Integrin
Integrins are transmembrane receptors that help cell–cell and cell–extracellular matrix adhesion. Upon ligand binding, integrins activate signal transduction pathways that mediate cellular signals such as regulation of the cell cycle, organization of the intracellular cytoskeleton, and movement of new receptors to the cell membrane. The presence of integrins allows rapid and flexible responses to events at the cell surface.
Several types of integrins exist, and one cell generally has multiple different types on its surface. Integrins are found in all animal cells while integrin-like receptors are found in plant cells.
Integrins work alongside other proteins such as cadherins, the immunoglobulin superfamily cell adhesion molecules, selectins and syndecans, to mediate cell–cell and cell–matrix interaction. Ligands for integrins include fibronectin, vitronectin, collagen and laminin.
Structure
Integrins are obligate heterodimers composed of α and β subunits. Several genes code for multiple isoforms of these subunits, which gives rise to an array of unique integrins with varied activity. In mammals, integrins are assembled from eighteen α and eight β subunits, in Drosophila five α and two β subunits, and in Caenorhabditis nematodes two α subunits and one β subunit. The α and β subunits are both class I transmembrane proteins, so each penetrates the plasma membrane once, and can possess several cytoplasmic domains.| gene | protein | synonyms |
| CD49a | VLA1 | |
| CD49b | VLA2 | |
| CD49c | VLA3 | |
| CD49d | VLA4 | |
| CD49e | VLA5 | |
| CD49f | VLA6 | |
| ITGA7 | FLJ25220 | |
| ITGA8 | ||
| ITGA9 | RLC | |
| ITGA10 | PRO827 | |
| ITGA11 | HsT18964 | |
| CD11D | FLJ39841 | |
| CD103 | HUMINAE | |
| CD11a | LFA1A | |
| CD11b | MAC-1 | |
| CD51 | VNRA, MSK8 | |
| CD41 | GPIIb | |
| CD11c |
| gene | protein | synonyms |
| CD29 | FNRB, MSK12, MDF2 | |
| CD18 | LFA-1, MAC-1, MFI7 | |
| CD61 | GP3A, GPIIIa | |
| CD104 | ||
| ITGB5 | FLJ26658 | |
| ITGB6 | ||
| ITGB7 | ||
| ITGB8 |
Variants of some subunits are formed by differential RNA splicing; for example, four variants of the beta-1 subunit exist. Through different combinations of the α and β subunits, 24 unique mammalian integrins are generated, excluding splice- and glycosylation variants.
Integrin subunits span the cell membrane and have short cytoplasmic domains of 40–70 amino acids. The exception is the beta-4 subunit, which has a cytoplasmic domain of 1,088 amino acids, one of the largest of any membrane protein. Outside the cell membrane, the α and β chains lie close together along a length of about 23 nm; the final 5 nm N-termini of each chain forms a ligand-binding region for the ECM. They have been compared to lobster claws, although they don't actually "pinch" their ligand, they chemically interact with it at the insides of the "tips" of their "pinchers".
The molecular mass of the integrin subunits can vary from to. Beta subunits have four cysteine-rich repeated sequences. Both α and β subunits bind several divalent cations. The role of divalent cations in the α subunit is unknown, but may stabilize the folds of the protein. The cations in the β subunits are more interesting: they are directly involved in coordinating at least some of the ligands that integrins bind.
Integrins can be categorized in multiple ways. For example, some α chains have an additional structural element inserted toward the N-terminal, the alpha-A domain. Integrins carrying this domain either bind to collagens, or act as cell-cell adhesion molecules. This α-I domain is the binding site for ligands of such integrins. Those integrins that don't carry this inserted domain also have an A-domain in their ligand binding site, but this A-domain is found on the β subunit.
In both cases, the A-domains carry up to three divalent cation binding sites. One is permanently occupied in physiological concentrations of divalent cations, and carries either a calcium or magnesium ion, the principal divalent cations in blood at median concentrations of 1.4 mM and 0.8 mM. The other two sites become occupied by cations when ligands bind—at least for those ligands involving an acidic amino acid in their interaction sites. An acidic amino acid features in the integrin-interaction site of many ECM proteins, for example as part of the amino acid sequence Arginine-Glycine-Aspartic acid.
Structure
Despite many years of effort, discovering the high-resolution structure of integrins proved to be challenging, as membrane proteins are classically difficult to purify, and as integrins are large, complex and highly glycosylated with many sugar 'trees' attached to them. Low-resolution images of detergent extracts of intact integrin GPIIbIIIa, obtained using electron microscopy, and even data from indirect techniques that investigate the solution properties of integrins using ultracentrifugation and light scattering, were combined with fragmentary high-resolution crystallographic or NMR data from single or paired domains of single integrin chains, and molecular models postulated for the rest of the chains.The X-ray crystal structure obtained for the complete extracellular region of one integrin, αvβ3, shows the molecule to be folded into an inverted V-shape that potentially brings the ligand-binding sites close to the cell membrane. Perhaps more importantly, the crystal structure was also obtained for the same integrin bound to a small ligand containing the RGD-sequence, the drug cilengitide. As detailed above, this finally revealed why divalent cations are critical for RGD-ligand binding to integrins. The interaction of such sequences with integrins is believed to be a primary switch by which ECM exerts its effects on cell behaviour.
The structure poses many questions, especially regarding ligand binding and signal transduction. The ligand binding site is directed towards the C-terminal of the integrin, the region where the molecule emerges from the cell membrane. If it emerges orthogonally from the membrane, the ligand binding site would apparently be obstructed, especially as integrin ligands are typically massive and well cross-linked components of the ECM. In fact, little is known about the angle that membrane proteins subtend to the plane of the membrane; this is a problem difficult to address with available technologies. The default assumption is that they emerge rather like little lollipops, but there is little evidence for this. The integrin structure has drawn attention to this problem, which may have general implications for how membrane proteins work. It appears that the integrin transmembrane helices are tilted, which hints that the extracellular chains may also not be orthogonal with respect to the membrane surface.
Although the crystal structure changed surprisingly little after binding to cilengitide, the current hypothesis is that integrin function involves changes in shape to move the ligand-binding site into a more accessible position, away from the cell surface, and this shape change also triggers intracellular signaling. There is a wide body of cell-biological and biochemical literature that supports this view. Perhaps the most convincing evidence involves the use of antibodies that only recognize integrins when they have bound to their ligands, or are activated. As the "footprint" that an antibody makes on its binding target is roughly a circle about 3 nm in diameter, the resolution of this technique is low. Nevertheless, these so-called LIBS antibodies unequivocally show that dramatic changes in integrin shape routinely occur. However, how the changes detected with antibodies look on the structure is still unknown.