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...

We used cryoelectron tomography to reveal the arrangements of photosystem II (PSII) and ATP synthase in vitreous sections of intact chloroplasts and plunge-frozen suspensions of isolated thylakoid membranes. We found that stroma and grana thylakoids are connected at the grana margins by staggered lamellar membrane protrusions. The stacking repeat of grana membranes in frozen-hydrated chloroplasts is 15.7 nm, with a 4.5-nm lumenal space and a 3.2-nm distance between the flat stromal surfaces. The chloroplast ATP synthase is confined to minimally curved regions at the grana end membranes and stroma lamellae, where it covers 20% of the surface area. In total, 85% of the ATP synthases are monomers and the remainder form random assemblies of two or more copies. Supercomplexes of PSII and light-harvesting complex II (LHCII) occasionally form ordered arrays in appressed grana thylakoids, whereas this order is lost in destacked membranes. In the ordered arrays, each membrane on either side of the stromal gap contains a two-dimensional crystal of supercomplexes, with the two lattices arranged such that PSII cores, LHCII trimers, and minor LHCs each face a complex of the same kind in the opposite membrane. Grana formation is likely to result from electrostatic interactions between these complexes across the stromal gap.

Aging is one of the most fundamental, yet least understood biological processes that affect all forms of eukaryotic life. Mitochondria are intimately involved in aging, but the underlying molecular mechanisms are largely unknown. Electron cryotomography of whole mitochondria from the aging model organism Podospora anserina revealed profound age-dependent changes in membrane architecture. With increasing age, the typical cristae disappear and the inner membrane vesiculates. The ATP synthase dimers that form rows at the cristae tips dissociate into monomers in inner-membrane vesicles, and the membrane curvature at the ATP synthase inverts. Dissociation of the ATP synthase dimer may involve the peptidyl prolyl isomerase cyclophilin D. Finally, the outer membrane ruptures near large contact-site complexes, releasing apoptogens into the cytoplasm. Inner-membrane vesiculation and dissociation of ATP synthase dimers would impair the ability of mitochondria to supply the cell with sufficient ATP to maintain essential cellular functions.subtomogram averaging | cell death

The archaellum is the macromolecular machinery that Archaea use for propulsion or surface adhesion, enabling them to proliferate and invade new territories. The molecular composition of the archaellum and of the motor that drives it appears to be entirely distinct from that of the functionally equivalent bacterial flagellum and flagellar motor. Yet, the structure of the archaellum machinery is scarcely known. Using combined modes of electron cryo-microscopy (cryoEM), we have solved the structure of the Pyrococcus furiosus archaellum filament at 4.2 Å resolution and visualise the architecture and organisation of its motor complex in situ. This allows us to build a structural model combining the archaellum and its motor complex, paving the way to a molecular understanding of archaeal swimming motion.DOI: http://dx.doi.org/10.7554/eLife.27470.001