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.