Subunit-Specific Association of Protein Kinase C and the Receptor for Activated C Kinase with GABA Type A Receptors

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Abstract
GABA receptors (GABAA) are the major sites of fast synaptic inhibition in the brain and can be assembled from five subunit classes: α, β, γ, δ, and ε. Receptor function can be regulated by direct phosphorylation of β and γ2 subunits, but how kinases are targeted to GABAAreceptors is unknown. Here we show that protein kinase C-βII (PKC-βII) is capable of directly binding to the intracellular domain of the receptor β1 and β3 subunits, but not to those of the α1 or γ2 subunits. Moreover, associating PKC-βII is capable of specifically phosphorylating serine 409 in β1 subunit and serines 408/409 within the β3 subunit, key residues for modulating GABAAreceptor function. The receptor for activated C kinase (RACK-1) was found also to bind to the β1 subunit intracellular domain, but PKC binding appeared to be independent of this protein. Using immunoprecipitation, the association of PKC isoforms and RACK-1 with neuronal GABAAreceptors was seen. Furthermore, PKC isoforms associating with neuronal receptors were capable of phosphorylating the receptor β3 subunit.Together, these observations suggest GABAAreceptors are intimately associated with PKC isoforms via a direct interaction with receptor β subunits. This interaction may serve to localize PKC activity to GABAAreceptors in neurons allowing the rapid regulation of receptor activity by cell-signaling pathways that modify PKC activity.