Delta-catenin was first identified because of its interaction with presenilin-1, and its aberrant expression has been reported in various human tumors and in patients with Cri-du-Chat syndrome, a form of mental retardation. However, the mechanism whereby delta-catenin is regulated in cells has not been fully elucidated. We investigated the possibility that glycogen-synthase kinase-3 (GSK-3) phosphorylates delta-catenin and thus affects its stability. Initially, we found that the level of delta-catenin was greater and the half-life of delta-catenin was longer in GSK-3beta(-/-) fibroblasts than those in GSK-3beta(+/+) fibroblasts. Furthermore, four different approaches designed to specifically inhibit GSK-3 activity, i.e. GSK-3-specific chemical inhibitors, Wnt-3a conditioned media, small interfering RNAs, and GSK-3alpha and -3beta kinase dead constructs, consistently showed that the levels of endogenous delta-catenin in CWR22Rv-1 prostate carcinoma cells and primary cortical neurons were increased by inhibiting GSK-3 activity. In addition, it was found that both GSK-3alpha and -3beta interact with and phosphorylate delta-catenin. The phosphorylation of DeltaC207-delta-catenin (lacking 207 C-terminal residues) and T1078A delta-catenin by GSK-3 was noticeably reduced compared with that of wild type delta-catenin, and the data from liquid chromatography-tandem mass spectrometry analyses suggest that the Thr(1078) residue of delta-catenin is one of the GSK-3 phosphorylation sites. Treatment with MG132 or ALLN, specific inhibitors of proteosome-dependent proteolysis, increased delta-catenin levels and caused an accumulation of ubiquitinated delta-catenin. It was also found that GSK-3 triggers the ubiquitination of delta-catenin. These results suggest that GSK-3 interacts with and phosphorylates delta-catenin and thereby negatively affects its stability by enabling its ubiquitination/proteosome-mediated proteolysis.