Control of cell surface expression of GABA receptors by a conserved region at the end of the N-terminal extracellular domain of receptor subunits.

Type A γ-aminobutyric acid receptors (GABARs) represent a family of pentameric GABA-gated Cl/HCO ion channels which mediate inhibitory transmission in the central nervous system. Cell surface expression of GABARs, a prerequisite for their function, is dependent on the appropriate assembly of the receptor subunits and their transient interactions with molecular chaperones within the endoplasmic reticulum (ER) and Golgi apparatus. Here, we describe a highly conserved amino acid sequence within the extracellular N-terminal domain of the receptor subunits adjoining the first transmembrane domain as a region important for GABAR processing within the ER. Modifications of this region in the α1, β3, and γ2 subunits using insertion or site-directed mutagenesis impaired GABAR trafficking to the cell surface in heterologous cell systems although they had no effect on the subunit assembly. We found that mutated receptors accumulated in the ER where they were shown to associate with chaperones calnexin, BiP, and Grp94. However, their surface expression was increased when ER-associated degradation or proteosome function was inhibited, while modulation of ER calcium stores had little effect. When compared to the wt, mutated receptors showed decreased interaction with calnexin, similar binding to BiP, and increased association with Grp94. Structural modeling of calnexin interaction with the wt or mutated GABAR revealed that disruption in structure caused by mutations in the conserved region adjoining the first transmembrane domain may impair calnexin binding. Thus, this previously uncharacterized region plays an important role in intracellular processing of GABARs at least in part by stabilizing their interaction with calnexin.