Protons and calcium modulate SV-type channels in the vacuolar-lysosomal compartment — channel interaction with calmodulin inhibitors
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- Schulz-Lessdorf, B. & Hedrich, R. Planta (1995) 197: 655. doi:10.1007/BF00191574
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Slowly activating vacuolar (SV-type; Hedrich and Neher 1987, Nature 329: 833–835) ion channels provide the predominant membrane conductance of the vacuolar-lysosomal compartment of Vicia faba L. guard cells and sugar beet (Beta vulgaris L.) taproots. Applying the patch-clamp technique to isolated vacuoles of both tissues, the electrical and pharmacological properties of guard-cell SV-type currents were studied and compared to the sugar beet channel with regard to its modulation by cytoplasmic Ca2+ and pH. This outward rectifier of V. faba guard cells showed a half-maximum activation at 55–60 mV with an apparent gating charge equivalent of z ≈ 4. Studies on the single-channel and whole-vacuole level revealed an extremely high conductance of 280 pS for the guard-cell channels at a mean density of 0.37 μm-2 compared to taproots (120–140 pS at about 0.16 channels per μm2). Guard-cell SV-type channels are weakly selective for cations over anions and lack saturation at KC1 concentrations of up to 1 M. Since in the absence of physiological K+ concentrations, Ca2+ is the major permeable ion, relative changes in the amounts of the two ions might control the permeation process. In spite of their different origins and physiological functions, in guard cells and beet taproot cells, cytoplasmic Ca2+ and protons, both considered as candidates for intracellular signalling in plants, modulate the voltage dependence of SV-type channels. While the two effectors do not alter the single-channel conductance, they strongly interact with the voltage sensor. The calmodulin (CaM) antagonists N-(6-aminohexyl)-5-chloro-l-naphthalenesulfonamide hydrochloride (W-7), trifluoperazine (TFP) and calmidazolium hydrochloride (R 24571) effectively blocked the channel in an antagonist-specific manner. In agreement with the properties of a Ca2+-permeable channel, CaM could be involved in the modulation of the activation threshold of the SV-type channel. We therefore conclude that guard-cell SV-type channels, which might be responsible for the release of K+, Cl- and to a smaller extent Ca2+ during stomatal closure, could serve as an intracellular sensor for changes in cytosolic calcium (calcium-CaM) and pH.
Key wordsCalcium Calmodulin antagonists pH SV-type channels (vacuole) Taproot, guard cells
- R 24571
slowly activating vacuolar