Modulation of the concentration of postsynaptic GABAA receptors contributes to functional plasticity of inhibitory synapses. The γ2 subunit of GABAA receptor is specifically required for clustering of these receptors, for recruitment of the submembrane scaffold protein gephyrin to postsynaptic sites, and for postsynaptic function of GABAergic inhibitory synapses. To elucidate this mechanism, we here have mapped the γ2 subunit domains required for restoration of postsynaptic clustering and function of GABAA receptors in γ2 subunit mutant neurons. Transfection of γ2-/- neurons with the γ2 subunit but not the α2 subunit rescues postsynaptic clustering of GABAA receptors, results in recruitment of gephyrin to postsynaptic sites, and restores the amplitude and frequency of miniature inhibitory postsynaptic currents to wild-type levels. Analogous analyses of chimeric γ2/α2 subunit constructs indicate, unexpectedly, that the fourth transmembrane domain of the γ2 subunit is required and sufficient for postsynaptic clustering of GABAA receptors, whereas cytoplasmic γ2 subunit domains are dispensable. In contrast, both the major cytoplasmic loop and the fourth transmembrane domain of the γ2 subunit contribute to efficient recruitment of gephyrin to postsynaptic receptor clusters and are essential for restoration of miniature IPSCs. Our study points to a novel mechanism involved in targeting of GABA A receptors and gephyrin to inhibitory synapses.
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