Molecular determinants for activation of G-protein-coupled inward rectifier K+ (GIRK) channels by extracellular acidosis

Synaptic cleft acidification occurs following vesicle release. Such a pH change may affect synaptic transmissions in which G-protein-coupled inward rectifier K(+) (GIRK) channels play a role. To elucidate the effect of extracellular pH (pH(o)) on GIRK channels, we performed experiments on heteromeric GIRK1/GIRK4 channels expressed in Xenopus oocytes. A decrease in pH(o) to 6.2 augmented GIRK1/GIRK4 currents by approximately 30%. The channel activation was reversible and dependent on pH(o) levels. This effect was produced by selective augmentation of single channel conductance without change in the open-state probability. To determine which subunit was involved, we took advantage of homomeric expression of GIRK1 and GIRK4 by introducing a single mutation. We found that homomeric GIRK1-F137S and GIRK4-S143T channels were activated at pH(o) 6.2 by approximately 20 and approximately 70%, respectively. Such activation was eliminated when a histidine residue in the M1-H5 linker was mutated to a non-titratable glutamine, i.e. H116Q in GIRK1 and H120Q in GIRK4. Both of these histidines were required for pH sensing of the heteromeric channels, because the mutation of one of them diminished but not abolished the pH(o) sensitivity. The pH(o) sensitivity of the heteromeric channels was completely lost when both were mutated. Thus, these results suggest that the GIRK-mediated synaptic transmission is determined by both neurotransmitter and protons with the transmitter accounting for only 70% of the effect on postsynaptic cell and protons released together with the transmitter contributing to the other 30%.