A PROCESSED FORM OF THE SPATZLE PROTEIN DEFINES DORSAL-VENTRAL POLARITY IN THE DROSOPHILA EMBRYO

Stein et al. (1991) identified a soluble, extracellular factor that in duces ventral structures at the site where it is injected in the extra cellular space of the early Drosophila embryo. This factor, called pol arizing activity, has the properties predicted for a ligand for the tr ansmembrane receptor encoded by the Toll gene. Using a bioassay to fol low activity, we purified a 24x10(3) M(r) protein that has polarizing activity. The purified protein is recognized by antibodies to the C-te rminal half of the Spatzle protein, indicating that this polarizing ac tivity is a product of the spatzle gene. The purified protein is small er than the primary translation product of spatzle, suggesting that pr oteolytic processing of Spatzle on the ventral side of the embryo is r equired to generate the localized, active form of the protein.