Gbetagamma binding increases the open time of IKACh: kinetic evidence for multiple Gbetagamma binding sites.

IKACh is an inwardly rectifying potassium channel that plays an important role in the regulation of mammalian heart rate. IKACh is activated by direct interaction with Gbetagamma subunits of pertussis toxin-sensitive heterotrimeric G-proteins. The stoichiometry of the Gbetagamma/channel complex is currently unknown, and kinetic analysis of the channel behavior has led to conflicting conclusions. Here, we analyze the kinetics of the native IKACh channel in inside-out cardiomyocyte patches activated directly by Gbetagamma. We conclude that the channel has at least two open states and that binding of Gbetagamma prolongs its mean open time duration. These findings imply the existence of at least two binding sites on the channel complex for Gbetagamma. We also show that the duration of the channel opening is negatively correlated with the duration of subsequent channel closing, which further constrains the possible kinetic models. A simple qualitative model describing the kinetic behavior of IKACh is presented.