Department of Molecular, Cellular & Developmental Biology, University of Colorado Boulder, Boulder, CO 80309, USA; Howard Hughes Medical Institute, Boulder, CO 80309, USA.
Department of Molecular, Cellular & Developmental Biology, University of Colorado Boulder, Boulder, CO 80309, USA; Howard Hughes Medical Institute, Boulder, CO 80309, USA.
Dev Cell. 2021 Jan 11;56(1):52-66.e7. doi: 10.1016/j.devcel.2020.12.014.
ER tubules form and maintain membrane contact sites (MCSs) with endosomes. How and why these ER-endosome MCSs persist as endosomes traffic and mature is poorly understood. Here we find that a member of the reticulon protein family, Reticulon-3L (Rtn3L), enriches at ER-endosome MCSs as endosomes mature. We show that this localization is due to the long divergent N-terminal cytoplasmic domain of Rtn3L. We found that Rtn3L is recruited to ER-endosome MCSs by endosomal protein Rab9a, which marks a transition stage between early and late endosomes. Rab9a utilizes an FSV region to recruit Rtn3L via its six LC3-interacting region motifs. Consistent with our localization results, depletion or deletion of RTN3 from cells results in endosome maturation and cargo sorting defects, similar to RAB9A depletion. Together our data identify a tubular ER protein that promotes endosome maturation at ER MCSs.
内质网小管与内体形成并维持膜接触位点（MCS）。这些内质网-内体 MCS 如何以及为何在内体运输和成熟时持续存在，目前了解甚少。在这里，我们发现网质蛋白家族的一个成员，Reticulon-3L（Rtn3L），在内体成熟时在内质网-内体 MCS 中富集。我们表明，这种定位是由于 Rtn3L 的长而发散的细胞质 N 端结构域。我们发现 Rtn3L 被内体蛋白 Rab9a 募集到内质网-内体 MCS，Rab9a 标志着早期和晚期内体之间的过渡阶段。Rab9a 通过其六个 LC3 相互作用区域基序利用 FSV 区域来募集 Rtn3L。与我们的定位结果一致，细胞中 RTN3 的耗竭或缺失导致内体成熟和货物分拣缺陷，类似于 RAB9A 的耗竭。总之，我们的数据确定了一种管状内质网蛋白，它在内质网 MCS 处促进内体成熟。
