ROMK channels are responsible for K+ secretion in kidney. The activity of ROMK is regulated by intracellular pH (pH(i)) with acidification causing channel closure (effective pK(a) ~6.9). Recently, we and others reported that a direct interaction of the channels with phosphatidyl-4,5-bisphosphate (PIP2) is critical for opening of the inwardly rectifying K+ channels. Here, we investigate the relationship between the mechanisms for regulation of ROMK by PIP2 and by pH(i). We find that disruption of PIP2-ROMK1 interaction not only decreases single-channel open probability (P(o)) but gives rise to a ROMK1 subconductance state. This state has an increased sensitivity to intracellular protons (effective pK(a) shifted to pH ~7.8), such that the subconductance channels are relatively quiescent at physiological pH(i). Open probability for the subconductance channels can then be increased by intracellular alkalinization to supra-physiological pH. This increase in P(o) for the subconductance channels by alkalinization is not associated with an increase in PIP2-channel interaction. Thus, direct interaction with PIP2 is critical for ROMK1 to open at full conductance. Disruption of this interaction increases pH(i) sensitivity for the channels via emergence of the subconductance state. The control of open probability of ROMK1 by pH(i) occurs via a mechanism distinct from the regulation by PIP2.
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