Since the Torso-Pelle fusion did not require a functional Tube to produce ventral forms, Pelle must act downstream of Tube. Their work with Torso fusion proteins indicated that membrane localization might be required to activate Tube and Pelle. But, since Torso is a membrane receptor that activates downstream targets, it could have been activating Tube and Pelle in a manner not requiring membrane localization. This fusion contains a myristilation signal that localizes it to the plasma membrane by a fatty acid side chain, but does not alter it in other ways. A. A wildtype embryo.
Arrows point to ventral denticles and filzkorper.
B. A gastrulation defective mutant, with no ventral forms. A gastrulation defective mutant injected with src-Tube has ectopic filzkorper. A gastrulation defective mutant injected with src-Pelle has ectopic ventral denticles and filzkorper. By inducing plasma membrane localization in a mutant background where there would otherwise be no Tube and Pelle activity, we can positively infer that localization leads to ventralization of the embryo. Together with their previous evidence that localization of Tube and Pelle to the membrane via Torso activity causes ventralization, it seems likely the membrane localization leads to Tube and Pelle activation and the creation of ventral morphology.