Contributions of N-linked glycosylation to the expression of a functional alpha7-nicotinic receptor in Xenopus oocytes.

The alpha7 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 amino-terminal extracellular domain of the alpha7 sequence has three consensus sites for asparagine-linked glycosylation (N46DS, N90MS, and N133AS). In this study, we show that alpha7 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 alpha7 receptor, wild-type and glycosylation-deficient alpha7 subunits were expressed in COS cells and oocytes. We examined biochemical and physiological properties of expressed receptors and found that alpha7 glycosylation mutations do not affect homooligomerization and surface protein expression of the alpha7 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 alpha7 receptor in oocytes.