Fig. 2. SR protein assembly onto exon 6D sequences. (A) Graphic representation of exon 6D and the location of the substrates used in the RNA affinity chromatography assay. The sequence of the wild-type substrate RNA used in the experiment is indicated together with PPE and U-to-C, U-to-A and U-to-G mutations. (B) SR protein binding to the exon 6D substrates. Exon 6D and control RNA substrates were covalently linked to agarose beads and incubated in HeLa cell nuclear extracts. Proteins bound to the substrates were eluted and immunoblotted with antibodies specific for all SR proteins (mAb 104) or antibodies specific for the SR proteins SC35 (mAb 532) and SF2/ASF (mAb AK-96). Lanes 1–7 contain the proteins eluted from beads covalently linked to the substrate RNAs indicated. Lane 8 contains 10 µl of HeLa cell nuclear extract.