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

G protein-coupled inwardly rectifying potassium (IRK) channels can be activ ated or inhibited by different classes of receptors, suggesting a role for C proteins in determining signaling specificity. Because C protein beta gam ma subunits containing either beta 1 or beta 2 with multiple G gamma subuni ts activate GIRK channels, we hypothesized that specificity might be impart ed by beta 3, beta 4,or beta 5 subunits. We used a transfection assay in ce ll lines expressing GIRK channels to examine effects of dimers containing t hese G beta subunits. Inwardly rectifying K+ currents were increased in cel ls expressing beta 3 or beta 4, with either gamma 2 or gamma 11. Purified, recombinant beta 3 gamma 2 and beta 4 gamma 2 bound directly to glutathione -S-transferase fusion proteins containing N- or C-terminal cytoplasmic doma ins of GIRK1 and GIRK4, indicating that beta 3 and beta 4, like beta 1, for m dimers that bind to and activate GIRK channels. By contrast beta 5-contai ning dimers inhibited GIRK channel currents. This inhibitory effect was obt ained with either beta 5 gamma 2 or beta 5 gamma 11, was observed with eith er GIRK1,4 or GIRK1,2 channels, and was evident in the context of either ba sal or agonist-induced currents, both of which were mediated by endogenous G beta gamma subunits. In cotransfection assays, beta 5 gamma 2 suppressed beta 1 gamma 2-activated GIRK currents in a dose-dependent manner consisten t with competitive inhibition. Moreover, we found that beta 5 gamma 2 could bind to the same GIRK channel cytoplasmic domains as other, activating G b eta gamma subunits. Thus, beta 5-containing dimers inhibit G beta gamma-sti mulated GIRK channels, perhaps by directly binding to the channels. This su ggests that beta 5-containing dimers could act as competitive antagonists o f other G beta gamma dimers on GIRK channels.