Ca(2+)-independent cell-adhesion activity of claudins, a family of integral membrane proteins localized at tight junctions. |
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Authors: | K Kubota M Furuse H Sasaki N Sonoda K Fujita A Nagafuchi S Tsukita |
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Institution: | Department of Cell Biology Faculty of Medicine Kyoto University Sakyo-ku, Kyoto, 606-8501, Japan. |
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Abstract: | In multicellular organisms, various compositionally distinct fluid compartments are established by epithelial and endothelial cellular sheets. For these cells to function as barriers, tight junctions (TJs) are considered to create a primary barrier for the diffusion of solutes through the paracellular pathway 1] 2] 3]. In ultrathin sections viewed under electron microscopy, TJs appear as a series of apparent fusions, involving the outer leaflets of plasma membranes of adjacent cells, to form the so-called kissing points of TJs, where the intercellular space is completely obliterated 4]. Claudins are a family of 16 proteins whose members have been identified as major integral membrane proteins localized exclusively at TJs 5] 6] 7] 8]. It remains unclear, however, whether claudins have the cell-adhesion activity that would explain the unusual intercellular adhesion at TJs. Using mouse L-fibroblast transfectants expressing various amounts of claudin-1, -2 or -3, we found that these claudins possess Ca(2+)-independent cell-adhesion activity. Using ultrathin-section electron microscopy, we observed many kissing points of TJs between adjacent transfectants. Furthermore, the cell-adhesion activity of occludin, another integral membrane protein localized at TJs 9] 10] 11], was negligible when compared with that of claudins. Thus, claudins are responsible for TJ-specific obliteration of the intercellular space. |
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