SRp20 and SC35 interact with U1 70K and U2AF35. (A) Plot of FRET efficiencies ± SD (mean for 7–14 cells) between ECFP and EYFP fusion proteins measured by FRET acceptor photobleaching. P-values were obtained from the t test comparing the FRET efficiencies with and without DRB treatment. (B) FRET between U1 70K and SC35 measured by FLIM. HeLa cells were transfected with EGFP-U1 70K and cotransfected with either mCherry-C1 or mCherry-SC35. Confocal images are of transfected cells and FLIM images are of the same cells, in which FRET efficiency and FRET amplitude are shown in pseudocolor. The color scale with the respective efficiency (%) is indicated. Top, EGFP-U1 70K + mCherry-C1; Middle, EGFP-U1 70K + mCherry-SC35; Bottom, EGFP-U1 70K + mCherry-SC35 in the presence of DRB. Arrowheads indicate high FRET within the nucleoplasm. Bars, 10 μm. (C) FRET efficiencies determined by FLIM for interaction of SC35 with U1 70K and U2AF35 in the presence and absence of DRB. Plot is of mean FRET efficiencies ± SD for 8–11 cells. P-values were obtained as described in A.