Characterization of Schnurri : an integral component of the Dpp-signaling pathway in "Drosophila melanogaster"

Signaling by the transforming growth factor-β (TGF-β) family of ligands plays a crucial role during development and tissue homeostasis in all multicellular organisms. The signal transduction pathway is highly conserved and relatively simple. Ligand mediated receptor activation induces heteromerization and nuclear translocation of the signal mediators, the Smad proteins. Once in the nucleus, the Smad complexes bind to and regulate directly a large number of target genes. Intriguingly, only a few Smad proteins mediate the majority of the complex cellular responses elicited by ligands of the TGF-β family by either transcriptional activation or repression, depending on their associated partners. A number of Smad interacting proteins and modulators have been identified, partly explaining the diversity of transcriptional outcome. Nevertheless, the biological consequences and their role in developmental aspects of TGF-β signaling are poorly understood. The best characterized member of the TGF-β family in Drosophila melanogaster is Decapentaplegic (Dpp). Dpp plays important roles during development and its cellular function has been extensively investigated in genetic experiments. One of the main Dpp-target genes during development encodes a repressor protein, Brinker. Brinker suppresses the transcription of Dpp-target genes and is itself negatively regulated by Dpp-signaling. This down-regulation of brinker by Dpp is essential for Dpp target genes to become activated. The molecular basis of the Dpp-mediated repression is elusive; however, genetic studies identified the gene schnurri (shn) to be required for this activity. In this thesis, the results of a detailed structure-function analysis of the nuclear zinc finger protein Schnurri are presented. It could be demonstrated that Schnurri protein, together with the Drosophila Smads Mad and Medea, forms a signal-dependant DNA-protein complex on short silencer elements within the brinker regulatory region. The recruitment of Schnurri to the silencer elements by Mad/Medea is required for the transcriptional regulatory activity of the complex in vivo. Two modules within the Schnurri protein were identified that are required and sufficient for Dpp-mediated brinker repression in vivo. The two entities, a protein-DNA complex formation domain and a domain containing repressor activity, can be separated from each while retaining their function. The Schnurri protein presents the first interaction partner for Mad and Medea in Drosophila. Furthermore, Schnurri is the first examples that link TGF-β signaling to tissue-unspecific repression rather than activation.