Interplay of Signal Mediators of Decapentaplegic (Dpp): Molecular Characterization of Mothers against dpp, Medea, and Daughters against dpp

Decapentaplegic (Dpp) plays an essential role in Drosophila development, and analyses of the Dpp signaling pathway have contributed greatly to understanding of the actions of the TGF-beta superfamily. Intracellular signaling of the TGF-beta superfamily is mediated by Smad proteins, which are now grouped into three classes. Two Smads have been identified in Drosophila. Mothers against dpp (Mad) is a pathway-specific Smad, whereas Daughters against dpp (Dad) is an inhibitory Smad genetically shown to antagonize Dpp signaling. Here we report the identification of a common mediator Smad in Drosophila, which is closely related to human Smad4. Mad forms a heteromeric complex with Drosophila Smad4 (Medea) upon phosphorylation by Thick veins (Tkv), a type I receptor for Dpp. Dad stably associates with Tkv and thereby inhibits Tkv-induced Mad phosphorylation. Dad also blocks hetero-oligomerization and nuclear translocation of Mad. We also show that Mad exists as a monomer in the absence of Tkv stimulation. Tkv induces homo-oligomerization of Mad, and Dad inhibits this step. Finally, we propose a model for Dpp signaling by Drosophila Smad proteins.