The three-dimensional structure of the complex formed by two plasma proteins, transthyretin and retinol-binding protein, was determined from x-ray diffraction data to a nominal resolution of 3.1 angstroms. One tetramer of transthyretin was bound to two molecules of retinol-binding protein. The two retinol-binding protein molecules established molecular interactions with the same transthyretin dimer, and each also made contacts with one of the other two monomers. Thus, the other two potential binding sites in a transthyretin tetramer were blocked. The amino acid residues of the retinol-binding protein that were involved in the contacts were close to the retinol-binding site.

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The crystalline State and isometric Operations 1 Symmetry elements 3 Axes of rotational symmetry 3 Axes of rototranslation or screw axes 5 Axes of inversion 5 Axes of rotorefiection 5 Refiection planes with translational component (glide planes) 6 Lattices 6 The rational properties of lattices 7 Crystallographic directions 7 Crystallographic planes 8 Symmetry restrictions due to the lattice periodicity and vice versa 9 Point groups and symmetry classes 11 Point groups in one and two dimensions 16 The Laue classes 17 The seven crystal Systems 17 The Bravais lattices 18 Plane lattices 18 Space lattices 19 The space groups 22 The plane and line groups 30 On the matrix representation of symmetry Operators 32 Appendices: LA The isometric transformations 35 l.B Some combinations of movements 37 l.C Wigner-Seitz cells 41 l.D The space-group rotation matrices l.E Symmetry groups l.F Symmetry generalization References Crystallographic Computing Carmelo Giacovazzo

Odorant binding protein (OBP) is the major odorant binding component of mammalian nasal mucosa. The two structures of bovine OBP reported in this paper (one crystallized as purified and one soaked in the presence of a selenium-containing odorant) show that: (i) the OBP dimer is composed of two compact domains related by an approximate two-fold axis of symmetry; (ii) between residues 122 and 123 the polypeptide chains cross from one domain to the other such that each domain is formed by residues from both monomers; (iii) purified OBP already contains two bound odorant molecules (one per monomer)-odorant binding occurs by replacement of these molecules with the added odorant; and (iv) the structure of the odorant binding site can explain OBP's extraordinarily broad odorant specificity.

play an essential role in the survival of the fetus.The family of the chicken RfBPs includes three wellThe crystal structure of chicken egg white riboflavinknown proteins that are the product of the same gene but binding protein, determined to a resolution of 2.5 Å, have undergone different post-translational modifications is the prototype of a family that includes other ribo- (White and Merrill, 1988). The proteins can be purified flavin-and folate-binding proteins. An unusual characfrom egg white (Rhodes et al., 1959), egg yolk (Ostrowski teristic of these molecules is their high degree of cross et al., 1962) and from the plasma of laying hens (Miller linking by disulfide bridges and, in the case of the et al., 1982a). Egg white RfBP is synthesized by the avian proteins, the presence of stretches of highly oviduct cells (Mandeles and Ducay, 1962), plasma RfBP phosphorylated polypeptide chain. The structure of is produced in the liver under estrogen control and yolk chicken egg white riboflavin-binding protein is characRfBP is the result of a proteolytic cleavage of the last terized by a ligand-binding domain and a phosphoryl-11-13 amino acids of plasma RfBP when the molecule ated motif. The ligand-binding domain has a fold that crosses the oocyte membrane (Norioka et al., 1985). The appears to be strongly conditioned by the presence three proteins have a single binding site for riboflavin of the disulfide bridges. The phosphorylated motif, with a dissociation constant of 1.3 nM in the pH range essential for vitamin uptake, is made up of two helices 6-9; removal of the vitamin from the holoprotein is found before and after the flexible phosphorylated accomplished at lower pH where the affinity for the region. The riboflavin molecule binds to the protein specific ligand is substantially reduced (Müller and van with the isoalloxazine ring stacked in between the rings Berkel, 1991