RESUMEN
A major current deficit in structural biology is the lack of high-resolution structures of eukaryotic membrane proteins, many of which are key drug targets for the treatment of disease. Numerous eukaryotic membrane proteins require specific lipids for their stability and activity, and efforts to crystallize and solve the structures of membrane proteins that do not address the issue of lipids frequently end in failure rather than success. To help address this problem, we have developed a sparse matrix crystallization screen consisting of 48 lipidic-sponge phase conditions. Sponge phases form liquid lipid bilayer environments which are suitable for conventional hanging- and sitting-drop crystallization experiments. Using the sponge phase screen, we obtained crystals of several different membrane proteins from bacterial and eukaryotic sources. We also demonstrate how the screen may be manipulated by incorporating specific lipids such as cholesterol; this modification led to crystals being recovered from a bacterial photosynthetic core complex.
Asunto(s)
Cristalización/métodos , Membrana Dobles de Lípidos/química , Proteínas de la Membrana/ultraestructura , Proteínas Bacterianas/química , Detergentes/química , Proteínas de la Membrana/química , Dispersión del Ángulo Pequeño , Difracción de Rayos XRESUMEN
Membrane proteins are embedded in a lipid bilayer and maintain strong interactions with lipid molecules. Tightly bound lipids are responsible for vertical positioning and integration of proteins in the membrane and for assembly of multisubunit complexes and occasionally act as substrates. In this work we present the lipidic sponge phase crystal structure of the reaction center from Blastochloris viridis to 1.86 A, which reveals lipid molecules interacting with the protein surface. A diacylglycerol molecule is bound, through a thioether bond, to the N-terminus of the tetraheme cytochrome c subunit. From the electron density recovered at the Q(B) site and the observed change in recombination kinetics in lipidic sponge phase-grown crystals, the mobile ubiquinone appears to be displaced by a monoolein molecule. A 36 A long electron density feature is observed at the interface of transmembrane helices belonging to the H- and M-subunits, probably arising from an unidentified lipid.
Asunto(s)
Lípidos/química , Proteínas del Complejo del Centro de Reacción Fotosintética/química , Proteínas Bacterianas/análisis , Proteínas Bacterianas/química , Proteínas Bacterianas/metabolismo , Sitios de Unión , Cristalografía , Diglicéridos/química , Cinética , Membrana Dobles de Lípidos , Modelos Moleculares , Proteínas del Complejo del Centro de Reacción Fotosintética/metabolismo , Unión Proteica , Conformación Proteica , Rhizobium/química , Ubiquinona/metabolismo , Difracción de Rayos XRESUMEN
Ultrafast time-resolved wide angle x-ray scattering from chemical reactions in solution has recently emerged as a powerful technique for determining the structural dynamics of transient photochemical species. Here we examine the structural evolution of photoexcited CH(2)I(2) in the nonpolar solvent cyclohexane and draw comparisons with a similar study in the polar solvent methanol. As with earlier spectroscopic studies, our data confirm a common initial reaction pathway in both solvents. After photoexcitation, CH(2)I(2) dissociates to form CH(2)I* + I*. Iodine radicals remaining within the solvent cage recombine with a nascent CH(2)I* radical to form the transient isomer CH(2)I-I, whereas those which escape the solvent cage ultimately combine to form I(2) in cyclohexane. Moreover, the transient isomer has a lifetime approximately 30 times longer in the nonpolar solvent. Of greater chemical significance is the property of time-resolved wide angle x-ray diffraction to accurately determine the structure of the of CH(2)I-I reaction intermediate. Thus we observe that the transient iodine-iodine bond is 0.07 A+/-0.04 A shorter in cyclohexane than in methanol. A longer iodine-iodine bond length for the intermediate arises in methanol due to favorable H-bond interaction with the polar solvent. These findings establish that time-resolved x-ray diffraction has sufficient sensitivity to enable solvent dependent structural perturbations of transient chemical species to be accurately resolved.
RESUMEN
Bicontinuous lipidic cubic phases can be used as a host for growing crystals of membrane proteins. Since the cubic phase is stiff, handling is difficult and time-consuming. Moreover, the conventional cubic phase may interfere with the hydrophilic domains of membrane proteins due to the limited size of the aqueous pores. Here, we introduce a new crystallization method that makes use of a liquid analogue of the cubic phase, the sponge phase. This phase facilitates a considerable increase in the allowed size of aqueous domains of membrane proteins, and is easily generalised to a conventional vapour diffusion crystallisation experiment, including the use of nanoliter drop crystallization robots. The appearance of the sponge phase was confirmed by visual inspection, small-angle X-ray scattering and NMR spectroscopy. Crystals of the reaction centre from Rhodobacter sphaeroides were obtained by a conventional hanging-drop experiment, were harvested directly without the addition of lipase or cryoprotectant, and the structure was refined to 2.2 Angstroms resolution. In contrast to our earlier lipidic cubic phase reaction centre structure, the mobile ubiquinone could be built and refined. The practical advantages of the sponge phase make it a potent tool for crystallization of membrane proteins.
