NrdR is a bacterial transcriptional repressor consisting of a zinc (Zn)-ribbon domain followed by an ATP-cone domain. Understanding its mechanism of action could aid the design of novel antibacterials. NrdR binds specifically to two “NrdR boxes” upstream of ribonucleotide reductase operons, of which Escherichia coli has three: nrdHIEF, nrdDG and nrdAB, in the last of which we identified a new box. We show that E. coli NrdR (EcoNrdR) has similar binding strength to all three sites when loaded with ATP plus deoxyadenosine triphosphate (dATP) or equivalent diphosphate combinations. No other combination of adenine nucleotides promotes binding to DNA. We present crystal structures of EcoNrdR–ATP–dATP and EcoNrdR–ADP–dATP, which are the first high-resolution crystal structures of an NrdR. We have also determined cryo-electron microscopy structures of DNA-bound EcoNrdR–ATP–dATP and novel filaments of EcoNrdR–ATP. Tetrameric forms of EcoNrdR involve alternating interactions between pairs of Zn-ribbon domains and ATP-cones. The structures reveal considerable flexibility in relative orientation of ATP-cones vs Zn-ribbon domains. The structure of DNA-bound EcoNrdR–ATP–dATP shows that significant conformational rearrangements between ATP-cones and Zn-ribbons accompany DNA binding while the ATP-cones retain the same relative orientation. In contrast, ATP-loaded EcoNrdR filaments show rearrangements of the ATP-cone pairs and sequester the DNA-binding residues of NrdR such that they are unable to bind to DNA. Our results, in combination with a previous structural and biochemical study, point to highly flexible EcoNrdR structures that, when loaded with the correct nucleotides, adapt to an optimal promoter-binding conformation.

Botulinum neurotoxins are the causative agents of botulism, a lethal paralytic disease, but are also one of the most commonly used therapeutics for the treatment of numerous neuromuscular conditions. These toxins recognise motor nerve terminals with high specificity and affinity by using a dual binding mechanism involving gangliosides and protein receptors. The initial recognition of gangliosides is crucial for the toxins’ potency. In this study, we employed a synaptosome-binding screening strategy to identify BoNT/A mutants with enhanced ganglioside-binding which translated into improved potency. X-ray crystallography and receptor-binding assays were used to elucidate the molecular mechanisms underlying the increased affinity or altered ganglioside selectivity of these mutants. Our findings provide a basis for the development of BoNT/A variants with enhanced therapeutic potential.

The base excision repair enzyme 8-oxoguanine DNA glycosylase 1 (OGG1) plays a central role in maintaining genome integrity and mediating cellular responses to oxidative stress. As such, it represents an attractive target for pharmaceutical modulation. Small-molecule organocatalytic switches (ORCAs) greatly enhance the rate of OGG1-catalyzed cleavage of DNA abasic sites, thereby accelerating DNA repair. Here, we present the discovery and hit-to-lead optimization of a novel class of highly potent serotonin-derived ORCAs with greatly improved pharmacokinetic properties. Biochemical assays, X-ray crystallography, and molecular dynamics simulations point toward a water-mediated mechanism of activation, distinct from previously proposed Brønsted base-assisted models. These findings establish serotonin-based ORCAs as promising chemical probes and potential leads for therapeutic modulation of OGG1 in oxidative stress-driven diseases.

Insecticidal bacterial proteins play key roles in insect-bacteria interactions and have been used as biopesticides. Here, we identify two insecticidal proteins in Paeniclostridium ghonii, designated PG-toxin 1 (PG1) and PG-toxin 2 (PG2), which are homologs of botulinum neurotoxins (BoNTs). Unlike BoNTs, PG1 and PG2 contain two separate proteins: One is the protease light chain (LC), and the other is the heavy chain containing the translocation domain and the receptor binding domain. Crystal and cryo–electron microscopy structures show a conserved BoNT-like architecture but without an interchain disulfide bond. Functional characterizations establish that the LCs of PG1 and PG2 cleave insect synaptosomal–associated protein 25 (SNAP25), but not human or rat SNAP25, and microinjection of PG1 and PG2 caused paralysis and death in Drosophila and Aedes mosquitoes. These findings identified unique two-component BoNT-like insecticidal proteins, revealing insights into the evolution of the BoNT family of toxins, and broadening our understanding of bacteria that can be used for biopest controls.

8-oxoGuanine DNA glycosylase 1 (OGG1) is the first known target of organocatalytic switches (ORCAs), which rewrite the biochemical function of the enzyme through redirection of its preferred substrate from 8-oxoG to AP sites. Previously, different ORCA chemotypes were shown to enhance the operational pH window for OGG1, possibly through direct involvement in proton transfer events during DNA strand cleavage. Accordingly, compound pK_a is a crucial and necessary consideration for the identification and application of future OGG1 ORCAs. Here, we identify a minimal structure of organocatalytic switches–4-anilino pyridines and 6-anilino pyrimidines–which are dimethyl-amino-pyridine (DMAP)-type Brønsted bases binding the active site of OGG1. Systematic interrogation of compound basicity through modulation of electron-withdrawing (EWG) and electron-donating (EDG) substituents reveals that a pK_a less or equal to the assay pH is a viable parameter for prediction of compound activity. The lead structure (AC₅₀ 13 nM, pK_a 7.0) was then identified as a potent scaffold from a screen in a patient-derived 3D model of metabolic dysfunction-associated steatohepatitis (MASH), where it reduced hepatic fibrosis by 35%. Collectively, these findings deepen the knowledge of this novel modulator class, with important implications for future enzyme targets and probe development.

