Role of Rab 13 and its effector in establishing cell polarity

紧密连接（Tight junction， TJ）和粘附连接（adherens junction， AJ）位于极化上皮细胞基底外膜的顶端。在极化过程中，上皮细胞首先在初始细胞-细胞接触的尖端形成斑点状的原始AJ，然后发展成带状的成熟AJ。在AJ成熟的同时，TJ也在AJ的顶端侧形成。虽然上皮极化需要协调的TJ和AJ组装和几个基本细胞过程之间的复杂相互作用，但整体TJ和AJ蛋白进出质膜的运输对于控制TJ和AJ组装至关重要。我们之前报道了rab13和与CasL-like 2 （MICAL-L2）相互作用的连接rab13结合蛋白(JRAB)/分子介导完整TJ蛋白occludin的内噬循环和功能性TJ的形成。在本研究中，我们研究了Rab13和JRAB/MICAL-L2在其他完整的TJ和AJ蛋白claudin-1和E-cadherin向质膜运输中的作用。虽然Rab13的敲低特异性地抑制了claudin-1和occludin，但不抑制E-cadherin的转运，但JRAB/MICAL-L2的敲低及其Rab13结合域（JRAB/MICAL-L2- c）的表达延缓了claudin-1、occludin和E-cadherin的转运。然后我们确定Rab8是另一个JRAB/ mical - l2 - c结合蛋白。敲低Rab8抑制了不依赖rab13的E-cadherin向质膜的转运。Rab13和Rab8相互竞争与JRAB/MICAL-L2结合，并分别在质膜和循环内体上与JRAB/MICAL-L2结合。这些结果表明，JRAB/MICAL-L2与Rab13和Rab8的相互作用在细胞极性的建立中起着至关重要的作用。

Tight junction (TJ) and adherens junction (AJ) are located at the apical end of the basolateral membrane of polarized epithelial cells. During polarization, epithelial cells first form spot-like primordial AJ at the tips of the initial cell-cell contacts and develop belt-like mature AJ. In parallel with the maturation of AJ, TJ is formed at the apical side of AJ. Whereas epithelial polarization entails a complex interplay between the coordinated TJ and AJ assembly and several fundamental cellular processes, the transport of integral TJ and AJ proteins to and/or from the plasma membrane is essential for the control of TJ and AJ assembly. We previously reported that Rab 13 and a junctional Rab13-binding protein(JRAB)/molecule interacting with CasL-like 2 (MICAL-L2) mediated the endocytic recycling of an integral TJ protein occludin and the formation of functional TJ. In the present study, we examined the role of Rab13 and JRAB/MICAL-L2 in the transport of other integral TJ and AJ proteins claudin-1 and E-cadherin to the plasma membrane. Although knockdown of Rab13 specifically inhibited claudin-1 and occludin but not E-cadherin transport, knockdown of JRAB/MICAL-L2 and expression of its Rab13-binding domain(JRAB/MICAL-L2-C)retarded claudin-1, occludin, and E-cadherin transport. We then identified Rab8 as another JRAB/MICAL-L2-C-binding protein. Knockdown of Rab8 inhibited the Rab13-independent transport of E-cadherin to the plasma membrane. Rab13 and Rab8 competed with each other for the binding to JRAB/MICAL-L2 and functionally associated with JRAB/MICAL-L2 at the plasma membrane and recycling endosome, respectively. These results suggest that the interactions of JRAB/MICAL-L2 with Rab13 and Rab8 play a crucial role in establishing cell polarity.