NMR Analyses of the Gβγ Binding and Conformational Rearrangements of the Cytoplasmic Pore of G Protein-activated Inwardly Rectifying Potassium Channel 1 (GIRK1)

G protein-activated inwardly rectifying potassium channel (GIRK) plays crucial roles in regulating heart rate and neuronal excitability in eukaryotic cells. GIRK is activated by the direct binding of heterotrimeric G protein beta gamma subunits (G beta gamma) upon stimulation of G protein-coupled receptors, such as M2 acetylcholine receptor. The binding of G beta gamma to the cytoplasmic pore (CP) region of GIRK causes structural rearrangements, which are assumed to open the transmembrane ion gate. However, the crucial residues involved in the G beta gamma binding and the structural mechanism of GIRK gating have not been fully elucidated. Here, we have characterized the interaction between the CP region of GIRK and G beta gamma, by ITC and NMR. The ITC analyses indicated that four G beta gamma molecules bind to a tetramer of the CP region of GIRK with a dissociation constant of 250 mu M. The NMR analyses revealed that the G beta gamma binding site spans two neighboring subunits of the GIRK tetramer, which causes conformational rearrangements between subunits. A possible binding mode and mechanism of GIRK gating are proposed.