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. Specifically expressed in heading panicles, spikelets and mature pollen grains. Not expressed in vegetative tissues. 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 (347-377) inactivates kinase activity under calcium-free conditions. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. CDPK subfamily.