Improvement of cryo-EM maps by density modification

Structure, assembly and inhibition of the Toxoplasma gondii respiratory chain supercomplex

The apicomplexan mitochondrial electron transport chain is essential for parasite survival and displays a divergent subunit composition. Here we report cryo-electron microscopy structures of an apicomplexan III2-IV supercomplex and of the drug target complex III2. The supercomplex structure reveals how clade-specific subunits form an apicomplexan-conserved III2-IV interface with a unique, kinked architecture, suggesting that supercomplexes evolved independently in different eukaryotic lineages. A knockout resulting in supercomplex disassembly challenges the proposed role of III2-IV in electron transfer efficiency as suggested for mammals. Nevertheless, knockout analysis indicates that III2-IV is critical for parasite fitness. The complexes from the model parasite Toxoplasma gondii were inhibited with the antimalarial atovaquone, revealing interactions underpinning species specificity. They were also inhibited with endochin-like quinolone (ELQ)-300, an inhibitor in late-stage preclinical development. Notably, in the apicomplexan binding site, ELQ-300 is flipped compared with related compounds in the mammalian enzyme. On the basis of the binding modes and parasite-specific interactions discovered, we designed more potent ELQs with subnanomolar activity against T. gondii. