Transmembrane proteins in the tight junction barrier

Three types of transmembrane proteins have been identified within the tight junction, but it remains to be determined how they provide the molecular b asis for regulating the paracellular permeability for water, solutes, and i mmune cells. Several of these proteins localize specifically within the con tinuous cell-to-cell contacts of the tight junction. One of these, occludin , is a cell adhesion molecule that has been demonstrated to influence ion a nd solute permeability. The claudins are a family of four-membrane spanning proteins; unexpectedly, other members of this family have already been cha racterized without recognizing their relationship to tight junctions. Junct ion adhesion molecule, the most recently identified tight junction componen t, is a member of the Ig superfamily and influences the paracellular transm igration of immune cells. A plaque of cytoplasmic proteins under the juncti on may be responsible for scaffolding the transmembrane proteins, creating a link to the perijunctional actin cytoskeleton and transducing regulatory signals that control the paracellular barrier.