REGULATION OF DORSAL ITS CULTURED-CELLS BY TOLL AND TUBE - TUBE FUNCTION INVOLVES A NOVEL MECHANISM

We described previously a transient cotransfection assay that allows u s to study regulation of the Drosophila Dorsal protein (dl) in culture d cells. For example, we showed that over-expression of the Toll trans membrane receptor was sufficient to cause relocalization of dl from th e cytoplasm to the nucleus. Here we present data that the tube protein , shown previously by genetic studies to act downstream of Toll, can f unction in a novel way to enhance dl activity. In the absence of dl, o r when dl is cytoplasmic, tube is also found in the cytoplasm of trans fected cells. But when dl is localized to the nucleus, so is tube. tub e can then function to enhance reporter gene expression, either by coo peration with dl or as a GAL4-tube fusion protein. tube thus appears c apable of acting both as a chaperon or escort for dl as it moves to th e nucleus, and then as a transcriptional coactivator. We also show tha t the intracytoplasmic domain of Toll, and specifically the region sha ring homology with the interleukin-l receptor, is sufficient to induce dl-tube nuclear translocation.