The ability to generate high-resolution views of cells with cryogenic electron microscopy (cryo-EM) can reveal the molecular mechanisms of biological processes in their native cellular context. The revolutionary impact of this strategy is limited by the difficulty of accurately annotating structures within these images. 2D template matching (2DTM), in which high-resolution structural models are used

The ADAMTS (a disintegrin-like and metalloproteinase domain with thrombospondin type 1 motifs) family of secreted metalloproteinases plays essential roles in extracellular matrix remodeling. ADAMTS-5 contributes to cartilage degradation, cleaving proteoglycans such as aggrecan and versican, and being involved in both physiological tissue turnover and pathological processes such as osteoarthritis and

Echinacea purpurea hydroxycinnamoyl-CoA:tartaric acid hydroxycinnamoyl transferase (EpHTT) is a cytosolic BAHD acyltransferase that catalyzes the transfer of caffeoyl groups to tartaric acid, a key step in chicoric acid biosynthesis. Understanding the structure of EpHTT is essential to elucidate the molecular basis of substrate recognition and catalytic specificity. Here, we report the crystal structure

Here, we describe the first ten years of the CCP-EM Spring Symposium, an annual conference to bring together the cryogenic-sample electron microscopy (cryoEM) community to present and discuss the latest methodological advances and applications. We quantify the growth of the event and provide a detailed breakdown of the demographics of the tenth edition.

More than 80% of protein structure models in the Protein Data Bank have been solved using X-ray crystallography. Despite continuous improvements in this experimental technique, crystallographic structure models may still present artifacts related to the crystallization process as well as errors introduced during model building and refinement, even in high-resolution cases. Such limitations can alter

Many fundamental biological processes, including those in photosynthetic reaction centers and enzyme active sites, involve charge and energy transfer, bond cleavage, protonation and hydrogen bonding. Because H atoms play such central roles in these reactions, accurately determining their positions is essential. Yet, conventional X-ray crystallography primarily resolves the heavy atoms in biological

Polo-like kinase 1 (PLK1) is a major regulator of cell division and has been pursued as a drug target for cancer therapy for a long time. Crystallization of the kinase domain has proven to be exceptionally challenging. Previously, we published a crystallization approach using a PLK1-specific designed ankyrin-repeat protein (DARPin) as a crystallization facilitator. Here, we report an alternative route:

Microcrystal electron diffraction (MicroED), also known as three-dimensional electron diffraction (3D ED), allows the collection of diffraction data from submicrometre-sized crystals under low electron-dose conditions. Despite having several advantages over conventional X-ray crystallographic techniques, susceptibility to radiation damage is a great challenge that remains to be solved in MicroED. Similar

The accessory secretion (aSec) system is a protein export pathway that is uniquely present in Gram-positive bacteria and is dedicated to the secretion of large, glycosylated cell wall-anchored adhesins called serine-rich repeat proteins (SRRPs). Strain-specific glycosylation of SRRPs has previously been reported in Limosilactobacillus reuteri and attributed to GtfC, a glycosyltransferase belonging

The explosion of cryo-electron microscopy (cryo-EM) over the last decade has brought with it a range of new approaches for gaining high-resolution structural information on previously inaccessible biological systems. Cryo-EM single-particle analysis (SPA) approaches typically entail overexpression and purification of the target protein. Larger and more complex molecular assemblies often require extensive

Methanol, a sustainable and cost-effective C1 compound, has been considered as a promising substrate for the biosynthesis of fuels and value-added chemicals. Synthetic methylotrophs have been developed by integrating natural methanol-assimilation pathways into non-native microbial hosts, with NAD+-dependent methanol dehydrogenases (MDHs) serving as attractive candidates for methanol oxidation. NAD+-dependent

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues to threaten global health. This underpins the need for novel therapeutics against this virus. Nonstructural protein 1 (Nsp1) of SARS-CoV-2 is a multifunctional protein with an essential role in viral replication. As such, it presents itself as an attractive target for drug discovery. Here, we describe two crystallographic fragment-screening

Particulate methane monooxygenase (pMMO) is an enzyme that converts methane into methanol at ambient temperature and pressure. Over the past three decades, the metal content and location of the active site have been highly controversial. Recent single-particle cryogenic electron-microscopy (cryo-EM) structures have furthered this debate. In this study, three cryo-EM structures (PDB entries 7s4h, 7s4j

Low-confidence regions in computational protein models have been identified to represent a largely untapped resource that may contain valuable structural information.

