We report a novel modification of spliceosome proteins Sin D1, Sm D3, and Sm B/B'. L292 mouse fibroblasts were labeled in vivo with [H-3]methionine. Sm D1, Sin D3, and Sm B/B' were purified from either nuclear extracts, cytosolic extracts or a cytosolic 6S complex by immunoprecipitation of the Sm protein-containing complexes and then separation by electrophoresis on a polyacrylamide gel containing urea. The isolated Sm D1, Sm D3 or Sm B/B' proteins were hydrolyzed to amino acids and the products were analyzed by high-resolution cation exchange chromatography. Sm D1, Sm D3, and Sin B/B' isolated from nuclear fractions were all found to contain omega-N-G-monomethylarginine and symmetric omega-N-G,N-G-dimethylarginine, modifications that have been previously described. In addition, Sm D1, Sm D3, and Sm B/B' were also found to contain asymmetric omega-N-G,N-G-dimethylarginine in these nuclear fractions. Analysis of Sm B/B' from cytosolic fractions and Sm B/B' and Sm D1 from cytosolic 6S complexes showed only the presence of omega-N-G-monomethylarginine and symmetric omega-N-G,N-G'-dimethylarginine. These results indicate that Sin D1, Sm D3, and Sin B/B' are asymmetrically dimethylated and that these modified proteins are located in the nucleus. In reactions in which Sm D1 or Sm D3 was methylated in vitro with a hemagglutinin-tagged PRMT5 purified from HeLa cells, we detected both symmetric omega-N-G,NG'-dimethylarginine and asymmetric co-N-G,N-G-dimethylarginine when reactions were done in a Tris/HCI buffer, but only detected symmetric omega-N-G,N-G'-dimethylarginine when a sodium phosphate buffer was used. These results suggest that the activity responsible for the formation of asymmetric dimethylated arginine residues in Sm proteins is either PRMT5 or a protein associated with it in the immunoprecipitated complex. (C) 2004 Elsevier Inc. All rights reserved.