G beta gamma binds directly to the G protein-gated K+ channel, IKACh

The cardiac G protein-gated K+ channel, IKACh, is activated by application of purified and recombinant beta and gamma subunits (G beta gamma) of heterotrimeric G proteins to excised inside-out patches from atrial membranes (Logothetis, D. E., Kurachi, Y., Galper, J., Neer, E., and Clapham, D. E. (1987) Nature 325, 321-326; Wickman, K., Iniguez-Lluhi, J., Davenport, P., Taussig, R. A., Krapivinsky, G. B., Linder, M. E., Gilman, A., and Clapham, D.E. (1994) Nature 368, 255-257). Cardiac IKACh is composed of two inward rectifier K+ channel subunits, GIRK1 and CIR (Krapivinsky, G., Gordon, E., Wickman, K., Velimirovic, B., Krapivinsky, L., and Clapham, D. E. (1995) Nature 374, 135-141). We show that G beta gamma directly binds to immunoprecipitated cardiac IKACh as well as to recombinant CIR and GIRK1 subunits, with dissociation constants (Kd) of 55, 50, and 125 nM, respectively. In each case, binding appeared specific as judged by competition of unlabeled G beta gamma with radiolabeled G beta gamma and inhibition of binding by antigenic peptide or G alpha-GDP, but not G alpha-GTP gamma S (guanosine 5'-3-O-(thio)triphosphate). In contrast, G alpha (GTP gamma S- or GDP-bound) did not bind to the native channel. We conclude that G beta gamma binds directly and specifically to IKACh via interactions with both CIR and GIRK1 subunits to gate the channel.