FUNCTIONAL INTERACTIONS BETWEEN THE PELLE KINASE, TOLL RECEPTOR, AND TUBE SUGGEST A MECHANISM FOR ACTIVATION OF DORSAL

A complex signal transduction pathway functions in the early Drosophil a embryo to establish dorsal-ventral polarity. Activation of this path way results in the nuclear transport of the protein dorsal (dl), a mem ber of the rel/NF-kappa B family of transcription factors. Genetic stu dies have identified three intracellular components whose activity is required for activation of dl: Toll, a transmembrane receptor; pelle ( pll), a serine/threonine protein kinase; and tube, a protein of unknow n function. Here we examine the activities of these proteins when coex pressed in Drosophila Schneider cells. Coexpression of pll with dl enh anced dl nuclear localization and resulted in a modest increase in tra nscriptional activity. However, when pll was coexpressed with a specif ic mutant derivative of Toll (TlNaeI), although not with wild-type Tol l, a striking synergistic activation of dl was detected. Unexpectedly, coexpression of ph plus TlNaeI, in the absence of dl, resulted in a s imilar synergistic activation of a GAL4-tube fusion protein. Based on these and other results, we propose a model in which pll receives a si gnal from activated Toll and phosphorylates tube, which then participa tes directly in dl activation.