Essential for differentiation and fruit morphogenesis. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] In Dictyostelium the holoenzyme is a dimer composed of a regulatory (R) and a catalytic (C) subunit. In the presence of cAMP it dissociates into the active C subunit and an R monomer. CAPK activity is low in vegetatively growing amoebae, increases during development of aggregation and reaches a maximum at culmination. Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family. cAMP subfamily.