Asunto(s)
Lípidos/química , Proteínas de la Membrana/química , Proteínas del Complejo del Centro de Reacción Fotosintética/química , Sitios de Unión , Cristalización , Estructura Molecular , Conformación Proteica , Rhodobacter sphaeroides/química , Difracción de Rayos XRESUMEN
Photosynthetic reaction centers convert the energy content of light into a transmembrane potential difference and so provide the major pathway for energy input into the biosphere. We applied time-resolved Laue diffraction to study light-induced conformational changes in the photosynthetic reaction center complex of Blastochloris viridis. The side chain of TyrL162, which lies adjacent to the special pair of bacteriochlorophyll molecules that are photooxidized in the primary light conversion event of photosynthesis, was observed to move 1.3 angstroms closer to the special pair after photoactivation. Free energy calculations suggest that this movement results from the deprotonation of this conserved tyrosine residue and provides a mechanism for stabilizing the primary charge separation reactions of photosynthesis.
Asunto(s)
Proteínas Bacterianas/química , Hyphomicrobiaceae/química , Luz , Proteínas del Complejo del Centro de Reacción Fotosintética/química , Proteínas Bacterianas/metabolismo , Bacterioclorofilas/química , Bacterioclorofilas/metabolismo , Cristalografía por Rayos X , Citocromos c/química , Citocromos c/metabolismo , Transporte de Electrón , Enlace de Hidrógeno , Concentración de Iones de Hidrógeno , Hyphomicrobiaceae/metabolismo , Modelos Moleculares , Simulación de Dinámica Molecular , Oxidación-Reducción , Proteínas del Complejo del Centro de Reacción Fotosintética/metabolismo , Conformación Proteica , Protones , Quinonas/química , Quinonas/metabolismo , TermodinámicaRESUMEN
Membrane protein structural biology is enjoying a steady acceleration in the rate of success. Nevertheless, numerous membrane protein targets are resistant to the traditional approach of directly crystallizing detergent solubilized and purified protein and the 'niche market' of lipidic phase crystallization is emerging as a powerful complement. These approaches, including lipidic cubic phase, lipidic sponge phase, and bicelle crystallization methods, all immerse purified membrane protein within a lipid rich matrix before crystallization. This environment is hypothesized to contribute to the protein's long-term structural stability and thereby favor crystallization. Spectacular recent successes include the high-resolution structures of the beta(2)-adrenergic G-protein-coupled receptor, the A(2A) adenosine G-protein-coupled receptor, and the mitochondrial voltage dependent anion channel. In combination with technical innovations aiming to popularize these methods, lipidic phase crystallization approaches can be expected to deliver an increasing scientific impact as the field develops.
Asunto(s)
Lípidos/química , Proteínas de la Membrana/química , Cristalización , HumanosRESUMEN
Bacteriorhodopsin and proteorhodopsin are simple heptahelical proton pumps containing a retinal chromophore covalently bound to helix G via a protonated Schiff base. Following the absorption of a photon, all-trans retinal is isomerized to a 13-cis conformation, initiating a sequence of conformational changes driving vectorial proton transport. In this study we apply time-resolved wide-angle X-ray scattering to visualize in real time the helical motions associated with proton pumping by bacteriorhodopsin and proteorhodopsin. Our results establish that three conformational states are required to describe their photocycles. Significant motions of the cytoplasmic half of helix F and the extracellular half of helix C are observed prior to the primary proton transfer event, which increase in amplitude following proton transfer. These results both simplify the structural description to emerge from intermediate trapping studies of bacteriorhodopsin and reveal shared dynamical principles for proton pumping.
Asunto(s)
Bacteriorodopsinas/química , Luz , Rodopsina/química , Conformación Proteica , Rodopsinas Microbianas , Dispersión de RadiaciónRESUMEN
Liquid phase time-resolved x-ray diffraction with 100 ps resolution has recently emerged as a powerful technique for probing the structural dynamics of transient photochemical species in solution. It is intrinsic to the method, however, that a structural signal is observed not only from the photochemical of interest but also from the embedding solvent matrix. To experimentally characterize the x-ray diffraction signal deriving from the solvent alone we performed time-resolved diffraction studies of a pure liquid sample over a time domain from -250 ps to 2.5 micros. Multiphoton excitation was used to rapidly heat liquid CH(2)Cl(2) using UV pulses of 100 fs duration. A significant x-ray diffraction signal is visible prior to the onset of thermal expansion, which characterizes a highly compressed superheated liquid. Liquid CH(2)Cl(2) then expands as a shock wave propagates through the sample and the temporal dependence of this phenomenon is in good agreement with theory. An unexpectedly slow initial release of energy into the liquid as heat is observed from multiphoton excited CH(2)Cl(2), revealing the presence of a metastable state of multiphoton excited CH(2)Cl(2).