Polypeptide components of human small nuclear ribonucleoproteins.

Small nuclear RNA molecules (snRNAs) are associated with polypeptides in vivo, forming small nuclear ribonucleoprotein complexes (snRNPs). These snRNP complexes are targets for certain autoimmune antisera. Antisera of the type anti-Sm precipitate (and therefore define) a class including U1, U2, U4, U5, and U6 snRNAs, whereas antisera of the anti-RNP type precipitate only U1 snRNPs. We used these two types of autoimmune antisera (from patients with systemic lupus erythematosus) to study the polypeptide components in human cells. Sequential immunoprecipitation of the complexes from nuclear extracts with anti-RNP and anti-Sm antibodies, along with radioimmunoassay of protein transfers, identified four polypeptides of 14,000 (P14), 17,000 (P17), 26,000 (P26), and 27,000 (P27) daltons that are present on all members of this class, whereas a 68,000-dalton (P68) polypeptide is present only on U1 snRNPs. Based on the radioimmunoassay, three of these polypeptides, P17, P26, and P27, are also the antigens for anti-Sm antisera, whereas P68 is the antigen for anti-RNP antisera. Long-term phosphate labeling experiments show that the only detectably phosphorylated polypeptide is P68, which contains phosphoserine.