The Axin scaffold protects the kinase GSK3β from cross-pathway inhibition

Multiple signaling pathways regulate the kinase GSK3{beta} by inhibitory phosphorylation at Ser9, which then occupies the GSK3{beta} priming pocket and blocks substrate binding. Since this mechanism should affect GSK3{beta} activity towards all primed substrates, it is unclear why Ser9 phosphorylation does not affect other GSK3{beta}-dependent pathways, such as Wnt signaling. We used biochemical reconstitution and cell culture assays to evaluate how Wnt-associated GSK3{beta} is insulated from cross-activation by other signals. We found that the Wnt-specific scaffold protein Axin allosterically protects GSK3{beta} from phosphorylation at Ser9 by upstream kinases, which prevents accumulation of pS9-GSK3{beta} in the Axin-GSK3{beta} complex. Scaffold proteins that protect bound proteins from alternative pathway reactions could provide a general mechanism to insulate signaling pathways from improper crosstalk.