Casein kinases are operationally defined by their preferential utilization of acidic proteins such as caseins as substrates. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Targeting to the vacuolar membrane may depend on AP-3 pathway. The YXXZ (Tyr-Xaa-Xaa-Zaa, where Zaa is a hydrophobic residue) motif mediates the targeting to the lysosomal compartments. Present with 1320 molecules/cell in log phase SD medium. Belongs to the protein kinase superfamily. CK1 Ser/Thr protein kinase family. Casein kinase I subfamily.