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. Gβγ subunits interact directly with G protein-gated inwardly rectifying K+ (GIRK) channels to stimulate their activity. In addition, free Gβγ 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 Gβγ-insensitive K+ channel, IRK1, we have identified a critical site of interaction of GIRK with Gβγ. Mutation of Leu339 to Glu within this site impaired agonist- induced sensitivity and decreased binding to Gβγ, without removing the Gβγ contribution 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 Gβγ subunits released through receptor stimulation.