Cellular adhesion is the binding of a cell to a surface, extracellular matrix or another cell using cell adhesion molecules such as integrins, cadherins, and selectins. Cell adhesion is critical for the genesis and maintenance of both three-dimensional structure and normal function in tissues. Cell adhesion receptors are typically transmembrane glycoproteins that mediate binding to extracellular matrix molecules (ECM) or to counter-receptors on other cells; these molecules determine the specificity of cell-to-cell or cell-to-ECM interaction.
The integrins are cell-surface glycoproteins that act as receptors for ECM proteins, or for membrane-bound counter-receptors on other cells. They also play a role in cell signaling and therefore regulate cellular shape, motility, and the cell cycle. Each integrin is a heterodimer that contains an α and a β subunit with each subunit having a large extracellular domain, a single membrane-spanning region, and in most cases, a short cytoplasmic domain.
The cadherins are a class of type-1, transmembrane proteins. They share an extracellular domain consisting of multiple repeats of a cadherin-specific motif. They play important roles in cell adhesion, ensuring that cells within tissues are bound together. Their name is derived from the fact that they are dependent on calcium (Ca2+) ions to function. This subfamily includes the N-, P-, R-, B-, and E-cadherins, also including 10 other members. These molecules localize in specialized sites of cell-to-cell adhesion that are termed adherence junctions; at these sites cadherins can establish linkages with the actin-containing cytoskeleton.
The selectins are lectin-like adhesion receptors composed of three members, L-, E-, and P-selectin. P-selectin is present in latent form in endothelial cells and platelets; it is rapidly translocated from secretory granules to the cell surface upon cell activation by thrombin or other agonists. E-selectin is synthesized and expressed on endothelial cells in response to inflammatory cytokines such as tumor necrosis factor (TNF) or IL-1. L-selectin is expressed constitutively on leukocytes, but its presentation at the cell surface may be regulated. All selectins are single-chain transmembrane glycoproteins that share similar properties to C-type lectins due to a related amino terminus and calcium-dependent binding.
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