HCC1 interacts with both subunits of the U2AF heterodimer in vitro and in vivo. (A) Extracts prepared from 293T cells transiently transfected with either EGFP-U2AF35 and pCG-T7-HCC1.4 (lanes 3 and 4) or EGFP-U2AF35 (lanes 5 and 6) were incubated with anti-T7 antibody bound to Sepharose beads. The bound proteins were analyzed by Western blotting with anti-GFP antibody. Alternatively, the assay was performed in the presence of RNase (lanes 4 and 6). (B) U2AF65 interacts with HCC1 in cultured mammalian cells. Extracts prepared from 293T were incubated with either anti-HCC1 antibody bound to Sepharose beads (lanes 2 and 4) or Sepharose beads alone (lanes 3 and 5). The bound proteins were analyzed by Western blotting with anti-U2AF65 antibody. Alternatively, the immunoprecipitate was treated with RNase (lanes 4 and 5). (C) Effect of DRB on interactions of HCC1 with U2AF35 and U2AF65. Plot is of FRET efficiencies ± SD (mean for 8–18 cells) between ECFP and EYFP fusion proteins measured by FRET acceptor photobleaching. P-values were obtained as described in the Fig. 5 legend.