Rab11 is essential for polarized post-Golgi vesicle trafficking to photosensitive membrane rhabdomeres in Drosophila photoreceptors. Here, we found that Parcas (Pcs), recently shown to have guanine nucleotide exchange (GEF) activity toward Rab11, co-localizes with Rab11 on the trans-side of Golgi units and post-Golgi vesicles at the base of the rhabdomeres in pupal photoreceptors. Pcs fused with the electron micrography tag APEX2 localizes on 150-300 nm vesicles at the trans-side of Golgi units, which are presumably fly recycling endosomes. Loss of Pcs impairs Rab11 localization on the trans-side of Golgi units and induces the cytoplasmic accumulation of post-Golgi vesicles bearing rhabdomere proteins, as observed in Rab11 deficiency. In contrast, loss of Rab11-specific subunits of the TRAPPII complex, another known Rab11-GEF, does not cause any defects in eye development nor the transport of rhabdomere proteins; however, simultaneous loss of TRAPPII and Pcs results in severe defects in eye development. These results indicate that both TRAPPII and Pcs are required for eye development, but Pcs functions as the predominant Rab11-GEF for post-Golgi transport to photosensitive membrane rhabdomeres.

Drosophila photoreceptors develop from polarized epithelial cells that have apical and basolateral membranes. During morphogenesis, the apical membranes subdivide into a united bundle of photosensory microvilli (rhabdomeres) and a surrounding supporting membrane (stalk). By EMS-induced mutagenesis screening, we found that the F-Bin/Amphiphysin/Rvs (F-BAR) protein syndapin is essential for apical membrane segregation. The analysis of the super-resolution microscopy, STORM and the electron microscopy suggest that syndapin localizes to the neck of the microvilli at the base of the rhabdomere. Syndapin and moesin are required to constrict the neck of the microvilli to organize the membrane architecture at the base of the rhabdomere, to exclude the stalk membrane. Simultaneous loss of syndapin along with the microvilli adhesion molecule chaoptin significantly enhanced the disruption of stalk-rhabdomere segregation. However, loss of the factors involving endocytosis do not interfere. These results indicated syndapin is most likely functioning through its membrane curvature properties, and not through endocytic processes for stalk-rhabdomere segregation. Elucidation of the mechanism of this unconventional domain formation will provide novel insights into the field of cell biology.

Rab11 and its effectors dRip11 and MyoV are essential for polarized post-Golgi vesicle trafficking to photosensitive membrane rhabdomeres in Drosophila photoreceptors. Here, we found that Parcas (Pcs), recently shown to have guanine-nucleotide-exchange (GEF) activity toward Rab11, co-localizes with Rab11 on the trans-side of Golgi units and post-Golgi vesicles at the base of the rhabdomeres in pupal photoreceptors. Pcs fused with the EM-tag APEX2 localizes on 150-300 nm vesicles at the trans-side of Golgi units, which are presumably fly recycling endosomes (RE). Loss of Pcs impairs Rab11 localization on the trans-side of Golgi units and induces the cytoplasmic accumulation of post-Golgi vesicles bearing rhabdomere proteins, as observed in Rab11-deficiency. In contrast, loss of the specific subunits of TRAPPII, another known Rab11-GEF, does not cause any defects on the eye development nor the transport of rhabdomere proteins, however, simultaneous loss of TRAPPII and Pcs shows severe defects on eye development. These results indicated that in pupal photoreceptors, Pcs is the predominant Rab11-GEF, and TRAPPII performs a function that is redundant but subsidiary to that of Pcs.