Isofocusing of antigenic small nuclear ribonucleoproteins. II. Preparative isolation

In a companion report (T.B. Okarma, W.S. Schrier, and R. Feinbaum, 1985, Anal. Biochem. 147, 27-37) the behavior of small nuclear ribonucleoproteins (snRNPs) in native isofocusing gels was characterized. This communication extends those findings and describes a gentle procedure for the preparative isolation of snRNPs in native form from cultured murine L-5178y leukemia cells using sucrose density gradient centrifugation, preparative isofocusing, and gel filtration chromatography. Isofocusing in granulated gels separated intact uridylic acid (U)-snRNPs from tRNA and La RNPs. The U-snRNPs remained immunoprecipitable by lupus antisera throughout fractionation. The final product obtained in 2% yield contained primarily U1 and U2 snRNAs and lesser amounts of U3, U4, U5, and U6, along with the core U-snRNP polypeptides A-G. The core polypeptides displayed apparent pI's which ranged from 4.5 to 9.5 when analyzed by two-dimensional gel electrophoresis. Proteins B (28,000), D (16,000), and E (13,000) exhibited isoelectric variants. The Sm determinant proteins B' (28,000) and E (13,000) isofocused as basic peptides with apparent pI's of 9.5 and 8.5, respectively. The purity of the final fractions compared well with that of immunoprecipitates and the procedure reproducibly generated yields of native snRNPs sufficient for in vitro studies of their biological function.