Figure 2
STAT5A binds to the phosphorylated activation loop of SFK. (A) Domain structure of vSrc-dsRed. Selected amino acids are highlighted. A multiple sequence alignment of the activation loop of SFK is shown. Autophosphorylation site is highlighted (red). (*) conserved amino acids, (:) similar properties [51]. Bold characters highlight peptide sequence used for precipitation. (B + C) HeLa T-REx FRT cells stably expressing STAT5A-eYFP were transfected with the indicated vSrc-dsRed variants. Phosphorylation was analyzed 24 h after transfection using antibodies against pY416-Src, Src, pY694/699-STAT5A/B and STAT5A. CTRL = untransfected cells. (D) Quantification of relative subcellular distribution of STAT5A in HeLa T-REx FRT stably expressing STAT5A-eYFP and the indicated vSrc-dsRed mutants. Mean fluorescence intensity (MFI) of eYFP-fluorescence in the cytoplasm and nucleus were determined using the Zen 2012 software and changes in the ratio between the compartments were plotted. Data show means ± SD of n = 10 cells and were statistically evaluated by Student’s t-test. ***p < 0.0005, **p < 0.005, *p < 0.05, n.s. = not significant. (E) HeLa T-REx FRT cells stably expressing STAT5A-eYFP were transfected with vSrcK295N-dsRed or vSrcY416F-dsRed. Subcellular distribution of STAT5A-eYFP was analyzed 24 h after transfection by confocal microscopy. Scale bars: 20 μm. (F + G) HeLa T-REx FRT cells were co-transfected with plasmids coding for vSrc-dsRed (Hck-dsRed), vSrcK295N-dsRed (HckK269N-dsRed) or vSrcY416F-dsRed (HckY390F-dsRed) and STAT5A-eYFP. STAT5-eYFP was immunoprecipitated from cell lysates using a GFP antibody and analyzed by immunoblotting for the presence of vSrc-dsRed (Hck-dsRed) 24 h after transfection. Expression and phosphorylation of STAT5A and vSrc proteins was analyzed in the WCL using antibodies against pY416-Src, Src, Hck, pY694/699-STAT5A/B and GFP. (s) short exposure, (l) long exposure. (H) HeLa T-REx FRT cells expressing STAT5A-eYFP or STAT5AR618Q-eYFP were lysed and incubated with a Src-peptide containing tyrosine- or phosphotyrosine 416. Precipitates and WCL were analyzed by immunoblotting using a GFP-specific antibody.