Integrin
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An integrin, or integrin receptor, is an integral membrane protein in the plasma membrane of cells. It plays a role in the attachment of a cell to the to the extracellular matrix (ECM) and in signal transduction from the ECM to the cell. There are many different types of integrin and many cells have multiple types on their surface. Integrins are of vital importance to human beings and many other organisms.
Mutations in the genes encoding for integrin can be found in certain types of cancer, for instance breast cancer. A failure of integrin to anchor a cell to the ECM can play a role in the metastasis of certain cancer cells.
Other types of protein that play a role in cell-cell/cell-matrix interaction and communication are cadherins, NCAMs and selectins.
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Structure
Integrins are heterodimers of two chains, called the α (alpha) and β (beta) subunits. Many different α and β subunits have been found.. Through different combinations of these, a great amount of unique integrin proteins can be created. The subunits each penetrate the plasma membrance individually, and join together about 12-15 nm to the outside of the plasma membrane, where they form the ligand-binding region. The molecular mass of the subunits varies from about 100,000 to 140,000 in different molecules. β subunits have four cysteine-rich repeated sequences. α subunits bind several divalent cations.
Function
The two main functions of integrins are:
- Attachment of the cell to the to the ECM.
- Signal transduction from the ECM to the cell.
Attachment of cell to the ECM
Integrins couple the ECM outside a cell to the cytoskeleton (in particular the microfilaments) inside the cell. Which ligand in the ECM the integrin can bind to is mainly decided by which α and β subunits the integrin is constructed of. Among the ligands of integrins are fibronectin and collagen. The connection between the cell and the ECM enables the cell to endure pulling forces without being ripped out of the ECM. The ability of a cell to create this kind of bond is also of vital importance in ontogeny.
The connections between integrin and the ligands in the ECM and the microfilaments inside the cell are indirect: they are linked via certain small proteins.
Signal transduction
Integrins play an important role in cell signaling. Connection with ECM molecules can cause a signal to be relayed into the cell through protein kinases that are connected with the intracellular end of the integrin molecule.
The signals the cell receives through the integrin can have relation to: