Drosophila Knickkopf and Retroactive are needed for epithelial tube growth and cuticle differentiation through their specific requirement for chitin filament organization.
2006
Precise epithelial tube diameters rely on coordinated cell shape changes
and apical membrane enlargement during tube growth. Uniform tube expansion in
the developing Drosophila trachea requires the assembly of a
transient intraluminal chitin matrix, where chitin forms a broad cable that
expands in accordance with lumen diameter growth. Like the chitinous
procuticle, the tracheal luminal chitin cable displays a filamentous structure
that presumably is important for matrix function. Here, we show that
knickkopf ( knk ) and retroactive ( rtv ) are
two new tube expansion mutants that fail to form filamentous chitin
structures, both in the tracheal and cuticular chitin matrices. Mutations in
knk and rtv are known to disrupt the embryonic cuticle, and
our combined genetic analysis and chemical chitin inhibition experiments
support the argument that Knk and Rtv specifically assist in chitin function.
We show that Knk is an apical GPI-linked protein that acts at the plasma
membrane. Subcellular mislocalization of Knk in previously identified tube
expansion mutants that disrupt septate junction (SJ) proteins, further suggest
that SJs promote chitinous matrix organization and uniform tube expansion by
supporting polarized epithelial protein localization. We propose a model in
which Knk and the predicted chitin-binding protein Rtv form membrane complexes
essential for epithelial tubulogenesis and cuticle formation through their
specific role in directing chitin filament assembly.
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