Biogenesis of endoplasmic reticulum membrane in rat liver cells. I. Intracellular sites of synthesis of cytochrome b5 and NADPH-cytochrome c reductase.

The sites of synthesis of microsomal membrane proteins, NADPH-cytochrome c reductase and cytochrome b5, were investigated by three methods; the in vitro synthesis of these proteins by isolated rough microsomes, the immunoprecipitation of polyribosomes carrying their nascent peptides, and the immunoprecipitation of in vivo-labeled nascent peptides. The in vitro incorporation experiment confirmed that the synthesis of these microsomal proteins was carried out by the bound polyribosomes of rough microsomes. When free and bound polyribosomes were separately examined by the other two methods, we found that NADPH-cytochrome c reductase was synthesized by both classes of polyribosomes whereas cytochrome b5 was synthesized only by bound polyribosomes.