Phospho-dependent binding of the clathrin AP2 adaptor complex to GABA _A receptors regulates the efficacy of inhibitory synaptic transmission

The efficacy of synaptic inhibition depends on the number of γ-aminobutyric acid type A receptors (GABA_ARs) expressed on the cell surface of neurons. The clathrin adaptor protein 2 (AP2) complex is a critical regulator of GABA_AR endocytosis and, hence, surface receptor number. Here, we identify a previously uncharacterized atypical AP2 binding motif conserved within the intracellular domains of all GABA_AR β subunit isoforms. This AP2 binding motif (KTHLRRRSSQLK in the β3 subunit) incorporates the major sites of serine phosphorylation within receptor β subunits, and phosphorylation within this site inhibits AP2 binding. Furthermore, by using surface plasmon resonance, we establish that a peptide (pepβ3) corresponding to the AP2 binding motif in the GABA_AR β3 subunit binds to AP2 with high affinity only when dephosphorylated. Moreover, the pepβ3 peptide, but not its phosphorylated equivalent (pepβ3-phos), enhanced the amplitude of miniature inhibitory synaptic current and whole cell GABA_AR current. These effects of pepβ3 on GABA_AR current were occluded by inhibitors of dynamin-dependent endocytosis supporting an action of pepβ3 on GABA_AR endocytosis. Therefore phosphodependen regulation of AP2 binding to GABA_ARs provides a mechanism to specify receptor cell surface number and the efficacy of inhibitory synaptic transmission.