Biosynthesis of intrinsic sarcoplasmic reticulum proteins during differentiation of the myogenic cell line L6.

Temporal patterns of biosynthesis and the rates of turnover of the 53,000- and 160,000-Da glycoproteins from the sarcoplasmic reticulum membrane were determined and compared with the rates of biosynthesis of the (Ca2+ + Mg2+)-dependent ATPase in the differentiating rat myogenic continuous cell line, L6. Cells were labeled at various stages of differentiation with [35S]methionine and sarcoplasmic reticulum proteins were isolated from Triton extracts of the cells by protein A-mediated immunoprecipitation with specific antibodies. The immunoprecipitates were separated by polyacrylamide gel electrophoresis and the radioactivity incorporated into particular protein bands was recorded by autoradiography. The patterns of biosynthesis and the rates of degradation of the three intrinsic sarcoplasmic reticulum proteins were similar, even though two of them were glycosylated and one was not. The glycoproteins and the ATPase were found in the microsomal fraction obtained from the cell homogenate but not in the cytosol. This suggests that the synthesis and processing of these intrinsic sarcoplasmic reticulum proteins occurred in membrane systems.