Ykt6 membrane-to-cytosol cycling regulates exosomal Wnt secretion

Abstract

Protein trafficking in the secretory pathway, for example the secretion of Wnt proteins, requires tight regulation. These ligands activate Wnt signaling pathways and are crucially involved in development and disease. Wnt is transported to the plasma membrane by its cargo receptor Evi, where Wnt/Evi complexes are endocytosed and sorted onto exosomes for long-range secretion. However, the trafficking steps within the endosomal compartment are not fully understood. The promiscuous SNARE Ykt6 folds into an auto-inhibiting conformation in the cytosol, but a portion associates with membranes by its farnesylated and palmitoylated C-terminus. Here, we demonstrate that membrane detachment of Ykt6 is essential for exosomal Wnt secretion. We identified conserved phosphorylation sites within the SNARE domain of Ykt6, which block Ykt6 cycling from the membrane to the cytosol. In Drosophila, Ykt6 RNAi-mediated block of Wg secretion is rescued by wildtype but not phosphomimicking Ykt6. We show that phosphomimicking Ykt6 accumulates at membranes, while wildtype Ykt6 regulates Wnt trafficking between the plasma membrane and multivesicular bodies in a dose-dependent manner. Taken together, we show that exosomal sorting of Wnts is fine-tuned by a regulatory switch in Ykt6 shifting it from a membrane-bound to cytosolic state at the level of endosomal maturation.