PROTONS AND CALCIUM MODULATE SV-TYPE CHANNELS IN THE VACUOLAR-LYSOSOMAL COMPARTMENT - CHANNEL INTERACTION WITH CALMODULIN INHIBITORS

Slowly activating vacuolar (SV-type; Hedrich and Neher 1987, Nature 32 9: 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 tech nique to isolated vacuoles of both tissues, the electrical and pharmac ological properties of guard-cell SV-type currents were studied and co mpared to the sugar beet channel with regard to its modulation by cyto plasmic Ca2+ and pH. This outward rectifier of V. faba guard cells sho wed a half-maximum activation at 55-60 mV with an apparent gating char ge equivalent of z approximate to 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 mu m(-2) compare d to taproots (120-140 pS at about 0.16 channels per mu m(2)) Guard-ce ll SV-type channels are weakly selective for cations over anions and l ack saturation at KCl concentrations of up to 1 M. Since in the absenc e of physiological K+ concentrations, Ca2+ is the major permeable ion, relative changes in the amounts of the two ions might control the per meation process. In spite of their different origins and physiological functions, in guard cells and beet taproot cells, cytoplasmic Ca2+ an d protons, both considered as candidates for intracellular signalling in plants, modulate the voltage dependence of SV-type channels. While the two effecters do not alter the single-channel conductance, they st rongly interact with the voltage sensor. The calmodulin (CaM) antagoni sts N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride ( W-7), trifluoperazine (TFP) and calmidazolium hydrochloride (R 24571) effectively blocked the channel in an antagonist-specific manner. In a greement 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, whic h might be responsible for the release of K+, Cl- and to a smaller ext ent Ca2+ during stomatal closure, could serve as an intracellular sens or for changes in cytosolic calcium (calcium-CaM) and pH.