Integrins are αβ heterodimeric receptors that mediate divalent cation-dependent cell-cell and cellmatrix adhesion through tightly regulated interactions with ligands. We have solved the crystal structure of the extracellular portion of integrin αVβ3 at 3.1 Å resolution. Its 12 domains assemble into an ovoid "head" and two "tails." In the crystal, αVβ3 is severely bent at a defined region in its tails, reflecting an unusual flexibility that may be linked to integrin regulation. The main inter-subunit interface lies within the head, between a seven-bladed β-propeller from αV and an A domain from β3, and bears a striking resemblance to the Gα/Gβ interface in G proteins. A metal ion-dependent adhesion site (MIDAS) in the βA domain is positioned to participate in a ligand-binding interface formed of loops from the propeller and βA domains. MIDAS lies adjacent to a calcium-binding site with a potential regulatory function.Integrins are large heterodimeric cell surface receptors found in many animal species ranging from sponges to mammals [reviewed in (1)]. These receptors are involved in fundamental cellular processes such as attachment, migration, proliferation, differentiation, and survival. Integrins also contribute to the initiation and/or progression of many common diseases including neoplasia, tumor metastasis, immune dysfunction, ischemia-reperfusion injury, viral infections, osteoporosis, and coagulopathies [reviewed in (2,3)]. An integrin is ~280 Å long and consists of one α (150 to 180 kD) and one β (~90 kD) subunit, both of which are type I membrane proteins. Eighteen α and eight β mammalian subunits are known, which assemble noncovalently into 24 different heterodimers. Contacts between the α and β subunits primarily involve their NH 2 -terminal halves [reviewed in (1)], which together form a globular head; the remaining portions form two rod-shaped tails (4-7) that span the plasma membrane.Like other receptors, integrins transmit signals to the cell interior (so-called "outside-in" signaling), which regulate organization of the cytoskeleton, activate kinase signaling cascades, and modulate the cell cycle and gene expression [reviewed in (8)]. Unlike other receptors, however, ligand binding with integrins is not generally constitutive but is regulated to reflect the activation state of the cell. This "inside-out" regulation of integrin affinity protects the host

The R V β 3 integrin is implicated in human tumor metastasis and in angiogenesis. The design of lowmolecular-mass R V β 3 antagonists by "spatial screening" led to the highly active peptides c(RGDFV) and c(RGDFV).Here the influence of the amino acids in positions 4 and 5 flanking the RGD-sequence on the inhibition of vitronectin and fibrinogen binding to the isolated R V β 3 and R IIb β 3 receptors was investigated. The influence of the side chain and the backbone conformation on activity and selectivity was studied. The compounds were divided into conformational classes. For each class at least one representative peptide was subjected to detailed structure determination in solution. The peptides of classes 1, 2, and 3 show a βII′/γ-turn arrangement with the D-amino acid in the i + 1 position of the βII′-turn. By contrast, the peptides of class 4 reveal a modified βII′/γ-turn pattern with glycine in the i + 1 position of the βII′-turn and the D-amino acid in the i + 1 position of the γ-turn. Class 1 is divided into two subclasses: besides the βII′/γ-turn arrangement a γ/γ-turn motif is found for two members of this class. Structure-activity relationship (SAR) investigations show that the amino acid in position 4 and the proton of the amide bond between residues 3 and 4 are essential for high biological activities toward R V β 3 . By contrast, the amino acid in position 5 has no influence on the activity. A bent conformation of the RGD-sequence, as observed for the peptides of classes 1 and 2, fits the R V β 3 better than the R IIb β 3 receptor and so increases the selectivity of these peptides.

