Studies on the Synthesis and Degradation of Proteins of the Endoplasmic Reticulum of Rat Liver

Abstract

The turnover of the protein constituents of the endoplasmic reticulum and certain cell fractions of rat liver has been examined by following the decay in specific radioactivity after a single administration of either uniformly labeled 14C-L-arginine or 14C-guanidino-L-arginine.Significantly shorter half-lives were obtained for all cell fractions when proteins were pulse-labeled with the guanidino-labeled arginine.The mean half-lie for both rough and smooth microsomal membrane fractions was 2.0 to 2.1 days with guanidino-labeled arginine compared to 5.2 to 5.8 days with uniformly labeled arginine.We ascribe such differences to differing degrees of isotope reutilization.A new technique using two administrations of the same amino acid but labeled with different isotopes was developed to examine relative rates of degradation of proteins of the microsomal membranes.Proteins of the membrane, including NADPH-cytochrome c reductase, cytochrome bs, and protein fractions solubilized by detergent and separated on diethylaminoethyl-cellulose, had different rates of degradation.The administration of phenobarbital to rats induces proliferation of endoplasmic reticulum in the liver.Phenobarbital increased the rate of synthesis of some but not all proteins of the endoplasmic reticulum.The rate of synthesis of NADPH-cytochrome c reductase was increased 3-to 4-fold over the basal rate; the rate of cytochrome bS synthesis was increased to an insignificant extent.The rate of synthesis of some membrane proteins was decreased during phenobarbital administration.Phenobarbital had no effect on the rate of degradation of total membrane protein.These results indicate that the endoplasmic reticulum is a dynamic system with a rapid rate of renewal and are consistent with a mosaic model of membrane biogenesis.