A NOVEL CHLORIDE CHANNEL IN VICIA-FABA GUARD-CELL VACUOLES ACTIVATED BY THE SERINE THREONINE KINASE, CDPK/

Calcium-Dependent Protein Kinases (CDPKs) in higher plants contain a C -terminal calmodulin-like regulatory domain. Little is known regarding physiological CDPK targets, Both kinase activity and multiple Ca2+-de pendent signaling pathways have been implicated in the control of stom atal guard cell movements, To determine whether CDPK or other protein kinases could have a role in guard cell signaling, purified and recomb inant kinases were applied to Vicia faba guard cell vacuoles during pa tch-clamp experiments. CDPK activated novel vacuolar chloride (VCL) an d malate conductances in guard cells, Activation was dependent on both Ca2+ and ATP, Furthermore, VCL activation occurred in the absence of Ca2+ using a Ca2+-independent, constitutively active, CDPK mutant. Pr otein kinase A showed weaker activation (22% as compared with CDPK). C urrent reversals in whole vacuole recordings shifted with the Nernst p otential for Cl- and vanished in glutamate. Single channel recordings showed a CDPK-activated 34 +/- 5 pS Cl- channel. VCL channels were act ivated at physiological potentials enabling Cl- uptake into vacuoles, VCL channels may provide a previously unidentified, but necessary, pat hway for anion uptake into vacuoles required for stomatal opening. CDP K-activated VCL currents were also observed in red beet vacuoles sugge sting that these channels may provide a more general mechanism for kin ase-dependent anion uptake.