DELETION OF AN AMINO-TERMINAL SEQUENCE STABILIZES BETA-CATENIN IN-VIVO AND PROMOTES HYPERPHOSPHORYLATION OF THE ADENOMATOUS POLYPOSIS-COLI TUMOR-SUPPRESSOR PROTEIN

Regulation of cell adhesion and cell signaling by beta-catenin occurs through a mechanism likely involving the targeted degradation of the p rotein. Deletional analysis was used to generate a beta-catenin refrac tory to rapid turnover and to examine its effects on complexes contain ing either cadherin or the adenomatous polyposis coli (APC) protein. T he results show that amino-terminal deletion of beta-catenin results i n a protein with increased stability that acts in a dominant fashion w ith respect to wild-type beta-catenin. Constitutive expression in AtT2 0 cells of a beta-catenin lacking 89 N-terminal amino acids (Delta N89 beta-catenin) resulted in severely reduced levels of the more labile wild-type beta-catenin. The mutant beta-catenin was expressed at endog enous levels but displaced the vast majority of wild-type beta-catenin associated with N-cadherin. The Delta N89 beta-catenin accumulated on the APC protein to a level 10-fold over that of wild-type beta-cateni n and recruited a kinase into the APC complex. The kinase was highly a ctive toward APC in vitro and promoted a sodium dodecyl sulfate gel ba nd shift that was also evident for endogenous APC from cells expressin g the mutant beta-catenin. Unlike wild-type beta-catenin, which partit ions solely as part of a high-molecular-weight complex, the Delta N89 mutant protein also fractionated as a stable monomer, indicating that it had escaped the requirement to associate with other proteins. That similar N-terminal mutants of beta-catenin have been implicated in cel lular transformation suggests that their abnormal association with APC may, in part, be responsible for this phenotype.