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

Abstract

The key protein in the canonical Wnt pathway is beta-catenin, which is phosphorylated both in absence and presence of Wnt signals by different kinases. Upon activation in the cytoplasm, beta-catenin can enter into the nucleus to transactivate target gene expression, many of which are cancer-related genes. The mechanism governing beta-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 beta-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 beta-catenin activity as well as localization were explored. beta-Catenin 56464D 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 p-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 beta-catenin S646D. Altogether, these results show that C-terminal phosphorylation of beta-catenin seems to be related with its nucleocytoplasmic transport but not transactivation activity. (C) 2015 Elsevier B.V. All rights reserved.