Quantitative analysis of mammalian GIRK2 channel regulation by G proteins, the signaling lipid PIP2 and Na+ in a reconstituted system

GIRK channels control spike frequency in atrial pacemaker cells and inhibitory potentials in neurons. By directly responding to G proteins, PIP2 and Na+, GIRK is under the control of multiple signaling pathways. In this study, the mammalian GIRK2 channel has been purified and reconstituted in planar lipid membranes and effects of G alpha, G beta gamma, PIP2 and Na+ analyzed. G beta gamma and PIP2 must be present simultaneously to activate GIRK2. Na+ is not essential but modulates the effect of G beta gamma and PIP2 over physiological concentrations. G alpha(i1)(GTP gamma S) has no effect, whereas G alpha(i1)(GDP) closes the channel through removal of G beta gamma. In the presence of G beta gamma, GIRK2 opens as a function of PIP2 mole fraction with Hill coefficient 2.5 and an affinity that poises GIRK2 to respond to natural variations of PIP2 concentration. The dual requirement for G beta gamma and PIP2 can help to explain why GIRK2 is activated by G(i/o), but not G(q) coupled GPCRs.