EGFP-U2AF35 interacts with SF2/ASF in cultured mammalian cells. (A) Cell extracts prepared from 293T cells either transiently transfected with EGFP-U2AF35 (lanes 3 and 4) or mock transfected (lanes 5 and 6) were incubated with anti-GFP antibody bound to Sepharose beads. The bound proteins were analyzed by Western blotting with an anti-SF2/ASF antibody. Alternatively, the assay was performed in the presence of RNase (lanes 4 and 6). NTC, nontransfected cells. (B) Effect of DRB on the interaction between ECFP-U2AF35 and EYFP-SF2/ASF. Plot is of FRET efficiencies ± SD (mean for 9–27 cells) between ECFP + EYFP fusion pairs before and after DRB treatment. P-values were obtained as described in the Fig. 1 legend.