Serine/threonine-protein kinase. Casein kinases are operationally defined by their preferential utilization of acidic proteins such as caseins as substrates. It can phosphorylate a large number of proteins. Participates in Wnt signaling (By similarity). Phosphorylates COL4A3BP/CERT, MTA1 and SMAD3. Involved in brain development and vesicular trafficking and neurotransmitter releasing from small synaptic vesicles. Regulates fast synaptic transmission mediated by glutamate. SMAD3 phosphorylation promotes its ligand-dependent ubiquitination and subsequent proteasome degradation, thus inhibiting SMAD3-mediated TGF-beta responses. Hyperphosphorylation of the serine-repeat motif of COL4A3BP/CERT leads to its inactivation by dissociation from the Golgi complex, thus down-regulating ER-to-Golgi transport of ceramide and sphingomyelin synthesis. Triggers PER1 proteasomal degradation probably through phosphorylation (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Stimulated by estrogen. Monomer. Interacts with MTA1 (short isoform) in the cytoplasm. Interacts with SMAD3 (By similarity). Testis. The phospho-regulated basic and hydrophobic (PRBH) motif is sufficient and important for interaction with phospholipids permitting cortical localization (By similarity). Phosphorylation of the PRBH motif by aPKC inhibits the association of the protein with the cortical membrane (By similarity). Autophosphorylated (PubMed:7759525). Phosphorylated by aPKC which promotes dissociation from the cell cortex (By similarity). Belongs to the protein kinase superfamily. CK1 Ser/Thr protein kinase family. Casein kinase I subfamily.