A conserved cytoplasmic region of ROMK modulates pH sensitivity, conductance, and gating

ROMK channels play a key role in overall K balance by controlling K secretion across the apical membrane of mammalian cortical collecting tubule. In contrast to the family of strong inward rectifiers (IRKs), ROMK channels are markedly sensitive to intracellular pH. Using Xenopus oocytes, we have confirmed this pH sensitivity at both the single-channel and whole cell level. Reduction of oocyte pH from 6.8 to 6.4 (using a permeant acetate buffer) reduced channel open probability from 0.76 +/- 0.02 to near zero (n = 8), without altering single-channel conductance. This was due to the appearance of a long-lived closed state at low internal pH. We have confirmed that a lysine residue (K61 on ROMK2; K80 on ROMK1), NH2 terminal to the first putative transmembrane segment (M1), is primarily responsible for conferring a steep pH sensitivity to ROMK (B. Fakler, J. Schultz, J. Yang, U. Schulte, U. Bråandle, H. P. Zenner, L. Y. Jan, and J. P. Ruppersberg. EMBO J. 15: 4093-4099, 1996). However, the apparent pKa of ROMK also depends on another residue in a highly conserved, mildly hydrophobic area: T51 on ROMK2 (T70 on ROMK1). Replacing this neutral threonine (T51) with a negatively charged glutamate shifted the apparent pKa for inward conductance from 6.5 +/- 0.01 (n = 8, wild type) to 7.0 +/- 0.02 (n = 5, T51E). On the other hand, replacing T51 with a positively charged lysine shifted the apparent pKa in the opposite direction, from 6.5 +/- 0.01 (n = 8, wild type) to 6.0 +/- 0.02 (n = 9, T51K). The opposite effects of the glutamate and lysine substitutions at position 51 (ROMK2) are consistent with a model in which T51 is physically close to K61 and alters either the local pH or the apparent pKa via an electrostatic mechanism. In addition to its effects on pH sensitivity, the mutation T51E also decreased single-channel conductance from 34.0 +/- 1.0 pS (n = 8, wild type) to 17.4 +/- 1 pS (n = 9, T51E), reversed the voltage gating of the channel, and significantly increased open-channel noise. These effects on single-channel currents suggest that the T51 residue, located in a mildly hydrophobic area of ROMK2, also interacts with the hydrophobic region of the permeation pathway.