The Scaffold Protein Axin Promotes Signaling Specificity within the Wnt Pathway by Suppressing Competing Kinase Reactions

Scaffold proteins are thought to promote signaling specificity by accelerating reactions between bound kinase and substrate proteins. To test the long-standing hypothesis that the scaffold protein Axin accelerates glycogen synthase kinase 3 beta (GSK3 beta)-mediated phosphorylation of beta-catenin in the Wnt signaling network, we measured GSK30 reaction rates with multiple substrates in a minimal, biochemically reconstituted system. We observed an unexpectedly small, similar to 2-fold Axin-mediated rate increase for the beta-catenin reaction when measured in isolation. In contrast, when both beta-catenin and non-Wnt pathway substrates are present, Axin accelerates the beta-catenin reaction by preventing competition with alternative substrates. At high competitor concentrations, Axin produces >10-fold rate effects. Thus, while Axin alone does not markedly accelerate the beta-catenin reaction, in physiological settings where multiple GSK3 beta substrates are present, Axin may promote signaling specificity by suppressing interactions with competing, non-Wnt pathway targets. This mechanism for scaffold-mediated control of competition enables a shared kinase to perform distinct functions in multiple signaling networks.