We report a novel modification of spliceosome proteins Sm D1, Sm D3, and Sm B/B 0. L292 mouse fibroblasts were labeled in vivo with [H]methionine. Sm D1, Sm D3, and Sm B/B 0 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 containin...