The selectivity filter of the tandem pore potassium channel TASK-1 and its pH-sensitivity and ionic selectivity

We have studied pH sensitivity and ionic selectivity of the tandem pore K+ channel TASK-1 heterologously expressed in Xenopus oocytes. We fit pH sensitivity assuming that only one of the two residues H98 need be protonated for channels to be shut. The effect of protons was weakly voltage dependent with a pK(a) of 6.02 at +40 mV. Replacement of His (H98D, H98N) reduced pH sensitivity but did not abolish it. Use of a concatameric channel permitted replacement of one His residue only; this concatamer was fully pH-sensitive. Increasing the number of His residues to 4 (mutant D204H) abolished pH sensitivity over the physiological range. The implication that D204 plays a role in pH-sensitivity was confirmed by the finding that pH sensitivity over the physiological range was also abolished in the mutant D204N. Ionic selectivity was also altered in D204H, D204N and H98D mutants. P-Rb/P-K was increased from 0.80+/-0.04 (n=19) in wild type to 1.06+/-0.04 (n=19) in D204H. H98D, D204H and D204N were permeable to Na+ with P-Na/P-K=0.39+/-0.03 (n=14) in H98D, 0.64+/-0.04 (n=18) in D204H and 0.33+/-0.07 (n=3) in D204N. Thus, the arrangement of ring of residues HDHD appears to optimise both pH sensitivity and ionic selectivity.