Role of Rab 13 and its effector in establishing cell polarity

紧密连接(TJ)和贴壁连接(AJ)位于极化上皮细胞基侧膜的顶端。在极化过程中，上皮细胞首先在最初的细胞-细胞接触的顶端形成点状的原始AJ，然后发展成带状的成熟AJ。与AJ的成熟平行，TJ在AJ的顶侧形成。尽管上皮细胞极化需要协调的TJ和AJ组装和几个基本的细胞过程之间的复杂相互作用，但完整的TJ和AJ蛋白向质膜和/或从质膜转运是控制TJ和AJ组装的关键。我们先前报道，Rab 13和与CASL-like 2相互作用的连接Rab13结合蛋白(JRAb)/分子(Mical-L2)介导了完整的TJ蛋白occludin的内吞循环和功能性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到质膜运输中的作用。虽然Rab13基因敲除特异性地抑制claudin-1和occludin的转运，但不抑制E-cadherin转运，但JRAb/Mical-L2及其Rab13结合域(JRAb/Mical-L2-C)的表达抑制claudin-1、occludin和E-cadherin的转运。然后我们鉴定了Rab8是另一种JRAb/Mical-L2-C结合蛋白。敲除Rab8基因可抑制Rab13非依赖的E-钙粘蛋白向质膜的转运。Rab13和Rab8分别与JRAb/Mical-L2和JRAb/Mical-L2在质膜和内体循环功能上相互竞争结合。这些结果表明，JRAb/Mical-L2与Rab13和Rab8的相互作用在建立细胞极性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.