Schnatwinkel Carsten, Christoforidis Savvas, Lindsay Margaret R, Uttenweiler-Joseph Sandrine, Wilm Matthias, Parton Robert G, Zerial Marino
Max-Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany.
PLoS Biol. 2004 Sep;2(9):E261. doi: 10.1371/journal.pbio.0020261. Epub 2004 Aug 24.
The small GTPase Rab5 is a key regulator of clathrin-mediated endocytosis. On early endosomes, within a spatially restricted domain enriched in phosphatydilinositol-3-phosphate [PI(3)P], Rab5 coordinates a complex network of effectors that functionally cooperate in membrane tethering, fusion, and organelle motility. Here we discovered a novel PI(3)P-binding Rab5 effector, Rabankyrin-5, which localises to early endosomes and stimulates their fusion activity. In addition to early endosomes, however, Rabankyrin-5 localises to large vacuolar structures that correspond to macropinosomes in epithelial cells and fibroblasts. Overexpression of Rabankyrin-5 increases the number of macropinosomes and stimulates fluid-phase uptake, whereas its downregulation inhibits these processes. In polarised epithelial cells, this function is primarily restricted to the apical membrane. Rabankyrin-5 localises to large pinocytic structures underneath the apical surface of kidney proximal tubule cells, and its overexpression in polarised Madin-Darby canine kidney cells stimulates apical but not basolateral, non-clathrin-mediated pinocytosis. In demonstrating a regulatory role in endosome fusion and (macro)pinocytosis, our studies suggest that Rab5 regulates and coordinates different endocytic mechanisms through its effector Rabankyrin-5. Furthermore, its active role in apical pinocytosis in epithelial cells suggests an important function of Rabankyrin-5 in the physiology of polarised cells.
小GTP酶Rab5是网格蛋白介导的内吞作用的关键调节因子。在早期内体上，在富含磷脂酰肌醇-3-磷酸[PI(3)P]的空间受限区域内，Rab5协调一个效应器复杂网络，这些效应器在膜拴系、融合和细胞器运动中发挥功能协同作用。在此，我们发现了一种新型的PI(3)P结合Rab5效应器Rabankyrin-5，它定位于早期内体并刺激其融合活性。然而，除了早期内体，Rabankyrin-5还定位于大的液泡结构，这些结构与上皮细胞和成纤维细胞中的巨吞饮小泡相对应。Rabankyrin-5的过表达增加了巨吞饮小泡的数量并刺激液相摄取，而其下调则抑制这些过程。在极化上皮细胞中，该功能主要局限于顶端膜。Rabankyrin-5定位于肾近端小管细胞顶端表面下方的大型胞饮结构，并且其在极化的Madin-Darby犬肾细胞中的过表达刺激顶端而非基底外侧的非网格蛋白介导的胞饮作用。在证明其在内体融合和（巨）吞饮作用中的调节作用时，我们的研究表明Rab5通过其效应器Rabankyrin-5调节和协调不同的内吞机制。此外，它在上皮细胞顶端胞饮作用中的积极作用表明Rabankyrin-5在极化细胞生理学中具有重要功能。
