Identification of a potassium channel site that interacts with G protein βγ subunits to mediate agonist-induced signaling

Activation of heterotrimeric GTP-binding (G) proteins by their coupled receptors, causes dissociation of the G protein alpha and beta gamma subunits, G(beta gamma) Subunits interact directly with G protein-gated inwardly rectifying K+ (GIRK) channels to stimulate their activity. In addition, free G(beta gamma) 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(beta gamma)-insensitive K+ channel, IRK1, we have identified a critical site of interaction of GIRK with G(beta gamma). Mutation of Leu(339) to Glu within this site impaired agonist-induced sensitivity and decreased binding to G(beta gamma), without removing the G(beta gamma) contribution to basal currents. Mutation of the corresponding residue in GIRK1 (Leu(333)) 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(beta gamma) subunits released through receptor stimulation.