Assembly of nicotinic α7 subunits in Xenopus oocytes is partially blocked at the tetramer level

Abstract

The assembly of nicotinic α1β1γδ, α3β4, and α7 receptors and 5-hydroxytryptamine 3A (5HT 3A) receptors was comparatively evaluated in Xenopus oocytes by blue native PAGE analysis. While α1βγδ subunits, α3β4 subunits, and 5HT 3A subunits combined efficiently to pentamers, α7 subunits existed in various assembly states including trimers, tetramers, pentamers, and aggregates. Only α7 subunits that completed the assembly process to homopentamers acquired complex-type carbohydrates and appeared at the cell surface. We conclude that Xenopus oocytes have a limited capacity to guide the assembly of α7 subunits, but not 5HT 3A subunits to homopentamers. Accordingly, ER retention of imperfectly assembled α7 subunits rather than inefficient routing of fully assembled α7 receptors to the cell surface limits surface expression levels of α7 nicotinic acetylcholine receptors.