THE DROSOPHILA-14-3-3 PROTEIN LEONARDO ENHANCES TORSO SIGNALING THROUGH D-RAF IN A RAS1-DEPENDENT MANNER

14-3-3 proteins have been shown to interact with Raf-1 and cause its a ctivation when overexpressed, However, their precise role in Raf-1 act ivation is still enigmatic, as they are ubiquitously present in cells and found to associate with Raf-1 in vivo regardless of its activation state, We have analyzed the function of the Drosophila 14-3-3 gene le onardo (leo) in the Torso (Tor) receptor tyrosine kinase (RTK) pathway , In the syncytial blastoderm embryo, activation of Tor triggers the R as/Raf/MEK pathway that controls the transcription of tailless (tll). We find that, in the absence of Tor, overexpression of leo is sufficie nt to activate tll expression, The effect of leo requires D-Raf and Ra s1 activities but not KSR or DOS, two recently identified essential co mponents of Drosophila RTK signaling pathways, Tor signaling is impair ed in embryos derived from females lacking maternal expression of leo. We propose that binding to 14-3-3 by Raf is necessary but not suffici ent for the activation of Raf and that overexpressed Drosophila 14-3-3 requires Ras1 to activate D-Raf.