The sensory neurons of the Drosophila melanogaster photoreceptor form apical photosensory membranes, called rhabdomeres, by means of polarised epithelial cell differentiation, amplification and specialisation. This process requires the successful trafficking of vesicles containing rhodopsins to rhabdomeres. Now, Akiko Satoh and co-workers (Otsuka et al., 2019) investigate how two guanine nucleotide exchange factors (GEFs), parcas (Pcs) and TRAPPII, regulate the function of the GTPase Rab11, which is required for the transport of rhodopsin Rh1 in Drosophila photoreceptors. They show that the absence of Pcs, similar to loss of Rab11, causes cytoplasmic accumulation of vesicles containing Rh1 in pupal retinas. Furthermore, they detect colocalisation of Pacs with Rab11 at recycling endosomes but also notice a shift in Pcs localisation towards the trans-Golgi network, suggesting that Pcs acts upstream of Rab11. Indeed, in the absence of Pcs, Rab11 cannot be detected at the Golgi-associated recycling endosome or at post-Golgi vesicles, indicating that Pcs functions as a Rab11-GEF. In contrast to Pcs mutants, TRAPPII loss-of-function mutants do not show any defects in the localisation of Rh1, although double mutants of pcs and a TRAPPII subunit show synthetic defects in eye development. Collectively, these results identify Pcs as the key Rab11-GEF responsible for transport of rhodopsin Rh1 to rhabdomeres in Drosophila photoreceptors.
