G βγ Binding Increases the Open Time of I KACh: Kinetic Evidence for Multiple G βγ Binding Sites

Abstract

I KACh is an inwardly rectifying potassium channel that plays an important role in the regulation of mammalian heart rate. I KACh is activated by direct interaction with G βγ subunits of pertussis toxin-sensitive heterotrimeric G-proteins. The stoichiometry of the G βγ/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 I KACh channel in inside-out cardiomyocyte patches activated directly by G βγ. We conclude that the channel has at least two open states and that binding of G βγ prolongs its mean open time duration. These findings imply the existence of at least two binding sites on the channel complex for G βγ. 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 I KACh is presented.