589 18.4 Forming complex pattern: establishing positional information tral midline) then binds to the TOLL transmembrane re-ceptor, encoded by the Toll gene, present uniformly in the oocyte plasma membrane (see Figure 18-17a). In a con-centration-dependent manner, the SPZ–TOLL complex triggers a signal-transduction pathway that ends up phos-phorylating the inactive DL and CACT cytoplasmic pro-teins of the DL–CACT complex (Figure 18-17b). Phos-phorylation of DL and CACT causes conformational changes that break apart the cytoplasmic complex. The free phosphorylated DL protein is then able to migrate into the nucleus, where it serves as a transcription factor that activates genes necessary for establishing the ventral fates. The crucial arrangement is the positioning of the SPZ ligand near the ventral midline, which ensures that only the DL on the ventral side of the embryo is able to activate the genes that create structures for the ventral part of the body. Dorsal
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