Polypeptide components of human small nuclear ribonucleoproteins.

Abstract

Small nuclear RNA molecules (snRNA) are associated with polypeptides in vivo, forming small nuclear ribonucleoprotein complexes (snRNP). 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 snRNA, whereas antisera of the anti-RNP type precipitate only U1 snRNP. These 2 types of autoimmune antisera (from patients with systemic lupus erythematosus) were used 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 4 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 snRNP. Based on the radioimmunoassay, 3 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.