The Wnt pathway scaffold protein Axin promotes signaling specificity by suppressing competing kinase reactions
GSK3{beta} is a multifunctional kinase that phosphorylates {beta}-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 {beta}-catenin phosphorylation, we measured GSK3{beta} reaction rates with multiple substrates in a minimal, biochemically-reconstituted system. We observed an unexpectedly small, ~2-fold Axin-mediated rate increase for the {beta}-catenin reaction. The much larger effects reported previously may have arisen because Axin can rescue GSK3{beta} from an inactive state that occurs only under highly specific conditions. Surprisingly, Axin significantly slows the reaction of GSK3{beta} with CREB, a non-Wnt pathway substrate. When both {beta}-catenin and CREB are present, Axin accelerates the {beta}-catenin reaction by preventing competition with CREB. Thus, while Axin alone does not markedly accelerate the {beta}-catenin reaction, in physiological settings where multiple GSK3{beta} substrates are present, Axin can promote signaling specificity by suppressing interactions with competing, non-Wnt pathway targets.