The structure of the light-harvesting chlorophyll a/b-protein complex, an integral membrane protein, has been determined at 3.4 A resolution by electron crystallography of two-dimensional crystals. Two of the three membrane-spanning alpha-helices are held together by ion pairs formed by charged residues that also serve as chlorophyll ligands. In the centre of the complex, chlorophyll a is in close contact with chlorophyll b for rapid energy transfer, and with two carotenoids that prevent the formation of toxic singlet oxygen.

The plant light-harvesting complex of photosystem II (LHC-II) collects and transmits solar energy for photosynthesis in chloroplast membranes and has essential roles in regulation of photosynthesis and in photoprotection. The 2.5 Å structure of pea LHC-II determined by X-ray crystallography of stacked two-dimensional crystals shows how membranes interact to form chloroplast grana, and reveals the mutual arrangement of 42 chlorophylls a and b, 12 carotenoids and six lipids in the LHC-II trimer. Spectral assignment of individual chlorophylls indicates the flow of energy in the complex and the mechanism of photoprotection in two close chlorophyll a-lutein pairs. We propose a simple mechanism for the xanthophyllrelated, slow component of nonphotochemical quenching in LHC-II, by which excess energy is transferred to a zeaxanthin replacing violaxanthin in its binding site, and dissipated as heat. Our structure shows the complex in a quenched state, which may be relevant for the rapid, pH-induced component of nonphotochemical quenching. The EMBO Journal (2005) 24, 919-928.

Advances in detector technology and image processing are yielding high-resolution electron cryo-microscopy structures of biomolecules. [Also see Report by Amunts et al. ]

ATP synthase converts the electrochemical potential at the inner mitochondrial membrane into chemical energy, producing the ATP that powers the cell. Using electron cryo-tomography we show that the ATP synthase of mammalian mitochondria is arranged in long B1-lm rows of dimeric supercomplexes, located at the apex of cristae membranes. The dimer ribbons enforce a strong local curvature on the membrane with a 17-nm outer radius. Calculations of the electrostatic field strength indicate a significant increase in charge density, and thus in the local pH gradient of B0.5 units in regions of high membrane curvature. We conclude that the mitochondrial cristae act as proton traps, and that the proton sink of the ATP synthase at the apex of the compartment favours effective ATP synthesis under proton-limited conditions. We propose that the mitochondrial ATP synthase organises itself into dimer ribbons to optimise its own performance.

We used electron cryotomography of mitochondrial membranes from wild-type and mutant Saccharomyces cerevisiae to investigate the structure and organization of ATP synthase dimers in situ. Subtomogram averaging of the dimers to 3.7 nm resolution revealed a V-shaped structure of twofold symmetry, with an angle of 86°between monomers. The central and peripheral stalks are well resolved. The monomers interact within the membrane at the base of the peripheral stalks. In wild-type mitochondria ATP synthase dimers are found in rows along the highly curved cristae ridges, and appear to be crucial for membrane morphology. Strains deficient in the dimer-specific subunits e and g or the first transmembrane helix of subunit 4 lack both dimers and lamellar cristae. Instead, cristae are either absent or balloon-shaped, with ATP synthase monomers distributed randomly in the membrane. Computer simulations indicate that isolated dimers induce a plastic deformation in the lipid bilayer, which is partially relieved by their side-by-side association. We propose that the assembly of ATP synthase dimer rows is driven by the reduction in the membrane elastic energy, rather than by direct protein contacts, and that the dimer rows enable the formation of highly curved ridges in mitochondrial cristae.membrane-protein oligomerization | membrane deformation | molecular dynamics simulations | bioenergetics | ATP synthesis T he F 1 F o ATP synthase is a highly conserved molecular machine that catalyses the production of ATP from ADP and P i in energy-converting membranes of eukaryotes and bacteria.

We used electron cryotomography to study the molecular arrangement of large respiratory chain complexes in mitochondria from bovine heart, potato, and three types of fungi. Long rows of ATP synthase dimers were observed in intact mitochondria and cristae membrane fragments of all species that were examined. The dimer rows were found exclusively on tightly curved cristae edges. The distance between dimers along the rows varied, but within the dimer the distance between F 1 heads was constant. The angle between monomers in the dimer was 70°or above. Complex I appeared as L-shaped densities in tomograms of reconstituted proteoliposomes. Similar densities were observed in flat membrane regions of mitochondrial membranes from all species except Saccharomyces cerevisiae and identified as complex I by quantumdot labeling. The arrangement of respiratory chain proton pumps on flat cristae membranes and ATP synthase dimer rows along cristae edges was conserved in all species investigated. We propose that the supramolecular organization of respiratory chain complexes as proton sources and ATP synthase rows as proton sinks in the mitochondrial cristae ensures optimal conditions for efficient ATP synthesis.cryoelectron tomography | subtomogram averaging | membrane curvature | membrane potential | mitochondrial ultrastructure M itochondria, the powerhouses of eukaryotic cells, generate ATP, the universal energy carrier in all life forms. The F 1 F o ATP synthase uses the energy stored in the electrochemical proton gradient across the inner mitochondrial membrane to produce ATP from ADP and phosphate. The proton gradient is established by the respiratory chain complexes I, III, and IV, which pump protons out of the mitochondrial matrix into the cristae space while transferring electrons from the electron donors NADH, FADH, or succinate (via complex II) to the final electron acceptor O 2 . The F 1 F o ATP synthase and complex I (NADH dehydrogenase) are the largest membrane protein complexes in mitochondria, composed of more than 20 or 40 individual protein subunits, respectively (1, 2). The 600-kDa ATP synthase consists of the F o part in the membrane that works like a proton-driven turbine, and the catalytic F 1 part on the matrix side. The two parts are held together by a static peripheral stalk and a rotating central stalk that transmits the torque from the rotor unit in the membrane to the catalytic F 1 head (3, 4). Complex I is an L-shaped molecule of approximately 1 MDa. Its membrane arm has three or four proton-pumping modules, while the matrix arm catalyzes electron transfer from NADH to the hydrophobic electron acceptor ubiquinol (5). The structures of both complexes have been determined by X-ray crystallography, either partially in the case of the F 1 F o ATP synthase (6), or at low resolution in the case of mitochondrial complex I (7, 8), but their relative organization in the mitochondrial inner membrane is largely unknown.The two large complexes occur at an approximate ratio of one molecule of complex I per 3.5 ATP...

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Biological energy conversion in mitochondria is carried out by the membrane protein complexes of the respiratory chain and the mitochondrial ATP synthase in the inner membrane cristae. Recent advances in electron cryomicroscopy have made possible new insights into the structural and functional arrangement of these complexes in the membrane, and how they change with age. This review places these advances in the context of what is already known, and discusses the fundamental questions that remain open but can now be approached.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-015-0201-x) contains supplementary material, which is available to authorized users.