Transactivation activity and nucleocytoplasmic transport of ß-catenin are independently regulated by its C-terminal end.

The key protein in the canonical Wnt pathway is ß-catenin, which is phosphorylated both in absence and presence of Wnt signals by different kinases. Upon activation in the cytoplasm, ß-catenin can enter into the nucleus to transactivate target gene expression, many of which are cancer-related genes. The mechanism governing ß-catenin's nucleocytoplasmic transport has been recently unvealed, although phosphorylation at its C-terminal end and its functional consequences are not completely understood. Serine 646 of ß-catenin is a putative CK2 phosphorylation site and lies in a region which has been proposed to be important for its nucleocytoplasmic transport and transactivation activity. This residue was mutated to aspartic acid mimicking CK2-phosphorylation and its effects on ß-catenin activity as well as localization were explored. ß-Catenin S6464D did not show significant differences in both transcriptional activity and nuclear localization compared to the wild-type form, but displayed a characteristic granular nuclear pattern. Three-dimensional models of nuclei were constructed which showed differences in number and volume of granules, being those from ß-catenin S646D more and smaller than the wild-type form. FRAP microscopy was used to compare nuclear export of both proteins which showed a slightly higher but not significant retention of ß-catenin S646D. Altogether, these results show that C-terminal phosphorylation of ß-catenin seems to be related with its nucleocytoplasmic transport but not transactivation activity.