May play a role in signal transduction pathways that involve calcium as a second messenger. Could act as a calcium sensor involved in drought- and salt stress-induced calcium signaling cascades. Mediates the phosphorylation and activation of the S-type anion efflux channel SLAC1. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated by calcium. Autophosphorylation may play an important role in the regulation of the kinase activity (By similarity). Interacts with SLAC1 and ABI1. A number of isoforms are produced. According to EST sequences. There are 3 contiguous domains conserved in the CDPK subfamily: a kinase domain, an autoinhibitory (junction) domain and a calmodulin-like domain. The autoinhibitory domain (332-362) inactivates kinase activity under calcium-free conditions (By similarity). Mutant cpk23-1 shows greatly enhanced tolerance to drought and salt stresses and displays reduced stomatal aperture and reduced K(+) content. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. CDPK subfamily.