To impart directionality to the motions of a molecular mechanism, one must overcome the random thermal forces that are ubiquitous on such small scales and in liquid solution at ambient temperature. In equilibrium without energy supply, directional motion cannot be sustained without violating the laws of thermodynamics. Under conditions away from thermodynamic equilibrium, directional motion may be achieved within the framework of Brownian ratchets, which are diffusive mechanisms that have broken inversion symmetry. Ratcheting is thought to underpin the function of many natural biological motors, such as the F1F0-ATPase, and it has been demonstrated experimentally in synthetic microscale systems and also in artificial molecular motors created by organic chemical synthesis. DNA nanotechnology has yielded a variety of nanoscale mechanisms, including pivots, hinges, crank sliders and rotary systems, which can adopt different configurations, for example, triggered by strand-displacement reactions or by changing environmental parameters such as pH, ionic strength, temperature, external fields and by coupling their motions to those of natural motor proteins. This previous work and considering low-Reynolds-number dynamics and inherent stochasticity led us to develop a nanoscale rotary motor built from DNA origami that is driven by ratcheting and whose mechanical capabilities approach those of biological motors such as F1F0-ATPase.
[image shows schematics of the DNA origami motor design]
Creating artificial macromolecular transport systems that can support the movement of molecules along defined routes is a key goal of nanotechnology. Here, we report the bottom- up construction of a macromolecular transport system in which molecular pistons diffusively move through micrometer-long, hollow filaments. The pistons can cover micrometer dis- tances in fractions of seconds. We build the system using multi-layer DNA origami and analyze the structures of the components using transmission electron microscopy. We study the motion of the pistons along the tubes using single-molecule fluorescence microscopy and perform Langevin simulations to reveal details of the free energy surface that directs the motions of the pistons. The tubular transport system achieves diffusivities and displacement ranges known from natural molecular motors and realizes mobility improvements over five orders of magnitude compared to previous artificial random walker designs. Electric fields can also be employed to actively pull the pistons along the filaments, thereby realizing a nanoscale electric rail system. Our system presents a platform for artificial motors that move autonomously driven by chemical fuels and for performing nanotribology studies, and it could form a basis for future molecular transportation networks.
[image shows an artist's impression of the molecular tunnel system, created by Chriss Hohmann]
Broad-spectrum antiviral platforms that can decrease or inhibit viral infection would alleviate many threats to global public health. Nonetheless, effective technologies of this kind are still not available. Here, we describe a programmable icosahedral canvas for the self-assembly of icosahedral shells that have viral trapping and antiviral properties. Programmable triangular building blocks constructed from DNA assemble with high yield into various shell objects with user-defined geometries and apertures. We have created shells with molecular masses ranging from 43 to 925 MDa (8 to 180 subunits) and with inter- nal cavity diameters of up to 280 nm. The shell interior can be functionalized with virus-specific moieties in a modular fash- ion. We demonstrate this virus-trapping concept by engulfing hepatitis B virus core particles and adeno-associated viruses. We demonstrate the inhibition of hepatitis B virus core interactions with surfaces in vitro and the neutralization of infectious adeno-associated viruses exposed to human cells.
[image shows a cryo EM reconstruction of hepatitis B virus core capsids trapped in between two half octahedral DNA origami shells]
The methods of DNA nanotechnology enable the rational design of custom shapes that self-assemble in solution from sets of DNA molecules. DNA origami, in which a long template DNA single strand is folded by many short DNA oligonucleotides, can be employed to make objects comprising hundreds of unique DNA strands and thousands of base pairs, thus in principle providing many degrees of freedom for modelling complex objects of defined 3D shapes and sizes. Here, we address the problem of accurate structural validation of DNA objects in solution with cryo-EM based methodologies. By taking into account structural fluctuations, we can determine structures with improved detail compared to previous work. To interpret the experimental cryo-EM maps, we present molecular-dynamics-based methods for building pseudo-atomic models in a semi-automated fashion. Among other features, our data allows discerning details such as helical grooves, single-strand versus double-strand crossovers, backbone phosphate positions, and single-strand breaks. Obtaining this higher level of detail is a step forward that now allows designers to inspect and refine their designs with base-pair level interventions.
[image shows the cryo EM map of a multi-layer DNA origami with and the corresponding atomic model (left), and some magnified details (right)]
Natural biomolecular assemblies such as molecular motors, enzymes, viruses and subcellular structures often form by self-limiting hierarchical oligomerization of multiple subunits. Large structures can also assemble efficiently from a few components by combining hierarchical assembly and symmetry, a strategy exemplified by viral capsids4. De novo protein design and RNA and DNA nanotechnology aim to mimic these capabilities, but the bottom-up construction of artificial structures with the dimensions and complexity of viruses and other subcellular components remains challenging. Here we show that natural assembly principles can be combined with the methods of DNA origami to produce gigadalton-scale structures with controlled sizes. DNA sequence information is used to encode the shapes of individual DNA origami building blocks, and the geometry and details of the interactions between these building blocks then control their copy numbers, positions and orientations within higher-order assemblies. We illustrate this strategy by creating planar rings of up to 350 nanometres in diameter and with atomic masses of up to 330 megadaltons, micrometre-long, thick tubes commensurate in size to some bacilli, and three-dimensional polyhedral assemblies with sizes of up to 1.2 gigadaltons and 450 nanometres in diameter. We achieve efficient assembly, with yields of up to 90 per cent, by using building blocks with validated structure and sufficient rigidity, and an accurate design with interaction motifs that ensure that hierarchical assembly is self-limiting and able to proceed in equilibrium to allow for error correction. We expect that our method, which enables the self-assembly of structures with sizes approaching that of viruses and cellular organelles, can readily be used to create a range of other complex structures with well defined sizes, by exploiting the modularity and high degree of addressability of the DNA origami building blocks used.
