Localization of nascent NADPH-cytochrome c reductase in rat liver microsomes

Rat liver microsomes incubated with [3H] puromycin in high salt buffer were digested with a mixture of protease, trypsin and chymotrypsin, in both the presence and absence of 1% deoxycholate. Our observations revealed that the proteolysis of peptidyl puromycin labeled with [3H] puromycin was at least partially protected by the presence of microsomal membrane. Immunochemical analyses have further shown that most of the nascent NADPH-cytochrome c reductase in the microsomes was digested with the proteases while serum albumin was effectively protected from the digestion. It is thus proposed that NADPH-cytochrome c reductase synthesized on the membrane bound ribosomes is not transported to the vesicular cavity but directly to the outer surface of the microsomal membrane in a form which is accessible to the proteases.