Drosophila TRAPPC8-Rab1 module regulates retrograde trafficking of Wingless and Evi/Wntless

Lipid-modified Wnt proteins are secreted from polarized epithelial cells through a multi-step pathway involving membrane delivery, internalization and resecretion. Internalization enables dissociation of Wnt from its cargo receptor Evi/Wntless in endosomes and promotes retromer-mediated recycling of Evi to the Golgi, however, the molecular basis of this retrograde Wnt trafficking remains poorly understood. Here, we identify an essential function of TRAPPC8, the TRAPPIII-specific subunit, in the trafficking of Drosophila Wingless (Wg), the Wnt1 homolog. Loss of TRAPPC8 impaired retrograde trafficking of both Wg and Evi, resulting in their intracellular accumulation in Wg-producing cells. Consistent with the role of TRAPPIII as a Rab1-specific GEF, inhibition of Rab1 phenocopied the trafficking defects caused by TRAPPC8 loss, whereas constitutively active Rab1 rescued these defects. Furthermore depletion of either TRAPPC8 or Rab1 increased the levels of Wg-unbound Evi, indicating that they act downstream of Evi-Wg dissociation. Together, these findings identify the TRAPPIII complex and its effector Rab1 as key regulators of retrograde Wg trafficking required for its efficient secretion.