There are four main types of junctions in multicellular organisms, Anchoring Junctions, Occluding junctions, Channel-forming junctions and Signal-relaying junctions.
Anchoring junctions include cell-to-cell adhesion and cell-to-matrix adhesions, and are bound to the cytoskeleton filaments inside of a cell. (Cadherin & integrin proteins)
Cadherins bind with actin filaments to form adherens junctions and bind with intermediate filaments to form desmosomes, both of which are cell-to-cell adhesions. Integrins can also bind with actin filaments to form actin-linked cell-matrix adhesions or bind with intermediate filaments forming hemidesmosomes, both of which are cell-to-matrix adhesions.
Cadherins are so called as they utilize calcium ions to work properly. Without Ca+, most would fall apart while others also need the inclusion of trypsin, a protease, to completely cleave the cell adhesion molecules and separate the cell. This is often used when animal cells are split after growing in culture. Cadherins are utilized during embryo development. If anti-cadherin antibodies are added to the growth media of an embryo, then the cells will fall apart. Also, if a mutation has occurred in E-cadherin, then early in development an embryo will fall apart and die.
The first three cadherin types were named after the tissue that they were found in, although they were later found to be present in other tissues. E-cadherin was found in epithelial tissue, P-cadherin was found in the placenta and epidermis, and N-cadherin was found in the nerves and muscle tissue.
Cadherins have an extracellular motif that vary in number between each type. They then have a transmembrane domain and varying intracellular domains. The intracellular ligands can have signalling molecules or can bind with the cytoskeletal matrix. These same intracellular domains are also found on other protein families.
The binding of cadherins is typically homophilic and those cadherins that bind together will typically bind to the same cytoskeletal proteins. The cadherins typically bind to each other at the end of their extracellular domains and Ca+ then binds on to further tighten the connection. Ca+ also binds along the extracellular regions between flexible hinge regions to make the whole structure more stiff. Most of the connections that are formed between cadherins have a relatively low affinity, but similarly to the additive effect of hydrogen bonds or van der waals interactions, many cadherins in a close area form a tight bond.
Cadherins are also utilized for epithelial-mesenchymal transitions. During embryonic development, epithelial cells can remove their cadherin expression to allow them to move location. They can then turn on cadherin expression to bind elsewhere, as is done to form the neural crest. This can also be a transition that occurs in cancer in order for a tumor to become malignant.
Macroscopically in animals, adheren junctions work with...