Permeant ion-dependent changes in gating of Kir2.1 inward rectifier potassium channels

We studied the effect of monovalent thallium ion (Tl+) on the gating of sin gle Kir2.1 channels, which open and close spontaneously at a constant membr ane potential. In cell-attached recordings of single-channel inward current , changing the external permeant ion from K+ to Tl+ decreases the mean open -time by similar to 20-fold. Furthermore, the channel resides predominantly at a subconductance level, which results from a slow decay (tau = 2.7 ms a t -100 mV) from the fully open level immediately following channel opening. Mutation of a pore-lining cysteine (C169) to valise abolishes the slow dec ay and subconductance level, and single-channel recordings from channels fo rmed by tandem tetramers containing one to three C169V mutant subunits indi cate that TV must interact with at least three C169 residues to induce thes e effects. However; the C169V mutation does not alter the single-channel cl osing kinetics of Tl+ current. These results suggest that TV ions change th e conformation of the ion conduction pathway during permeation and alter ga ting by two distinct mechanisms. First, they interact with the thiolate gro ups of C169 lining the cavity to induce conformational changes of the ion p assageway; and thereby produce a slow decay of single-channel current and a dominant subconductance state. Second, they interact. more strongly than K + with the main chain carbonyl oxygens lining the selectivity filter to des tabilize the open state of the channel and, thus, alter the open/close kine tics of gating. In addition to altering gating, Tl+ greatly diminishes Ba2 block. The unblocking rate of Ba2+ is increased by > 22-fold when the exte rnal permeant ion is switched from K+ to Tl+ regardless of the direction of Ba2+ exit. This effect cannot be explained solely by ion-ion interactions, but is consistent with the notion that Tl+ induces conformational changes in the selectivity filter.