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The RecA protein forms a hexameric ring that is similar to the core of the F1-ATPase. Several lines of evidence suggest that this hexamer may be a structural homologue of ring helicases.

Electron microscopic image analysis and protein–DNA crosslinking show that DNA binds asymmetrically to the hexameric bacteriophage T7 gp4b helicase, and binds to only one or two subunits

Cannulae are heat-resistant protein nanotubes found on the surface of thermophilic archaea. Here, the authors report the structures of cannulae at the atomic level with insight into their high stability and mechanism of assembly, which has potential impact for biomaterials design.

Tumor suppressor PALB2 is known to interact with BRCA1 and BRCA2, and to be required for the latter's localization to sites of DNA damage. Now PALB2 is shown to bind directly to DNA, to recombinase RAD51 and its accessory factor RAD51AP1. PALB2 also stimulates D loop formation by RAD51 in a synergistic manner with RAD51AP1.

B. thuringiensis spores contain uncharacterized protein filaments that extend from the surface of the exosporium. Here, the authors show that these filaments feature conserved β-barrel neck domains and promote spore clustering through protein contacts and filament bundling, and reveal a mechanism for biofilm-like spore aggregation.

Chromophore supramolecular assemblies have long been studied for their exotic photophysical properties arising from their local geometry and long-range sensitive excitonic couplings. Now a high-resolution structure of a model nanotubular system has revealed a uniform brick-layer molecular arrangement and a non-biological supramolecular motif—interlocking sulfonates—enabling clear understanding of supramolecular structure–excitonic property relationships.

Here, the authors structurally characterise a protein filament species composed of the spore coat protein GerQ using cryo-EM. The nine-subunit assembly sheds light on spore biology and hints at an interaction with CwlJ, a protein required for spore germination.

Type IV pili (T4P) are long surface appendages assembled from small pilin proteins that participate in diverse functions such as adhesion and biofilm formation in archaea. Here, the authors show that the same pilin polypeptide can adopt four conformations and assemble two distinct T4P structures under different growth conditions.

Despite advances in machine learning approaches, de novo design of collagen-based materials remains difficult. In this study, based on the natural structure of the defense collagen family of proteins, designed triple helical peptide assemblies are found to form ribbons and a variety of bundled, porous architectures.

Sonani et al. report the cryo-EM structures of the Milano phage tail, unraveling the mechanism of its flexible-to-rigid transformation, the inter-chain disulfide network hindering tail contraction and the structural organization of receptor binding proteins.

Many actin-binding proteins contain calponin homology (CH) domains, but the manner in which these domains interact with F-actin has been controversial. Electron microscopy analyses show that the tandem CH domains of α-actinin bind to F-actin in an open conformation and that opening of these domains might be a key regulatory mechanism for proteins with tandem CH domains.

Flexible filamentous plant viruses, which cause substantial crop damage worldwide, have eluded structural characterization so far. The cryo-EM structure of BaMV now reveals the virus architecture and the structural basis of its flexibility.

Creation of cell spheroids by using triggered d-peptide self-assembly is reported. Peptides are dephosphorylated by transcytosis in cells and intercellularly assembled to facilitate fibronectin fibrillogenesis and subsequent spheroid formation.

Structural analyses reveal that Geobacter sulfurreducens produces two different cytochrome filaments, comprised of either OmcS or OmcE, that share almost identical haem packing while having no recognizable sequence or structural similarity.

The disease causing L253P mutation in the actin-binding domain (ABD) of β-III-spectrin drastically increases actin-binding affinity. Here, the authors present the cryo-EM structure of F-actin complexed with the ABD mutant and double electron–electron resonance measurements show how the mutation affects the ABD conformational state.

Bacteria can exchange DNA through extracellular appendages (‘mating pili’) in a process known as conjugation. Here, Beltran et al. determine atomic structures by cryo-electron microscopy of a bacterial conjugative pilus and two archaeal pili, showing that the archaeal pili are homologous to bacterial mating pili.

It has been suggested that the outer domains of bacterial flagellins are not needed for motility. Here, the authors show that flagellar filament outer domains from some bacteria have unique structures which can alter the motility of the bacteria.

A dissipative self-assembly system was developed by re-engineering a naturally hexameric adenosine 5′-triphosphatase (ATPase) enzyme (filamentous temperature-sensitive protease H) to form transient one-dimensional tubular structures in an ATP-dependent fashion. These tubular structures in turn exert negative feedback on the ATP-dependent activation of the protein building blocks.

Archaeal type IV pili (T4P) mediate adhesion to surfaces and are receptors for hyperthermophilic archaeal viruses. Here, the authors present the cryo-EM structures of two archaeal T4P from Pyrobaculum arsenaticum and Saccharolobus solfataricus and discuss evolutionary relationships between bacterial T4P, archaeal T4P and archaeal flagellar filaments.

The electron cryo-microscopy structure of Sulfolobus islandicus pili enabled the identification of SiL_2606 as the main pilin in these filaments and revealed that the pili are glycosylated, which probably explains how these structures remain soluble and stable even when cells grow at pH 3 and 80 °C.

