Lipidic Sponge Phase Crystal Structure of a Photosynthetic Reaction Center Reveals Lipids on the Protein Surface

Abstract

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.

The Significance of G Protein-Coupled Receptor Crystallography for Drug Discovery

“…This approach met with success in deter-922 mining a 2.2-Å structure for the photosynthetic reaction center of Rhodobacter sphaeroides, offering improvements in resolution and order over a previous structure determined from LCP grown crystals. However, this approach has not yet been employed successfully in GPCR structure determination (Wadsten et al, 2006;Johansson et al, 2009;Wöhri et al, 2009).…”

Methods for Studying Interactions of Detergents and Lipids with α‐Helical and β‐Barrel Integral Membrane Proteins

“…The structurally characterized integral membrane proteins with the largest number of distinct lipids and inferred lipid functions are photosynthetic reaction centers and electron transport complexes from energy-transducing membranes: the bacterial photosynthetic reaction center (Jones, 2007; Wohri et al, 2009), photosynthetic reaction centers II (Guskov et al, 2009; Umena et al, 2011) and I (Jordan et al, 2001; Kern et al, 2009), bovine (Shinzawa-Itoh et al, 2007) and bacterial cytochrome oxidase (Qin et al, 2007), and cytochrome bc 1 (Palsdottir and Hunte, 2004) and b 6 f (Hasan et al, 2011) complexes.…”

Opportunities and challenges for time-resolved studies of protein structural dynamics at X-ray free-electron lasers

“…Unlike nano/microcrystals of photosystem I [15], the microcrystals of the Blastochloris viridis photosynthetic reaction centre were not isomorphous to their larger crystal phase form [40], packing in a new space-group and having one very long (398 Å) cell axis. When shorter wavelength X-rays became available at the coherent X-ray imaging (CXI) beamline [41] of the LCLS these microcrystals diffracted to 2.8 Å resolution and data were processed and the structure refined to 3.5 Å resolution [20].…”

“…On the other hand, XFEL-based SFX will ultimately be judged against the extent to which new structural insights emerge that would not be accessible using synchrotron radiation or single particle cryo-EM. To this end, the first proof-of-principle studies at the LCLS using model systems of known structure have been important, but higher resolution X-ray structures of photosystem I [45], photosystem II [46], the photosynthetic reaction centre [40] and lysozyme [47] have all been solved using synchrotron radiation. Given the pace of developments, examples will surely soon emerge where diffraction data recorded at an XFEL is of higher quality than that attainable at a synchrotron source.…”