Regulation of dorsal in cultured cells by Toll and tube: tube function involves a novel mechanism.

We described previously a transient cotransfection assay that allows us to study regulation of the Drosophila Dorsal protein (dl) in cultured cells. For example, we showed that over-expression of the Toll transmembrane receptor was sufficient to cause relocalization of dl from the cytoplasm to the nucleus. Here we present data that the tube protein, shown previously by genetic studies to act downstream of Toll, can function in a novel way to enhance dl activity. In the absence of dl, or when dl is cytoplasmic, tube is also found in the cytoplasm of transfected cells. But when dl is localized to the nucleus, so is tube. tube can then function to enhance reporter gene expression, either by cooperation with dl or as a GAL4-tube fusion protein. tube thus appears capable of acting both as a chaperon or escort for dl as it moves to the nucleus, and then as a transcriptional coactivator. We also show that the intracytoplasmic domain of Toll, and specifically the region sharing homology with the interleukin-1 receptor, is sufficient to induce dl-tube nuclear translocation.