Activation and inhibition of G protein-coupled inwardly rectifying potassium (Kir3) channels by G protein βγ subunits

<jats:p> 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 βγ subunits containing either β1 or β2 with multiple Gγ subunits activate GIRK channels, we hypothesized that specificity might be imparted by β3, β4, or β5 subunits. We used a transfection assay in cell lines expressing GIRK channels to examine effects of dimers containing these Gβ subunits. Inwardly rectifying K <jats:sup>+</jats:sup> currents were increased in cells expressing β3 or β4, with either γ2 or γ11. Purified, recombinant β3γ2 and β4γ2 bound directly to glutathione- <jats:italic>S</jats:italic> -transferase fusion proteins containing N- or C-terminal cytoplasmic domains of GIRK1 and GIRK4, indicating that β3 and β4, like β1, form dimers that bind to and activate GIRK channels. By contrast, β5-containing dimers <jats:italic>inhibited</jats:italic> GIRK channel currents. This inhibitory effect was obtained with either β5γ2 or β5γ11, was observed with either GIRK1,4 or GIRK1,2 channels, and was evident in the context of either basal or agonist-induced currents, both of which were mediated by endogenous Gβγ subunits. In cotransfection assays, β5γ2 suppressed β1γ2-activated GIRK currents in a dose-dependent manner consistent with competitive inhibition. Moreover, we found that β5γ2 could bind to the same GIRK channel cytoplasmic domains as other, activating Gβγ subunits. Thus, β5-containing dimers inhibit Gβγ-stimulated GIRK channels, perhaps by directly binding to the channels. This suggests that β5-containing dimers could act as competitive antagonists of other Gβγ dimers on GIRK channels. </jats:p>