Identification of a Potassium Channel Site That Interacts with G Protein βγ Subunits to Mediate Agonist-induced Signaling

Abstract

Activation of heterotrimeric GTP-binding (G) proteins by their coupled receptors, causes dissociation of the G protein α and βγ subunits. G_βγ subunits interact directly with G protein-gated inwardly rectifying K⁺ (GIRK) channels to stimulate their activity. In addition, free G_βγ subunits, resulting from agonist-independent dissociation of G protein subunits, can account for a major component of the basal channel activity. Using a series of chimeric constructs between GIRK4 and a G_βγ-insensitive K⁺ channel, IRK1, we have identified a critical site of interaction of GIRK with G_βγ. Mutation of Leu³³⁹ to Glu within this site impaired agonist-induced sensitivity and decreased binding to G_βγ, without removing the G_βγcontribution to basal currents. Mutation of the corresponding residue in GIRK1 (Leu³³³) resulted in a similar phenotype. Both the GIRK1 and GIRK4 subunits contributed equally to the agonist-induced sensitivity of the heteromultimeric channel. Thus, we have identified a channel site that interacts specifically with G_βγsubunits released through receptor stimulation.