Increased synthesis of Apolipoprotein A-I (ApoA-I) and HDL is believed to provide a new approach to treating atherosclerosis through the stimulation of reverse cholesterol transport. RVX-208 increases the production of ApoA-I in hepatocytes in vitro, and in vivo in monkeys and humans, which results in increased HDL-C, but the molecular target was not previously reported. Using binding assays and X-ray crystallography, we now show that RVX-208 selectively binds to bromodomains of the BET (Bromodomain and Extra Terminal) family, competing for a site bound by the endogenous ligand, acetylated lysine, and that this accounts for its pharmacological activity. siRNA experiments further suggest that induction of ApoA-I mRNA is mediated by BET family member BRD4. These data indicate that RVX-208 increases ApoA-I production through an epigenetic mechanism and suggests that BET inhibition may be a promising new approach to the treatment of atherosclerosis.

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To demonstrate that crystallographic methods can be applied to index and interpret diffraction patterns from well-ordered quasicrystals that display non-crystallographic 5-fold symmetry, we have characterized the properties of a series of periodic two-dimensional lattices built from pentagons, called Fibonacci pentilings, which resemble aperiodic Penrose tilings. The computed diffraction patterns from periodic pentilings with moderate size unit cells show decagonal symmetry and are virtually indistinguishable from that of the infinite aperiodic pentiling. We identify the vertices and centers of the pentagons forming the pentiling with the positions of transition metal atoms projected on the plane perpendicular to the decagonal axis of quasicrystals whose structure is related to crystalline phase alloys. The characteristic length scale of the pentiling lattices, evident from the Patterson (autocorrelation) function, is ϳ 2 times the pentagon edge length, where is the golden ratio. Within this distance there are a finite number of local atomic motifs whose structure can be crystallographically refined against the experimentally measured diffraction data.Five-fold symmetry has been associated with magic and mysticism since ancient times. Kepler, in his Mysterium Cosmigraphicum, published 400 years ago, described how he ingeniously found the symmetry of the five Platonic polyhedra in the structure of the solar system. Book II of his Harmonices Mundi (1), on the congruence of harmonic figures, is a pinnacle in the history of geometry, combining imaginative mathematical mysticism with profound insights into the symmetry of polyhedra and polygonal tilings of the plane. Kepler's exploration of orderly arrangements of plane pentagons has been viewed (2) as an anticipation of Penrose's aperiodic tilings (3), which have served as models for the geometry of quasicrystal structures.Quasicrystallography has developed into an elaborate discipline since 1984 when Shechtman et al. (4) first reported crystal-like diffraction patterns with forbidden icosahedral symmetry from aluminum-manganese alloys, and Levine and Steinhardt (5) coined the name quasicrystals for the class of quasiperiodic structures. Exposition of the results of many experimental studies on these novel alloys, and of the efforts of physicists to model their properties are presented in the book Quasicrystals: A Primer, by Janot (6); the mathematical concepts involved in the construction of aperiodic lattices are described in Quasicrystals and Geometry, by Senechal (2).In their endeavors, quasicrystallographers have used a variety of mathematically sophisticated but physically unrealistic models to analyze aperiodic lattices with icosahedral or decagonal symmetry. Quasicrystal structures have been represented as projections into two-or three-dimensional space from periodic models in five-or six-dimensional space. For example, such procedures have been applied by Steurer and his colleagues to calculate five-dimensional Fourier maps from three-dimensional x-...

Crystallographic analysis at 2-A resolution of the selective binding of dihalogenated methane, ethane, and ethylene compounds in the cavity on the cubic insulin dimer axis provides a model for anesthetic-protein interactions. At pH 6-11, 1,2-dichloroethane binds isomorphically in the righthanded cis-conformation, displacing four water molecules from the invariant cavity. Lowering the pH to 5.7 in 1 M Na2SO4 without dihaloalkanes induces a cooperative structural transition in which the dyad cavities between B13 glutamate pairs are constricted, and SO2-ions are bound by rearranged triads of Bi NH' groups. In the presence of dichloroethane at pH 5-5.5, the equilibrium is shifted to a mixture of the ligand-bound and ligand-excluding cavity structures, with halfoccupancy of the sulfate sites, exemplifying how a volatile anesthetic can act as an allosteric effector. Measurements at pH 9 of the occupancies of structurally similar dihaloalkanes demonstrate a high degree of binding selectivity. Induced polarization of the ligand and bound water by the charge distribution in the binding cavity apparently provides the selective electrostatic interactions that discriminate between dihaloalkanes of comparable size and polarity.Insulin (51 amino acids, 5.8 kDa) crystallizes from zinc-free alkaline solutions as a symmetric dimer arranged in orthogonal cross-connected rows in the cubic space group I213 (a = 78.9 A) (1, 2). The crystal lattice, which contains 65% solvent volume, is stable from pH 7-10 in 0.1 M monovalent cation salt solutions (3) and from pH 5-11 in 1 M Na2SO4. The pH-dependent local conformational changes and coupled binding of monovalent cations, which have been crystallographically characterized in pH range 7-11 (4), demonstrate the usefulness of this system for studying the effects of altered electrostatic interactions on the protein and solvent structure. Following our observation (4) that cubic insulin crystals in alkaline 0.1-1 M salt solutions bind 1,2-dichloroethane (ClH2C-CH2CI) with unit occupancy at a site on the dimer axis ( Fig. 1), we have characterized this binding at pH 5-11 and compared the binding of similar dihaloalkanes at pH 9. § Such binding exemplifies the type of hydrophobic ligand-protein interactions that may underlie the action of inhalational general anesthetics (6, 7). MATERIALS AND METHODSCrystal Preparation and X-Ray Data Collection. Cubic Zn-free crystals of bovine insulin, grown from phosphate buffer at pH 9 (1, 4), were equilibrated by dialysis against sodium salt solutions that varied in pH from 5 to 11 and subsequently equilibrated with liquid dihaloalkanes. Dihaloalkane binding was discovered by exposing the crystals to 1,2-dichloroethane as solvent for Formvar, which was applied in the capillary to cast an encasing film that prevents crystal slippage (8). Binding at the dyad site, as detected by x-ray diffraction, was complete after 30-sec direct exposure to liquid dichloroethane or after overnight soaking in a saturated aqueous solution. The x-ray data were collect...