Wingless secretion requires endosome-to-Golgi retrieval of Wntless/Evi/Sprinter by the retromer complex

The glycolipoproteins of the Wnt family raise interesting trafficking issues, especially with respect to spreading within tissues. Recently, the retromer complex has been suggested to participate in packaging Wnts into long-range transport vehicles. Our analysis of a Drosophila mutant in Vps35 show that, instead, the retromer complex is required for efficient progression of Wingless (a Drosophila Wnt) through the secretory pathway. Indeed expression of senseless, a short-range target gene, is lost in Vps35-deficient imaginal discs. In contrast, Vps35 is not required for Hedgehog secretion, suggesting specificity. Overexpression of Wntless, a transmembrane protein known to be specifically required for Wingless secretion overcomes the secretion block of Vps35-mutant cells. Furthermore, biochemical evidence confirms that Wntless engages with the retromer complex. We propose that Wntless accompanies Wingless to the plasma membrane where the two proteins dissociate. Following dissociation from Wingless, Wntless is internalized and returns to the Golgi apparatus in a retromer-dependent manner. Without the retromer-dependent recycling route, Wingless secretion is impaired and, as electron microscopy suggests, Wntless is diverted to a degradative compartment.