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Structure[edit source]
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. 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.

N-Terminal
The N-Terminal is usually very short, ranging between 1-3 amino acids. It is located in the cytoplasm where it is thought to help to contribute to cell signaling, cytoskeletal organization and other possible functions.

C-Terminal
The C-Terminal generally has a longer chain of amino acids. The length varies between 21-63 amino acids and it is located in the cytoplasm. It is thought that it may play a role in cell signaling. The C-Terminal end is used to help with protein degradation but the mechanism is unknown. All human claudins (with the exception of Claudin 12) have domains that let them bind to PDZ domains of scaffold proteins. This is very important for localization of the proteins.

Transmembrane Domain
The transmembrane domain is the amino acids that cross the cellular membrane. The transmembrane domain is important for cis-interaction of claudins.

First Extracellular Loop
The first extracellular loop has a range of 42-56 amino acids and is longer than the second extracellular loop. This loop has charged amino acids that may be the predictor for the charge selectivity of tight junctions. The first extracellular loop plays a role in trans-interaction of claudins of adjacent cells.

Second Extracellular Loop
The second extracellular loop on average has 24 amino acids. In this short chain of amino acids there is three hydrophobic resides. These three residues are suspected to be a contributor to the trans-interaction of proteins between adjacent cells.

History[edit source]
Claudins were first named in 1998 by Japanese researchers Mikio Furuse and Shoichiro Tsukita at Kyoto University. The name claudin comes from Latin word claudere ("to close"), suggesting the barrier role of these proteins.

Studies
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.

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 the structure and functions of claudins.

Genes[edit source]
There are 23 genes found in the human genome for claudin proteins and there are 27 transmembrane domains across mammals. The conservation is not observed on a genetic level. Despite the genetic level not being conserved across claudins their structural conservation are very similar.