Identification of a titratable lysine residue that determines sensitivity of kidney potassium channels (ROMK) to intracellular pH

Potassium (K+) homeostasis is controlled by the secretion of K+ ions across the apical membrane of renal collecting duct cells through a low-conductance inwardly rectifying K+ channel, The sensitivity of this channel to intracellular pH is particularly high and assumed to play a key role in K+ homeostasis, Recently, the apical K+ channel has been cloned (ROMK1, 2, 3 = K(ir)1.1a, K(ir)1.1b and K(ir)1.1c) and the pH dependence of ROMK1 was shown to resemble closely that of the native apical K+ channel, It is reported here that the steep pH dependence of ROMK channels is determined by a single amino acid residue located in the N-terminus close to the first hydrophobic segment M1. Changing lysine (K) at position 80 to methionine (M) removed the sensitivity of ROMK1 channels to intracellular pH. In pH-insensitive IRK1 channels, the reverse mutation (M84K) introduced dependence on intracellular pH similar to that of ROMK1 wild-type, A detailed mutation analysis suggests that a shift in the apparent pK(a) of K80 underlies the pH regulation of ROMK1 channels in the physiological pH range.