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In eucaryotic cells, the mechanism whereby proteins newly synthesized in the cytoplasm are transferred across cellular membranes is generally thought to involve synthesis of these proteins on ribosomes attached to membranes, primarily of the rough endoplasmic reticulum (RER). Proteins falling into this category, therefore, include those destined for secretion, for transport to inner compartments of other cellular organelles, e.g., lysosomes, Golgi complex, peroxisomes, and mitochondria, and those membrane proteins which form the tightly bound class of integral proteins or which are deposited asymmetrically toward the noncytoplasmic face of membranes. Such conclusions derive originally from studies (14, 17-19) which analyzed the directional discharge of polypeptides synthesized in vivo and in vitro on RER. This process has since become the subject of a large literature (recently reviewed, references 14 and 22), and now some of the details pertaining to the mechanism of this process have been elucidated, especially by experiments designed to test the "signal hypothesis" (2, 3, 5). It still remains to be determined, however, whether transfer of nascent polypeptides into or across membranes is the underlying feature common to all proteins synthesized on bound ribosomes. It has been suggested, for example, that a large fraction of the proteins synthesized by RER in nonsecretory tissues is released from bound ribosomes directly to the cytosol (1). Serine dehydratase, an inducible enzyme located exclusively in the cytosol, is apparently synthesized on RER in rat liver (12). Rothman (20) has argued, on kinetic grounds, that secreted hydrolases in the pancreas may be released into both the ER and the cytosol after their synthesis on bound ribosomes. There has even been a recent proposal (9), based on cytochemical observations in rat liver, that newly formed albumin is released from bound ribosomes to the cytosol and cytoplasmic surface of RER where it is subsequently transported (but see references 4, 7, and 19). In all of these studies, however, the question is whether or not such polypeptides represent bona fide populations of proteins which are normally released from bound ribosomes directly to the cytosol or to the cytoplasmic side of RER in vivo. The present study, therefore, considered this question for a tissue (rat liver) whose RER is well known to function in the synthesis of a heterogeneous population of both secreted and nonsecreted proteins. The fate of polypeptides synthesized in vitro on hepatic rough microsomes (RM) was followed under conditions which led to synthesis and release of primarily full-sized products. Such conditions were achieved by incubating RM in a potent messenger-dependent system derived from rabbit reticulocytes. The results described here clearly indicate that most, if not all, of these proteins are released either deep into or across this membrane.

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