In vitro synthesis, glycosylation, and membrane insertion of the four subunits of Torpedo acetylcholine receptor

Abstract

We have characterized the early biosynthetic forms of the Torpedo electroplax acetylcholine receptor by using a cell-free protein synthesizing system. We obtained primary translation products of ≈38, 50, 49, and 60 kilodaltons for the α, ß, γ, and δ polypeptides, respectively, by using immunoprecipitation with subunit-specific antisera. These chains could each be labeled by the formylated initiator [35S]Met-tRNA. On cotranslational incubation with pancreatic rough microsomes, glycosylated forms of each subunit were obtained that had molecular weights close to those of their mature authentic counterparts. Extensive trypsinization reduced the glycosylated forms of the receptor subunits to glycosylated membrane-protected fragments of ≈35(α), 37(ß), 45(γ), and 44(δ) kilodaltons. In this system, then, each receptor chain spans the membrane at least once. This in vitro-synthesized material apparently exhibited neither oligomeric assembly nor -bungarotoxin binding.