Post-translational amino acid conversion in photosystem II as a possible origin of photosynthetic oxygen evolution

How photosynthetic oxygen evolution is originated on ancient Earth is unknown. Here, the authors find that some amino acid residues at the ligand sites of the Mn cluster can be posttranslationally converted to the original carboxylate residues, which could have contributed to the evolutionary process of photosynthetic oxygen evolution

Crystal Structure of Cruxrhodopsin-3 from <i>Haloarcula vallismortis</i>

<div><p>Cruxrhodopsin-3 (cR3), a retinylidene protein found in the claret membrane of <i>Haloarcula vallismortis</i>, functions as a light-driven proton pump. In this study, the membrane fusion method was applied to crystallize cR3 into a crystal belonging to space group <i>P321</i>. Diffraction data at 2.1 Å resolution show that cR3 forms a trimeric assembly with bacterioruberin bound to the crevice between neighboring subunits. Although the structure of the proton-release pathway is conserved among proton-pumping archaeal rhodopsins, cR3 possesses the following peculiar structural features: 1) The DE loop is long enough to interact with a neighboring subunit, strengthening the trimeric assembly; 2) Three positive charges are distributed at the cytoplasmic end of helix F, affecting the higher order structure of cR3; 3) The cytoplasmic vicinity of retinal is more rigid in cR3 than in bacteriorhodopsin, affecting the early reaction step in the proton-pumping cycle; 4) the cytoplasmic part of helix E is greatly bent, influencing the proton uptake process. Meanwhile, it was observed that the photobleaching of retinal, which scarcely occurred in the membrane state, became significant when the trimeric assembly of cR3 was dissociated into monomers in the presence of an excess amount of detergent. On the basis of these observations, we discuss structural factors affecting the photostabilities of ion-pumping rhodopsins.</p></div

Spectroscopic properties of cR3-rich claret membrane and the <i>P321</i> crystal of cR3.

<p><i>A</i>) Absorption spectra of dark- and light-adapted states of cR3 in claret membrane suspension at pH 7. Insert: Difference spectrum associated with the light adaptation of cR3. B) Absorption spectra of the <i>P321</i> crystal of cR3 at pH 4. The two spectra were recorded when the polarization plane of the measuring light was parallel (brawn) and perpendicular (cyan) to the crystal c-axis. Insert: Photograph of the <i>P321</i> crystal of cR3.</p

Photobleaching of the retinal chroophore in cR3 and bR at 20°C.

<p>The solubilized sample was prepared by incubating cR3-rich claret membrane or the purple membrane of <i>H. salinarum</i> in a solution containing 0.01 <i>M</i> HEPES (pH 7) and 13–52 m<i>M</i> nonylglucoside (cmc ∼6 m<i>M</i>) at 30°C for 2–24 hours); unsolubilized membranes were removed by centrifugation at 100,000 rpm. The membrane suspension or the solubilized sample was exposed to strong orange light (570–700 nm, 80 mW/cm²). Absorption changes at 580 nm are plotted against the exposure time. Here, A(0) is the absorbance (∼0.35) observed just after the light-adaptation (i.e., after the pre-illumination for ∼5 minutes).</p

Structural comparison between cR3 and bR.

<p>A) The structure of the cytoplasmic vicinity of retinal in cR3 (green), bR (skyblue, PDB entry 1IW6) and aR2 (magenta, PDB entry 1EI4). B) The structure of cR3 (green) is compared with those of the unphotolyzed state (white, PDB entry 1IW6) and the L state (orange, PDB entry 1UCQ) of bR. C) The structure of the cytoplasmic ends of helices E and F in cR3 (green) and bR (white).</p

Photoreaction kinetis of cR3 and bR.

<p>A) Photoreaction kinetics of cR3 in the claret membrane of <i>Haloarcula vallismortis</i> suspended in 1 <i>M</i> KCl and 0.05 <i>M</i> HEPES at pH 8 and at 24°C. Flash-induced absorption changes measured at various wavelengths were fitted with four exponential components; in this panel, the amplitude of each component is plotted against the wavelength of the measuring light. B) Photoreaction kinetics of bR in the purple membrane of <i>Halobacterium salinarum</i> suspended in 1 <i>M</i> KCl and 0.05 <i>M</i> HEPES at pH 7.5 and at 24°C <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0108362#pone.0108362-Swamy1" target="_blank">[49]</a>.</p

Crystal packing of cR3 in the <i>P321</i> crystal.

<p>A, B) the crystal packing at pH 5, viewed along the c-axis (a) and the a*-axis (<b>b</b>). C) The trimeric structure of cR3 in complex with bacterioruberin (yellow). D) 2<i>F₀-F_c</i> map around bacterioruberin, contoured at 1 σ. E) The DE loop in one subunit (green) extending towards a neighboring subunit (cyan).</p