Guard Cells Possess a Calcium-Dependent Protein Kinase That Phosphorylates the KAT1 Potassium Channel

Increasing evidence suggests that changes in cytosolic Ca²⁺ levels and phosphorylation play important roles in the regulation of stomatal aperture and as ion transporters of guard cells. However, protein kinases responsible for Ca²⁺ signaling in guard cells remain to be identified. Using biochemical approaches, we have identified a Ca²⁺-dependent protein kinase with a calmodulin-like domain (CDPK) in guard cell protoplasts of Vicia faba. Both autophosphorylation and catalytic activity of CDPK are Ca²⁺ dependent. CDPK exhibits a Ca²⁺-induced electrophoretic mobility shift and its Ca²⁺-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 Ca²⁺ concentrations stimulate phosphorylation of several proteins from guard cells; cyclosporin A, a specific inhibitor of the Ca²⁺-dependent protein phosphatase calcineurin enhances the Ca²⁺-dependent phosphorylation of several soluble proteins. CDPK from guard cells phosphorylates the K⁺ channel KAT1 protein in a Ca²⁺-dependent manner. These results suggest that CDPK may be an important component of Ca²⁺ signaling in guard cells.