Listeria monocytogenes is a ubiquitous, psychrotrophic human pathogen that can cause listeriosis, a serious illness for vulnerable populations. Some foods, such as Hispanic-style fresh cheeses like queso fresco, pose a specific risk because there are no widely accepted or available methods for L. monocytogenes mitigation that are both effective and able to maintain the properties of the products. Listeria-specific bacteriophages encode endolysins that can cleave the peptidoglycan layer of L. monocytogenes cells externally, showing promise as a potential solution to this problem. PlyP100, from the GRAS Listeria phage P100, is one such endolysin that can prevent the growth of L. monocytogenes in both lab culture conditions and a miniaturized queso fresco model. In this work, we aimed to understand the structural and functional properties of PlyP100. An AlphaFold prediction suggested the presence of three separate domains (D1, D2, and D3). By solving the crystal structure of D1 and assessing various domain truncations, we present evidence that D1 is responsible for catalytic activity, D3 is sufficient for cell wall binding, and D2 is necessary for full function of the enzyme against live cells. Additionally, we performed point mutations in D1 and compared PlyP100 to proteins with similar structures, including Streptococcus pneumoniae LytA and Listeria endolysin Ply511, to understand its specific enzymatic mechanism and target strain specificity. These insights into the structure and function of PlyP100 will aid future work aiming to engineer better endolysins as safe food antimicrobials.

Galactolipids are characteristic lipids of the photosynthesis membranes of higher plants and cyanobacteria. Due to their close relationship to the stability of the photosystem protein complexes, the biogenesis of galactolipids has been intensively studied on the genetic and molecular levels. There are two major types of galactolipids in chloroplastic membranes: monogalactosyldiacylglycerol and digalactosyldiacylglycerol (DGDG). Under phosphate-limiting conditions, the amount of DGDG increases dramatically to allow for phosphate salvage from phospholipids. In Arabidopsis thaliana, the membrane-associated glycosyltransferase digalactosyldiacylglycerol synthase 2 (atDGD2) is highly responsive to phosphate starvation and is significantly upregulated during such conditions. The lipid galactosylation reactions are also fundamentally interesting as they require a catalyst that is capable of bringing a hydrophilic and lipophilic substrate together at the solution-membrane phase border. Here, we present the X-ray crystal structure of atDGD2, which is the first reported DGDG synthase structure. AtDGD2 is most structurally similar to functionally unrelated GT-B enzymes. Interestingly, in spite of significant donor substrate binding differences, we identified four amino acids (Gly22, His151, Lys243, and Glu321, atDGD2 numbering) which were entirely conserved between the structurally similar enzymes. We also investigated the membrane interaction kinetics and membrane anchoring mechanism of atDGD2. This demonstrated that atDGD2 is membrane-bound but also showed that membrane binding is highly dynamic. Furthermore, our structural information in context of previous biophysical studies highlights regions of the enzyme exhibiting a high degree of structural plasticity, which we propose to be important for allowing atDGD2 to quickly adapt its activity based on the membrane lipid environment.

Botulinum neurotoxin serotype B1 (BoNT/B) is a highly potent neurotoxin and therapeutic agent. Here, we present the structure of the complete 14-subunit (780 kDa) progenitor toxin complex (L-PTC) and of five subcomplexes. The structures show how the toxin interacts with its associated components in their role to protect and deliver BoNT/B across epithelial barriers. Each subcomplex, including the M-PTC, M-PTC-HA70, NTNH-HA70, and HA70 trimer, provides detailed understanding of the assembly mechanism, in which the NTNH-nLoop adopts a unique fold that locks the M-PTC into a central pore formed by HA70. The HA subcomplex presents a tripod architecture with flexible legs that may adapt to the rugged cell surface. Mass photometry reveals the pH dependence of BoNT/B release from the complex which is unexpectedly influenced by the presence of HA70. This study provides the complete L-PTC structure, offering insights into its assemblage and supporting the development of countermeasures and therapeutic applications.

Human MutT homolog 1 (hMTH1) removes damaged nucleotides from the nucleotide pool, preventing their incorporation into DNA. Due to its potential as an anticancer drug target, hMTH1 has been the focus of several inhibitor development studies. Unexpectedly, we show that the anabolic steroid stanozolol (Stz) is a potent nanomolar inhibitor of hMTH1. We present the structure of hMTH1 in complex with Stz, which indicates a unique core scaffold that could be exploited for future inhibitor development. Comparison with human protein structures bound with dihydrotestosterone (DHT) shows hMTH1 is entirely unrelated in terms of its structure. As these DHT binding proteins are all involved in steroid regulation, this makes the identification of Stz as a potent hMTH1 inhibitor all the more unusual.

Fragment-based screening can catalyze drug discovery by identifying novel scaffolds, but this approach is limited by the small chemical libraries studied by biophysical experiments and the challenging optimization process. To expand the explored chemical space, we employ structure-based docking to evaluate orders-of-magnitude larger libraries than those used in traditional fragment screening. We computationally dock a set of 14 million fragments to 8-oxoguanine DNA glycosylase (OGG1), a difficult drug target involved in cancer and inflammation, and evaluate 29 highly ranked compounds experimentally. Four of these bind to OGG1 and X-ray crystallography confirms the binding modes predicted by docking. Furthermore, we show how fragment elaboration using searches among billions of readily synthesizable compounds identifies submicromolar inhibitors with anti-inflammatory and anti-cancer effects in cells. Comparisons of virtual screening strategies to explore a chemical space of 10²² compounds illustrate that fragment-based design enables enumeration of all molecules relevant for inhibitor discovery. Virtual fragment screening is hence a highly efficient strategy for navigating the rapidly growing combinatorial libraries and can serve as a powerful tool to accelerate drug discovery efforts for challenging therapeutic targets.