Search

Inward rectifier K+(Kir) channels regulate cell membrane potential. Different Kir channels respond to unique ligands, but all are regulated by phosphatidylinositol 4,5bisphosphate (PI(4,5)P2). Using planar lipid bilayers, we show that Kir2.2 exhibits bursts of openings separated by long quiescent interburst periods. Increasing PI(4,5)P2 concentration shortens the Kir2.2 interburst duration and lengthens the burst duration without affecting dwell times within a burst. From this, we propose that burst and interburst durations correspond to the cytoplasmic domain (CTD)–docked and CTDundocked conformations observed in the presence and absence of PI(4,5)P2 in atomic structures. We also studied the effect of different phosphatidylinositol lipids on Kir2.2 activation and conclude that the 50 phosphate is essential to Kir2.2 pore opening. Other phosphatidylinositollipidscan compete with PI(4,5)P2 but cannot activate Kir2.2without the 50 phosphate. PI(4)P, which is directly interconvertible to and from PI(4,5)P2, might thus be a regulator of Kir channels in the plasma membrane. [ABSTRACT FROM AUTHOR]

Sour taste is detected by a subset of taste cells on the tongue and palate epithelium that respond to acids with trains of action potentials. Entry of protons through a Zn2+-sensitive proton conductance that is specific to sour taste cells has been shown to be the initial event in sour taste transduction. Whether this conductance acts in concert with other channels sensitive to changes in intracellular pH, however, is not known. Here, we show that intracellular acidification generates excitatory responses in sour taste cells, which can be attributed to block of a resting K+ current. We identify KIR2.1 as the acid-sensitive K+ channel in sour taste cells using pharmacological and RNA expression profiling and confirm its contribution to sour taste with tissuespecific knockout of the Kcnj2 gene. Surprisingly, acid sensitivity is not conferred on sour taste cells by the specific expression of KIR2.1, but by the relatively small magnitude of the current, which makes the cells exquisitely sensitive to changes in intracellular pH. Consistent with a role of the K+ current in amplifying the sensory response, entry of protons through the Zn2+-sensitive conductance produces a transient block of the KIR2.1 current. The identification in sour taste cells of an acid-sensitive K+ channel suggests a mechanism for amplification of sour taste and may explain why weak acids that produce intracellular acidification, such as acetic acid, taste more sour than strong acids. [ABSTRACT FROM AUTHOR]

The tetrameric K channel ROMK provides an important pathway for K secretion by the mammalian kidney, and the gating of this channel is highly sensitive to changes in cytosolic pH. Although charge-charge interactions have been implicated in pH sensing by this K channel tetramer, the molecular mechanism linking pH sensing and the gating of ion channels is poorly understood. The x-ray crystal structure KirBac1.1, a prokaryotic ortholog of ROMK, has suggested that channel gating involves intermolecular inter- actions of the N- and C-terminal domains of adjacent subunits. Here we studied channel gating behavior to changes in pH using giant patch clamping of Xenopus laevis oocytes expressing WI or mutant ROMK, and we present evidence that no single charged residue provides the pH sensor. Instead, we show that N-C- and C-C- terminal subunit-subunit interactions form salt bridges, which function to stabilize ROMK in the open state and which are modified by protons. We identify a highly conserved C-C-terminal arginine-glutamate (R-E) ion pair that forms an intermolecular salt bridge and responds to changes in proton concentration. Our results support the intermolecular model for pH gating of inward rectifier K channels. [ABSTRACT FROM AUTHOR]