Claudins are 20-27kDa transmembrane proteins of tight junctions (TJ) spanning the bilayer four times, where the N-termini and C-termini are oriented towards the cytoplasm and there are two extracellular loop (ECL) domains. Claudins, together with occludins, are the first detected TJ integral proteins and TJ are the most apical type of cell-cell contact existing in the lateral membrane between polarised cells. Therefore, claudins can determine the barrier properties of cell-cell contact among the plasma membranes of two adjacent cells, such as those occurring in endothelia or epithelia. They are expressed in a tissue-specific combination, which leads to tissue-specific barrier characteristics. Claudins can tighten the paracellular cleft for solutes completely and form paracellular ion pores. Mammals express approximately 24 distinct claudins and sequence analysis has fallen them into two groups, designated as classic claudins (1-10, 14, 15, 17, 19) and non-classic claudins (11-13, 16, 18, 20-24), according to their sequence similarity.
Mammalian intracellular claudins exhibit around 7 N-terminal amino acids, 12 loop amino acids, and 25 to 55 C-terminal amino acids, excluding claudin-12, claudin-16, and claudin-23. The ECL 1 is composed of about 50 amino acids with two conserved cysteines implicated in the barrier function. Negative and positive charges in ECL1 contribute to pore formation. The ECL2 generally has approximately 25 amino acids, but fewer in claudin-11 and more in claudin-18. The ECL2 has been less investigated and it probably associates with itself and possesses a holding function. Here show several proteins of the claudin family in humans as bellow.
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