Structural Basis of the Interaction between Human Axin2 and SIAH1 in the Wnt/β-Catenin Signaling Pathway

Area of Science:

• Molecular Biology
• Biochemistry
• Structural Biology

Background:

• Axin is a key scaffolding protein in Wnt signaling, regulating the β-catenin destruction complex.
• Dysfunctional Axin is linked to carcinogenesis, and its regulation involves phosphorylation, ubiquitination, and poly-ADP-ribosylation.
• The E3 ubiquitin ligase SIAH1 targets proteins for degradation and is involved in Axin2 regulation, though the mechanism is unclear.

Purpose of the Study:

• To elucidate the molecular mechanism by which SIAH1 interacts with Axin2.
• To identify the specific binding domain and interaction interface between Axin2 and SIAH1.
• To explore the potential of this interaction as a drug-binding site for modulating Wnt/β-catenin signaling.

Main Methods:

• GST pull-down assay to confirm the sufficiency of the Axin2-GSK3 binding domain (GBD) for SIAH1 binding.
• X-ray crystallography to determine the high-resolution structure of the Axin2/SIAH1 complex (2.53 Å).

Main Results:

• The Axin2-GBD is sufficient for binding to SIAH1.
• The crystal structure revealed a 1:1 complex where Axin2 binds SIAH1 via its GBD.
• A conserved peptide (EMTPVEPA) within Axin2-GBD forms a loop that binds to a deep groove on SIAH1, mediated by specific amino acid residues and a VxP motif.

Conclusions:

• A novel binding mode between Axin2 and SIAH1 has been characterized at the molecular level.
• The identified interaction interface, particularly the Axin2 peptide and SIAH1 groove, represents a potential druggable target.
• This structural insight provides a foundation for developing therapeutics aimed at regulating the Wnt/β-catenin signaling pathway.