Early Translational Events in the Synthesis of Acetylcholine Receptor

Abstract

Early events in the biosynthesis of the Torpedo acetylcholine receptor (AChR) have been studied, using a cell-free protein synthesizing system supplemented with canine pancreas microsomal membranes. Using subunit-specific antisera, it has been demonstrated that each AChR subunit is translated from a separate mRNA, and is integrated into the endoplasmic reticulum membrane by a process that results in a transmembrane orientation for each polypeptide. The in vitro synthesized subunits are glycosylated, but are apparently not assembled into a functional quaternary complex. Purified signal recognition protein (SRP), previously shown to be obligatory for the translocation of nascent secretory proteins across the microsomal membrane, was shown to be required as well for the integration of AChR subunits into the lipid bilayer. Therefore, AChR subunits apparently undergo membrane integration by the same mechanism established in the case of the simple viral transmembrane protein, vesicular stomatitis virus (VSV) G protein.