ABSTRACT
Size .....................................................................................................75 4.4.3 Relationship of Junctional Depth and Junctional Permeability ........77
4.5 Junctional Water Permeability .......................................................................77 4.5.1 Relationship between Junctional Ion and Water Permeability..........78 4.5.2 Junctional Permeation of Large Hydrophilic Solutes .......................79 4.5.3 Estimates of Junctional Water Permeability......................................79 4.5.4 Solute-Solvent Coupling and Electrokinetic Phenomena.................81
4.6 Conclusions ....................................................................................................82 References................................................................................................................83
The issue of the chemical composition of the tight junctions (TJs) appears to be near resolution. Recent studies carried out principally by Tsukita’s group (reviewed in Tsukita et al., 1999; Tsukita and Furuse, 2000) indicate that several hydrophobic proteins are components of the junctional strands. The claudins (integral membrane proteins of molecular mass ~28 kDa, cytoplasmic amino and carboxy termini, four transmembrane domains, over 20 isoforms described; Furuse et al., 1998a,b; see also Chapter 10) are essential. Occludin (integral membrane protein of molecular mass ~60 kDa, similar topology to occludins, one known gene product), the first integral junctional protein to be identified (Furuse et al., 1993; Ando-Akatsuka et al., 1996), proved nonessential in gene-knockout and heterologous-expression experiments (Saito et al., 1998; Furuse et al., 1998b). In addition, the junctions contain numerous peripheral proteins (see Chapter 11) that appear to play roles in targeting the junctional proteins themselves, association of TJs with the actin cytoskeleton, intracellular signaling, and vesicle targeting.
