Tight junction

Tight junctions, or otherwise known as zona occludens, are junctional structures between the plasma membrane of two adjacent cells and control paracellular movement of molecules through epithelial layers.  They are easily seen in intestinal epithelial cells and are also abundant in the ducts and cavities of glands, such as the liver pancreas and urinary bladder. Tight junctions form a continuous belt around the apical ends of the lateral surfaces of each cell, preventing the movement of fluids, ions and molecules between cells. Tight junctions also block the lateral movement of larger molecules like lipids and proteins within the membrane. The different concentrations of different kinds of integral proteins in the plasma membrane can then be maintained.

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Tight junctions consist of three major proteins. Occludin is a 65kDa tetra-spanning protein with two short extracellular domains and a highly extended C terminus. Claudin family of proteins are the major proteins in tight junctions and are 23kDa tetra-spanning proteins with two short extracellular domains. Both proteins have cytosolic C and T terminals and there are over 20 Claudin proteins. Junctional adhesion molecule is also involved in the formation of tight junctions. It has one membrance span and to immunoglobulin folds in its extracellular domain. Together these proteins for a "zipper" across the cleft between two adjacent cells.

The composition of the Claudin proteins determines whether the junction is tight or leaky. The introduction of Claudin 2 into the junction produces a leaky junction. Heterodimers that include Claudin 2 do not fit tightly together and, therefore, allow molecules to pass through.