Overexpression of wild-type ATR potentiates the phosphorylation of BRCA1 at Ser 1423 in intact cells. (A) Overexpression of FLAG–ATRWT stimulates the HU-induced phosphorylation of Ser 1423 in HEK 293T cells. HEK 293T cells were transfected with an expression vector encoding AU1-BRCA1–ΔN together with empty expression vector (−) or plasmids encoding FLAG–ATRWT, or FLAG–ATRKI. After 24 h, the transfected cells were left untreated or exposed to 2 mM HU, and harvested 1 h later. Cellular extracts were separated by SDS-PAGE and subjected to immunoblot analysis with the α-pS-1423 antibody to detect Ser 1423-phosphorylated BRCA1. Immunoblotting with α-FLAG or α-AU1 antibodies was performed to monitor expression levels of the FLAG–ATR and AU1-BRCA1–ΔN constructs, respectively. α-, β-, and γ-phosphorylated forms of AU1-BRCA1–ΔN are marked. (B) Overexpression of FLAG–ATRWT stimulates UV light-induced phosphorylation of Ser 1423 in HEK 293T cells. The experiment was performed as described in A, except that the transfected HEK 293T cells were exposed to 50 J/m2 UV light, and then harvested 1 h later. Cell lysates were subjected to SDS-PAGE and immunoblot analysis with the indicated antibodies. (C) Relative contributions of ATM and ATR to the IR-induced phosphorylation of BRCA1 on Ser 1423. HEK 293T cells were cotransfected with expression plasmids encoding FLAG-tagged BRCA1 (1351–1552) together with an empty expression vector (−), FLAG–ATR, or FLAG–ATM plasmids. The FLAG–ATR and FLAG–ATM expression constructs encoded either wild-type (WT) or catalytically inactive (KI) proteins. After 24 h, the cells were either left untreated, or exposed to 25 Gy of IR and harvested 1 h later. Cell lysates were analyzed by SDS-PAGE and immunoblot analysis with the α-pS-1423 antibody to detect Ser 1423-phosphorylated BRCA1. Expression levels of FLAG–ATR, FLAG–ATM, and FLAG–BRCA1 (1351–1552) were monitored by immunoblotting with α-FLAG, and are presented at top and bottom, respectively.