Assembly of the sarcoplasmic reticulum. Synthesis of calsequestrin and the Ca2+ + Mg2+ -adenosine triphosphatase on membrane-bound polyribosomes.

Membrane-bound and free polyribosomes were isolated from skeletal muscle of neonatal rats and messages were translated in a rabbit reticulocyte lysate treated with Ca2+ -dependent nuclease to reduce endogenous messenger translation. Newly synthesized calsequestrin and adenosine triphosphatase (ATPase) sere isolated by antibody precipitation, followed by separation of the precipitates in SDS-polyacrylamide gels. Radioactivity in calsequestrin and the ATPase were counted in gel slices. Calsewuestrin and the ATPase were both found to be synthesized on membrane-bound polyribosomes. Since calsequestrin is a glycoprotein, localized in Golgi regions in early stages of muscle cell differentiation, it is probable that its synthesis follows the pathway for synthesis of secreted proteins except that its destination is the luminal space of a cellular organelle. The disposition of the ATPase during synthesis is, as yet, unknown.