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Fig. 8. Specific developmental functions are carried out by different partnerships
between interacting LIM-HOM and HOM proteins. (A) Ap function in the wing is
carried out by a complex of Ap and Chip. This unit dimerises to form a
tetrameric complex comprising two molecules of Ap bridged by a Chip dimer. The
relative stoichiometry of the two proteins is important for the formation of
these complexes. Dlmo regulates Ap function by sequestering Chip into
non-functional complexes. (B) Ap-Chip complexes are also necessary for the
proper development of Ap motoneurones. However, balanced amounts of Chip and
Ap are not required for tetrameric complex formation indicating that the
limiting factor is Ap. In addition, there is no regulation by Dlmo. (C) In the
fourth tarsal segment, Ap function might be achieved by a multiprotein
complex, comprising Ap, Bar and Chip proteins. Our experiments indicate that
the limiting factor in the formation of functional complexes is Bar, whereas
Ap and Chip are more abundant. Bar interacts with Chip but not through the OID
domain. This Ap-Chip-Bar functional unit could dimerise to produce a hexamer,
or could consist of a molecule of each Ap and Bar bridged by a dimer of Chip.
(D) High levels of Bar expression are required for the development of the
fifth tarsal segment. As loss of Chip also affects the fifth tarsal segment,
it is possible that a heterodimer of Bar and Chip is the functional unit in
tarsus five. This unit could dimerise to produce a tetramer. (E) Synergism
between Al and Lim1 is required for pretarsal development. Lim1 and Chip
interact through their LIM and LID domains, respectively, and Al is also able
to interact with Chip. In addition, genetic experiments show that Chip, Al and
Lim1 are required in balanced amounts, suggesting that the functional unit in
the pretarsus involves these three proteins simultaneously.
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