Regulation of the inward rectifying properties of G-protein-activated inwardly rectifying K+ (GIRK) channels by Gβγ subunits

Gbetagamma subunits are known to bind to and activate G-protein-activated inwardly rectifying K+ channels (GIRK) by regulating their open probability and bursting behavior. Studying G-protein regulation of either native GIRK (I-KACh) channels in feline atrial myocytes or heterologously expressed GIRK1/4 channels in Chinese hamster ovary cells and HEK 293 cells uncovered a novel Gbetagamma subunit mediated regulation of the inwardly rectifying properties of these channels. I-KACh activated by submaximal concentrations of acetylcholine exhibited a similar to2.5-fold stronger inward rectification than I-KACh activated by saturating concentrations of acetylcholine. Similarly, the inward rectification of currents through GIRK1/4 channels expressed in HEK cells was substantially weakened upon maximal stimulation with co-expressed Gbetagamma subunits. Analysis of the outward current block underlying inward rectification demonstrated that the fraction of instantaneously blocked channels was reduced when Gbetagamma was over-expressed. The Gbetagamma induced weakening of inward rectification was associated with reduced potencies for Ba2+ and Cs+ to block channels from the extracellular side. Based on these results we propose that saturation of the channel with Gbetagamma leads to a conformational change within the pore of the channel that reduced the potency of extracellular cations to block the pore and increased the fraction of channels inert to a pore block in outward direction.