Integrins are proteins that function mechanically, by attaching the cell cytoskeleton to the extracellular matrix (ECM), and biochemically, by sensing whether adhesion has occurred. The integrin family of proteins consists of alpha and beta subtypes, which form transmembrane heterodimers. Integrins function as adhesion receptors for extracellular ligands and transduce biochemical signals into the cell, through downstream effector proteins. Remarkably, they function bidirectionally, meaning they can transmit information both outside-in and inside-out (reviewed in ).
Each integrin heterodimer consists of an alpha (α) and a beta (β) subunit associated by noncovalent interactions forming an extracellular ligand-binding head, two multi-domain `legs’, two single-pass transmembrane helices and two short cytoplasmic tails. The α and β groups show no homology to each other,however, conserved regions are found among subtypes of both groups.
The α subunit leg consists of a thigh and 2 calf domains that support the ligand binding head formed by a β-propeller domain with 7 repeats forming the blades (shown as a cylinder in the figure below). Some of the propeller blade domains contain calcium binding EF-hand domains on the lower side; these allosterically affect ligand binding . An additional αI (interactive) domain containing ~200 residues is present in some vertebrate α chains  (nine human α subtypes) between the propeller repeats 2 and 3 . This contains a metal-ion dependent adhesion site (MIDAS) which is important for ligand binding.
The β subunit comprises of 4 cysteine-rich epidermal growth factor (EGF) repeats, a hybrid domain (split in sequence), an I-like domain (βI) and a plexin-sempahorin-integrin (PSI) domain. Similar to αI, the contains βI domain contains a MIDAS site for ligand binding and additional regulatory site “adjacent to MIDAS” or ADMIDAS, inhibited by Ca2+ and activated by Mn2+  for ligand binding.
Figure 3. Integrin Dimer Structure: Globular domain structures of α and β subunits in a stable dimer. Ligand binding happens at the interface of the αI (left panel) or β-propeller (right panel) and the βI domain.
Dimerization occurs via the β-propeller surface on the α chain and the hybrid domain in the β chain in the cytoplasm . The sequences at these interacting surfaces seem to control the specificity of chain selection. The dimers have been shown to be stabilized and remain inactive by hydrophobic interactions and electrostatic salt bridges at the outer- and inner-membrane proximal regions respectively .
The cytoplasmic tail of β-chain is known to bind to protein adaptors through NPxY/F motifs ; this activates the integrins by breaking the salt bridge between the dimer (reviewed in ). In general, the adaptor proteins promote linkage to actin , however intermediate filaments have also been implicated via vimentin .
Catalytic adaptors (e.g. focal adhesion kinase, integrin-linked kinase, Src) propagate signal transduction from adhesion sites. Interactions via α-tail are not well established due to sequence variability, however, α-tail is implicated in the cell-type specific integrin activation through binding proteins that modulate downstream signaling . Phosphorylation state of cytoplasmic tail residues modulate the competition between adaptors for binding and hence the subsequent cytoskeletal interactions of integrins and response (reviewed in ).
The role of protein structure in ligand affinity modulation, signaling and dynamics of surface distribution of integrins is reviewed in .
Humans have at least 18 α subtypes and 8 β subtypes which together generate 24 known binding pairs for the integrins heterodimer (reviewed in ). The α subunit of the integrin heterodimer especially the αI domain determines the ligand specificity for cell-ECM adhesion (reviewed in ). The characteristic of integrin subunits is their ability to bind diverse matrix molecules imparted by the heterogeneity of the monomers; this plasticity is instrumental for cell-ECM binding and subsequent mechanotransduction events.
The amino acid sequence: arginine-glycine-aspartic acid, or RGD motif, is commonly accepted as a general integrin-binding motif on target ligands, however, individual integrins can also recognize other protein-specific motifs (reviewed in ).
Common ECM components that are bound by integrins (with respective recognition sequence) are
Immunologically important integrin ligands are the inter-cellular adhesion molecules (ICAMs), immunoglobulin superfamily members present on inflamed endothelium and antigen-presenting cells.
