Notum deacylates Wnt proteins to suppress signalling activity

Area of Science:

• Biochemistry
• Developmental Biology
• Molecular Biology

Background:

• Wnt protein signaling is crucial for development and homeostasis, and its dysregulation is linked to cancer.
• Notum is a secreted feedback antagonist that regulates Wnt signaling, previously thought to act as a phospholipase.
• The phospholipase model failed to explain the specificity of Notum's action on glypicans and Wnt proteins.

Purpose of the Study:

• To investigate the precise mechanism by which Notum antagonizes Wnt signaling.
• To determine if Notum cleaves the glycophosphatidylinositol anchor of glypicans.
• To elucidate the enzymatic activity and substrate specificity of Notum.

Main Methods:

• Genetic analysis in Drosophila to study Notum-glypican interactions.
• Structural analyses of human and Drosophila Notum.
• Kinetic and mass spectrometric analyses of human Notum activity.

Main Results:

• Notum requires glypicans to suppress Wnt signaling but does not cleave their anchor.
• Structural studies revealed glycosaminoglycan binding sites and a hydrophobic pocket accommodating palmitoleate.
• Notum functions as a carboxylesterase, removing palmitoleate from Wnt proteins.

Conclusions:

• Notum is the first identified extracellular protein deacylase, specifically targeting Wnt proteins.
• This deacylation mechanism explains Notum's specificity in regulating Wnt signaling.
• The findings revise the understanding of Notum's role in Wnt pathway regulation and tissue homeostasis.