Increasing evidence suggests that changes in cytosolic Ca 2+ levels and phosphorylation play important roles in the regulation of stomatal aperture and as ion transporters of guard cells. However, protein kinases responsible for Ca 2+ signaling in guard cells remain to be identified. Using biochemical approaches, we have identified a Ca 2+ -dependent protein kinase with a calmodulin-like domain (CDPK) in guard cell protoplasts of Vicia faba . Both autophosphorylation and catalytic activity of CDPK are Ca 2+ dependent. CDPK exhibits a Ca 2+ -induced electrophoretic mobility shift and its Ca 2+ -dependent catalytic activity can be inhibited by the calmodulin antagonists trifluoperazine and N -(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide. Antibodies to soybean CDPKα cross-react with CDPK. Micromolar Ca 2+ concentrations stimulate phosphorylation of several proteins from guard cells; cyclosporin A, a specific inhibitor of the Ca 2+ -dependent protein phosphatase calcineurin enhances the Ca 2+ -dependent phosphorylation of several soluble proteins. CDPK from guard cells phosphorylates the K + channel KAT1 protein in a Ca 2+ -dependent manner. These results suggest that CDPK may be an important component of Ca 2+ signaling in guard cells. Abbreviations: AAPK ABA-activated protein kinase CDPK calcium-dependent protein kinase containing a calmodulin-like domain CsA cyclosporin A GCP guard cell protoplast KAT1 a potassium channel cDNA from Arabidopsis TFP trifluoperazine W-5 N -(6-aminohexyl)-1-naphthalenesulfonamide W-7 N -(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide
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