The acquisition of distinct branch sizes and shapes is a central aspect in tubular organ morphogenesis and function. In the Drosophila airway tree, the interplay of apical extracellular matrix (ECM) components with the underlying membrane and cytoskeleton controls tube elongation, but the link between ECM composition with apical membrane morphogenesis and tube size regulation is elusive. Here, we characterized Emp (epithelial membrane protein), a Drosophila CD36 homolog belonging to the scavenger receptor class B protein family. emp mutant embryos fail to internalize the luminal chitin deacetylases Serp and Verm at the final stages of airway maturation and die at hatching with liquid filled airways. Emp localizes in apical epithelial membranes and shows cargo selectivity for LDLr-domain containing proteins. emp mutants also display over elongated tracheal tubes with increased levels of the apical proteins Crb, DE-cad, and phosphorylated Src (p-Src). We show that Emp associates with and organizes the βH-Spectrin cytoskeleton and is itself confined by apical F-actin bundles. Overexpression or loss of its cargo protein Serp lead to abnormal apical accumulations of Emp and perturbations in p-Src levels. We propose that during morphogenesis, Emp senses and responds to luminal cargo levels by initiating apical membrane endocytosis along the longitudinal tube axis and thereby restricts airway elongation.

The respiratory system of Drosophila melanogaster, the trachea, is widely used as a model system to study the development of branched tubular organs. Airway maturation in Drosophila entails a series of sequential events: first, a massive apical secretion of luminal proteins initiates diametric tube expansion, second an endocytic wave removes luminal proteins and finally liquid clearance and gas filling converts the nascent epithelial tubes to a functional respiratory organ. Several studies have highlighted the importance of luminal protein clearance by endocytosis, however cargo specific receptors mediating protein uptake remain unknown. We characterized the Drosophila homologue of CD36 (scavenger receptor protein class B), Emp (epithelial membrane protein). Embryos lacking emp display over elongated tracheal tubes and fail to clear a subset of luminal chitin binding proteins before hatching. Emp shows a dynamic localization between endosomes and apical/sub-apical regions of epithelial cells during tube maturation. The apical punctate localization of Emp depends on intact F-actin filaments. In turn, Emp organizes the apical cytoskeleton through its binding to β_H-Spectrin. In emp mutants the apical membrane levels of Crb and DE-cad are perturbed and phosphorylation of Src42A (pSrc) at the adherens epithelial junctions are increased. We show that Emp is a downstream target of Yki and physically interacts with Diap1 (Death-associated inhibitor of apoptosis 1) thereby controlling the activation of DrICE (Death related ICE-like caspase). A Y2H screen and co-immunoprecipitation, revealed that Emp interacts with the GTPase activating protein, CG32506. We speculate that CG32506 acts as an antagonist of Emp in regulating Src42A phosphorylation and the downstream targets of the Yki pathway Diap1 and DrICE. Overall, we show that Emp is a selective scavenger receptor for endocytosis, regulating apical cytoskeletal integrity. We defined intracellular components of Emp-mediated endocytosis and cargo recycling route including effectors of apoptosis and components of the Hippo signaling pathway.