Cell surface expression of homomeric GABAA receptors depends on single residues in subunit transmembrane domains

Cell surface expression of type A GABA receptors (GABA(A)Rs) is a critical determinant of the efficacy of inhibitory neurotransmission. Pentameric GABA(A)Rs are assembled from a large pool of subunits according to precise co-assembly rules that limit the extent of receptor structural diversity. These rules ensure that particular subunits, such as 1 and 3, form functional cell surface ion channels when expressed alone in heterologous systems, whereas other brain-abundant subunits, such as and , are retained within intracellular compartments. Why some of the most abundant GABA(A)R subunits fail to form homomeric ion channels is unknown. Normally, surface expression of and subunits requires co-assembly with subunits via interactions between their N-terminal sequences in the endoplasmic reticulum. Here, using molecular biology, imaging, and electrophysiology with GABA(A)R chimeras, we have identified two critical residues in the transmembrane domains of and subunits, which, when substituted for their 1 counterparts, permit cell surface expression as homomers. Consistent with this, substitution of the 1 transmembrane residues for the subunit equivalents reduced surface expression and altered channel gating, highlighting their importance for GABA(A)R trafficking and signaling. Although not ligand-gated, the formation of and homomeric ion channels at the cell surface was revealed by incorporating a mutation that imparts the functional signature of spontaneous channel activity. Our study identifies two single transmembrane residues that enable homomeric GABA(A)R subunit cell surface trafficking and demonstrates that and subunits can form functional ion channels.