SBDS cosediments with the 60S ribosomal precursor subunit and associates with the 28S ribosomal RNA. (A) HeLa cell lysates were fractionated on a 10% to 30% sucrose gradient by ultracentrifugation. Absorbance at 254 nM across the gradient is shown (top panel). Proteins were precipitated from equal aliquots of each fraction and immunoblotted for SBDS (middle panel). RNA was extracted from equal volumes of each fraction and analyzed by agarose gel electrophoresis and ethidium bromide staining (bottom panel). Data shown are representative of results obtained from 3 independent experiments. (B) Endogenous SBDS was immunoprecipitated from nuclear extracts of HeLa cells, healthy control lymphoblasts (control), or DF277 lymphoblasts derived from an SDS patient. Preimmune serum was used as a negative control for the immunoprecipitation (lane 2). RNA was extracted from the immunoprecipitates and analyzed by agarose gel electrophoresis and ethidium bromide staining. Data shown are representative of results obtained from 3 independent experiments.