Data from: Rab6 is required for multiple apical transport pathways but not the basolateral transport pathway in Drosophila photoreceptors

Polarized membrane trafficking is essential for the construction and
maintenance of multiple plasma membrane domains of cells. Highly polarized
Drosophila photoreceptors are an excellent model for studying polarized
transport. A single cross-section of Drosophila retina contains many
photoreceptors with 3 clearly differentiated plasma membrane domains: a
rhabdomere, stalk, and basolateral membrane. Genome-wide high-throughput
ethyl methanesulfonate screening followed by precise immunohistochemical
analysis identified a mutant with a rare phenotype characterized by a loss
of 2 apical transport pathways with normal basolateral transport. Rapid
gene identification using whole-genome resequencing and single nucleotide
polymorphism mapping identified a nonsense mutation of Rab6 responsible
for the apical-specific transport deficiency. Detailed analysis of the
trafficking of a major rhabdomere protein Rh1 using blue light-induced
chromophore supply identified Rab6 as essential for Rh1 to exit the Golgi
units. Rab6 is mostly distributed from the trans-Golgi network to a
Golgi-associated Rab11-positive compartment that likely recycles endosomes
or transport vesicles going to recycling endosomes. Furthermore, the Rab6
effector, Rich, is required for Rab6 recruitment in the trans-Golgi
network. Moreover, a Rich null mutation phenocopies the Rab6 null mutant,
indicating that Rich functions as a guanine nucleotide exchange factor for
Rab6. The results collectively indicate that Rab6 and Rich are essential
for the trans-Golgi network–recycling endosome transport of cargoes
destined for 2 apical domains. However, basolateral cargos are sorted and
exported from the trans-Golgi network in a Rab6-independent manner.

极化膜运输（polarized membrane trafficking）对于细胞多种质膜结构域的构建与维持至关重要。高度极化的果蝇感光细胞（Drosophila photoreceptors）是研究极化运输的绝佳模型。果蝇视网膜的单个横截面中包含大量感光细胞，其具有3个清晰分化的质膜结构域：微绒毛区（rhabdomere）、柄区（stalk）以及基底侧膜（basolateral membrane）。通过全基因组高通量乙基甲磺酸（ethyl methanesulfonate）诱变筛选，并结合精准免疫组织化学分析，研究人员鉴定出一株具有罕见表型的突变体：该突变体的两条顶端运输通路缺失，但基底侧运输功能正常。借助全基因组重测序与单核苷酸多态性定位技术快速进行基因鉴定，研究人员发现Rab6的无义突变是导致顶端特异性运输缺陷的原因。通过蓝光诱导生色团供应技术，对主要微绒毛区蛋白Rh1的运输过程进行详细分析后证实，Rab6是Rh1脱离高尔基体所必需的蛋白。Rab6主要分布于反式高尔基网络（trans-Golgi network）至与高尔基体相关的Rab11阳性区室之间，该区室大概率为循环内体或是靶向循环内体的运输囊泡。此外，Rab6效应蛋白Rich是反式高尔基网络中招募Rab6所必需的因子。不仅如此，Rich无效突变的表型与Rab6无效突变体完全一致，这表明Rich可作为Rab6的鸟苷酸交换因子（guanine nucleotide exchange factor）发挥功能。综合以上研究结果可知，Rab6与Rich对于靶向两个顶端结构域的货物蛋白的反式高尔基网络-循环内体运输通路至关重要。但基底侧货物蛋白的分选与从反式高尔基网络的输出过程则不依赖于Rab6。