DETERMINATION OF THE SUBUNIT STOICHIOMETRY OF AN INWARDLY RECTIFYING POTASSIUM CHANNEL

Inwardly rectifying K+ channels are distantly related to their voltage -gated counterparts and possess a structural motif of only two putativ e transmembrane segments in each subunit. They are formed by the assem bly of an unknown number of subunits. We have examined the subunit sto ichiometry oi a strongly rectifying K+ channel, IRK1, by linking toget her the coding sequence of three or four subunits and distinguishing c hannels with different numbers of subunits carrying a double mutation that alters inward rectification and single-channel properties. We fin d that IRK1 channels, like voltage-gated K+ channels, are tetrameric c hannels. Interestingly, the high sensitivity to Mg2+ and polyamines, c ations that produce inward rectification by blocking the channel pore from the cytoplasmic side, is largely retained in a channel containing only one wild-type subunit and three subunits bearing mutations that abolish high affinity Mg2+ and polyamine block.