Cell adhesion molecule
Cell adhesion molecules (CAMs) are proteins located on the cell surface involved in binding with other cells or with the extracellular matrix (ECM) in the process called cell adhesion. In essence, cell adhesion molecules help cells stick to each other and to their surroundings.
These proteins are typically transmembrane receptors and are composed of three domains: an intracellular domain that interacts with the cytoskeleton, a transmembrane domain, and an extracellular domain that interacts either with other CAMs of the same kind (homophilic binding) or with other CAMs or the extracellular matrix (heterophilic binding).
Families of CAMs
One classification system involves the distinction between calcium-independent CAMs and calcium-dependent CAMs.
Integrins and the Ig-superfamily CAMs do not depend on Ca2+ while cadherins and selectins depend on Ca2+. In addition, integrins participate in cell-matrix interactions, while other CAM families participate in cell-cell interactions.
Integrins, one of the major classes of receptors within the ECM, mediates cell-ECM interactions with collagen, fibrinogen, fibronectin, and vitronectin. Integrins provide essential links between the extracellular environment and the intracellular signalling pathways, which can play roles in cell behaviours such as apoptosis, differentiation, survival, and transcription.
Integrins are heterodimeric, as they consist of an alpha and beta subunit. There are currently 18 alpha subunits and 8 beta subunits, which combine to make up 24 different integrin combinations. Within each of the alpha and beta subunits there is a large extracellular domain, a transmembrane domain and a short cytoplasmic domain. The extracellular domain is where the ligand binds through the use of divalent cations. In general, Mn2+
increases affinity, Mg2+
promotes adhesion to cells, and Ca2+
decreases cell adhesion. Integrins regulate their activity within the body by changing conformation. Most exist at rest in a low affinity state, which can be altered to high affinity through an external agonist which causes a conformational change within the integrin, increasing their affinity.
An example of this is the aggregation of platelets; Agonists such as thrombin or collagen trigger the integrin into its high affinity state, which causes increased fibrinogen binding, causing platelet aggregation.
Immunoglobulin superfamily CAMs (IgSF CAMs) are either homophilic or heterophilic and bind integrins or different IgSF CAMs.
Lymphocyte homing receptors
The cadherins are homophilic Ca2+
-dependent glycoproteins. The classic cadherins (E-, N- and P-) are concentrated at the intermediate cell junctions, which link to the actin filament network through specific linking proteins called catenins.
Each cadherin exhibits a unique pattern of tissue distribution, such as epithelial (E-cadherins), placental (P-cadherins), neural (N-cadherins), retinal (R-cadherins), brain (B-cadherins and T-cadherins), and muscle (M-cadherins). Many cell types express combinations of cadherin types.
The extracellular domain has major repeats called extracellular cadherin domains (ECD). Sequences involved in Ca2+
binding between the ECDs are necessary for cell adhesion. The cytoplasmic domain has specific regions where catenin proteins bind.
The selectins are a family of heterophilic CAMs that bind fucosylated carbohydrates, e.g., mucins. The three family members are E-selectin (endothelial), L-selectin (leukocyte), and P-selectin (platelet). The best-characterized ligand for the three selectins is P-selectin glycoprotein ligand-1 (PSGL-1), which is a mucin-type glycoprotein expressed on all white blood cells.
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- Cell Adhesion Molecules at the US National Library of Medicine Medical Subject Headings (MeSH)
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