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Fig. 8. Models of Dsh activation and targeting in the vegetal region of the unfertilized egg or early embryo. We propose that Dsh is activated specifically in the vegetal region (see Discussion). A putative, vegetally localized activator is shown in red, egg vesicles in yellow, and Dsh in blue. Activated Dsh is shown by a blue star-burst symbol. The activator has not been identified but might be a maternally localized kinase such as Par-1 (Sun et al., 2001). Alternatively, a repressor of Dsh activation might be localized (or activated) in the animal region of the embryo. In Model A, local activation of Dsh triggers targeting to cortical vesicles. This targeting requires a phospholipid-binding motif in the DIX domain of the protein. Although the yellow vesicles are shown localized in the vegetal region of the cell, they might be more widely distributed and Dsh may associate only with those in the vegetal region. In Model B, unactivated Dsh in the vegetal region of the embryo associates with vesicles through the DIX domain, and this association triggers activation of Dsh by the localized activator, which might be associated with vesicles or the vegetal cortex of the egg.

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