Contributions of N-linked glycosylation to the expression of a functional α7-nicotinic receptor in Xenopus oocytes

The alpha 7 subunit of the neuronal nicotinic acetylcholine receptor, when expressed in Xenopus oocytes, forms homooligomeric ligand-gated ion channels that are blocked by a snake toxin, alpha-bungarotoxin. The aminoterminal extracellular domain of the alpha 7 sequence has three consensus sites for asparagine-linked glycosylation (N46DS, N90MS, and N133AS). In this study, we show that alpha 7 expressed either in vivo or in vitro is a glycoprotein of 57 kDa. In addition, we demonstrate by site-directed mutagenesis that all three consensus sites are used for glycosylation. To elucidate the role(s) of asparagine-linked glycosylation in the formation and function of the alpha 7 receptor, wild-type and glycosylation-deficient alpha 7 subunits were expressed in COS cells and oocytes. We examined biochemical and physiological properties of expressed receptors and found that alpha 7 glycosylation mutations do not affect homooligomerization and surface protein expression of the alpha 7 receptor but do affect surface expression of alpha-bungarotoxin binding sites and the function of the receptor. Our data indicate that asparagine-linked glycosylation is required for the expression of a functional alpha 7 receptor in oocytes.