Fig. 6. A model for redistribution of Dorsal between cytoplasm and nucleus as a dynamic equilibrium between the activated Toll receptors and the nucleus in Drosophila. Dorsal and Cactus are recruited and phosphorylated by active Toll membrane receptor complexes on the ventral plasma membrane. Phosphorylated Cactus is degraded and phosphorylated Dorsal translocates to the nucleus. Within the nucleus it becomes dephosphorylated and is exported, re-entering the cytoplasmic pool binding de novo synthesized Cactus. Dorsal is again recruited to active receptor complexes and the cycle repeats itself. Partial compartmentalization of the cytoplasm assures integration of the signal from only local active signaling complexes and buffering from signaling complexes associated with the plasma membrane in close proximity to adjacent nuclei.