PP1 and developmental signalling. L.S. Alphey , D. Bennett , S. Gross , B. Szöõr. Dept Zoology, Univ Oxford, Oxford, England.

   Genetic analysis of pattern formation in Drosophila has identified a number of secreted signalling proteins which cells use to communicate positional information and to establish patterns within apparently undifferentiated groups of cells. These signalling pathways are repeatedly used and reused in different contexts throughout development. Remarkably similar homologous signalling molecules and pathways are found in mammals, e.g. wingless/Wnt, dpp/TGFb/BMP/activin and hedgehog/shh. The receptors for these molecules and their intracellular signal transduction pathways have been intensively studied in flies and mammals. In each pathway, at least part of the intracellular signal transduction is regulated by reversible phosphorylation. Many of the protein kinases are known, but in almost no case has the relevant protein phosphatase been identified. For example, in the case of the dpp pathway, the extracellular signal (dpp) binds to the type II receptor, which is a Ser/Thr protein kinase. Binding of the ligand leads to phosphorylation and activation of the type I receptor, which is also a Ser/Thr protein kinase. This then phosphorylates a pathway-specific Smad (Mad), causing it to dissociate from the receptor complex, bind to another Smad (Medea), translocate to the nucleus and activate transcription. This nuclear translocation can be blocked by phosphorylation of the Smad by MAP kinase. None of the relevant protein phosphatases have been identified. We have explored the role of PP1 in developmental signalling in Drosophila, using both genetic and molecular (two-hybrid) approaches. We have genetic evidence that PP1 is involved in the regulation of the wingless/Wnt signalling pathway, and both genetic and two-hybrid evidence that PP1 is involved in dpp/TGFbBMP/activin signalling. We are currently attempting to identify the role and possible substrate(s) of PP1 in these pathways.