Immunoprecipitation of U2 snRNP particles with antibody B2.4–1. (A) Coomassie blue-stained SDS gel after immunoprecipitation of the 17S, 15S, and 12S U2 snRNP fractions in the absence (−) or presence (+) of antibody B2.4–1. Arrows indicate the 146-kDa protein. R, reference proteins as shown in Figure 2. (B) Immunoblot of a parallel SDS gel incubated with antibody B2.4–1. (C) After stripping of the membrane the same blot was incubated with mAb 66. The arrowhead indicates SF3a66. (D) A Northern blot of snRNAs immunoprecipitated from the 17S, 15S, and 12S U2 snRNP fractions (IP, input) in the absence of antibody (−) or in the presence of antibody B2.4–1 (146) or mAb 66 (66) was probed with a radiolabeled antisense U2 snRNA transcript. The input corresponds to the amount of fraction used for the immunoprecipitations.