Department of Anatomy and Structural Biology, Albert Einstein College of Medicine, Bronx, New York;
Am J Physiol Gastrointest Liver Physiol. 2014 Mar 1;306(5):G412-24. doi: 10.1152/ajpgi.00118.2013. Epub 2014 Jan 9.
We previously reported that Rab1a is associated with asialoorosomucoid (ASOR)-containing early endocytic vesicles, where it is required for their microtubule-based motility. In Rab1a knockdown (KD) cell lines, ASOR failed to segregate from its receptor and, consequently, did not reach lysosomes for degradation, indicating a defect in early endosome sorting. Although Rab1 is required for Golgi/endoplasmic reticulum trafficking, this process was unaffected, likely due to retained expression of Rab1b in these cells. The present study shows that Rab1a has a more general role in endocytic vesicle processing that extends to EGF and transferrin (Tfn) trafficking. Compared with results in control Huh7 cells, EGF accumulated in aggregates within Rab1a KD cells, failing to reach lysosomal compartments. Tfn, a prototypical example of recycling cargo, accumulated in a Rab11-mediated slow-recycling compartment in Rab1a KD cells, in contrast to control cells, which sort Tfn into a fast-recycling Rab4 compartment. These data indicate that Rab1a is an important regulator of early endosome sorting for multiple cargo species. The effectors and accessory proteins recruited by Rab1a to early endocytic vesicles include the minus-end-directed kinesin motor KifC1, while others remain to be discovered.
我们之前曾报道 Rab1a 与含有唾液酸糖蛋白（ASOR）的早期内吞小泡相关联，在该小泡中 Rab1a 对于其基于微管的运动是必需的。在 Rab1a 敲低（KD）细胞系中，ASOR 不能与其受体分离，因此不能到达溶酶体进行降解，表明早期内体分选存在缺陷。虽然 Rab1 对于高尔基体/内质网运输是必需的，但这个过程没有受到影响，这可能是由于这些细胞中 Rab1b 的保留表达。本研究表明，Rab1a 在涉及 EGF 和转铁蛋白（Tfn）运输的内吞小泡处理中具有更普遍的作用。与对照 Huh7 细胞的结果相比，EGF 在 Rab1a KD 细胞中聚集在颗粒内，无法到达溶酶体区室。Tfn 是回收货物的典型范例，在 Rab1a KD 细胞中积聚在 Rab11 介导的慢回收 Rab11 隔间中，而对照细胞则将 Tfn 分拣到快速回收 Rab4 隔间中。这些数据表明 Rab1a 是多种货物的早期内体分选的重要调节剂。Rab1a 招募到早期内吞小泡的效应物和辅助蛋白包括负向定向驱动蛋白 KifC1，而其他蛋白仍有待发现。
