Reconstitution of Torso signaling in cultured cells suggests a role for both Trunk and Torsolike in receptor activation

Abstract

Formation of the Drosophila embryonic termini is controlled by the localized activation of the receptor tyrosine kinase, Torso. Both Torso and Torso's presumed ligand, Trunk, are expressed uniformly in the early embryo. Polar activation of Torso requires Torsolike, which is expressed by follicle cells adjacent to the ends of the developing oocyte. We find that Torso expressed at high levels in cultured Drosophila cells is activated by individual application of Trunk, Torsolike or another known Torso ligand, Prothoracicotropic Hormone. In addition to assays of downstream signaling activity, Torso dimerization was detected using bimolecular fluorescence complementation. Trunk and Torsolike were active when co-transfected with Torso and when presented to Torso-expressing cells in conditioned medium. Trunk and Torsolike were also taken up from conditioned medium specifically by cells expressing Torso. At low levels of Torso, similar to those present in the embryo, Trunk and Torsolike alone were ineffective but acted synergistically to stimulate Torso signaling. Our results suggest that Torso interacts with both Trunk and Torsolike, which cooperate to mediate dimerization and activation of Torso at the ends of the Drosophila embryo.