Identification of the 146-kDa protein in fractions of HeLa cell nuclei enriched in splicing factors and U2 snRNPs. (A) Coomassie blue-stained gel after SDS-PAGE of total nuclear extract (lane 1) and fractions enriched in splicing factors SF3a and SF3b (lane 2), SF3a (lane 3), SF3b (lane 4), and fractions enriched in U2 snRNPs of different composition: 17S, i.e., U2 snRNP associated with SF3a and SF3b (lane 5); 15S, containing U2 snRNP associated with SF3b (lane 6); and 12S, U2 snRNP lacking SF3a and SF3b (lane 7). R, reference proteins as shown in Figure 2. (B) Corresponding immunoblot with antibody B2.4–1 directed against the 146-kDa protein. (C) In parallel, a second filter was probed with mAb 66 directed against the 66-kDa component of SF3a.