Figure 4

Binding of STAT5A to Src kinases interferes with dimerization. (A) HeLa T-REx HA-EpoR cells stably expressing STAT5A-eYFP were transfected with STAT5A-FLAG. The cells were treated with 5 ng/ml doxyxcyline for 24 h to induce the expression of the HA-tagged EpoR and stimulated with 5 U/ml Epo for 30 minutes or left untreated. HeLa T-REx vSrc-dsRed cells stably expressing STAT5A-eYFP were transfected with STAT5A-FLAG. The expression of vSrc-dsRed was induced for 8 h with 5 ng/ml doxycycline or the cells were left untreated. STAT5A-eYFP was immunoprecipitated from cell lysates using a GFP antibody and analyzed by immunoblotting for the presence of STAT5A-FLAG. The expression and phosphorylation of STAT5A-eYFP and STAT5A-FLAG was analyzed in the WCL using antibodies against pY^694/699-STAT5A/B, GFP and the FLAG-tag. (B) HeLa T-REx HA-EpoR cells stably expressing STAT5A-eYFP were treated with 5 ng/ml doxyxcyline for 24 h to induce the expression of the human EpoR and stimulated with 5 U/ml Epo for 30 minutes or left untreated. HeLa T-REx vSrc-dsRed cells stably expressing STAT5A-eYFP or a STAT5A^S710F-eYFP were treated with 5 ng/ml doxyxcline for 8 h or the cells were left untreated. Cellular extracts were prepared under native conditions and STAT5A-eYFP dimers were separated from monomers by blue native PAGE electrophoresis (NP). STAT5A-eYFP dimer complexes were measured by the detection of the eYFP fluorescence. The cellular extracts were subjected to immunoblotting using antibodies against pY^694/699-STAT5A/B, STAT5A, Src and the HA-tag of the EpoR. (C) Confocal microscopy analysis of HeLa T-REx FRT cells co-expressing vSrc-dsRed together with STAT5A^S710F-eYFP (upper panel), a serine phosphorylation mimicking mutant STAT5A^S710D-eYFP (middle panel) or a serine phosphorylation deficient mutant STAT5A^S710A-eYFP (lower panel). Methanol fixation was performed 24 h after transfection. Scale bars: 20 μm.