Claudin

Group of proteins forming tight junctions between cells

Claudins are a family of proteins which, along with occludin, are the most important components of the tight junctions (zonulae occludentes). Tight junctions establish the paracellular barrier that controls the flow of molecules in the intercellular space between the cells of an epithelium. They have four transmembrane domains, with the N terminus and the C terminus in the cytoplasm.

Claudins are small (20–24/27 kilodalton (kDa)) transmembrane proteins which are found in many organisms, ranging from nematodes to human beings. They all have a very similar structure. Claudins span the cellular membrane 4 times, with the N terminal end and the C terminal end both located in the cytoplasm, and two extracellular loops which show the highest degree of conservation. Claudins have both cis and trans interactions between cell membranes. Cis interactions is when claudins on the same membrane interact, one way they interact is by transmembrane domain having molecular interactions. Trans interaction is when claudins of neighboring cells interact through their extracellular loops. Cis interactions is also known as side to side interactions and trans interactions is also known as head to head interactions. Generally the tight junction is known for its impermeability. However, depending on the type of claudin and their interactions there is selective permeability. This includes charge selectivity and size selectivity.

The C terminal has a longer chain and is located in the cytoplasm. It varies in length from 21 to 63 and is necessary for the localization of these proteins in the tight junctions. It is thought that it may play a role in cell signaling. The name claudin comes from Latin word claudere ("to close"), suggesting the barrier role of these proteins.

A recent review discusses evidence regarding the structure and function of claudin family proteins using a systems approach to understand evidence generated by proteomics techniques. A chimeric claudin was synthesized to help enhance the understanding of both the structure and function of the tight junction. Computational modeling is also another technique being used to help enhance research into the structure and functions of claudins. CLDN11, CLDN12, CLDN13, CLDN14, CLDN15, CLDN16, CLDN17, CLDN18, CLDN19, CLDN20, CLDN21, CLDN22, CLDN23