Purification of the major UsnRNPs from broad bean nuclear extracts and characterization of their protein constituents.

Small nuclear ribonucleoprotein particles containing the five major nucleoplasmic snRNAs U1, U2, U4, U5 and U6 as well as two smaller sized snRNAs were purified from broad bean nuclear extracts by anti-m3G, monoclonal antibody, immunoaffinity chromatography. We have so far defined 13 polypeptides of approximate mol. wts. of 11 kd, 11.5 kd, 12.5 kd, 16 kd, 17 kd, 17.5 kd, 18.5 kd, 25 kd (double band), 30 kd, 31 kd, 35 kd, 36 kd and 54 kd. Upon fractionation of the UsnRNPs by anion exchange chromatography, essentially pure U5 snRNPs were obtained, containing the 11 kd, 11.5 kd, 12.5 kd, 16 kd, 17 kd, 17.5 kd, 35 kd and 36 kd polypeptides. These may therefore represent the common snRNP polypeptides and which may also be present in the other snRNPs. By immunoblotting studies, using anti-Sm sera and mouse monoclonal antibodies we show that the 35 kd and 36 kd proteins are immunologically related to the mammalian common B/B' proteins. The broad bean 16 kd and 17 kd proteins appear to share structural elements with the mammalian D protein. The three proteins of mol. wts. 11 kd, 11.5 kd and 12.5 kd probably represent the broad bean polypeptides E, F, and G. Cross-reactivity of proteins of mol. wts of 30 kd and 31 kd with Anti-(U1/U2)RNP antibodies suggests that they may represent the broad bean A and B" polypeptides. The 54 kd protein and the 18.5 kd protein could be candidates for the U1 specific 70 k and C polypeptides. Our results demonstrate a strong similarity between the overall structure of broad bean and mammalian snRNPs.