Electron cryo-microscopy images of actin filaments (F-actin) allow the visualization of different conformational states, which can be classified into different groups. Further analyses reveal the plasticity of the subdomain 2 region and suggest that missense mutations related to disease affect F-actin structural dynamics.

Type IV pili are present on a wide range of bacterial pathogens and mediate diverse functions. Here the authors report a high resolution crystal structure of the pilin subunit PilE, and a cryoEM reconstruction of the Type IV pilus filament from N. meningitidisthat offer insight into pilus assembly and functions.

New detector technology has improved the resolution of cryo-electron microscopy (cryo-EM), but tools for structure determination from high-resolution maps have lagged behind. Wang et al. describe a de novo approach for structure determination from high-resolution cryo-EM maps. Also in this issue, DiMaio et al. report structure determination using a homologous structure as a starting model.

Dynamic supramolecular systems can be designed to adapt phases in a pre-programmable way. Here the transient nature of a gel system is exploited, in combination with the application of mechanical stimuli, to obtain soft materials with aligned fibres in a controllable way.

Only a few archaeal filamentous viruses have been structurally characterized. Here the authors describe the membrane-enveloped Sulfolobus filamentous virus 1 that infects Sulfolobus shibatae and present its 3.7 Å resolution cryo-EM structure, which reveals that major coat proteins are structurally conserved among archaeal filamentous viruses.

Combining conjugation and structural analyses, the authors show that TraN-OMP pairings determine bacterial conjugation species specificity, with implications in resistance plasmid distribution within Enterobacteriaceae.

Bacterial flagellar filaments are composed almost entirely of a single protein—flagellin—which can switch between different supercoiled states in a highly cooperative manner. Here the authors present near-atomic resolution cryo-EM structures of nine flagellar filaments, and begin to shed light on the molecular basis of filament switching.

Structural insight into TIR-domain interactions, which are essential for the recruitment of signaling adapters to Toll-like receptors during innate immune responses, demonstrates a conserved interaction mode involved in both TLR and IL-1R signaling.

Peptide-based filamentous assemblies are successfully used for generation of structurally ordered materials, but their de novo design and structural characterization is challenging. Here, the authors provide a strategy for the design of self-assembling peptide nanotubes based on modifications of an arginine clasp interaction motif, and report the cryo-EM structures of seven designed nanotubes.

AIM2-ASC inflammasomes are filamentous signalling platforms that play a central role in host innate defence. Here, the authors present the filament cryo-EM structure of the inflammasome receptor AIM2, which is very similar to the adaptor ASC filament structure. By employing Rosetta and Molecular Dynamics simulations the authors provide further insights into the directionality and recognition mechanisms of the individual AIM2 and ASC filaments, which is further validated with biochemical and cellular experiments.

Torsins are unusual AAA + ATPases of unknown function that reside in the endoplasmic reticulum of all animals. Here the authors report that TorsinA forms tubular helical filaments with an unusual periodicity and that filamentous TorsinA directly interacts with membranes to form tubular protrusions.

A new cryo-EM structure of the bacterial flagellar hook, which connects the motor to the flagellar filament, reveals 11 distinctive conformations of the subunit. The cooperative dynamic switching among these states offers a dramatic extension of two-state models of protein allostery.

Crystal structures of proteins not only shed light on how those proteins work. By revealing previously hidden similarities, they can also force a re-evaluation of what other proteins are predicted to do.

Cryo-EM, biochemical and cell-based assays reveal strong allosteric autoinhibition of the actin-binding domains of plastins. Physiological tuning of this engagement explains the plastins’ ability to organize actin assemblies of different morphologies.

The AIM2 inflammasome complex is essential for defence against a number of human pathogens but how it assembles upon recognition of foreign DNA remains incompletely understood. Here Morrone et al.suggest the AIM2 pyrin domain acts in both DNA binding and filament assembly to generate a structural template for complex assembly.

New detector technology has improved the resolution of cryo-electron microscopy (cryo-EM), but tools for structure determination from high-resolution maps have lagged behind. DiMaio et al. report structure determination from high-resolution cryo-EM maps using a homologous structure as a starting model. Also in this issue, Wang et al. describe a de novo approach for structure determination that does not require a starting model.

The first crystal structure of an actin molecule not associated with another protein has been solved. Remarkably, the most variable part of actin's structure is also the most conserved part of its sequence.

An atomic model for a type 2 secretion system pseudopilus from Klebsiella oxytoca reveals a comprehensive network of inter-subunit contacts while mutational and functional analyses highlight the role of calcium in PulG folding and stability.

RosettaES, an algorithm that uses a fragment-based sampling strategy, improves macromolecular structure modeling from cryo-EM data at 3–5-Å resolution.

Cryo-EM structures of the neck, neck-capsid and neck-tail junctions, and the capsid of the phage Milano reveal a unique collar structure and an extensive network of disulfide bonds providing structural stability to the phage.