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.

Muscle contraction involves the cyclic interaction of the myosin cross-bridges with the actin filament, which is coupled to steps in the hydrolysis of ATP. While bound to actin each cross-bridge undergoes a conformational change, often referred to as the "power stroke", which moves the actin filament past the myosin filaments; this is associated with the release of the products of ATP hydrolysis and a stronger binding of myosin to actin. The association of a new ATP molecule weakens the binding again, and the attached cross-bridge rapidly dissociates from actin. The nucleotide is then hydrolysed, the conformational change reverses, and the myosin cross-bridge reattaches to actin. X-ray crystallography has determined the structural basis of the power stroke, but it is still not clear why the binding of actin weakens that of the nucleotide and vice versa. Here we describe, by fitting atomic models of actin and the myosin cross-bridge into high-resolution electron cryo-microscopy three-dimensional reconstructions, the molecular basis of this linkage. The closing of the actin-binding cleft when actin binds is structurally coupled to the opening of the nucleotide-binding pocket.

Elucidation of the molecular contacts between actin and myosin is central to understanding the force-generating process in muscle and other cells. Actin, a highly conserved globular protein found in all eukaryotes, polymerizes into filaments (F-actin) for most of its biological functions. Myosins, which are more diverse in sequence, share a conserved globular head of about 900 amino acids in length (subfragment-1 or S1) at the N-terminal end of the molecule. S1 contains all the elements necessary for mechano-chemical force transduction in vitro. Here we report an atomic model for the actomyosin complex produced by combining the atomic X-ray structure of F-actin and chicken myosin S1 with a three-dimensional reconstruction from electron micrographs of frozen-hydrated F-actin decorated with recombinant Dictyostelium myosin S1. The accuracy of the reconstruction shows the position of actin and myosin molecules unambiguously.

To increase efficiency of bulk heterojunctions for photovoltaic devices, the functional morphology of active layers has to be understood, requiring visualization and discrimination of materials with very similar characteristics. Here we combine high-resolution spectroscopic imaging using an analytical transmission electron microscope with nonlinear multivariate statistical analysis for classification of multispectral image data. We obtain a visual representation showing homogeneous phases of donor and acceptor, connected by a third composite phase, depending in its extent on the way the heterojunction is fabricated. For the first time we can correlate variations in nanoscale morphology determined by material contrast with measured solar cell efficiency. In particular we visualize a homogeneously blended phase, previously discussed to diminish charge separation in solar cell devices.

Molecular motors produce force when they interact with their cellular tracks. For myosin motors, the primary force-generating state has MgADP tightly bound, whereas myosin is strongly bound to actin. We have generated an 8-Å cryoEM reconstruction of this state for myosin V and used molecular dynamics flexed fitting for model building. We compare this state to the subsequent state on actin (Rigor). The ADP-bound structure reveals that the actin-binding cleft is closed, even though MgADP is tightly bound. This state is accomplished by a previously unseen conformation of the β-sheet underlying the nucleotide pocket. The transition from the force-generating ADP state to Rigor requires a 9.5°rotation of the myosin lever arm, coupled to a β-sheet rearrangement. Thus, the structure reveals the detailed rearrangements underlying myosin force generation as well as the basis of strain-dependent ADP release that is essential for processive myosins, such as myosin V.T he actin-based molecular motor, myosin, generates force and movement via a series of structural transitions when bound to filamentous actin (F-actin). Among these structural states, the most important contribution to force production comes from the myosin ADP states that bind strongly to actin. Release of ADP from the motor is rapidly followed by MgATP binding, which then leads to myosin dissociation from actin. Once the dissociation occurs, myosin can undergo a structural change that allows it to prime its mechanical element (known as the lever arm; Fig. 1) and hydrolyze ATP. Myosin rebinding to actin triggers release of phosphate and reentry into the force-generating states on actin. Thus, force is produced by MgADP-bound states of myosin bound to actin.In the absence of strain, a strong-binding MgADP state bound to actin can exist for varying durations, depending on the type of myosin. Myosins that are designed to spend the majority of their catalytic cycle bound to actin in force-generating states in the absence of load, such as myosin V a , primarily occupy a state that is characterized by having both a high affinity for MgADP and a high affinity for actin. The myosin motor cycle is summarized in Fig. 1. By not rapidly releasing MgADP, the myosin motors can dwell in a force-generating state on actin that cannot be detached from actin by ATP binding. This is the primary force-generating state for all myosins, and its lifetime on actin is prolonged under load (for a review, see ref. 1).Although the nucleotide-free myosin V X-ray structure (2) (Rigor-like state) provides an atomic level model for the actomyosin state that is formed after ADP is released, and to which ATP can rapidly bind (2), we only have low-resolution EM reconstructions of myosin bound to actin in a MgADP state. Visualization of this state at the EM level was first performed for smooth muscle myosin II (3) and provided the first structural evidence that the myosin lever arm swings as the motor progresses through its actinbound, force-generating states. It was postulated at the time that th...

