Phosphatidylinositol 4,5-bisphosphate and intracellular pH regulate the ROMK1 potassium channel via separate but interrelated mechanisms

ROMK channels are responsible for K+ secretion in kidney. The activity of R OMK is regulated by intracellular pH (pH(i)) with acidification causing cha nnel closure (effective pK(a) similar to 6.9). Recently, we and others repo rted that a direct interaction of the channels with phosphatidyl-1,5-bispho sphate (PIP2) is critical for opening of the inwardly rectifying K+ channel s. Here, we investigate the relationship between the mechanisms for regulat ion of ROMK by PIP2 and by pH(i), We find that disruption of PIP2-ROMK1 int eraction not only decreases single-channel open probability (P-o) but gives rise to a ROMK1 subconductance state. This state has an increased sensitiv ity to intracellular protons (effective pK(a) shifted to pH similar to 7.8) , such that the subconductance channels are relatively quiescent at physiol ogical pH(i) Open probability for the subconductance channels can then be i ncreased by intracellular alkalinization to supra-physiological pH. This in crease in P-o for the subconductance channels by alkalinization is not asso ciated with an increase in PIP2-channel interaction. Thus, direct interacti on with PIP2 is critical for ROMK1 to open at full conductance. Disruption of this interaction increases pH(i) sensitivity for the channels via emerge nce of the subconductance state. The control of open probability of ROMK1 b y pH(i) occurs via a mechanism distinct from the regulation by PIP2.