Streptomyces griseolus CYP105A1 exhibits monooxygenase activity towards a wide variety of structurally diverse substrates with regiospecificity and stereospecificity, making it suitable for broad applications. Our previous studies have demonstrated that both wild-type CYP105A1 and its mutants metabolize vitamin D3 and its derivatives, as well as 12 types of nonsteroidal anti-inflammatory drugs (NSAIDs)

AlphaFold2 protein structure predictions are widely available for structural biology uses. These predictions, especially for eukaryotic proteins, frequently contain extensive regions predicted below the pLDDT = 70 level, the rule-of-thumb cutoff for high confidence. This work identifies major modes of behavior within low-pLDDT regions through a survey of human proteome predictions provided by the AlphaFold

Heterogeneity in cryoEM is essential for capturing the structural variability of macromolecules, reflecting their functional states and biological significance. However, estimating heterogeneity remains challenging due to particle misclassification and algorithmic biases, which can lead to reconstructions that blend distinct conformations or fail to resolve subtle differences. Furthermore, the low

Marc Hebrant provides a review of the second editon of Biophysical Chemistry by Dagmar Klostermeier and Markus G. Rudolph.

The c-Src SH3 domain is one of the best-characterized modular domains from a biophysical and structural point of view. This SH3 domain displays noncanonical alternative folding, forming 3D domain-swapped oligomers and amyloid fibrils. These features make this small protein an ideal model for studying these phenomena. Residues in the regions that favour unfolding of the monomer and those in the hinge

Fragment-based lead discovery (FBLD) is an efficient and effective method for identifying novel chemical scaffolds that have advantages in drug development. X-ray crystallography has an inherent advantage in recognizing low-affinity fragments and integrates fragment identification with complex structure determination, making it an increasingly important tool for screening fragment compounds. Here,

Rengachary Parthasarathy is remembered.

β2-Microglobulin (β2M) is an essential component of major histocompatibility complex class I (MHC-I) molecules, with a well established canonical role in immune surveillance. Beyond its classical functions, accumulating evidence has highlighted β2M as a multifaceted biomarker, with elevated serum levels closely associated with disease burden and prognosis in metabolic disorders, malignancies, autoimmune

An introduction to a set of three related papers in this issue describing 216 ligand-bound fatty acid-binding protein structure determinations and the pitfalls that can arise in such a study.

FABP4 has been implicated as a therapeutic target for treating diabetes and atherosclerosis. Structure-based drug design (SBDD) based on initial hits from high-throughput and fragment screens yielded 216 ligand-bound structures of human FABP3, FABP4 and FABP5 isoforms, many of which were at resolutions of better than 1.2 Å. An estimated 15% of the ligands had a different chemical composition to that

Fatty acid-binding protein isoforms 4 and 5 are potential diabetes and atherosclerosis targets. During a drug-design program aiming at dual isoform-specific FABP4/5 inhibitors with little or no affinity for FABP3, a set of crystal structures with a median resolution of 1.2 Å was generated. The chemical space of the ligands covers various series in which the carboxylate and aliphatic groups of the natural

Fatty acid-binding proteins (FABPs) are involved in the uptake and intracellular trafficking of fatty acids for metabolic and gene-regulatory purposes. FABPs are known to associate with membranes and also enter the nucleus. Using NMR and a human FABP4 (hFABP4) preparation completely free of endogenous ligands, we studied the influence of fatty acids and inhibitors on the conformational flexibility

Three-dimensional (3D) structure determination by single-particle analysis of cryo-electron microscopy (cryo-EM) images requires ab initio 3D reconstruction of density volume(s) from 2D images (particles). This large-scale inverse problem requires the determination of many million degrees of freedom from extremely noisy experimental measurements. Here, we introduce a new approach to probabilistic multi-volume

Factor XIIa (FXIIa) is generated from its zymogen factor XII (FXII) by contact with polyanions such as inorganic polyphosphates. FXIIa cleaves the substrates prekallikrein and factor XI, triggering inflammatory cascades and plasma coagulation. From the N-terminus, FXII has fibronectin type II (FnII), epidermal growth factor-1 (EGF1), fibronectin type I (FnI), EGF2 and kringle domains. The N-terminal

The proteins SFPQ (splicing factor proline- and glutamine-rich) and NONO (non-POU domain-containing octamer-binding protein) are members of the Drosophila behaviour/human splicing (DBHS) protein family, sharing 76% sequence identity in their conserved DBHS domain. These proteins are critical for elements of pre- and post-transcriptional regulation in mammals and are primarily located in paraspeckles:

Two-dimensional X-ray mesh (raster) scans are broadly used in macromolecular crystallography to identify positions on the sample holder at which diffraction images can be collected. Based on a single mesh scan, the commonly known method Mesh&Collect allows the collection of small angular wedges of diffraction data from multiple crystals identified in the mesh-scan area. Recently, the capabilities of

Multi-crystal processing of X-ray diffraction data has become highly automated to keep pace with the current high-throughput capabilities afforded by beamlines. A significant challenge, however, is the automated clustering of such data based on subtle differences such as ligand binding or conformational shifts. Intensity-based hierarchical clustering has been shown to be a viable method of identifying

The SARS-CoV-2 helicase NSP13 is a highly conserved and essential component of the viral replication machinery, making it a promising target for antiviral drug development. Here, we present the 2 Å resolution crystal structure of NSP13 bound to the natural flavonoid myricetin, revealing a conserved allosteric binding site. Guided by these structural findings, a virtual screening campaign identified

The focused issue on Image-processing methods for electron microscopy of biological specimens is introduced. The virtual issue is available at https://journals.iucr.org/special_issues/2025/imageprocessing.

