THE SPATZLE GENE ENCODES A COMPONENT OF THE EXTRACELLULAR SIGNALING PATHWAY ESTABLISHING THE DORSAL-VENTRAL PATTERN OF THE DROSOPHILA EMBRYO

spatzle is a maternal effect gene required in the signal transduction pathway that establishes the dorsal-ventral pattern of the Drosophila embryo. spatzle acts immediately upstream of the membrane protein Toll in the genetic pathway, suggesting that spatzle could encode the ventrally localized ligand that activates the receptor activity of Toll. The spatzle gene encodes a novel secreted protein that appears to require activation by a proteolytic processing reaction, which is controlled by the genes that act upstream of spatzle in the genetic pathway. We propose that proteolytic processing of the spatzle protein is confined to the ventral side of the embryo and that the localization of processed spatzle determines where the receptor, Toll, is active.