Inhibition of glycosylation with tunicamycin blocks assembly of newly synthesized acetylcholine receptor subunits in muscle cells.

We have characterized the oligosaccharide chains of the alpha subunit of acetylcholine receptor of the clonal mouse muscle cell line BC3H-1 by their sensitivity to end-beta-N-acetylglucosaminidase H and by comparison of the native glycosylated polypeptide with the nonglycosylated form made in tunicamycin-treated cells. These studies indicate that the native alpha subunit has a single N-asparagine-linked oligosaccharide chain of the "high mannose" or "simple" type. Furthermore, these results considered in light of our previous characterization of the alpha subunit synthesized in vitro suggest that the alpha subunit contains no "complex"-type N-linked oligosaccharide chains. We have investigated the role of glycosylation in the biogenesis of the acetylcholine receptor. Receptor biogenesis in normal cells involves the assembly of newly synthesized alpha subunits into a form active for binding alpha-bungarotoxin. This process is only 30% efficient and is complete by 30 min postsynthesis. When glycosylation is inhibited by tunicamycin, alpha subunit synthesis is inhibited only slightly but assembly into an alpha-bungarotoxin binding species is reduced dramatically.