Activation and Inhibition of G Protein-Coupled Inwardly Rectifying Potassium (kir3) Channels by G Protein β γ Subunits

G protein-coupled inwardly rectifying potassium (GIRK) channels can be activated or inhibited by different classes of receptors, suggesting a role for G proteins in determining signaling specificity. Because G protein β γ subunits containing either β 1 or β 2 with multiple Gγ subunits activate GIRK channels, we hypothesized that specificity might be imparted by β 3, β 4, or β 5 subunits. We used a transfection assay in cell lines expressing GIRK channels to examine effects of dimers containing these Gβ subunits. Inwardly rectifying K+currents were increased in cells expressing β 3 or β 4, with either γ 2 or γ 11. Purified, recombinant β 3γ 2 and β 4γ 2 bound directly to glutathione-S-transferase fusion proteins containing N- or C-terminal cytoplasmic domains of GIRK1 and GIRK4, indicating that β 3 and β 4, like β 1, form dimers that bind to and activate GIRK channels. By contrast, β 5-containing dimers inhibited GIRK channel currents. This inhibitory effect was obtained with either β 5γ 2 or β 5γ 11, was observed with either GIRK 1,4 or GIRK 1,2 channels, and was evident in the context of either basal or agonist-induced currents, both of which were mediated by endogenous Gβ γ subunits. In cotransfection assays, β 5γ 2 suppressed β 1γ 2-activated GIRK currents in a dose-dependent manner consistent with competitive inhibition. Moreover, we found that β 5γ 2 could bind to the same GIRK channel cytoplasmic domains as other, activating Gβ γ subunits. Thus, β 5-containing dimers inhibit Gβ γ -stimulated GIRK channels, perhaps by directly binding to the channels. This suggests that β 5-containing dimers could act as competitive antagonists of other Gβ γ dimers on GIRK channels.