Receptor for activated C kinase-1 facilitates protein kinase C-dependent phosphorylation and functional modulation of GABAA receptors with the activation of G-protein-coupled receptors

Abstract

GABAA receptors are the principal sites of fast synaptic inhibition in the brain. These receptors are hetero-pentamers that can be assembled from a number of subunit classes: α(1-6), β(1-3), γ(1-3), δ(1), ε, θ, and π, but the majority of receptor subtypes is believed, however, to be composed of α, β, and γ2 subunits. A major mechanism for modulating GABAA receptor function occurs via the phosphorylation of residues within the intracellular domains of receptor subunits by a range of serine/threonine and tyrosine kinases. However, how protein kinases are targeted to these receptors to facilitate functional modulation remains unknown. Here we demonstrate that the receptor for activated C kinase (RACK-1) and protein kinase C (PKC) bind to distinct sites on GABAA receptor β subunits. Although RACK-1 is not essential for PKC binding to GABAA receptor β subunits, it enhances the phosphorylation of serine 409, a residue critical for the phospho-dependent modulation of GABAA receptor function in the β1 subunit by anchored PKC. Furthermore, RACK-1 also enhances GABAA receptor functional modulation in neurons by a PKC-dependent signaling pathway with the activation of muscarinic acetylcholine receptors (mAChRs). This PKC-dependent modulation of neuronal GABAA receptors was mirrored by an increase in the phosphorylation of GABAA receptor β subunits with the activation of mAChRs. Our results suggest a central role for RACK-1 in potentiating PKC-dependent phosphorylation and functional modulation of GABAA receptors. Therefore, RACK-1 will enhance functional cross talk between GABAA receptors and G-protein-coupled receptors and therefore may have profound effects on neuronal excitability.