Functional domains of axin - Importance of the C terminus as an oligomerization domain

To understand the mechanism of how Axin acts as an inhibitory molecule in t he Wnt pathway, we generated a series of mutated forms of Axin. From the bi nding experiments, we defined the domains of Axin that bind glycogen syntha se kinase-3 beta (GSK-3 beta) and beta-catenin. We also examined the abilit y of each Axin mutant to inhibit lymphoid enhancer factor-1 (Lef-1) reporte r activity in a cell line expressing high levels of beta-catenin. Axin muta nts that did not bind GSK-3 beta or beta-catenin were ineffective in suppre ssing Lef-1 reporter activity. Binding GSK-3 beta and beta-catenin was not sufficient for this inhibitory effect of Axin. Axin mutants with C-terminal truncations lacked the ability to inhibit Lef-1 reporter activity, even th ough they bound GSH-3 beta and beta-catenin. The C-terminal region was requ ired for binding to Axin itself. Substitution of the C-terminal, region wit h an unrelated dimerizing molecule, the retinoid X receptor restored its in hibitory effect on Lef-1-dependent, transcription. The oligomerization of A xin through its C terminus is important for its function in regulation of b eta-catenin-mediated response.