Cactus and other IκB family members associate with Rel proteins via the ankyrin repeats ( 12, 16, 19, 20). In the nucleus, Dorsal regulates the asymmetric expression of a set of zygotic genes ( 17, 18). At the syncytial blastoderm stage, the Dorsal–Cactus complex is disrupted in response to the activated spatial signal transmitted through Toll, resulting in the graded nuclear import of Dorsal. Dorsal is initially retained in the cytoplasm by its inhibitor Cactus, a member of the IκB family ( 12, 16). Like other Rel proteins, Dorsal is regulated at the level of nuclear translocation ( 12– 15). The N-terminal domain 1 contains the motif RXXRXRXXC that is important for DNA binding, and the C-terminal domain 2 contains the dimerization and Cactus binding sequences ( 8– 11). Structure and function analyses have revealed two distinct subdomains of the RHR. All members of this family share a conserved 300-amino acid region, the Rel homology region (RHR) ( 1). We propose a model in which Tube, Pelle, Cactus, and Dorsal form a multimeric complex that represents an essential aspect of signal transduction.ĭorsal belongs to the Rel family of transcription factors ( 7). Genetic experiments indicate that Tube–Dorsal interaction is necessary for normal signal transduction. Domain 1 has been found to be necessary for Dorsal nuclear targeting. Tube and Pelle bind Dorsal in the N-terminal domain 1 of the Dorsal Rel homology region rather than at the Cactus binding site. Tube interacts with Dorsal via its C-terminal domain, whereas full-length Pelle is required for Dorsal binding. We confirmed these interactions in an in vitro binding assay. In a yeast two-hybrid assay, we found that both Tube and Pelle interact with Dorsal. ![]() ![]() Tube and Pelle are required to relay the signal from Toll to the Dorsal–Cactus complex. Upon activation of the transmembrane receptor Toll, Dorsal dissociates from its cytoplasmic inhibitor Cactus and enters the nucleus. The intracellular part of the Rel signal transduction pathway in Drosophila is encoded by Toll, tube, pelle, dorsal, and cactus, and it functions to form the dorsal–ventral axis in the Drosophila embryo.
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