Mitochondrial ATP synthase plays a key role in inducing membrane curvature to establish cristae. In Apicomplexa causing diseases such as malaria and toxoplasmosis, an unusual cristae morphology has ...been observed, but its structural basis is unknown. Here, we report that the apicomplexan ATP synthase assembles into cyclic hexamers, essential to shape their distinct cristae. Cryo-EM was used to determine the structure of the hexamer, which is held together by interactions between parasite-specific subunits in the lumenal region. Overall, we identified 17 apicomplexan-specific subunits, and a minimal and nuclear-encoded subunit-a. The hexamer consists of three dimers with an extensive dimer interface that includes bound cardiolipins and the inhibitor IF
. Cryo-ET and subtomogram averaging revealed that hexamers arrange into ~20-megadalton pentagonal pyramids in the curved apical membrane regions. Knockout of the linker protein ATPTG11 resulted in the loss of pentagonal pyramids with concomitant aberrantly shaped cristae. Together, this demonstrates that the unique macromolecular arrangement is critical for the maintenance of cristae morphology in Apicomplexa.
The mitochondrial ATP synthase fuels eukaryotic cells with chemical energy. Here we report the cryo-EM structure of a divergent ATP synthase dimer from mitochondria of
, a member of the phylum ...Euglenozoa that also includes human parasites. It features 29 different subunits, 8 of which are newly identified. The membrane region was determined to 2.8 Å resolution, enabling the identification of 37 associated lipids, including 25 cardiolipins, which provides insight into protein-lipid interactions and their functional roles. The rotor-stator interface comprises four membrane-embedded horizontal helices, including a distinct subunit
. The dimer interface is formed entirely by phylum-specific components, and a peripherally associated subcomplex contributes to the membrane curvature. The central and peripheral stalks directly interact with each other. Last, the ATPase inhibitory factor 1 (IF
) binds in a mode that is different from human, but conserved in Trypanosomatids.
The mitochondrial contact site and cristae organizing system (MICOS) is a conserved multi-subunit complex crucial for maintaining the characteristic architecture of mitochondria. Studies with ...deletion mutants identified Mic10 and Mic60 as core subunits of MICOS. Mic60 has been studied in detail; however, topogenesis and function of Mic10 are unknown. We report that targeting of Mic10 to the mitochondrial inner membrane requires a positively charged internal loop, but no cleavable presequence. Both transmembrane segments of Mic10 carry a characteristic four-glycine motif, which has been found in the ring-forming rotor subunit of F1Fo-ATP synthases. Overexpression of Mic10 profoundly alters the architecture of the inner membrane independently of other MICOS components. The four-glycine motifs are dispensable for interaction of Mic10 with other MICOS subunits but are crucial for the formation of large Mic10 oligomers. Our studies identify a unique role of Mic10 oligomers in promoting the formation of inner membrane crista junctions.
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•MICOS core subunit Mic10 is targeted into mitochondria by a charged internal loop•Two conserved glycine motifs are required to form large Mic10 oligomers•Overexpression of Mic10 strongly alters inner membrane extent and architecture•Mic10 oligomers play a central role in the formation of crista junctions
The mitochondrial inner membrane displays a complex architecture with tubular cristae invaginations and narrow openings, termed crista junctions. Barbot et al. and Bohnert et al. show that the inner membrane protein Mic10 forms large oligomers and induces membrane curvature to orchestrate the formation of crista junctions in mitochondria.
Mitochondrial ATP synthases form functional homodimers to induce cristae curvature that is a universal property of mitochondria. To expand on the understanding of this fundamental phenomenon, we ...characterized the unique type III mitochondrial ATP synthase in its dimeric and tetrameric form. The cryo-EM structure of a ciliate ATP synthase dimer reveals an unusual U-shaped assembly of 81 proteins, including a substoichiometrically bound ATPTT2, 40 lipids, and co-factors NAD and CoQ. A single copy of subunit ATPTT2 functions as a membrane anchor for the dimeric inhibitor IF
. Type III specific linker proteins stably tie the ATP synthase monomers in parallel to each other. The intricate dimer architecture is scaffolded by an extended subunit-a that provides a template for both intra- and inter-dimer interactions. The latter results in the formation of tetramer assemblies, the membrane part of which we determined to 3.1 Å resolution. The structure of the type III ATP synthase tetramer and its associated lipids suggests that it is the intact unit propagating the membrane curvature.
Abstract
Mitochondrial energy conversion requires an intricate architecture of the inner mitochondrial membrane
1
. Here we show that a supercomplex containing all four respiratory chain components ...contributes to membrane curvature induction in ciliates. We report cryo-electron microscopy and cryo-tomography structures of the supercomplex that comprises 150 different proteins and 311 bound lipids, forming a stable 5.8-MDa assembly. Owing to subunit acquisition and extension, complex I associates with a complex IV dimer, generating a wedge-shaped gap that serves as a binding site for complex II. Together with a tilted complex III dimer association, it results in a curved membrane region. Using molecular dynamics simulations, we demonstrate that the divergent supercomplex actively contributes to the membrane curvature induction and tubulation of cristae. Our findings highlight how the evolution of protein subunits of respiratory complexes has led to the I–II–III
2
–IV
2
supercomplex that contributes to the shaping of the bioenergetic membrane, thereby enabling its functional specialization.
