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Fig. 5. Model of Bmp signaling and the effect of Sax modulation. (A)
A model reconciling both the antagonistic and the signaling functions of Sax
in wing disc cells. Dpp and Gbb have different binding preferences (indicated
by the thickness of the arrows) for receptor complexes with different
combinations of type I receptors. Tkv-Sax receptor complexes contribute more
significantly to signaling (indicated by the thickness of the black arrow)
than Tkv-Tkv, whereas Sax-Sax complexes fail to phosphorylate Mad. (B)
tkv-null cells lack p-Mad. (C) sax-null clones with
receptor complexes solely of the Tkv-Tkv type are not as efficient at
signaling as wild-type cells. (D) Wild-type cells adjacent to
sax-null cells exhibit a higher level of pMad than normal. The
antagonistic function of Sax is preferentially directed at Gbb and the high
affinity of Gbb for Sax ensures that in the absence of Sax, a region of the
Gbb pool is available for signaling not only in the mutant cells via Tkv-Tkv
but also in wild-type neighboring cells. (E) Gene expression domains
for brk (green) and omb (orange) in the wing pouch of
wild-type (top) and sax-null wing disc (bottom). Gbb derived from
cells along the AP boundary (black vertical line) is required to repress
brk expression in the lateral wing pouch of both compartments
(Bangi and Wharton, 2006). Sax
limits the range of Gbb by acting antagonistically on the ability of Gbb to
signal; thus, in the absence of Sax, the range of Gbb signaling expands,
repressing brk expression throughout the wing pouch.
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