Drosophila Knickkopf and Retroactive are needed for epithelial tube growth and cuticle differentiation through their specific requirement for chitin filament organization

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 Kink 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.