Integrins

**Figure 5:** Schematic representation of the molecular structure of integrins. They are transmembrane glycoproteins. They anchor the cell to proteins on the basement membrane, they also function in cell to cell inter-communication (recognition and interaction). They are heterodimers consisting of a larger $\alpha$ and a smaller $\beta$ subunit. They come in families based upon identical $\beta$ subunits with varying $\alpha$ subunits
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Integrins are a family of heterodimeric transmembrane glycoproteins that attach cells to extracellular proteins on the basement membrane. They consist of large $\alpha$ (120-170 kD) and small $\beta$ ( 90-110 kD) subunits. Some integrins are important for direct cell to cell recognition and interaction, see figure 5. They contain binding sites for divalent cations (Mg²⁺ and Ca²⁺) which are required. Mamallian integrins form several families sharing common $\beta$ subunits with different $\alpha$ subunits.

**Figure 6:** Schematic representation of the molecular structure of $\beta _2$ integrins. a) They are transmembrane glycoproteins exclusively expressed on leucocytes. Their main ligands are members of the immunoglobulin superfamily expressed on the endothelium, such as ICAM-1. They are heterodimers consisting of a larger $\alpha$ and a smaller $\beta$ subunit. The $\alpha$ subunit is CD11a, b, c or d and the $\beta$ subunit CD18. b) Phosphorylation of the $\beta$ subunit results in a conformational change and activates the integrin.
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The $\beta _2$ integrins are exclusively expressed on leucocytes. The common $\beta$ unit is CD18 and the various $\alpha$ subunits are CD11a,b,c, and d. Thus the four $\beta _2$ integrins are CD11a / CD18, CD11b / CD18, CD11c / CD18 and CD11d / CD18, see figure 6. $\beta _2$ integrins are activated by phosphorylation which results in a conformational change in the molecule. A mutation in the gene encoding CD18, the $\beta$ subunit of $\beta _2$ integrins results in the genetic disorder leukocyte adhesion deficiency (LAD). Patients with LAD are prone to recurrent bacterial infections due to an inability to recruit granulocytes in response to infection.

The main ligands for $\beta _2$ integrins are members of the immunoglobulin superfamily such as ICAM-1.