Site-directed mutagenesis of the conserved N-glycosylation site on the nicotinic acetylcholine receptor subunits.
The role of the conserved N-glycosylation site on each subunit of the Torpedo acetylcholine receptor (AChR) in the biogenesis and function of the receptor was examined by expressing site-directed mutant subunits in Xenopus oocytes. Different mutant subunits caused different effects. The most striking effect was seen with the mutant gamma subunit which, when co-expressed with alpha, beta, and delta subunits, caused degradation of all the subunits. N-Glycosylation of the other subunits appears to contribute to stability of the subunits and/or efficient insertion of the receptor into the plasma membrane, but is not required for assembly. The AChRs containing the mutant alpha subunit formed functional ion channels in the plasma membrane and their activity could be blocked by alpha-bungarotoxin (alpha-BuTX). Thus, attachment of a carbohydrate moiety at the conserved N-glycosylation site is not an absolute requirement for the formation of ACh and alpha-BuTX binding sites.