Integrins are cell surface receptors that interact with the extracellular matrix (ECM) and mediate various intracellular signals. They define cellular shape, mobility, and regulate the cell cycle. These integral membrane proteins are attached to the cellular plasma membrane through a single transmembrane helix.

Integrin plays a role in the attachment of cells to other cells, and also plays a role in the attachment of a cell to the material part of a tissue that is not part of any cell (the extracellular matrix). Besides the attachment role, integrin also plays a role in signal transduction, a process by which a cell transforms one kind of signal or stimulus into another.

The integrins are unusual membrane proteins because the signals they convert travel in both outside-in: transducing information from the ECM to the cell, and inside-out: "revealing" the status of the cell to the extracellular world. This allows cells to make rapid and flexible responses (see the example of platelets, in the Function section). It is more common for cells to make new receptors on their surfaces, or remove them if they need to alter their ability to respond to the environment.

There are many types of integrin, and many cells have multiple types on their surface. Integrins are of vital importance to all animals and have been found in all animals tested, from sponges to mammals. Integrins have been extensively studied in humans.

Other types of protein that play a role in cell-cell and cell-matrix interaction and communication are cadherins, CAMs and selectins.

Structure

Integrins are obligate heterodimers containing two distinct chains, called the α (alpha) and β (beta) subunits. In mammals, 19 α and 8 β subunits have been characterized, whereas the Drosophila and Caenorhabditis genomes encode only five α and two β subunits.

• Alpha: Gene: ITGA1 (CD49a), Gene: ITGA2 (CD49b), Gene: ITGA2B (CD41), Gene: ITGA3 (CD49c), Gene: ITGA4 (CD49d), Gene: ITGA5 (CD49e), Gene: ITGA6 (CD49f), Gene: ITGA7, Gene: ITGA8, Gene: ITGA9, Gene: ITGA10, Gene: ITGA11, Gene: ITGAD (CD11d), Gene: ITGAE (CD103), Gene: ITGAL (CD11a), Gene: ITGAM (CD11b), Gene: ITGAV (CD51), Gene: ITGAW, Gene: ITGAX (CD11c)
• Beta: Gene: ITGB1 (CD29), Gene: ITGB2 (CD18), Gene: ITGB3 (CD61), Gene: ITGB4 (CD104), Gene: ITGB5, Gene: ITGB6, Gene: ITGB7, Gene: ITGB8

In addition, variants of some of the subunits are formed by differential splicing, for example 4 variants of the beta-1 subunit exist. Through different combinations of these alpha and beta subunits, some 24 unique integrins are generated, although the number varies according to different studies.

Integrin subunits span the plasma membrane and in general have very short cytoplasmic domains of about 40-70 amino acids. The exception is the beta-4 subunit which has a cytoplasmic domain of 1088 amino acids, one of the largest known cytoplasmic domains of any membrane protein. Outside the cell plasma membrane, the alpha and beta chains lie close together along a length of about 23nm, the final 5nm N-termini of each chain form a ligand-binding region for the ECM, or extracellular matrix.