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

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Abstract
Membrane-bound and free polyribosomes were isolated from skeletal muscle of neonatal rats and messages were translated in a rabbit reticulocyte lysate treated with a Ca2+-dependent nuclease to reduce endogenous messenger translation. Newly synthesized calsequestrin and ATPase were isolated by antibody precipitation, followed by separation of the precipitates in SDS[sodium dodecyl sulfate]-polyacrylamide gels. Radioactivity in calsequestrin and the ATPase were counted in gel slices. Calsequestrin and the ATPase were both 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 unknown.