May play a role in signal transduction pathways that involve calcium as a second messenger (By similarity). Involved in the activation of sucrose synthase 2 (SUS2) in developing seeds. Phosphorylates SUS2 at Ser-10 in a calcium-dependent manner. Phosphorylation of Ser-10 leads to the activation of SUS2, which supplies substrates for the biosynthesis of storage products in immature seeds (PubMed:11910009). 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 endosperm of developing seeds. By UV-C. 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 (337-367) inactivates kinase activity under calcium-free conditions. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. CDPK subfamily.