Casein kinases are operationally defined by their preferential utilization of acidic proteins such as caseins as substrates. Can phosphorylate casein on serine and threonine residues, and poly(Glu,Tyr) in vitro. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Monomer. Expressed in leaves, stems and flowers. Autophosphorylated on serine, threonine and tyrosine residues. Belongs to the protein kinase superfamily. CK1 Ser/Thr protein kinase family. Casein kinase I subfamily. Sequencing errors. Sequencing errors.