A cDNA encoding an inward rectifier K
+ channel (IRK1) was isolated by expression cloning. This channel conducts inward K
+ current below the K
+ equilibrium potential but passes little outward K
+ current. By sequence homology with IRK1, a cDNA for a muscarinic K
+ channel, another inward rectifying K
+ channel which is activated by direct interaction with G-protein was isolated. In contrast with the voltage-gated K
+ channels which have six transmembrane segments, the IRK1 channel and the GIRK1 channel contains only two putative transmembrane segments per subunit and corresponds to the inner core structure of voltagegated K
+ channels. The IRK1 channel and the GIRK1 channel show extensive sequence similarity and constitute a new superfamily.
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