Activation of the cloned muscarinic potassium channel by G protein βγ subunits

ACETYLCHOLINE released during parasv mpathetic stimulation of the vagal nerve slows the heart rate through the activation of muscarinic receptors and subsequent opening of an inwardly rectifying potassium channel¹. The activation of these muscarinic potassium channels is mediated by a pertussis toxin-sensitive heterotrimeric GTP-binding protein (G protein)^2,3. It has not been resolved whether exogenously applied G_α^4,5 or G_βγ^6,7, or both, activate the channel. Using a heterologous expression system, we have tested the ability of different G protein subunits to activate the cloned muscarinic potassium channel, GIRK1^8,9. We report here that coexpression of GIRK1 with G_βγ but not G_αβγ in Xenopus oocytes results in channel activity that persists in the absence of cytoplasmic GTP. This activity is reduced by fusion proteins of the (β-adrenergic receptor kinase and of recombinant G_αi-GDP, both of which are known to interact with G_βγ^10,11. Moreover, application of recombinant G_βγ, but not G_αi-GTP-γS, activates GIRK1 channels. Thus G_βγ appears to be sufficient for the activation of GIRK1 muscarinic potassium channels.