TASK-3, a novel tandem pore domain acid-sensitive K+ channel - An extracellular histidine as pH sensor

Tandem pore domain acid-sensitive K+ channel 3 (TASK-3) is a new member of the tandem pore domain potassium channel family. A cDNA encoding a 365-amin o acid polypeptide with four putative transmembrane segments and two pore r egions was isolated from guinea pig brain. An orthologous sequence was clon ed hom a human genomic library. Although TASK-3 is 62% identical to TASK-1, the cytosolic C-terminal sequence is only weakly conserved. Analysis of th e gene structure identified an intron within the conserved GYG motif of the first pore region. Reverse transcriptase-polymerase chain reaction analysi s showed strong expression in brain but very weak mRNA levels in other tiss ues. Cell-attached patch-clamp recordings of TASK-3 expressed in HER293 cel ls showed that the single channel current-voltage relation was inwardly rec tifying, and open probability increased markedly with depolarization. Remov al of external divalent cations increased the mean single channel current m easured at -100 mV from -2.3 to -5.8 pA. Expression of TASK-3 in Xenopus oo cytes revealed an outwardly rectifying RC current that was strongly decreas ed in the presence of lower extracellular pH. Substitution of the histidine residue His-98 by asparagine or tyrosine abolished pH sensitivity. This hi stidine, which is located at the outer part of the pore adjacent to the sel ectivity filter, may be an essential component of the extracellular pH sens or.