The Mn4Ca cofactor of photosystem II (PSII), which is found in its oxygen-evolving center (OEC), catalyzes the oxidation of water. Spectroscopic studies performed on dark-adapted PSII samples have led to two mutually incompatible hypotheses about the oxidation states of these manganese ions: Mn(III)4 or Mn(III)2Mn(IV)2. It should be possible to determine which is correct crystallographically because

Common bean (Phaseolus vulgaris) encodes three class 2 L-asparaginase enzymes: two potassium-dependent enzymes [PvAIII(K)-1 and PvAIII(K)-2] and a potassium-independent enzyme (PvAIII). Here, we present the crystal structure of PvAIII, which displays a rare P2 space-group symmetry and a unique pseudosymmetric 41-like double-helical packing. The asymmetric unit contains 32 protein chains (16 αβ units

Endo-galactosaminidases are an underexplored family of enzymes involved in the degradation of galactosaminogalactan (GAG) and other galactosamine-containing cationic exopolysaccharides produced by fungi and bacteria. These exopolysaccharides are part of the cell wall and extracellular matrix of microbial communities. Currently, these galactosaminidases are found in three distinct CAZy families: GH114

The structure of the humanized Fab from murine monoclonal antibody 5E5 specific for tumor antigen Tn-MUC1 has been determined to 1.57 Å resolution. Despite undertaking thousands of crystallization trials of the humanized 5E5 (h-5E5) Fab in the presence of either the singly or doubly glycosylated peptide antigens corresponding to Tn-MUC1, the Fab is only observed unliganded in the crystal. The conformations

The surface protein P113 serves as a membrane-anchored protein that tethers the Plasmodium falciparum RH5 complex, including its associated partners CyRPA and RIPR, to the parasite surface. This anchoring mechanism ensures the proper localization and stabilization of RH5, facilitating its critical interaction with the host erythrocyte receptor basigin during erythrocyte invasion. Here, the helical-rich

Despite the parallels between problems in computer vision and cryo-electron microscopy (cryo-EM), many state-of-the-art approaches from computer vision have yet to be adapted for cryo-EM. Within the computer-vision research community, implicits such as neural radiance fields (NeRFs) have enabled the detailed reconstruction of 3D objects from few images at different camera-viewing angles. While other

Green-to-red photoconvertible fluorescent proteins (PCFPs) serve as key players in single-molecule localization super-resolution imaging. As an early engineered variant, mEos3.2 has limited applications, mostly due to its slow maturation rate. The recent advent of a novel variant, pcStar, obtained by the simple mutation of only three amino acids (D28E/L93M/N166G) in mEos3.2, exhibits significantly

A global analysis of protein crystal structures in the Protein Data Bank (PDB) using a newly developed computational approach reveals many pairs with (nearly) identical main-chain coordinates. Such cases are identified and analyzed, showing that duplication is possible since the PDB does not currently have tools or mechanisms that would detect potentially duplicate submissions. Some duplicated entries

Resolving continuous conformational heterogeneity in single-particle cryo-electron microscopy (cryo-EM) is a field in which new methods are now emerging regularly. Methods range from traditional statistical techniques to state-of-the-art neural network approaches. Such ongoing efforts continue to enhance the ability to explore and understand the continuous conformational variations in cryo-EM data

Microorganisms are known to secrete copious amounts of extracellular polymeric substances (EPS) that form complex matrices around the cells to shield them against external stresses, to maintain structural integrity and to influence their environment. Many microorganisms also secrete enzymes that are capable of remodelling or degrading EPS in response to various environmental cues. One key enzyme class

The structure of the N-terminal actin-binding domain of human dystrophin was determined at 1.94 Å resolution. Each chain in the asymmetric unit exists in a `closed' conformation, with the first and second calponin homology (CH) domains directly interacting via a 2500.6 Å2 interface. The positioning of the individual CH domains is comparable to the domain-swapped dimer seen in previous human dystrophin

