Studies of the molecular basis of selectivity and gating in the inward rectifier potassium channel Kir2.1

1. The molecular basis of selectivity and gating were investigated in wild-type and mutant forms of the inward rectifier K+ channel Kir2.1 (IRK1).;2. Kir2.1 channels show characteristic time-dependent gating kinetics due to a reversible voltage-dependent channel block by cytoplasmic polyamines. Mutations at Asp 172 in the M2 domain revealed that a negative charge at this position is the main criterion of time-dependent gating kinetics in Kir2.1.;3. Kir channels are blocked by Cs+ and Rb+ in a voltage-dependent manner, characteristic of many Kir channels. Rb+ and Cs+ block in Kir2.1 was abolished by replacing Asp 172 by Asn, but was re-established by a change to Gln, narrowing the pore. However, blocking affinity was reduced by the mutation to Gln.;4. When Asp 172 was mutated to Glu, narrowing the pore but retaining the negative charge, block by both Cs+ and Rb+ was increased relative to wild-type.;5. Replacing Gly 168 in M2 by Ala was suggested to widen the pore at position 172. The effect of this mutation on Cs+ and Rb+ block was relatively small.;6. There appears to be a balance between charge and pore size in determining whether icons block or permeate. A major part of the selectivity of Kir2.1 is associated with Asp 172 in the M2 domain. This site also determines the time-dependent activation gating of the channel.;7. Mutation of Asp 172 to Ser were predicted to abolish both Cs+ and Rb+ block in Kir2.1, but blocking affinity was similar to WT. Therefore, other properties of the pore must contribute to Cs+ and Rb+ block at position 172 in addition to the electrostatic and steric effects identified in this study.