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Extracellular Ba 2+ and voltage interact to gate Ca 2+ channels at the plasma membrane of stomatal guard cells
Author(s) -
Hamilton David W.A.,
Hills Adrian,
Blatt Michael R.
Publication year - 2001
Publication title -
febs letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.593
H-Index - 257
eISSN - 1873-3468
pISSN - 0014-5793
DOI - 10.1016/s0014-5793(01)02176-7
Subject(s) - guard cell , divalent , biophysics , extracellular , chemistry , membrane , membrane potential , patch clamp , gating , ion transporter , cytosol , analytical chemistry (journal) , biochemistry , chromatography , biology , receptor , organic chemistry , enzyme
Ca 2+ channels at the plasma membrane of stomatal guard cells contribute to increases in cytosolic free [Ca 2+ ] ([Ca 2+ ] i ) that regulate K + and Cl − channels for stomatal closure in higher‐plant leaves. Under voltage clamp, the initial rate of increase in [Ca 2+ ] i in guard cells is sensitive to the extracellular divalent concentration, suggesting a close interaction between the permeant ion and channel gating. To test this idea, we recorded single‐channel currents across the Vicia guard cell plasma membrane using Ba 2+ as a charge carrying ion. Unlike other Ca 2+ channels characterised to date, these channels activate at hyperpolarising voltages. We found that the open probability ( P o ) increased strongly with external Ba 2+ concentration, consistent with a 4‐fold cooperative action of Ba 2+ in which its binding promoted channel opening in the steady state. Dwell time analyses indicated the presence of a single open state and at least three closed states of the channel, and showed that both hyperpolarising voltage and external Ba 2+ concentration prolonged channel residence in the open state. Remarkably, increasing Ba 2+ concentration also enhanced the sensitivity of the open channel to membrane voltage. We propose that Ba 2+ binds at external sites distinct from the permeation pathway and that divalent binding directly influences the voltage gate.