The physiological inertness of synthetic implant materials often results in insufficient implant integration and limited acceptance of implants in tissues. After implantation the implant surface is often separated from the surrounding healthy and regenerating tissue, for example by a fibrous capsule. To avoid this host-versus-graft reaction, a strong mechanical contact between tissue and implant must be ensured. An enhanced contact between graft and the surrounding tissue can be provided by coating the implant with cell-adhesive molecules. The highly active and alpha(v)beta(3)- and alpha(v)beta(5)-integrin-selective peptide c(-RGDfK-) (f=D-phenylalanine) was functionalized with various linker molecules containing an acrylamide end group by using the lysine side chain of c(-RGDfK-). The acrylamide group can be used to bind the peptide covalently to poly(methyl methacrylate) (PMMA) surfaces. The coated surfaces effectively bind to murine osteoblasts as well as human osteoblasts in vitro when a minimum distance of 3.5 nm between surface and the constrained RGD sequence is provided. In contrast to osteoblasts in cell suspension, surface-bound osteoblasts show no apoptosis but proliferate by a factor of 10 over a 22 d period. Coating of inert implant surfaces with highly active and alpha(v)-selective peptides affords a marked improvement in osteoblast binding over current technologies. In vivo studies show that peptide-coated PMMA pellets implanted into the patella groove of rabbits are integrated into the regenerating bone tissue faster and more strongly than uncoated pellets.

Integrin interactions with extracellular matrix proteins are mediated by brief oligopeptide recognition sequences, and synthetic peptides containing such sequences can inhibit integrin binding to the matrix. The RGD peptide motif is recognized by many integrins including ␣v␤6, a specific receptor for fibronectin thought to support epithelial cell proliferation during wound healing and carcinoma progression. We report here the discovery of an unexpected non-RGD recognition motif for integrin ␣v␤6. We compared the recognition profiles of recombinant ␣v␤6 and ␣v␤3 integrins by using phage display screening employing 7-mer and 12-mer peptide libraries. As predicted, phages binding strongly to ␣v␤3 contained ubiquitous RGD sequences. However, on ␣v␤6, in addition to RGD-containing phages, one-quarter of the population from the 12-mer library contained the distinctive consensus motif DLXXL. A synthetic DLXXL peptide, RTDLDSLRTYTL, selected from the phage sequences (clone-1) was a selective inhibitor of RGD-dependent ligand binding to ␣v␤6 in isolated receptor assays (IC 50 ‫؍‬ 20 nM), and in cell adhesion assays (IC 50 ‫؍‬ 50 M). DLXXL peptides were highly specific inhibitors of ␣v␤6-fibronectin interaction as synthetic scrambled or reversed DLXXL peptides were inactive. NH 2 -and COOH-terminal modifications of the flanking amino acids suggested that the preceding two and a single trailing amino acid were also involved in interaction with ␣v␤6. The DLXXL sequence is present in several matrix components and in the ␤ chain of many integrins. Although there is as yet no precise biological role known for DLXXL, it is clearly a specific inhibitory sequence for integrin ␣v␤6 which has been unrecognized previously.Integrins are a family of heterodimeric class I transmembrane receptors involved in numerous cell-matrix and cell-cell adhesion phenomena (1). They can be grouped roughly into three classes: the ␤1 series, which are ubiquitous receptors for extracellular matrix (2); the ␤2 series, which are activatable on leukocytes and are triggered during the inflammatory response (3); and the ␣v series, which bind and mediate the cell response to provisional extracellular matrices found during wound repair and other pathological processes (4).The integrins ␣5␤1 (5), ␣IIb␤3 (6), ␣8␤1 (7), ␣v␤1 (8), ␣v␤3(9), and ␣v␤6 (10) all bind the Arg-Gly-Asp-(RGD) peptide sequence in fibronectin, where it is presented in a constrained loop (11). Soluble RGD-containing peptides can inhibit the interaction of each of these integrins with fibronectin. However, to analyze the function of individual fibronectin receptors in a particular cellular environment it is useful to have more specific inhibitors, and a variety of inhibitory antibodies, modified peptides, and non-peptidic substances has been developed. However, as yet no inhibitor has been discovered specific for ␣v␤6. ␣v␤6 is a rare integrin, induced during repair processes in epithelia (10, 12). Its only known specificities are for fibronectin (10), where it can be the dominant receptor me...