Fig. 2.

Terminal patterning system in the early Drosophila embryo. (A) Torso receptors (purple) are uniformly distributed along the plasma membrane of the embryo. Inactive ligand (Trunk) is distributed uniformly in the extracellular matrix; it is converted into an active and diffusible form (blue) by Torsolike (yellow), a protein localized at the poles of the embryo. The Torso-Trunk complex signals through the MAPK signalin cascade, which leads to MAPK phosphorylation and nuclear import. (B) Quantified pattern of MAPK phosphorylation. (Left) Fluorescent image of the anterior of the embryo; nuclei are stained in green, and phosphorylated MAPK is stained in red. (Right) Gradients of nuclear and cytoplasmic phosphorylated MAPK.