The sorting nexin Snx21 promotes flotillin-mediated endocytosis.

Upregulation of flotillins, the defining proteins of an incompletely characterized clathrin-independent form of endocytosis, is reported in a variety of human pathologies such as cancer, neurodegeneration, and infection. Several sorting nexin (Snx) proteins have established roles in the endosomal network, but the function of many members of this phospholipid-binding protein family such as Snx21 is still poorly investigated. Here, we show that Snx21 binds to the phospholipid PI(3)P enriched in endosomal membranes, and it localizes to the endosomal system in various Drosophila tissues. We find that Snx21 also binds to and colocalizes with flotillins in Drosophila. We propose that overexpression of Snx21 facilitates endocytic trafficking toward late endosomes/lysosomes leading to a large increase in the size of late endosomes, which requires Flotillin2. In parallel, Snx21 is required for multiple alterations caused by Flotillin2 overexpression: both the intracellular accumulation of Wingless/Wnt morphogen and ectopic wing vein formation that are induced by Flotillin2 overexpression in the developing wing depend on Snx21. Taken together, we identify Snx21 as a previously undescribed physical and functional interacting partner of flotillins, and our data point to the role of Snx21 as a potential positive regulator of flotillin-mediated endocytic trafficking.