The Four Main Adhesion Molecule Families

November 12, 2012

The Four Main Adhesion Molecule Families

An adhesion molecule found on the surface of a cell happens to play a very important role in various cellular processes, such as cell growth, embryogenesis, differentiation, cancer metastasis, and immune cell response and transmigration. It can also send information to cells from extracellular matrices. In general, there are four main adhesion molecule families out there, and they are as follows:

Ig Superfamily CAMs

Ig superfamily CAMs refer to transmembrane glycoproteins that are independent of calcium and include the ICAMs, the VCAM-1, the PECAM-1, and the NCAM. Every single Ig superfamily CAM comes with an extracellular domain that has various intrachain loops that are bonded with disulfide and are similar to Ig. They also have intracellular domains, which interact with the cytoskeleon. Usually, they bind integrins together and are used in neuronal patterning. The endothelial CAMs, on the other hand, play a vital role in inflammation and immune responses.

Integrins

Integrins refer to alpha and beta subunit heterodimers that aren’t covalently linked. On the contrary, they are constitutively expressed transmembrane proteins that need to be activated, so that they can bind their ligands. Right now, there are 15 a subunits, as well as 8 b subunits in total, which can be combined in different ways in order to create various kinds of integrin receptors. Integrins seem to have various activation states, as well: basal avidity, high avidity and low avidity. Plus, their cells will change their expression of different integrin receptors based on their activation state, lineage or maturity. Integrins can also bind divalent cations together like calcium, manganese, and magnesium, the two latter ones of which have the power to activate integrins.

Cadherins

A cadherin is basically an adhesion molecule that is dependent on calcium. The most common cadherins would be the neural-cadherins, the epithelial-cadherins and the placental-cadherins, all of which are part of the traditional cadherin subfamily. Proto-cadherins and desmosomal cadherins exist, too, though. Either way, they are all a part of tissue organization and embryonic development and showcase homophilic adhesion. There are various cadherin repeats in the extracellular domain, as well, all of which can bind calcium ions. Whenever this happens, the extracellular domain ends up looking like a rigid rod.

Selectins

Selectins refer to glycoproteins that are dependent on divalent cations. These proteins can bind carbohydrates and mucins together and they have the following family members: the endothelial-selectins, the leukocyte-selectins, and the platelet-selectins. Each of their extracellular domains has a motif for carbohydrate recognition, a motif for epidermal growth factors, and numerous domains linked to complement-regulatory proteins. As mentioned earlier, an adhesion molecule is vital to various cellular responses and processes. Plus, it plays a part in numerous disease states, as well, such as leukocyte transendothelial migration and tumorigenesis.

An adhesion molecule also plays a big role in evading the immune system through pathogenic microorganisms, as well as in establishing blood-brain barriers and helping them penetrate into immune cells. The most important adhesion molecules for this process would be the integrins and selections, though.

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