THE ROLE OF A SINGLE ASPARTATE RESIDUE IN IONIC SELECTIVITY AND BLOCKOF A MURINE INWARD RECTIFIER K+ CHANNEL KIR2.1

1. The effects of Rb+ and Cs+ as blocking ions were investigated on wi ld-type and mutant forms of the inward rectifier K+ channel, IRK1 (Kir 2.1). 2. In wild-type channels, Rb+ blockage was voltage dependent, in creasing and then falling with increasing hyperpolarization. 3. Rb+ bl ockage was abolished by replacing Asp172 in the M2 domain of the pore- forming subunit by Asn, but was re-established by a change to Gln, nar rowing the pore. Blocking affinity was reduced in D172Q, and was also reduced by replacing Gly 168 in M2 by Ala. 4. Cs+ blockage was also ab olished in D172N but was re-established in D172Q. 5. There appears to be a balance between charge and pore size in determining whether ions block or permeate. A major part of the selectivity of Kir2.1 is associ ated with Asp172 in the M2 domain.