Cryo-electron tomography is a rapidly developing field for studying macromolecular complexes in their native environments and has the potential to revolutionize our understanding of protein function. However, fast and accurate identification of particles in cryo-tomograms is challenging and represents a significant bottleneck in downstream processes such as subtomogram averaging. Here, we present tomoCPT

Predicting the 3D structure of RNA is a significant challenge despite ongoing advancements in the field. Although AlphaFold has successfully addressed this problem for proteins, RNA structure prediction raises difficulties due to the fundamental differences between proteins and RNA, which hinder its direct adaptation. The latest release of AlphaFold, AlphaFold3, has broadened its scope to include multiple

P-clusters have been statistically analysed using the bond-valence sum (BVS) method together with weighting schemes. The crystallographic data come from the VFe proteins deposited in the Protein Data Bank (PDB) with high resolutions of better than 1.35 Å. Calculations show that the formal oxidation state of a P1+ cluster can be assigned as 2Fe3+6Fe2+ with high electron delocalization, giving the same

The apicomplexan AP2 (ApiAP2) proteins are the best characterized family of DNA-binding proteins in Plasmodium spp. malaria parasites. Apart from the AP2 DNA-binding domain, there is little sequence similarity between ApiAP2 proteins. However, a conserved AP2-coincident domain mostly at the C-terminus (ACDC domain) is observed in a subset of the ApiAP2 proteins. The structure and function of this domain

Recent advances in low-emittance synchrotron X-ray technology and highly sensitive photon-counting detectors have revolutionized protein micro-crystallography in structural biology. These developments and improvements to sample-exchange robots and beamline control have paved the way for automated and efficient unattended data collection. This study analyzed protein crystal structures such as type 2

Molecular replacement (MR) is highly effective for biomolecular crystal structure determination, increasingly so as the database of known structures has increased. For candidates without recognizable similarity to known structures, however, crystal structure analyses have nearly always required experiments for de novo phase evaluation. Now, with the unprecedented accuracy of AlphaFold predictions of

The Guest Editors introduce the special issue based on talks at the CCP4 Study Weekend 2023. The virtual issue is available at https://journals.iucr.org/special_issues/2024/CCP42023/.

A comment on how easy (or difficult) it is to find a stucture of interest and some suggestions on what could be done to start to address the problem.

Metals are essential components for the structure and function of many proteins. However, accurate modelling of their coordination environments remains a challenge due to the complexity and diversity of metal-coordination geometries. To address this, a method is presented for extracting and analysing coordination information, including bond lengths and angles, from the Crystallography Open Database

Recently, the conclusions drawn from crystallographic data about the number of oxygen ligands associated with the CaMn4 cofactor in the oxygen-evolving center (OEC) of Thermosynechococcus vulcanus photosystem II (PSII) have been called into question. Here, using OEC-omit, metal ion-omit and ligand-omit electron-density maps, it is shown that the number of oxygen ligands ranges from three in the functional

The type IV pilus is a diverse molecular machine capable of conferring a variety of functions and is produced by a wide range of bacterial species. The ability of the pilus to perform host-cell adherence makes it a viable target for the development of vaccines against infection by human pathogens such as Pseudomonas aeruginosa. Here, the 1.3 Å resolution crystal structure of the N-terminally truncated

For protein crystals in which more than two thirds of the volume is occupied by solvent, the featureless nature of the solvent region often generates a constraint that is powerful enough to allow direct phasing of X-ray diffraction data. Practical implementation relies on the use of iterative projection algorithms with good global convergence properties to solve the difficult nonconvex phase-retrieval

Pollution from plastics is a global problem that threatens the biosphere for a host of reasons, including the time scale that it takes for most plastics to degrade. Biodegradation is an ideal solution for remediating bioplastic waste as it does not require the high temperatures necessary for thermal degradation and does not introduce additional pollutants into the environment. Numerous organisms can

The availability of highly accurate protein structure predictions from AlphaFold2 (AF2) and similar tools has hugely expanded the applicability of molecular replacement (MR) for crystal structure solution. Many structures can be solved routinely using raw models, structures processed to remove unreliable parts or models split into distinct structural units. There is therefore an open question around

β-Glucosidase from the thermophilic bacterium Caldicellulosiruptor saccharolyticus (Bgl1) has been denoted as having an attractive catalytic profile for various industrial applications. Bgl1 catalyses the final step of in the decomposition of cellulose, an unbranched glucose polymer that has attracted the attention of researchers in recent years as it is the most abundant renewable source of reduced

During the automatic processing of crystallographic diffraction experiments, beamstop shadows are often unaccounted for or only partially masked. As a result of this, outlier reflection intensities are integrated, which is a known issue. Traditional statistical diagnostics have only limited effectiveness in identifying these outliers, here termed Not-Excluded-unMasked-Outliers (NEMOs). The diagnostic