The structural basis for the divalent cation-dependent binding of heterodimeric alphabeta integrins to their ligands, which contain the prototypical Arg-Gly-Asp sequence, is unknown. Interaction with ligands triggers tertiary and quaternary structural rearrangements in integrins that are needed for cell signaling. Here we report the crystal structure of the extracellular segment of integrin alphaVbeta3 in complex with a cyclic peptide presenting the Arg-Gly-Asp sequence. The ligand binds at the major interface between the alphaV and beta3 subunits and makes extensive contacts with both. Both tertiary and quaternary changes are observed in the presence of ligand. The tertiary rearrangements take place in betaA, the ligand-binding domain of beta3; in the complex, betaA acquires two cations, one of which contacts the ligand Asp directly and the other stabilizes the ligand-binding surface. Ligand binding induces small changes in the orientation of alphaV relative to beta3.

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

We propose that the contact between five-coordinated and six-coordinated pentamers may help to generate a six-pentamer nucleus, with which further pentamers can assemble to generate the complete particle. Calcium ions probably stabilize the structure of the assembled particle, rather than direct its assembly.