F₁Fₒ-ATP synthases are universal energy-converting membrane protein complexes that synthesize ATP from ADP and inorganic phosphate. In mitochondria of yeast and mammals, the ATP synthase forms ...V-shaped dimers, which assemble into rows along the highly curved ridges of lamellar cristae. Using electron cryotomography and subtomogram averaging, we have determined the in situ structure and organization of the mitochondrial ATP synthase dimer of the ciliate Paramecium tetraurelia. The ATP synthase forms U-shaped dimers with parallel monomers. Each complex has a prominent intracrista domain, which links the c-ring of one monomer to the peripheral stalk of the other. Close interaction of intracrista domains in adjacent dimers results in the formation of helical ATP synthase dimer arrays, which differ from the loose dimer rows in all other organisms observed so far. The parameters of the helical arrays match those of the cristae tubes, suggesting the unique features of the P. tetraurelia ATP synthase are directly responsible for generating the helical tubular cristae. We conclude that despite major structural differences between ATP synthase dimers of ciliates and other eukaryotes, the formation of ATP synthase dimer rows is a universal feature of mitochondria and a fundamental determinant of cristae morphology.
Abstract
Mitochondrial ATP synthase forms stable dimers arranged into oligomeric assemblies that generate the inner-membrane curvature essential for efficient energy conversion. Here, we report ...cryo-EM structures of the intact ATP synthase dimer from
Trypanosoma brucei
in ten different rotational states. The model consists of 25 subunits, including nine lineage-specific, as well as 36 lipids. The rotary mechanism is influenced by the divergent peripheral stalk, conferring a greater conformational flexibility. Proton transfer in the lumenal half-channel occurs via a chain of five ordered water molecules. The dimerization interface is formed by subunit-
g
that is critical for interactions but not for the catalytic activity. Although overall dimer architecture varies among eukaryotes, we find that subunit-
g
together with subunit-
e
form an ancestral oligomerization motif, which is shared between the trypanosomal and mammalian lineages. Therefore, our data defines the subunit-
g/e
module as a structural component determining ATP synthase oligomeric assemblies.
We used electron cryotomography and subtomogram averaging to determine the in situ structures of mitochondrial ATP synthase dimers from two organisms belonging to the phylum euglenozoa: Trypanosoma ...brucei, a lethal human parasite, and Euglena gracilis, a photosynthetic protist. At a resolution of 32.5 Å and 27.5 Å, respectively, the two structures clearly exhibit a noncanonical F₁ head, in which the catalytic (αβ)₃ assembly forms a triangular pyramid rather than the pseudo-sixfold ring arrangement typical of all other ATP synthases investigated so far. Fitting of known X-ray structures reveals that this unusual geometry results from a phylum-specific cleavage of the α subunit, in which the C-terminal αC fragments are displaced by ∼20 Å and rotated by ∼30° from their expected positions. In this location, the αC fragment is unable to form the conserved catalytic interface that was thought to be essential for ATP synthesis, and cannot convert γ-subunit rotation into the conformational changes implicit in rotary catalysis. The new arrangement of catalytic subunits suggests that the mechanism of ATP generation by rotary ATPases is less strictly conserved than has been generally assumed. The ATP synthases of these organisms present a unique model system for discerning the individual contributions of the α and β subunits to the fundamental process of ATP synthesis.
Summary
Apicomplexan parasites cause diseases such as malaria and toxoplasmosis. The apicomplexan mitochondrion shows striking differences from common model organisms, including fundamental processes ...such as mitochondrial translation. Despite evidence that mitochondrial translation is essential for parasite survival, it is largely understudied. Progress has been restricted by the absence of functional assays to detect apicomplexan mitochondrial translation, a lack of knowledge of proteins involved in the process and the inability to identify and detect mitoribosomes. We report the localization of 12 new mitochondrial proteins, including 6 putative mitoribosomal proteins. We demonstrate the integration of three mitoribosomal proteins in macromolecular complexes, and provide evidence suggesting these are apicomplexan mitoribosomal subunits, detected here for the first time. Finally, a new analytical pipeline detected defects in mitochondrial translation upon depletion of the small subunit protein 35 (TgmS35), while other mitochondrial functions remain unaffected. Our work lays a foundation for the study of apicomplexan mitochondrial translation.
The apicomplexan mitochondrion is divergent and essential yet poorly studied. Mitochondrial translation is predicted to utilise ribosomes assembled from fragmented rRNA but this was never shown. Lack of tools to study mitochondrial translation in apicomplexan has been a long‐standing challenge. Tagging and depletion of T. gondii mitoribosomal proteins enabled the first detection of a ribosomal complex, and provided proof of principle for a new mitochondrial translation analytic pipeline.
Electron cryo-tomography is a powerful tool in structural biology, capable of visualizing the three-dimensional structure of biological samples, such as cells, organelles, membrane vesicles, or ...viruses at molecular detail. To achieve this, the aqueous sample is rapidly vitrified in liquid ethane, which preserves it in a close-to-native, frozen-hydrated state. In the electron microscope, tilt series are recorded at liquid nitrogen temperature, from which 3D tomograms are reconstructed. The signal-to-noise ratio of the tomographic volume is inherently low. Recognizable, recurring features are enhanced by subtomogram averaging, by which individual subvolumes are cut out, aligned and averaged to reduce noise. In this way, 3D maps with a resolution of 2 nm or better can be obtained. A fit of available high-resolution structures to the 3D volume then produces atomic models of protein complexes in their native environment. Here we show how we use electron cryo-tomography to study the in situ organization of large membrane protein complexes in mitochondria. We find that ATP synthases are organized in rows of dimers along highly curved apices of the inner membrane cristae, whereas complex I is randomly distributed in the membrane regions on either side of the rows. By subtomogram averaging we obtained a structure of the mitochondrial ATP synthase dimer within the cristae membrane.
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