[image shows a 3D printed model of a 1.2 Gigadalton defined-size dodecahedron that integrates the equivalent of 220 DNA origami.]
DNA nanotechnology, in particular DNA origami, enables the bottom-up self-assembly of micrometre-scale, three-dimensional structures with nanometre-precise features. These structures are customizable in that they can be site-specifically functionalized or constructed to exhibit machine-like or logic-gating behaviour. Their use has been limited to applications that require only small amounts of material (of the order of micrograms), owing to the limitations of current production methods. But many proposed applications, for example as therapeutic agents or in complex materials could be realized if more material could be used. In DNA origami, a nanostructure is assembled from a very long single-stranded scaffold molecule held in place by many short single-stranded staple oligonucleotides. Only the bacteriophage-derived scaffold molecules are amenable to scalable and efficient mass production; the shorter staple strands are obtained through costly solid-phase synthesis or enzymatic processes. Here we show that single strands of DNA of virtually arbitrary length and with virtually arbitrary sequences can be produced in a scalable and cost-efficient manner by using bacteriophages to generate single-stranded precursor DNA that contains target strand sequences interleaved with self-excising ‘cassettes’, with each cassette comprising two Zn2+-dependent DNA-cleaving DNA enzymes. We produce all of the necessary single strands of DNA for several DNA origami using shaker-flask cultures, and demonstrate end-to-end production of macroscopic amounts of a DNA origami nanorod in a litre-scale stirred-tank bioreactor. Our method is compatible with existing DNA origami design frameworks and retains the modularity and addressability of DNA origami objects that are necessary for implementing custom modifications using functional groups. With all of the production and purification steps amenable to scaling, we expect that our method will expand the scope of DNA nanotechnology in many areas of science and technology.
[image shows an artists impression of future DNA-origami-containing medical tablets]
We describe an approach to bottom-up fabrication that allows integration of the functional diversity of proteins into designed three-dimensional structural frameworks. A set of
custom staple proteins based on transcription activator–like effector proteins folds a double- stranded DNA template into a user-defined shape. Each staple protein is designed to recognize and closely link two distinct double-helical DNA sequences at separate positions on the template. We present design rules for constructing megadalton-scale DNA-protein hybrid shapes; introduce various structural motifs, such as custom curvature, corners, and vertices; and describe principles for creating multilayer DNA-protein objects with enhanced rigidity. We demonstrate self-assembly of our hybrid nanostructures in one-pot mixtures that include the genetic information for the designed proteins, the template DNA, RNA polymerase, ribosomes, and cofactors for transcription and translation.
Revealing the energy landscape for nucleosome association may contribute to the understanding of higher-order chromatin structures and their impact on genome regulation. We accomplish this in a direct measurement by integrating two nucleosomes into a DNA origami–based force spectrometer, which enabled subnanometer-resolution measurements of nucleosome-nucleosome distance frequencies via single-particle electron microscopy imaging. From the data, we derived the Boltzmann-weighted distance-dependent energy landscape for nucleosome pair interactions. We find a shallow but long-range (~6 nm) attractive nucleosome pair potential with a minimum of −1.6 kcal/mol close to direct contact distances. The relative nucleosome orientation had little influence, but histone H4 acetylation or removal of histone tails drastically decreased the interaction strength. Because of the weak and shallow pair potential, higher-order nucleosome assemblies will be compliant and experience dynamic shape fluctuations in the absence of additional cofactors. Our results contribute to a more accurate description of chromatin and our force spectrometer provides a powerful tool for the direct and high-resolution study of molecular interactions using imaging techniques.
We directly measured at the single-molecule level the forces and lifetimes of DNA base-pair stacking interactions for all stack sequence combinations. Our experimental approach combined dual-beam optical tweezers with DNA origami components to allow positioning of blunt-end DNA helices so that the weak stacking force could be isolated. Base-pair stack arrays that lacked a covalent backbone connection spontaneously dissociated at average rates ranging from 0.02 to 500 per second, depending on the sequence combination and stack array size. Forces in the range from 2 to 8 piconewtons that act along the helical direction only mildly accelerated the stochastic unstacking process. The free-energy increments per stack that we estimate from the measured forward and backward kinetic rates ranged from –0.8 to –3.4 kilocalories per mole, depending on the sequence combination. Our data contributes to understanding the mechanics of DNA processing in biology, and it is helpful for designing the kinetics of DNA-based nanoscale devices according to user specifications.
