Fig. 1.

The canonical Wnt and TGFβ pathways in D. melanogaster. a Wnt pathway. In the absence of a Wnt signal, the destruction complex composed of Axin, APC, and Zw3 phosphorylates Arm (pArm) to tag it for degradation via the ubiquitin–proteasome pathway. In order to activate the pathway, a secreted Wnt ligand binds to unrelated Fz and Arr receptors. Wnt binding leads to the phosphorylation of the Dsh signal transducer. Dsh then inhibits the antagonistic activity of the destruction complex (a double negative mechanism of action), and Arm accumulates in the nucleus. There Arm joins transcription–activation complexes composed of one or more of the transcription factors Pygo, Lgs, and TCF. Arrows indicate positive, and T-bars negative effects on information transfer. b TGFβ pathway: The Dpp ligand binds to Punt a Type II transmembrane receptor serine–threonine kinase, which then recruits the related Tkv Type I receptor and phosphorylates it. Tkv then phosphorylates the R-Smad Mad (pMad). pMad then translocates to the nucleus as a heteromeric complex with the Co-Smad Medea. This multi-Smad complex then regulates the expression of target genes in cooperation with tissue-specific activators and repressors (a linear positive mechanism of action)