Activation of Toll receptor results from binding of:
Activation of Toll depends on extracellular cleavage of Spatzle precursor in perivitelline space. Ventrally restricted Pipe sulfotransferase modifies vitelline membrane enabling protease cascade comprising Gastrulation defective, Snake, Easter that processes Spatzle into active fragment. Active Spatzle binds Toll leucine-rich repeats inducing dimerization and recruitment of MyD88, Tube, Pelle. Gurken acts earlier in follicle cells, Dorsal acts downstream transcriptionally, Pipe modifies eggshell indirectly. Spatzle represents only physiological ligand whose ventral processing converts eggshell polarity into embryonic Toll activity and Dorsal gradient essential for mesoderm induction.
Ref: Nature, Spatzle cleavage by Easter protease as Toll ligand - ventral activation of Toll receptor in Drosophila.