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How Tight?
In metazoans, sheets of epithelial cells separate different tissue spaces and control their composition. Tight junctions are cell-cell adhesion structures in these cell sheets that form a seal between cells but also provide some selective permeability to ions and small molecules. Claudins are the main constituents of tight junctions, and mutations in claudins cause inherited human disorders involving the disruption of ionic balance. Suzuki et al. (p. 304) report the structure of mouse claudin-15 at 2.4 angstrom resolution, which shows an extracellular β-sheet domain anchored to a transmembrane four-helix bundle. The electrostatic distribution on the claudin surface reveals a negatively charged groove in the extracellular domain that may provide a pathway for positive ions.
Abstract
Tight junctions are cell-cell adhesion structures in epithelial cell sheets that surround organ compartments in multicellular organisms and regulate the permeation of ions through the intercellular space. Claudins are the major constituents of tight junctions and form strands that mediate cell adhesion and function as paracellular barriers. We report the structure of mammalian claudin-15 at a resolution of 2.4 angstroms. The structure reveals a characteristic β-sheet fold comprising two extracellular segments, which is anchored to a transmembrane four-helix bundle by a consensus motif. Our analyses suggest potential paracellular pathways with distinctive charges on the extracellular surface, providing insight into the molecular basis of ion homeostasis across tight junctions.
