Tightjunction Complex

Many structural components of the tight junctions (TJs) have been defined since 1992 [85-97]. Lutz and Siahaan [95] reviewed the protein structural components of the TJ. Figure 2.7 depicts the occludin protein complex that makes the water pores so restrictive. Freeze-fracture electronmicrographs of the constrictive region of the TJ show net-like arrays of strands (made partly of the cytoskeleton) circumscribing the cell, forming a division between the apical and the basolateral

TABLE 2.1 Intracellular pH Environment

Intracellular Compartment pH

Mitocondria 8.0

Cytosol 7.2-7.4

Endoplasmic reticulum 7.1-7.2

Endosomes 5.5-6.0

Secretory granules 5.0-6.0

Lysosomes 4.5-5.0

sides. A region 10 strands wide forms junctions that have very small pore openings; fewer strands produce leakier junctions. The actual cell-cell adhesions occur in the cadheren junctions, located further away from the apical side. Apparently three calciums contiguously link 10-residue portions of cadheren proteins spanning from two adjoining cell walls, as depicted in Fig. 2.7 [95]. Calcium-binding agents can open the junctions by interactions with the cadheren complex.

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