The Wnt pathway scaffold protein Axin promotes signaling specificity by suppressing competing kinase reactions

GSK3β is a multifunctional kinase that phosphorylates β-catenin in the Wnt signaling network and also acts on other protein targets in response to distinct cellular signals. To test the long-standing hypothesis that the scaffold protein Axin specifically accelerates β-catenin phosphorylation, we measured GSK3β reaction rates with multiple substrates in a minimal, biochemically-reconstituted system. We observed an unexpectedly small, ~2-fold Axin-mediated rate increase for the β-catenin reaction. The much larger effects reported previously may have arisen because Axin can rescue GSK3β from an inactive state that occurs only under highly specific conditions. Surprisingly, Axin significantly slows the reaction of GSK3β with CREB, a non-Wnt pathway substrate. When both β-catenin and CREB are present, Axin accelerates the β-catenin reaction by preventing competition with CREB. Thus, while Axin alone does not markedly accelerate the β-catenin reaction, in physiological settings where multiple GSK3β substrates are present, Axin can promote signaling specificity by suppressing interactions with competing, non-Wnt pathway targets.