Altered patterns ofN-linked glycosylation of theTorpedo acetylcholine receptor expressed inXenopus oocytes

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Abstract
The nicotinic acetylcholine receptor (AChR) fromTorpedo electroplax is an oligomeric transmembrane glycoprotein made up of four highly homologous subunits in a stoichiometry of α2βγδ. The role ofN-linked glycosylation of the AChR has been studied in several cell lines and these studies have suggested that the addition of carbohydrate may be important for receptor expression. WhileXenopus oocytes have proven to be an invaluable tool for studying the AChR, little is known aboutN-linked glycosylation of the oocyte-expressed receptor. The present report demonstrates that the oocyte-expressed AChR is glycosylated and contains the same number of oligosaccharide residues per subunit as the native receptor. However, unlike the nativeTorpedo receptor which contains both high mannose and complex oligosaccharides, the oocyte-expressed AChR contains only high mannose oligosaccharide modifications. However, as has been well documented, theTorpedo AChR expressed in oocytes is fully functional, demonstrating that the precise nature of the oligosaccharide modification is not critical for receptor function. The role of the oligosaccharide component of the AChR in receptor function was examined using tunicamycin (TM) to inhibitN-linked protein glycosylation. TM treatment resulted in a 70–80% inhibition of AChR expression in oocytes. Functional, unglycosylated receptors were not expressed; receptors expressed in TM-treated oocytes were functional wild-type, glycosylated AChR, formed only during the initial 12 hr of TM exposure. These data suggest that while glycosylation of the oocyte-expressedTorpedo AChR is required for assembly of subunits into a functional receptor, as has been demonstrated in other cells, oocyte modification of normalTorpedo glycosylation patterns does not affect receptor function or assembly.