Serine 1191 is the phosphorylation site that serves as a docking site for Pin1 in BRCA1 upon DNA damage. Coimmunoprecipitation (IP) of endogenous BRCA1 and Flag-tagged Pin1 in MCF7 (A) and T47D (B). Cells were irradiated one hour prior to cell lysis and IP. C, Coimmunoprecipitation of endogenous Pin1 with HA-tagged BRCA1 transfected into HEK293 cells. Cells were pretreated as in A and B. D, GST-pulldown of endogenous BRCA1 with GST-Pin1 in MCF7 cells. MCF7 cells were irradiated as indicated. E, The BRCA1 ORF has nine candidate SP-motifs and six candidate TP-motifs. Sketch depicts Myc-tagged fragments that were created to determine Pin1-binding sites. F, GST-Pin1 pulldown of Myc-tagged BRCA1 fragments as indicated in E with GST-Pin1 in HEK293 cells. Cells were transfected with BRCA1 fragments and irradiated 48 hours later, followed by lysis 1 hour after irradiation. G, GST-Pin1 pulldown of S→A mutations in candidate SP-motifs in fragment 5. HEK293 cells were transfected with the respective fragments, irradiated, and lysed 1 hour later for GST-Pin1 pulldown.