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Fig. 6. Model for how polarization of Fat activity might influence Warts
signaling. A proposed model for how differences in Dachsous (Ds) or
Four-jointed (Fj) expression might affect both planar cell polarity (PCP) and
Warts signaling pathways (Rogulja et al.,
2008). (A) A cell that encounters higher levels of Ds on
the cell to its left and lower levels of Ds on the cell to its right. Ds
gradients are associated with the polarization of Dachs localization, which is
mediated by Fat (Mao et al.,
2006). The establishment of polarized protein localizations,
including that of Dachs, but presumably also of other proteins, may initiate
the cellular polarization associated with PCP. Dachs also inhibits Warts. In
the model, this occurs locally, such that when Dachs is polarized, Warts could
be degraded and rendered inactive (colorless oval) on one side of a cell
(right, in this case), but abundant and active (colored oval) on the other
side. Where Warts is present and active, it would phosphorylate and inhibit
Yorkie (Yki), but where Warts is missing or inactive, Yki would not be
phosphorylated and hence could enter the nucleus. (B) A cell that
encounters higher levels of Fj expressed in the cell to its left and lower
levels of Fj expressed in the cell to its right. The opposing influences of Fj
and Ds on PCP and Dachs localization suggest that this is functionally
equivalent to a situation in which Ds levels are higher in the cell to the
right and lower in the cell to the left. This polarizes the cell in the
opposite direction, such that Dachs now accumulates on the membrane on the
left side of the cell, rather than on the right side. Even though the cell is
polarized in the opposite direction, the transcriptional response associated
with failure to locally phosphorylate Yki could be the same for A and B.
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