Conformational switches of AXIN in Wnt/catenin signalling

The Wnt/beta-catenin signalling network is highly conserved and controls important events in embryonic development as well as tissue homeostasis throughout life. The magnitude of Wnt/beta-catenin signalling is determined by the levels and activity of the transcriptional coactivator beta-catenin which also, in its second prominent role, mediates cell-cell adhesion, in this case acting at the plasma membrane. The levels of transcriptionally active beta-catenin are constantly being limited through the action of the beta-catenin destruction complex, a multi-protein complex that captures beta-catenin and then sequentially phosphorylates and ubiquitinates it to target it for proteasomal degradation.

AXIN is the central scaffolding protein of the destruction complex. It contains two domains, an N-terminal RGS (regulator of G-protein signalling) domain and a C-terminal DIX (Dishevelled and AXIN) domain with an extended linker connecting the two domains. The conformational states of AXIN, their regulated transition and the impact the conformations have on interactions of AXIN with other signalling components are poorly understood. Moreover, it remains unknown whether conformational switching affects the ability of AXIN to polymerise.

Using the insect cell/baculovirus system, we have successfully expressed and purified all full-length components of the beta-catenin destruction complex, including AXIN1. This opens up the possibility to study the mutual interactions of destruction complex components and the regulation of these binding events in mechanistic detail. The aim of this project is to understand how post-translational modification of AXIN controls AXIN conformation and oligomerisation status and thereby its ability to assemble protein complexes by binding to components of the beta-catenin destruction complex and Wnt signalosome.

The project aims to assess how AXIN conformation is determined by phosphorylation and PARylation, and how in turn these post-translational modifications determine interactions of AXIN with other Wnt/beta-catenin pathway components to control beta-catenin-dependent transcription.