Regulation of translation and proteolysis during the development of embryonic dorso-ventral polarity in Drosophila. Homology of easter proteinase with Limulus proclotting enzyme and translational activation of Toll receptor synthesis.

The generation of dorso-ventral polarity during Drosophila embryogenesis is regulated by the action of 12 maternally expressed gene products, the dorsal group. These products act together to form a dorso-ventral nuclear gradient of the transcription factor dorsal. At least three of the dorsal group genes (snake, easter and gastrulation defective) encode secreted serine proteinases which probably function during early development in the perivitelline compartment of the embryo. Here, we report that the easter proteinase is homologous in its light chain sequence to the haemocyte proclotting enzyme (PCE) of the Japanese horseshoe crab Tachypleus tridentatus. PCE is the terminal member of a proteolytic cascade activated in response to microbial polysaccharides and acts to cleave coagulogen, an invertebrate equivalent of fibrinogen. On the basis of this homology we are able to predict with confidence the overall primary structure of the easter proteinase, its mode of activation and its substrate specificity. The result also suggests that easter functions zygotically in haemocytes in a Drosophila defence response analogous to that found in Tachypleus. We also show here that the Toll receptor protein is absent in early cleavage embryos but accumulates rapidly at the syncitial blastoderm stage, the developmental stage at which its function is required. This finding suggests that translation of Toll mRNA is regulated in response to fertilisation and egg deposition. These two observations are consistent with a model of dorso-ventral pattern formation in which a proteolytic cascade is activated uniformly in the perivitelline compartment of the embryo and causes the release of ventrally localised ligands of the Toll receptor. A possible alternative model in which a proteolytic cascade is activated in response to a ventrally restricted signal is also discussed.