Fig. 3. Amino acid determinants present in the N-terminal 125 residues of Cactus are required for polarized degradation. The distribution of full-length Cactus-lacZ (A), or deleted derivatives lacking the first 51(B), 76(C), 101(D) or 125(E) amino acids, expressed under the control of the alpha-tubulin 67C promoter in cact^D13/+ females, visualized by ß-galactosidase activity. Deletion of the first 125 amino acid residues resulted in loss of regulated degradation (E) and the production of dominantly dorsalized embryos, as assessed in the cuticular pattern (F). While Cactus-lacZ deletion derivatives that lacked as many as 101 amino acids gave rise to a normal Dorsal protein nuclear gradient (G), the deletion of 125 amino acids resulted in a Cactus-lacZ protein that retains Dorsal in the cytoplasm (H).