A Quantitative Model of the GIRK1/2 Channel Reveals That Its Basal and Evoked Activities Are Controlled by Unequal Stoichiometry of Gα and Gβγ.

G protein-gated K+ channels (GIRK; Kir3), activated by G beta gamma subunits derived from G(i/o) proteins, regulate heartbeat and neuronal excitability and plasticity. Both neurotransmitter-evoked (I-evoked) and neurotransmitter-independent basal (I-basal) GIRK activities are physiologically important, but mechanisms of I-basal and its relation to I-evoked are unclear. We have previously shown for heterologously expressed neuronal GIRK1/2, and now show for native GIRK in hippocampal neurons, that I-basal and I-evoked are interrelated: the extent of activation by neurotransmitter (activation index, R-a) is inversely related to I-basal. To unveil the underlying mechanisms, we have developed a quantitative model of GIRK1/2 function. We characterized single-channel and macroscopic GIRK1/2 currents, and surface densities of GIRK1/2 and G beta gamma expressed in Xenopus oocytes. Based on experimental results, we constructed a mathematical model of GIRK1/2 activity under steady-state conditions before and after activation by neurotransmitter. Our model accurately recapitulates I-basal and I-evoked in Xenopus oocytes, HEK293 cells and hippocampal neurons; correctly predicts the dose-dependent activation of GIRK1/2 by coexpressed G beta gamma and fully accounts for the inverse I-basal-R-a correlation. Modeling indicates that, under all conditions and at different channel expression levels, between 3 and 4 G beta gamma dimers are available for each GIRK1/2 channel. In contrast, available G alpha(i/o) decreases from similar to 2 to less than one G alpha per channel as GIRK1/2's density increases. The persistent G beta gamma/channel (but not G alpha/channel) ratio support a strong association of GIRK1/2 with G beta gamma, consistent with recruitment to the cell surface of G beta gamma, but not G alpha, by GIRK1/2. Our analysis suggests a maximal stoichiometry of 4 G beta gamma but only 2 G alpha(i/o) per one GIRK1/2 channel. The unique, unequal association of GIRK1/2 with G protein subunits, and the cooperative nature of GIRK gating by G beta gamma, underlie the complex pattern of basal and agonist-evoked activities and allow GIRK1/2 to act as a sensitive bidirectional detector of both G beta gamma and G alpha.