Retrograde trafficking of apical extracellular matrix protein regulates epithelial tube geometry. Bo Dong¹, Ken Kakihara^1,2, Tetsuhisa Otani¹, Housei Wada¹, Shigeo Hayashi^1,2. 1) Riken CDB, Kobe, Japan; 2) Department of Biology, Kobe University Graduate School of Science.

   Apical extracellular matrix filling the lumen controls the morphology and geometry of epithelial tubes, yet the regulation of luminal protein composition and its role in tube morphogenesis are not well understood. Here, we show that an endosomal-retrieval machinery consisting of Rab9, retromer, and actin nucleator WASH regulates selective recycling of the luminal protein Serpentine in the Drosophila trachea. Secreted Serpentine is endocytosed and sorted into late endosome. Vps35, WASH, and actin filaments differentially localize at the Rab9-enriched subdomains of the endosomal membrane, where Serpentine containing vesicles bud off. In Rab9, Vps35 and WASH mutants, Serpentine was secreted normally into the tracheal lumen, but the luminal quantities were depleted at later stages, resulting in excessively elongated tubes. In contrast, secretion of many luminal-proteins was unaffected, suggesting that retrograde trafficking of a specific class of luminal proteins is a pivotal rate-limiting mechanism for continuous tube-length regulation.