The G protein-gated atrial K+ channel is stimulated by three distinct Gi alpha-subunits

The guanine nucleotide-binding protein, Gi, which inhibits adenylyl cyclase, has recently been shown to have three subtypes of the alpha-subunit, termed Gi alpha-1, Gi alpha-2 and Gi alpha-3. They share 87-94% amino-acid sequence homology and so are difficult to separate from one another. Among other functions, purified preparations activate K+ channels but there is confusion over which of the subtypes activates the muscarinic K+ channels of the atrial muscle of the heart: Gi alpha-3, also termed Gk, has been shown to activate this channel but it is not clear whether Gi alpha-1 does or does not. To clarify this problem, we expressed the subtypes separately in Escherichia coli to eliminate contamination by other subtypes and tested the recombinant alpha- chains on atrial muscarinic K+ channels. Although we anticipated that only Gi alpha-3 would have Gk activity, to our surprise all three recombinant subtypes were active, from which we deduce that the Gi subtypes are multifunctional.