Extracellular Ba²⁺ and voltage interact to gate Ca²⁺ channels at the plasma membrane of stomatal guard cells

Hamilton, D. W.A., Hills, A. and Blatt, M. R. (2001) Extracellular Ba²⁺ and voltage interact to gate Ca²⁺ channels at the plasma membrane of stomatal guard cells. FEBS Letters, 491(1-2), pp. 99-103. (doi: 10.1016/S0014-5793(01)02176-7)

Abstract

Ca2+ channels at the plasma membrane of stomatal guard cells contribute to increases in cytosolic free [Ca²⁺] ([Ca²⁺]_i) that regulate K⁺ and Cl⁻ channels for stomatal closure in higher-plant leaves. Under voltage clamp, the initial rate of increase in [Ca²⁺]_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 <i>Vicia</i> guard cell plasma membrane using Ba2+ as a charge carrying ion. Unlike other Ca²⁺ channels characterised to date, these channels activate at hyperpolarising voltages. We found that the open probability (P_o) increased strongly with external Ba²⁺ concentration, consistent with a 4-fold cooperative action of Ba²⁺ 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 Ba²⁺ concentration prolonged channel residence in the open state. Remarkably, increasing Ba²⁺ concentration also enhanced the sensitivity of the open channel to membrane voltage. We propose that Ba²⁺ binds at external sites distinct from the permeation pathway and that divalent binding directly influences the voltage gate.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Blatt, Professor Michael
• Authors: Hamilton, D. W.A., Hills, A., and Blatt, M. R.
• College/School: College of Medical Veterinary and Life Sciences > School of Molecular Biosciences
• Journal Name: FEBS Letters
• Publisher: Elsevier BV
• ISSN: 0014-5793
• ISSN (Online): 1873-3468
• Published Online: 20 February 2001