May play a role in signal transduction pathways that involve calcium as a second messenger. 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. Expressed in root tips, leaf veins, mesophyll cells, flower reproductive organs and mature pollen grains. Down-regulated by white light in leaves. 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 (348-378) inactivates kinase activity under calcium-free conditions. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. CDPK subfamily.