Subunit-specific association of protein kinase C and the receptor for activated C kinase with GABA type A receptors

GABA receptors (GABA(A)) are the major sites of fast synaptic inhibition in the brain and can be assembled from five subunit classes: alpha, beta, gam ma, delta, and epsilon. Receptor function can be regulated by direct phosph orylation of beta and gamma 2 subunits, but how kinases are targeted to GAB A(A) receptors is unknown. Here we show that protein kinase C-beta II (PKC- beta II) is capable of directly binding to the intracellular domain of the receptor beta 1 and beta 3 subunits, but not to those of the alpha 1 or gam ma 2 subunits. Moreover, associating PKC-beta II is capable of specifically phosphorylating serine 409 in beta 1 subunit and serines 408/409 within th e beta 3 subunit, key residues for modulating GABA(A) receptor function. Th e receptor for activated C kinase (RACK-1) was found also to bind to the be ta 1 subunit intracellular domain, but PKC binding appeared to be independe nt of this protein. Using immunoprecipitation, the association of PKC isofo rms and RACK-1 with neuronal GABA(A) receptors was seen. Furthermore, PKC i soforms associating with neuronal receptors were capable of phosphorylating the receptor beta 3 subunit. Together, these observations suggest GABA(A) receptors are intimately assoc iated with PKC isoforms via a direct interaction with receptor b subunits. This interaction may serve to localize PKC activity to GABA(A) receptors in neurons allowing the rapid regulation of receptor activity by cell-signali ng pathways that modify PKC activity.