Molecular determinants of intracellular pH modulation of human Kv1.4 N-type inactivation

A-type K⁺ currents serve important functions in neural and cardiac physiology. The human A-type Kv1.4 channel (hKv1.4) shows fast N-type inactivation when expressed in Xenopus laevis oocytes. We found that intracellular pH (pH_i) regulated the macroscopic inactivation time constant (T) and current amplitude (I_peak), producing a 2-fold change with each pH unit change in the physiologically relevant range of 8.0 to 6.0. These effects of pH_i were completely abolished by a large deletion in the hKv-1.4 N terminus. Site-directed mutagenesis identified a histidine (H16) in the inactivation ball domain as a critical H⁺ titratable site mediating the pH effects on N-type inactivation between pH 7.0 and 9.0. Substituting this histidine with arginine not only accelerated the time course of macroscopic channel inactivation but also eliminated the H⁺ effects on hKv1.4. In addition, a glutamic acid (E2) in the ball domain constitutes another H⁺ titratable site that mediates the pH effects in the more acidic pH range of 5.0 to 7.0. These results suggest that N-type inactivation in hKv1.4 is regulated by pH_i in the physiologic range through ionization of specific amino acid residues in the ball domain. Such pH_i effects may represent an important fundamental mechanism for physiological regulation of excitable tissue function.