Hewat Elizabeth A, Durmort Claire, Jacquamet Lilian, Concord Evelyne, Gulino-Debrac Danielle
Laboratoire de Microscopie Electronique Structurale, Institut de Biologie Structurale Jean-Pierre Ebel, UMR 5075, CEA-CNRS-UJF, 41 rue Jules Horowitz, 38027 Grenoble Cedex 1, France.
J Mol Biol. 2007 Jan 19;365(3):744-51. doi: 10.1016/j.jmb.2006.10.052. Epub 2006 Oct 21.
Vascular endothelial-cadherin (VE-cadherin) is the major constituent of the adherens junctions of endothelial cells and plays a key role in angiogenesis and vascular permeability. The ectodomains EC1-4 of VE-cadherin are known to form hexamers in solution. To examine the mechanism of homotypic association of VE-cadherin, we have made a 3D reconstruction of the EC1-4 hexamer using electron microscopy and produced a homology model based on the known structure of C-cadherin EC1-5. The hexamer consists of a trimer of dimers with each N-terminal EC1 module making an antiparallel dimeric contact, and the EC4 modules forming extensive trimeric interactions. Each EC1-4 molecule makes a helical curve allowing some torsional flexibility to the edifice. While there is no direct evidence for the existence of hexamers of cadherin at adherens junctions, the model that we have produced provides indirect evidence since it can be used to explain some of the disparate results for adherens junctions. It is in accord with the X-ray and electron microscopy results, which demonstrate that the EC1 dimer is central to homotypic cadherin interaction. It provides an explanation for the force measurements of the interaction between opposing cadherin layers, which have previously been interpreted as resulting from three different interdigitating interactions. It is in accord with observations of native junctions by cryo-electron microscopy. The fact that this hexameric model of VE-cadherin can be used to explain more of the existing data on adherens junctions than any other model alone argues in favour of the existence of the hexamer at the adherens junction. In the context of the cell-cell junction these cis-trimers close to the membrane, and trans-dimers from opposing membranes, would increase the avidity of the bond.
血管内皮钙黏蛋白（VE-钙黏蛋白）是内皮细胞黏附连接的主要成分，在血管生成和血管通透性中起关键作用。已知VE-钙黏蛋白的胞外结构域EC1-4在溶液中形成六聚体。为了研究VE-钙黏蛋白同型结合的机制，我们利用电子显微镜对EC1-4六聚体进行了三维重建，并基于C-钙黏蛋白EC1-5的已知结构构建了同源模型。该六聚体由三聚体二聚体组成，每个N端EC1模块形成反平行二聚体接触，EC4模块形成广泛的三聚体相互作用。每个EC1-4分子呈螺旋状弯曲，使整个结构具有一定的扭转灵活性。虽然没有直接证据表明黏附连接处存在钙黏蛋白六聚体，但我们构建的模型提供了间接证据，因为它可用于解释黏附连接的一些不同结果。它与X射线和电子显微镜结果一致，这些结果表明EC1二聚体是同型钙黏蛋白相互作用的核心。它为相对钙黏蛋白层之间相互作用的力测量提供了解释，此前这些测量结果被解释为由三种不同的相互交错相互作用导致。它与低温电子显微镜对天然连接的观察结果一致。与其他任何模型相比，VE-钙黏蛋白的这种六聚体模型能够解释更多关于黏附连接的现有数据，这支持了黏附连接处存在六聚体的观点。在细胞间连接的背景下，这些靠近膜的顺式三聚体和来自相对膜的反式二聚体将增加键的亲和力。
