Component of the extracellular signaling pathway that establishes the dorsal-ventral pathway of the embryo. Three proteases; ndl, gd and snk process easter to create active easter. Active easter defines cell identities along the dorsal-ventral continuum by activating the spz ligand for the Tl receptor in the ventral region of the embryo. Nudel, pipe and windbeutel together trigger the protease cascade within the extraembryonic perivitelline compartment which induces dorsoventral polarity of the Drosophila embryo