May play a role in signal transduction pathways that involve calcium as a second messenger. Functions in abscisic acid (ABA) regulation of guard cell S-type anion- and Ca(2+)-permeable channels and stomatal closure. 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 GHR1. Expressed in both guard cells and mesophyll cells. 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 (342-372) inactivates kinase activity under calcium-free conditions (By similarity). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. CDPK subfamily.