Activation and inhibition of G protein-coupled inwardly rectifying potassium (Kir3) channels by G protein bg 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 bg subunits containing either b1 or b2 with multiple Gg subunits activate GIRK channels, we hypothesized that specificity might be imparted by b3, b4, or b5 subunits. We used a transfection assay in cell lines expressing GIRK channels to examine effects of dimers containing these Gb subunits. Inwardly rectifying K1 currents were increased in cells expressing b3 or b4, with either g2 or g11. Purified, recombinant b3g2 and b4g2 bound directly to glutathioneS-transferase fusion proteins containing Nor C-terminal cytoplasmic domains of GIRK1 and GIRK4, indicating that b3 and b4, like b1, form dimers that bind to and activate GIRK channels. By contrast, b5containing dimers inhibited GIRK channel currents. This inhibitory effect was obtained with either b5g2 or b5g11, 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 Gbg subunits. In cotransfection assays, b5g2 suppressed b1g2-activated GIRK currents in a dose-dependent manner consistent with competitive inhibition. Moreover, we found that b5g2 could bind to the same GIRK channel cytoplasmic domains as other, activating Gbg subunits. Thus, b5-containing dimers inhibit Gbg-stimulated GIRK channels, perhaps by directly binding to the channels. This suggests that b5-containing dimers could act as competitive antagonists of other Gbg dimers on GIRK channels.