Integrins are broadly grouped into four categories based on their ligand-specificity (reviewed in ):
The concentration of β subunits is generally high while that of α subunit is the factor limiting the amount of heterodimers that localize on the membrane . Liganded integrins diffuse along the membrane and are known to preferentially couple actin at the leading edge , cluster, get pulled backward as the cell spreads or moves forward and subsequently cycled during motility .
Integrins are found in specialized cell-cell adhesions and in most cell-matrix adhesions of static cells as well as motility structures such as filopodia, lamellipodia and podosomes. While the β1 subfamily is commonly found on a wide variety of cells, some integrins are limited to certain cell types or tissues  as listed in the table.
Clustering occurs by integrin diffusion, multivalent ligand binding leading to transmembrane homodimerization or inside-out signals. For example, in lymphocytes Rap1 GTPase and its effector RAPL (regulator for cell adhesion and polarization enriched in lymphoid tissues) have been shown to regulate patchy distribution of integrin LFA-1 . Nanolithographic study provides strong evidence that controlled spatial organization of liganded integrins in nanoclusters is essential for effective signaling and is independent of global density . Also, the minimum cluster area required for stable adhesion formation and force transduction is determined by the adhesive force, cytoskeletal tension and the force-transmitting structural linkage. Thus, it is not a constant and has a dynamic threshold .
Nanoclustering is driven by the interaction of N-terminal of vinculin with talin , which in turn is promoted by contractile forces. Some talin-integrin interactions are also essential to prevent re-association of the separated integrin tails and maintain the activated integrins in a clustering-competent form . Increasing the avidity (valency) of ligand binding and clustering also contributes to adhesion strength and outside-in signaling .
Whether clustering triggers outside-in signalling to facilitate integrin activation, or whether clustering occurs after integrin activation has yet to be fully ascertained (reviewed in ). In one study, however the early clustering of integrins was observed after integrin was activated by its binding to Arg-Gly-Asp (RGD) peptides. In this case, clustering occurred in several phases. In the earliest phase integrin binding to RGD led to the recruitment of additional integrin molecules, as well as the recruitment of talin, paxillin and FAK. This occurred via lateral diffusion and capture independently of mechanical force . In a later phase, following local actin polymerization and the recruitment of myosin, the aggregation of distant integrin clusters was observed. This resulted in larger adhesions, and was associated with the recruitment of vinculin and stimulation of a Src kinase-dependent lamellipodial extension. Here, the inward movement of integrin clusters was attributed to the generation of force following myosin-mediated retraction of actin filaments .
For Liganded integrin to function as a bidirectional signal transmitter,
Upon activation, integrins are capable of triggering a variety of signal transduction cascades. The combination of α and β subtypes, for example, will affect different in vivo functions. As demonstrated by knockout mouse studies, and highlighted in the table below, these include cell behaviour and tissue organization (reviewed in ).
Which signalling pathway is initiated by integrin activation is based on the biological context, as well as the ligands bound (matrix components/growth factors). Depending on the combination of these factors, a variety of short-term and long-term responses may result . Substrate stiffness has also been shown to affect the type of adhesion structure formed following integrin activation.
One study revealed the development of podosome-like adhesion structures in non-transformed fibroblasts grown on fluid, membrane based substrates. In this case, integrin was activated by membrane bound RGD (Arg-Gly-Asp) peptides. When grown on rigid surfaces, RGD-activated integrin would normally initiate the formation of focal adhesions . The adhesion structures formed on the softer substrates had a similar morphology and makeup to classic podosomes found in macrophages. However, despite also being protrusive, the physiological function of these podosome-like structures remained unknown. The formation of these podosome-like structures in the absence of forces was mediated by p85beta recruitment and local PIP3 enrichment at the adhesion sites; both of which are not observed in focal adhesion formation. The increased production of PIP3 then caused N-WASP activation and RhoA -GAP ARAP3 recruitment, which downregulates RhoA-GTP level in podosome-forming cells.