Cell Systems
Volume 10, Issue 6, 24 June 2020, Pages 515-525.e5
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The Scaffold Protein Axin Promotes Signaling Specificity within the Wnt Pathway by Suppressing Competing Kinase Reactions

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Highlights

  • The Axin scaffold protein directs the kinase GSK3β between alternative substrates

  • Axin does not dramatically boost rate constants for GSK3β reacting with β-catenin

  • Axin suppresses GSK3β reactions with competing substrates

  • When multiple substrates are present, Axin accelerates the β-catenin reaction

Summary

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 3β (GSK3β)-mediated phosphorylation of β-catenin in the Wnt signaling network, 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 when measured in isolation. In contrast, when both β-catenin and non-Wnt pathway substrates are present, Axin accelerates the β-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 β-catenin reaction, in physiological settings where multiple GSK3β 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.

Keywords

scaffold protein
Wnt signaling
glycogen synthase kinase 3
GSK3
Axin
cell signaling

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These authors contributed equally

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