Assembly rules for GABA_A receptor complexes in the brain

GABA_A receptor (GABA_AR) pentamers are assembled from a pool of 19 subunits, and variety in subunit combinations diversifies GABA_AR functions to tune brain activity. Pentamers with distinct subunit compositions localize differentially at synaptic and non-synaptic sites to mediate phasic and tonic inhibition, respectively. Despite multitudes of theoretical permutations, limited subunit combinations have been identified in the brain. Currently, no molecular model exists for combinatorial GABA_AR assembly in vivo. Here, we reveal assembly rules of native GABA_AR complexes that explain GABA_AR subunit subcellular distributions using mice and Xenopus laevis oocytes. First, a subunits possess intrinsic signals to segregate into distinct pentamers. Second, γ² is essential for GABA_AR assembly with Neuroligin-2 (NL2) and GARLHs, which localize GABA_ARs at synapses. Third, δ suppresses α6 synaptic localization by preventing assembly with GARLHs/NL2. These findings establish the first molecular model for combinatorial GABA_AR assembly in vivo and reveal an assembly pathway regulating GABA_AR synaptic localization.