Fate of polypeptides synthesized on rough microsomal vesicles in a messenger-dependent rabbit reticulocyte system.

Abstract

A system for measuring protein synthesis in vitro on hepatic RM [rough microsomes] is described. RM were incubated in a potent messenger-dependent rabbit reticulocyte lysate under conditions where polypeptide chain elongation on RM polyribosomes led to completion and release of full-sized products. When subfractionation of reaction mixtures was subsequently performed, virtually all the products, including albumin, were recovered in the vesicles, not in the extravesicular space (cytoplasmic compartment). Polypeptides synthesized by RM were relatively resistant to proteolysis when reaction mixtures were incubated with trypsin and chymotrypsin, whereas these enzymes degraded most of the product syntehsized by the use of membrane-free RNA. Most, if not all, proteins normally synthesized on rat liver RER [rough endoplasmic reticulum] are passed either deep into or across this membrane after their release from bound ribosomes. Few, if any, proteins appear to be released directly to the cytosol.