Structure Integrin

variants of subunits formed differential rna splicing; example, 4 variants of beta-1 subunit exist. through different combinations of α , β subunits, around 24 unique integrins generated.

integrin subunits span cell membrane , have short cytoplasmic domains of 40–70 amino acids. exception beta-4 subunit, has cytoplasmic domain of 1,088 amino acids, 1 of largest of membrane protein. outside cell membrane, α , β chains lie close along length of 23 nm; final 5 nm n-termini of each chain forms ligand-binding region ecm. have been compared lobster claws, although don t pinch ligand, chemically interact @ insides of tips of pinchers .

the molecular mass of integrin subunits can vary 90 kda 160 kda. beta subunits have 4 cysteine-rich repeated sequences. both α , β subunits bind several divalent cations. role of divalent cations in α subunit unknown, may stabilize folds of protein. cations in β subunits more interesting: directly involved in coordinating @ least of ligands integrins bind.

integrins can categorized in multiple ways. example, α chains have additional structural element (or domain ) inserted toward n-terminal, alpha-a domain (so called because has similar structure a-domains found in protein von willebrand factor; termed α-i domain). integrins carrying domain either bind collagens (e.g. integrins α1 β1, , α2 β1), or act cell-cell adhesion molecules (integrins of β2 family). α-i domain binding site ligands of such integrins. integrins don t carry inserted domain have a-domain in ligand binding site, a-domain found on β subunit.

in both cases, a-domains carry 3 divalent cation binding sites. 1 permanently occupied in physiological concentrations of divalent cations, , carries either calcium or magnesium ion, principal divalent cations in blood @ median concentrations of 1.4 mm (calcium) , 0.8 mm (magnesium). other 2 sites become occupied cations when ligands bind—at least ligands involving acidic amino acid in interaction sites. acidic amino acid features in integrin-interaction site of many ecm proteins, example part of amino acid sequence arginine-glycine-aspartic acid ( rgd in one-letter amino acid code).

structure

despite many years of effort, discovering high-resolution structure of integrins proved challenging, membrane proteins classically difficult purify, , integrins large, complex , linked many sugar trees ( highly glycosylated ). low-resolution images of detergent extracts of intact integrin gpiibiiia, obtained using electron microscopy, , data indirect techniques investigate solution properties of integrins using ultracentrifugation , light scattering, combined fragmentary high-resolution crystallographic or nmr data single or paired domains of single integrin chains, , molecular models postulated rest of chains.

despite these wide-ranging efforts, x-ray crystal structure obtained complete extracellular region of 1 integrin, αvβ3, determined in 2001 laboratory of dr. m. amin arnaout, md, @ massachusetts general hospital , harvard medical school, surprise. showed molecule folded inverted v-shape potentially brings ligand-binding sites close cell membrane. perhaps more importantly, crystal structure obtained same integrin bound small ligand containing rgd-sequence, drug cilengitide. detailed above, revealed why divalent cations (in a-domains) critical rgd-ligand binding integrins. interaction of such sequences integrins believed primary switch ecm exerts effects on cell behaviour.

the structure poses many questions, regarding ligand binding , signal transduction. ligand binding site directed towards c-terminal of integrin, region molecule emerges cell membrane. if emerges orthogonally membrane, ligand binding site apparently obstructed, integrin ligands typically massive , cross-linked components of ecm. in fact, little known angle membrane proteins subtend plane of membrane; problem difficult address available technologies. default assumption emerge rather little lollipops, evidence sweet supposition noticeable absence. integrin structure has drawn attention problem, may have general implications how membrane proteins work. appears integrin transmembrane helices tilted (see activation below), hints extracellular chains may not orthogonal respect membrane surface.

although crystal structure changed surprisingly little after binding cilengitide, current hypothesis integrin function involves changes in shape move ligand-binding site more accessible position, away cell surface, , shape change triggers intracellular signaling. there wide body of cell-biological , biochemical literature supports view. perhaps convincing evidence involves use of antibodies recognize integrins when have bound ligands, or activated. footprint antibody makes on binding target circle 3 nm in diameter, resolution of technique low. nevertheless, these so-called libs (ligand-induced-binding-sites) antibodies unequivocally show dramatic changes in integrin shape routinely occur. however, how changes detected antibodies on structure still unknown.

activation

when released cell membrane, newly synthesized integrin dimers speculated found in same bent conformation revealed structural studies described above. 1 school of thought claims bent form prevents them interacting ligands, although bent forms can predominate in high-resolution em structures of integrin bound ecm ligand. therefore, @ least in biochemical experiments, integrin dimers must apparently not unbent in order prime them , allow binding ecm. in cells, priming accomplished protein talin, binds β tail of integrin dimer , changes conformation. α , β integrin chains both class-i transmembrane proteins: pass plasma membrane single transmembrane alpha-helices. unfortunately, helices long, , recent studies suggest that, integrin gpiibiiia, tilted respect both 1 , plane of membrane. talin binding alters angle of tilt of β3 chain transmembrane helix in model systems , may reflect stage in process of inside-out signalling primes integrins. moreover, talin proteins able dimerize , thought intervene in clustering of integrin dimers leads formation of focal adhesion. recently, kindlin-1 , kindlin-2 proteins have been found interact integrin , activate it.