Integrins are αβ heterodimeric cell surface receptors that mediate transmembrane signaling by binding extracellular and cytoplasmic ligands. The ectodomain of integrin αVβ3 crystallizes in a bent, genuflexed conformation considered to be inactive (unable to bind physiological ligands in solution) unless it is fully extended by activating stimuli. We generated a stable, soluble complex of the Mn²⁺-bound αVβ3 ectodomain with a fragment of fibronectin (FN) containing type III domains 7 to 10 and the EDB domain (FN7-EDB-10). Transmission electron microscopy and single particle image analysis were used to determine the three-dimensional structure of this complex. Most αVβ3 particles, whether unliganded or FN-bound, displayed compact, triangular shapes. A difference map comparing ligand-free and FN-bound αVβ3 revealed density that could accommodate the RGD-containing FN10 in proximity to the ligand-binding site of β3, with FN9 just adjacent to the synergy site binding region of αV. We conclude that the ectodomain of αVβ3 manifests a bent conformation that is capable of stably binding a physiological ligand in solution.

We have used electron cryo-microscopy and image analysis to examine the native structure of immature, protease-deficient (PR ⁻ ) and mature, wild-type (WT) Moloney murine leukemia virus (MuLV). Maturational cleavage of the Gag polyprotein by the viral protease is associated with striking morphological changes. The PR ⁻ MuLV particles exhibit a rounded central core, which has a characteristic track-like shell on its surface, whereas the WT MuLV cores display a polygonal surface with loss of the track-like feature. The pleomorphic shape and inability to refine unique orientation angles suggest that neither the PR ⁻ nor the WT MuLV adheres to strict icosahedral symmetry. Nevertheless, the PR ⁻ MuLV particles do exhibit paracrystalline order with a spacing between Gag molecules of ≈45 Å and a length of ≈200 Å. Because of the pleomorphic shape and paracrystalline packing of the Gag–RNA complexes, we raise the possibility that assembly of MuLV is driven by protein–RNA, as well as protein–protein, interactions. The maturation process involves a dramatic reorganization of the packing arrangements within the ribonucleoprotein core with disordering and loosening of the individual protein components.

Integrins are a large family of heterodimeric transmembrane signaling proteins that affect diverse biological processes such as development, angiogenesis, wound healing, neoplastic transformation, and thrombosis. We report here the three-dimensional structure at 20-Å resolution of the unliganded, low-affinity state of the human platelet integrin α _IIb β ₃ derived by electron cryomicroscopy and single particle image reconstruction. The large ectodomain and small cytoplasmic domains are connected by a rod of density that we interpret as two parallel transmembrane α-helices. The docking of the x-ray structure of the α _V β ₃ ectodomain into the electron cryomicroscopy map of α _IIb β ₃ requires hinge movements at linker regions between domains in the crystal structure. Comparison of the putative high- and low-affinity conformations reveals dramatic conformational changes associated with integrin activation.