Effector contributions to G beta gamma-mediated signaling as revealed by muscarinic potassium channel gating

Receptor-mediated activation of heterotrimeric G proteins leading to dissociation of the G alpha subunit from G beta gamma is a highly conserved signaling strategy used by numerous extracellular stimuli. Although G beta gamma subunits regulate a variety of effecters, including kinases, cyclases, phospholipases, and ion channels (Clapham, D.E., and E.J. Neer. 1993. Nature (Lond.). 365:403-406), few tools exist for probing instantaneous G beta gamma-effector interactions, and little is known about the kinetic contributions of effecters to the signaling process. Ln this study, we used the atrial muscarinic K+ channel, which is activated by direct interactions with G beta gamma subunits (Logothetis, D.E., Y. Kurachi,J. Galper, E.J. Neer, and D.E. Clap. 1981. Nature (Lend.). 325:321-326; Wickman, K.,J.A. Iniguez-Liuhi, P.A. Davenport, R. Taussig, G.B. Krapivinsky, M.E. Linder, A.G. Gilman, and D.E. Clapham. 1994. Nature (Lond.). 366: 654-663; Huang, C.-L., P.A. Slesinger, P.J. Casey, Y.N.Jan, and L.Y.Jan. 1995. Neuron. 15:1133-1143), as a sensitive reporter of the dynamics of G beta gamma-effector interactions. Muscarinic K+ channels exhibit bursting behavior upon G protein activation, shifting between three distinct functional modes, characterized by the frequency of channel openings during individual bursts. Acetylcholine concentration (and by inference, the concentration of activated G beta gamma) controls the fraction of time spent in each mode without changing either the burst duration or channel gating within individual modes. The picture which emerges is of a G beta gamma effector with allosteric regulation and an intrinsic ''off'' switch which serves to limit its own activation. These two features combine to establish exquisite channel sensitivity to changes in G beta gamma concentration, and may be indicative of the factors regulating other G beta gamma-modulated effecters.