Characterization of maternal and zygotic D-raf proteins: dominant negative effects on Torso signal transduction.

The maternal D-raf serine/threonine kinase acts downstream of Torso (Tor) for specification of cell fates at the embryonic termini. D-raf activity is also required in other signal transduction pathways and consistent with its pleiotropic role, we find accumulation of a 90-kD D-raf protein throughout embryonic development. We also characterize the accumulation of maternal D-raf proteins in 0-2-hr embryos derived from females with germ cells lacking D-raf activity. Accumulation of a 90-kD or truncated mutant D-raf protein is observed for some of these embryos, while others lack the maternal D-raf protein. Then to determine whether rescue of the Tor pathway is influenced by pools of nonfunctional maternal D raf. wild-type D-raf mRNA was injected into embryos that inherit maternal stores of inactive 90-kD of truncated D-raf protein. For embryos lacking the maternal D-raf protein, a high level of terminal rescue is obtained. In contrast, rescue is reduced or not observed for embryos that accumulate mutant maternal D-raf proteins. These findings suggest that mutant forms of D-raf may deplete the embryo of a positive activator and/or form inactive protein complexes that affect rescue of the Tor pathway.