Cryo-electron microscopy provides the means to quantitatively study macromolecules in their native state. However, the original mass distribution of the macromolecule is distorted by the contrast transfer function (CTF) of the electron microscope. In addition, the zeros of the CTF put a practical limit on the resolution that can be achieved. Substantial improvement to the quality of the results can be accomplished by collecting the data using a series of defocus settings. Such data sets can be combined and the resolution can be extended beyond the first zero of the CTF. This procedure can be applied either at the stage of raw data, or more effectively at the stage of reconstructed volumes which have a high signal-to-noise ratio as a result of averaging over many projections. A method of threedimensional (3D) reconstruction that combines an algebraic, iterative 3D reconstruction technique with CTF correction is proposed. The potential to incorporate a priori knowledge into the reconstruction process is discussed. This approach was used to obtain a 3D reconstruction of the E. coli 70S ribosome from energy-filtered cryo-images.

Decorated actin provides a model system for studying the strong interaction between actin and myosin. Cryo-energy-filter electron microscopy has recently yielded a 14 Å resolution map of rabbit skeletal actin decorated with chicken skeletal S1. The crystal structure of the cross-bridge from skeletal chicken myosin could not be fitted into the three-dimensional electron microscope map without some deformation. However, a newly published structure of the nucleotide-free myosin V cross-bridge, which is apparently already in the strong binding form, can be fitted into the three-dimensional reconstruction without distortion. This supports the notion that nucleotide-free myosin V is an excellent model for strongly bound myosin and allows us to describe the actin-myosin interface. In myosin V the switch 2 element is closed although the lever arm is down (post-power stroke). Therefore, it appears likely that switch 2 does not open very much during the power stroke. The myosin V structure also differs from the chicken skeletal myosin structure in the nucleotide-binding site and the degree of bending of the backbone b-sheet. These suggest a mechanism for the control of the power stroke by strong actin binding.

Future lightweight, flexible, and wearable electronics will employ visible‐light‐communication schemes to interact within indoor environments. Organic photodiodes are particularly well suited for such technologies as they enable chemically tailored optoelectronic performance and fabrication by printing techniques on thin and flexible substrates. However, previous methods have failed to address versatile functionality regarding wavelength selectivity without increasing fabrication complexity. This work introduces a general solution for printing wavelength‐selective bulk‐heterojunction photodetectors through engineering of the ink formulation. Nonfullerene acceptors are incorporated in a transparent polymer donor matrix to narrow and tune the response in the visible range without optical filters or light‐management techniques. This approach effectively decouples the optical response from the viscoelastic ink properties, simplifying process development. A thorough morphological and spectroscopic investigation finds excellent charge‐carrier dynamics enabling state‐of‐the‐art responsivities >102 mA W−1 and cutoff frequencies >1.5 MHz. Finally, the color selectivity and high performance are demonstrated in a filterless visible‐light‐communication system capable of demultiplexing intermixed optical signals.