Inhibition of the Wnt signaling pathway by the PR61 subunit of protein phosphatase 2A

Axin, a negative regulator of the Wnt signaling pathway, forms a complex with glycogen synthase kinase-3 beta (GSK-3 beta), beta -catenin, adenomatous polyposis coli (APC) gene product, and Dvl, and it regulates GSK-3 beta -dependent phosphorylation in the complex and the stability of beta -catenin. Using yeast two-hybrid screening, we found that regulatory subunits of protein phosphatase 2A, PR61 beta and -gamma, interact with Axin. PR61 beta or -gamma formed a complex with Axin in intact cells, and their interaction was direct. The binding site of PR61 beta on Axin was different from those of GSK-3 beta, beta -catenin, APC, and Dvl. Although PR61 beta did not affect the stability of beta -catenin, it inhibited Dvl- and beta -catenin-dependent T cell factor activation in mammalian cells. Moreover, it suppressed beta -catenin-induced axis formation and expression of siamois, a Wnt target gene, in Xenopus embryos, suggesting that PR61 beta acts either at the level of beta -catenin or downstream of it. Taken together with the previous observations that PR61 interacts with APC and functions upstream of beta -catenin, these results demonstrate that PR61 regulates the Wnt signaling pathway at various steps.