Imatinib-responsive phosphoproteins. (A) The 4 cell lines that were used for targeted analysis (BV173, K562, KU812, and SUP-B15) were treated with diluent (CON) or imatinib (IM) to inhibit Bcr-Abl kinase activity. Samples were immunoblotted using antiphosphotyrosine antibody. (B) Cells treated as described in panel A were immunoblotted for phosphoproteins that exhibited a change in phosphorylation level, as determined by SILAC analysis in K562 cells. Phospho-Gab1 (Y406) was identified by mass spectrometry, but a phosphospecific antibody that recognizes this site was not available; therefore, we used a p-Gab1 (Y627) antibody. Protein loading was normalized with the use of β-actin.