G beta gamma binding increases the open time of I-KACh: Kinetic evidence for multiple G beta gamma binding sites

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 dire ct interaction with G beta gamma subunits of pertussis toxin-sensitive hete rotrimeric G-proteins, The stoichiometry of the G beta gamma/channel comple x is currently unknown, and kinetic analysis of the channel behavior has le d to conflicting conclusions. Here, we analyze the kinetics of the native I -KACh channel in inside-out cardiomyocyte patches activated directly by G b eta gamma. We conclude that the channel has at least two open states and th at binding of G beta gamma prolongs its mean open time duration. These find ings imply the existence of at least two binding sites on the channel compl ex for G beta gamma. We also show that the duration of the channel opening is negatively correlated with the duration of subsequent channel closing, w hich further constrains the possible kinetic models. A simple qualitative m odel describing the kinetic behavior of I-KACh is presented.