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. 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. 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. Repressed by 5-iodotubercidin (DB04604). Monomer (By similarity). Interacts with MTA1 (short isoform) in the cytoplasm. Interacts with SMAD3. Testis. The phospho-regulated basic and hydrophobic (PRBH) motif is sufficient and important for interaction with phospholipids permitting cortical localization (PubMed:26481050). Phosphorylation of the PRBH motif by aPKC inhibits the association of the protein with the cortical membrane (PubMed:26481050). Autophosphorylated (PubMed:18794808). Phosphorylated by aPKC which promotes dissociation from the cell cortex (PubMed:26481050). Belongs to the protein kinase superfamily. CK1 Ser/Thr protein kinase family. Casein kinase I subfamily.