We report a nanoscale rotary mechanism that reproduces some of the dynamic properties of biological rotary motors in the absence of an energy source, such as random walks on a circle with dwells at docking sites. Our mechanism is built modularly from tight-fitting components that were self-assembled using multilayer DNA origami. The apparatus has greater structural complexity than previous mechanically interlocked objects and features a well-defined angular degree of freedom without restricting the range of rotation. We studied the dynamics of our mechanism using single-particle experiments analogous to those performed previously with actin-labeled adenosine triphosphate synthases. In our mechanism, rotor mobility, the number of docking sites, and the dwell times at these sites may be controlled through rational design. Our prototype thus realizes a working platform toward creating synthetic nanoscale rotary motors. Our methods will support creating other complex nanoscale mechanisms based on tightly fitting, sterically constrained, but mobile, DNA components.
Molecular self-assembly with DNA relies on building blocks that are commensurate to those of biological macromolecular machines and should therefore be capable of delivering the atomic-scale placement accuracy known today only from natural and designed proteins. So far, few objects afford a design accuracy better than 5 nm, and the subnanometre scale has been reached only within the unit cells of designed DNA crystals. Here, we report a molecular positioning device made from a hinged DNA origami object in which the angle between the two structural units can be controlled with adjuster helices. We rationally adjusted the average distance between fluorescent molecules and reactive groups from 1.5 to 9 nm in 123 discrete displacement steps. The smallest displacement step possible was 0.04 nm, which is slightly less than the Bohr radius. The fluctuation amplitudes in the distance coordinate were also small (±0.5 nm), and within a factor of two to three of the amplitudes found in protein structures.
Discrete three-dimensional (3D) DNA components can specifically self-assemble in solution on the basis of shape-complementarity and without base pairing. Using this principle, we produced homo- and heteromultimeric objects, including micrometer-scale one- and two-stranded filaments and lattices, as well as reconfigurable devices, including an actuator, a switchable gear, an unfoldable nanobook, and a nanorobot. These multidomain assemblies were stabilized via short-ranged nucleobase stacking bonds that compete against electrostatic repulsion between the components’ interfaces. The balance between attractive and repulsive interactions, and thus the conformation of the assemblies, may be finely controlled by global parameters such as cation concentration or temperature and by an allosteric mechanism based on strand-displacement reactions.
A key goal for nanotechnology is to design synthetic objects that may ultimately achieve functionalities known today only from natural macromolecular complexes. Molecular self-assembly with DNA has shown potential for creating user-defined 3D scaffolds, but the level of attainable positional accuracy has been unclear. Here we report the cryo-EM structure and a full pseudoatomic model of a discrete DNA object that is almost twice the size of a prokaryotic ribosome. The structure provides a variety of stable, previously undescribed DNA topologies for future use in nano- technology and experimental evidence that discrete 3D DNA scaf- folds allow the positioning of user-defined structural motifs with an accuracy that is similar to that observed in natural macromole- cules. Thereby, our results indicate an attractive route to fabricate nanoscale devices that achieve complex functionalities by DNA- templated design steered by structural feedback.
We demonstrate that, at constant temperature, hundreds of DNA strands can cooperatively fold a long template DNA strand within minutes into complex nanoscale objects. Folding occurred out of equilibrium along nucleation-driven pathways at temperatures that could be influenced by the choice of sequences, strand lengths, and chain topology. Unfolding occurred in apparent equilibrium at higher temperatures than those for folding. Folding at optimized constant temperatures enabled the rapid production of three-dimensional DNA objects with yields that approached 100%. The results point to similarities with protein folding in spite of chemical and structural differences. The possibility for rapid and high-yield assembly will enable DNA nanotechnology for practical applications.
We created nanometer-scale transmembrane channels in lipid bilayers by means of self-assembled DNA-based nanostructures. Scaffolded DNA origami was used to create a stem that penetrated and spanned a lipid membrane, as well as a barrel-shaped cap that adhered to the membrane, in part via 26 cholesterol moieties. In single-channel electrophysiological measurements, we found similarities to the response of natural ion channels, such as conductances on the order of 1 nanosiemens and channel gating. More pronounced gating was seen for mutations in which a single DNA strand of the stem protruded into the channel. Single-molecule translocation experiments show that the synthetic channels can be used to discriminate single DNA molecules. The work was done in collaboration with the Simmel lab at TUM.
We developed DNA origami nanoplates that function with solid-state nanopores, which can be fabricated through standard electron beam lithography. The DNA origami nanoplates are permeable to small ions, but the passage of macromolecules can be controlled by including custom apertures. The chemical addressability of the DNA nanoplates enables bait–prey single-molecule sensing experiments, as highlighted in our study by the sequence-specific detection of DNA snippets and genomic phage DNA. Applications in biomolecular interaction screens and for detecting DNA sequences by hybridization are readily conceivable. High-resolution sensing applications, such as electrical DNA sequencing will require reducing both the leakage current and current fluctuations.