Protein inhibiting dorsal nuclear localization:
Cactus protein shares homology with vertebrate I-kappa B inhibitor binding Rel domain factors. Cactus binds Dorsal covering nuclear localization signal, retaining complex cytoplasmically in future dorsal cells. Ventral Toll signaling activates scaffold Tube and kinase Pelle phosphorylating Cactus prompting ubiquitin-mediated degradation. Free Dorsal then enters nuclei establishing gradient. Spatzle activates Toll upstream, Tube and Pelle promote Cactus degradation indirectly. Cactus therefore serves as direct molecular brake on Dorsal nuclear entry, levels inversely correlating with Dorsal concentrations and defining ventralizing activity during DV patterning and mesoderm induction.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Cactus I-kB homolog inhibiting Dorsal nuclear localization.