Selective membrane transport pathways are essential for cells in situ to construct and maintain a polarized structure comprising multiple plasma membrane domains, which is essential for their specific cellular functions. Genetic screening in Drosophila photoreceptors harboring multiple plasma membrane domains enables the identification of genes involved in polarized transport pathways. Our genome-wide high-throughput screening identified a Rab6-null mutant with a rare phenotype characterized by a loss of 2 apical transport pathways with an intact basolateral transport. Although the functions of Rab6 in the Golgi apparatus are well known, its function in polarized transport is unexpected. The mutant phenotype and localization of Rab6 strongly indicate that Rab6 regulates transport between the trans-Golgi network (TGN) and recycling endosomes (REs): basolateral cargos are segregated at the TGN before Rab6 functions, but cargos going to multiple apical domains are sorted at REs. Both the medial-Golgi resident protein Metallophosphoesterase (MPPE) and the TGN marker GalT::CFP exhibit diffused co-localized distributions in Rab6-deficient cells, suggesting they are trapped in the retrograde transport vesicles returning to trans-Golgi cisternae. Hence, we propose that Rab6 regulates the fusion of retrograde transport vesicles containing medial, trans-Golgi resident proteins to the Golgi cisternae, which causes Golgi maturation to REs.
Keywords: Photoreceptor; Rab6; recycling endosome; retrograde transport; trans-Golgi network.