Integrins are
receptors that mediate attachment between a
cell and the tissues surrounding it, which may be other cells or the
extracellular matrix (ECM). They also play a role in
cell signaling and thereby define cellular shape, mobility, and regulate the
cell cycle.
Typically, receptors inform a cell of the molecules in its environment and the cell evokes a response. Not only do integrins perform this outside-in signalling, but they also operate an inside-out mode. Thus, they transduce information from the ECM to the cell as well as reveal the status of the cell to the outside, allowing rapid and flexible responses to changes in the environment, for example to allow blood coagulation by platelets.
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 investigated, from sponges to mammals. Integrins have been extensively studied in humans.
Integrins work alongside other proteins such as cadherins, cell adhesion molecules and selectins to mediate cell-cell and cell-matrix interaction and communication. Integrins bind cell surface and ECM components such as fibronectin, vitronectin, collagen, and laminin.
Structure
Integrins are
obligate heterodimers containing two distinct chains, called the α (alpha) and β (beta) subunits. In
mammals, eighteen α and eight β subunits have been characterized, whereas the
Drosophila genome encodes only five α and two β subunits, and
Caenorhabditis nematodes possess genes for two α subunits and one β. The α and β subunits each contain two separate tails, both of which penetrate the plasma membrane and possess small cytoplasmic domains.
alpha
beta
In addition, variants of some of the subunits are formed by differential splicing; for example four variants of the beta-1 subunit exist. Through different combinations of these α and β 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 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 extracellular matrix (ECM).
The molecular mass of the integrin subunits can vary from 90 kDa to 160 kDa. Beta subunits have four cysteine-rich repeated sequences. Both α and β subunits bind several divalent cations. The role of the α subunit is unknown, but it may stabilize the folds of the protein. The β subunits are more interesting: they are directly involved in coordinating at least some of the ligands that integrins bind.
There are various ways of categorizing the integrins. For example, a subset of the α chains has an additional structural element (or "domain") inserted toward the N-terminal, the alpha-A domain (so called because it has a similar structure to the A-domains found in the protein von Willebrand factor; it is also termed the α-I domain). Integrins carrying this domain either bind to collagens (e.g. integrins α1 β1, and α2 β1), or act as cell-cell adhesion molecules (integrins of the β2 family). 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 (calcium) and 0.8 mM (magnesium). 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 ("RGD" in the one-letter amino acid code).
High resolution structure
Despite many years of effort, discovering the high-resolution structure of integrins proved to be challenging: membrane proteins are classically difficult to purify, and integrins are also large, complex and linked to many sugar trees ("highly
glycosylated"). 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.
Despite these wide-ranging efforts, the X-ray crystal structure obtained for the complete extracellular region of one integrin, αvβ3, was a surprise. It showed the molecule to be folded into an inverted V-shape that 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 (in the A-domains) are critical for RGD-ligand binding to integrins. The interaction with such sequences 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 the evidence for this sweet supposition is noticeable by its absence. The integrin structure has drawn attention to this problem, which may have implications for how membrane proteins work.
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 (Ligand-Induced-Binding-Sites) antibodies unequivocally show that dramatic changes in integrin shape routinely occur.
Activation
When released into the cell membrane, newly synthesized integrin dimers are found in a "bent" conformation that prevents them from interacting with their ligands. Therefore integrin dimers must be activated in order to bind to the
ECM. This is accomplished by a protein named
Talin, which binds to the β tail of the integrin dimer and changes its conformation. Moreover, talin proteins are able to dimerize and thus are thought to intervene in the clustering of integrin dimers which leads to the formation of a
focal adhesion. Recently, the
Kindlin-1 and
Kindlin-2 proteins have also been found to interact with integrin and activate it.
Function
Integrins have two main functions:
Attachment of the cell to the ECM
Signal transduction from the ECM to the cell
However, they are also involved in a wide range of other biological activities, including immune patrolling, cell migration, and binding to cells by certain viruses, such as adenovirus, echovirus, hantavirus, and foot and mouth disease viruses.
A prominent function of the integrins is seen in the molecule GPIIbIIIa, an integrin on the surface of blood platelets (thrombocytes) responsible for attachment to fibrin within a developing blood clot. This molecule dramatically increases its binding affinity for fibrin/fibrinogen through association of platelets with exposed collagen in the wound site. Upon association with the collagen, GPIIbIIIa undergoes a conformational change in its structure, allowing it to bind with fibrin and other blood components to form the clot matrix and stop blood loss.
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 made of. Among the
ligands of integrins are
fibronectin,
vitronectin,
collagen, and
laminin. The connection between the cell and the ECM may help 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.
Cell attachment to the ECM is a basic requirement to build a multicellular organism. Integrins are not simply hooks, but give the cell critical signals about the nature of its surroundings. Together with signals arising from receptors for soluble growth factors like VEGF, EGF, and many others, they enforce a cellular decision on what biological action to take, be it attachment, movement, death, or differentiation. Thus integrins lie at the heart of many cellular biological processes. The attachment of the cell takes place through formation of cell adhesion complexes, which consist of integrins and many cytoplasmic proteins such as talin, vinculin, paxillin, and alpha-actinin. These act by regulating kinases such as FAK (focal adhesion kinase) and Src kinase family members to phosphorylate substrates such as p130CAS thereby recruiting signaling adaptors such as CRK. These adhesion complexes attach to the actin cytoskeleton. The integrins thus serve to link two networks across the plasma membrane: the extracellular ECM and the intracellular actin filamentous system.
Focal adhesion consists of molecular complexes containing a ligand, integrin molecule, and associate plaque proteins. Binding is propelled by changes in free energy. As previously stated, these complexes connect the extracellular matrix to actin bundles. Cryo-electron tomography reveals that the adhesion is controlled through particles on the cell membrane with diameter of 25 +/- 5 nm and spaced at approximately 45 nm. Treatment with Rho-kinase inhibitor Y-27632 reduces the size of the particle, suggesting that it is extremely mechanosensitive.
One of the most important functions of surface integrins is their role in cell migration. Cells adhere to a substrate through their integrins. During movement, the cell makes new attachments to the substrate at its front and concurrently releases those at its rear. When released from the substrate, integrin molecules are taken back into the cell by endocytosis; they are transported through the cell to its front by the endocytic cycle where they are added back to the surface. In this way they are cycled for reuse, enabling the cell to make fresh attachments at its leading front.
Signal transduction
Integrins play an important role in cell signalling. Connection with ECM molecules can cause a signal to be relayed into the cell through protein
kinases that are indirectly and temporarily connected with the intracellular end of the integrin molecule, likely following shape changes directly stimulated by ECM binding.
The signals the cell receives through the integrin can have relation to:
cell growth,
cell division,
cell survival,
cellular differentiation, and
apoptosis (programmed cell death).
Vertebrate integrins
The following are some of the integrins found in vertebrates:
{| class="wikitable"
|
Name ||
Synonyms ||
Distribution ||
Ligands
|-
|
α1β1 || || Many ||
Collagens,
laminins
|-
|
α2β1 || || Many || Collagens, laminins || activated endothelial cells, melanoma, glioblastoma||
vitronectin,
References
External links
The Integrin Protein
Talin substrate for calpain – PMAP The Proteolysis Map animation.
Category:Transmembrane proteins
Category:Cell adhesion proteins
