Muscarine-gated K+ channel: Subunit stoichiometry and structural domains essential for G protein stimulation

Coexpression in Xenopus oocytes of the cloned cardiac inward rectifier subunits K-ir 3.1 and K-ir 3.4 results in G protein-stimulated channel activity closely resembling the muscarinic channel underlying the inwardly rectifying K+ current in atrial myocytes. To determine the stoichiometry and relative subunit positions within the channel, K-ir 3.1 and K-ir 3.4 were coexpressed in varying ratios with cloned G beta(1 gamma 2) subunits and also as tandemly linked tetramers with different relative subunit positions. The results reveal that the most efficient channel comprises two subunits of each type in an alternating array within the tetramer. To localize regions important for subunit coassembly and G protein sensitivity, chimeric subunits containing domains from either K-ir 3.1, K-ir 3.4, or the G protein-insensitive subunit K-ir 4.1 were expressed. The results demonstrate that the transmembrane domains dictate the potentiation of the coassembled channels and that, although the NH4- or COOH-termini of both subunits alone can confer G protein sensitivity, both termini are required for maximal stimulation by G beta(1 gamma 2).