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Fig. 7. A speculative model of the Ds system. The A compartment, anterior is
towards the left. Ft is indicated in blue and Ds in red. The long arrows
indicate the polarity of each cell: normal in black and reversed in red. In
the wild type (top), there is evidence for a gradient of Ds (Ds, light red)
increasing from anterior to posterior, and of opposing gradients of Fj and Ft
activity (Casal et al., 2002),
as indicated by the size of the letters. Although there is no gradient of Ft
protein (Ft, light blue), we envisage a gradient of Ft activity (Ft, dark
blue), driven by the action of Fj on Ft. Active Ft could become stabilised in
the membrane of one cell so that it can form trans-heterodimers with Ds in the
next cell (provided that sufficient Ds is present there). Only those molecules
of Ft and Ds that form trans-heterodimers are shown; free Ft and Ds, as well
as other possible forms of Ds and Ft (e.g. cis-complexes) are not shown, even
though they may be in excess (the Ds protein gradient peaks posteriorly, but
the gradient of Ds molecules engaged in trans-heterodimers peaks anteriorly).
The polarity of a cell might depend on a comparison between the number of Ds
molecules (red numbers above the cells) that are engaged in trans-heterodimers
on the anterior and posterior faces of the cell, with the polarity of that
cell pointing down the differential (from high to low, as shown). The
probability of forming trans-heterodimers might depend on the availability of
active free Ft, as well as on free Ds on abutting cell surfaces, which in turn
could depend on graded Fj activity (driving the production of active Ft), on
graded Ds protein accumulation, and even the possibility that Ds and Ft might
form cis-heterodimers on the same cell surface. The middle row shows the
effect of a ft- cell, in which all Ds will be available to
make trans-heterodimers with Ft on the facing (anterior) membrane of
the wild-type cell on its right. Consequently, in this wild-type cell, Ft
engagement in trans-heterodimers will be promoted along the anterior face.
Conversely, the absence of Ft protein in the ft- cell will
deprive Ds on the surface of the abutting wild-type cell of binding partners,
and allow abnormally high levels of Ds to be recruited into trans-heterodimers
on the opposite (posterior) face. This excess of Ds molecules will then bind
to Ft in the next most (more posterior) cell, and again, by depleting Ds from
its anterior face, will repolarise it. This effect will weaken from cell to
cell. The lower row shows a UAS.ft cell that will attract more Ds to
the facing membrane (posterior) of the neighbour on its left, thereby
polarising that cell, the effect spreading anteriorwards.
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