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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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90% of new protein
synthesis. (F) Control for GFP photobleaching. Fertilized eggs were injected
with mRNA encoding GFP and allowed to develop to the 8-cell stage. Embryos
were treated with 100 µM emetine for 30 minutes and then imaged with 4-D
confocal microscopy under conditions identical to those shown in
Fig. 1. These representative
data from a single embryo show that GFP fluorescence (mean fluorescent pixel
intensity measured over the entire embryo) remained constant over the period
of the experiment. 0 on the x-axis corresponds to 30 minutes after
addition of emetine. (G) Decay in GFP fluorescence is dependent on
ß-catenin phosphorylation. Fertilized eggs were injected with
Xl-pt-ß-catenin-GFP mRNA. At the 8-cell stage, embryos were treated with
100 µM emetine for 30 minutes and then imaged with 4-D confocal microscopy
under conditions identical to those shown in
Fig. 1. These representative
data from a single embryo show that GFP fluorescence (mean fluorescent pixel
intensity measured over the entire embryo) remained constant over the period
of the experiment. (H) Summary of Xl-wt-ß-catenin-GFP half-life
measurements. Half-life was measured in different cell lineages of cleavage
stage embryos and data were pooled from 8-, 16- and 64-cell stage embryos. On
average, within any individual embryo, the half-life of ß-catenin-GFP in
the vegetal-most blastomeres (the micromere territory) was more than 8-fold
greater than in the animal blastomeres (the mesomere territory).
