells are complemented with human Mre11 cDNA (WT). Right, ATM control (ATM+/+) and knockout (ATM−/−) murine embryonic fibroblasts (MEFs). (b) Analyses of MEFs with endogenous Mre11 alleles are as follows: wild-type (Mre11+); nuclease deficient (Mre11H129N) and null (Mre11−). CtIP deficiency in Mre11−/− cells is observed in the cyclin A–positive population (S and G2 phases). MEFs were synchronized at G0 and G1 and released from serum starvation for the indicated time. (c) 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). A 54-amino-acid C-terminal tag on Mre11 causes CtIP deficiency (left). A 78-amino-acid C-terminal deletion of Mre11 causes CtIP deficiency (right). (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 (+).