Basic Integrin Info

Integrins are receptor proteins which are of crucial importance. They are the main way that cells both bind to and respond to the extracellular matrix and are involved in a variety of cellular functions such as wound healing, cell differentiotion, homing of tumor cells and apoptosis. 

They are part of a large family of cell adhesion receptors which are involved in cell-extracellular matrix and cell-cell interactions. Functional integrins consist of two transmembrane glycoprotein subunits, called alpha and beta, that are non-covalently bound. The alpha subunits all share some homology to each other, as do the beta subunits. The receptors always contain one alpha chain and one beta chain

and are thus called heterodimeric. Both of the subunits contribure to the binding of ligand. Until now 16 alpha and 8 beta subunits have been identified. From these subunits some 22 integrins are formed in nature, which implicates that not all possible combinations exist. The beta-4 subunit for instance can only form a heterodimer with the alpha-6 subunit. On the other hand the beta-1 subunit can form heterodimers with ten different alpha subunits. Because not all the beta-1 alpha heterodimers have the same ligand specificities, it is believed that the alpha chain is at least partly involved in the ligand specificity. The diversity of the integrins is increased by the alternative splicing of some integrin messenger RNA's

Integrins differ from other cell-surface receptors in that they bind their ligands with a low affinity (10^6-10^9 liters/mole) and that they are usually present at 10-100 fold higher concentration on the cell surface. The integrins however can only bind their ligands when they exceed a certain minimal number of integrins at certain places, called focal contacts and hemidesmosomes. So when the integrins are diffusely distributed over the cell surface, no adhesion will be present, but when after a certain stimuli these integrins cluster for example in focal contacts their combined weak affinities give rise to a spot on the cell surface which has enough adhesive (sticking) capacity to adhere to the extracellular matrix. This is a very usefull situation, because in this way cells can bind simultaniously but weakly to large numbers of matrix molecules and still have the uppertunity to explore their environment without losing all attachment to it by building or breaking down focal contacts. If the receptors were to bind strongly to their ligands, cells would probably be irreversably bound to the matrix, depriving it from motility. This problem does not arise when attachment depends on multiple weak adhesions.

Integrin-ligand interactions are accompanied by clustering and activation of the integrins on the cell surface, which is also accompanied by the transduction of signals into intracellular signal transduction pathways that mediate a number of intracellular events. Signaling through integrins depands on the formation of focal adhesions, dynamic sites in which cytoskeletal and other proteins are concentrated and which regulate migration and the shape of a cell (Scwartz, 1992).

Focal adhesion kinase (FAK) is a tyrosine kinase which is commonly found in integrin mediated focal adhesions (Schaller et al. 1992). Upon activation and phosphorylation of FAK, this kinase may phosphorylate other signaling proteins in a signal transduction cascade. Paxillin for example, a protein involved in cytoskeletal reorganization, is a target of FAK (Tremblay et al. 1996).

Activation of mitogen activated protein kinase (MAPK) occurs after integrin-ligand binding (RGD peptides, fibronectin, laminin), resulting in the translocation of MAPK from the cytoplasm to the nucleus (Morino et al. 1995). MAPK can also be activated by integrin linked kinase (ILK) in a FAK independent pathway (Weaver et al. 1997).

Induction of tyrosine phosphorylation of phospholipase C-gamma (PLC-gamma) and its recruitment to focal adhesions has been reported upon beta2-integrin activation (Kanner et al. 1993). Activation of PLC-gamma, results in the hydrolysis of the phospholipid phosphatidylinositol diphosphate (PIP2) into diacyl glycerol (DAG) and inositol triphosphate (IP3). DAG is an activator protein kinase C (PKC), while IP3 mediates the release of calcium from mitochondrial calcium stores (Cole et al. 1994; Whitfield 1992).

A number of integrins mediate the binding of a cell to the C-terminal globular domain of laminin. Alpha6beta1 has been shown to associate with proteins of the C-terminal globular domain of the laminin A1 chain (Nakahara et al. 1996). Alpha3beta1 has also been demonstrated to bind the globular domain of the laminin A chain (Gehlsen et al. 1992). Alpha7beta1 binds specificly to laminin (Kramer et al. 1991).

Integrins can adhere (bind) an array of ligands. Common ligands are for example fibronectin and laminin, which are both part of the extracellular matrix or basal lamina’s. Both of the ligands mentioned above are recognized by multiple integrins. For adhesion to ligands both integin subunits are needed, as is the presence of cations. The alpha chain is the part that has cation binding sites.

Integrins are composed of long extracellular domains which adhere to their ligands, and short cytoplasmic domains that link the receptors to the cytoskeleton of the cell. There is however one exception to this rule, which is the beta-4 subunit. Beta-4 contains a very large cytoplasmic domain of some 1000 amino acids, while the other integrins only have cytoplasmic domains of up to 60 amino acids.

The structure between the alpha subunits is very similar. All contain 7 homologous repeats of 30-40 amino acids in their extracellular domain, spaced by stretches of 20-30 amino acids. The three or four repeats that are most extracellular, contain sequences with cation-binding properties. These sequences are thought to be involved in the binding of ligands, because the interaction of integrins with their ligand is cation-dependent. All the alpha subunits share the 5 amino acid motif GFFKR, which is located directly under the transmembrane region. The precise function of this motif is not known yet, but a protein called calreticulin has been isolated on a collumn loaded with this fragment. Also an association between calreticulin and integrins (alpha-3 and alpha-5) has been demonstrated. The protein may play a role in the assembly of integrin dimers.

The alpha subunits are subdevided into two groups based on some structural differences. The first group is formed by alpha-1, alpha-2, alpha-L, alpha-M and alpha-X. The members of the first group all contain a so-called inserted domain (I-domain). This domain of about 180 amino acids is situated between the second and the third repeat. The function of this domain is not known, but it has been speculated that it is involved in ligand binding, because the domain has a resemblance with domains found in von Willebrand factor which are important for it's binding to collagen for example.
 

The second group is formed by alpha-3, alpha-5, alpha-6, alpha-7, alpha-8, alpha-IIb, alpha-V and alpha-IEL. Members of this group all share a post translational cleavage of their precursor into a heavy and a light chain. The light chain is composed of the cytoplasmic domain, the transmembrane region and a part of the extracellular domain (about 25 kD), while the heavy chain contains the rest on the extracellular domain (about 120 kD).

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