Members of the Cys-loop superfamily of ligand-gated ion channels, which mediate fast synaptic transmission in the nervous system, are assembled as heteropentamers from a large repertoire of neuronal subunits. Although several motifs in subunit N-terminal domains are known to be important for subunit assembly, increasing evidence points toward a role for C-terminal domains. Using a combination of flow cytometry, patch clamp recording, endoglycosidase H digestion, brefeldin A treatment, and analytic centrifugation, we identified a highly conserved aspartate residue at the boundary of the M3-M4 loop and the M4 domain that was required for binary and ternary ␥-aminobutyric acid type A receptor surface expression. Mutation of this residue caused mutant and partnering subunits to be retained in the endoplasmic reticulum, reflecting impaired forward trafficking. Interestingly although mutant and partnering wild type subunits could be coimmunoprecipitated, analytic centrifugation studies demonstrated decreased formation of pentameric receptors, suggesting that this residue played an important role in later steps of subunit oligomerization. We thus conclude that C-terminal motifs are also important determinants of Cys-loop receptor assembly.The Cys-loop superfamily of ligand-gated ion channels, which includes ␥-aminobutyric acid type A (GABA A ) 2 and type C (GABA C ), nicotinic acetylcholine, glycine, and 5-hydroxytryptamine type 3 receptors, mediates fast synaptic transmission in the nervous system. Mutations that alter Cys-loop receptor surface density by affecting receptor biogenesis have been associated with idiopathic generalized epilepsies (1-4), congenital myasthenic syndromes (5), and psychiatric disorders (6). Unfortunately because the structural and cellular determinants of receptor biogenesis are poorly understood, development of effective treatment strategies remains a significant challenge.A wealth of evidence suggests that Cys-loop receptors are assembled as heteropentamers from a large repertoire of neuronal subunits (7-10). Subunits share a similar topology that includes an extracellular N-terminal domain, four transmembrane domains, three loops including a large cytoplasmic loop, and a variable length extracellular C-terminal tail (7, 11). Receptor assembly is thought to occur in the endoplasmic reticulum (ER) following glycosylation and folding of de novo synthesized subunits (12-15). Assembly is closely monitored by ER quality control machinery, and consequently subunits that fail to assemble properly are retained and degraded (15)(16)(17). Although N-terminal motifs are known to be important for subunit assembly (18,19), recent studies in nicotinic acetylcholine receptors (nAChRs) suggest that C-terminal motifs may also play a role (20).GABA A receptors are the most abundant Cys-loop receptor in the mammalian brain and are responsible for the majority of fast inhibitory neurotransmission. Like other Cys-loop superfamily receptors, they are pentamers assembled from combinations of 16 subunit subtypes ...