Phosphorylation on Ser-359 of the α2 subunit in GABA type A receptors down-regulates their density at inhibitory synapses

GABA type A receptors (GABAARs) mediate fast synaptic inhibition and are trafficked to functionally diverse synapses. However, the precise molecular mechanisms that regulate the synaptic targeting of these receptors are unclear. Whereas it has been previously shown that phosphorylation events in a4, b,
and g subunits of GABAARs govern their function and trafficking, phosphorylation of other subunits has not yet been demonstrated. Here, we show that the a2 subunit of GABAARs is phosphorylated at Ser-359 and enables dynamic regulation of GABAAR binding to the scaffolding proteins gephyrin and collybistin. We initially identified Ser-359 phosphorylation by MS analysis, and additional experiments revealed that it is regulated by the activities of cAMP-dependent protein kinase (PKA) and the protein phosphatase 1 (PP1) and/or PP2A. GST-based pulldowns and coimmunoprecipitation experiments demonstrate preferential binding of both gephyrin and collybistin to WT and an S359A phosphonull variant, but not to an S359D phosphomimetic variant. Furthermore, the decreased capacity of the a2 S359D variant to bind collybistin and gephyrin decreased the density of synaptic a2-containing GABAAR clusters and caused an absence of a2 enrichment in the axon initial segment. These results suggest that PKA-mediated phosphorylation and PP1/PP2A-dependent dephosphorylation of the a2 subunit play a role in the dynamic regulation of GABAAR accumulation at inhibitory synapses, thereby regulating the strength of synaptic inhibition. The MS data have been deposited to ProteomeXchange, with the data set identifier PXD019597.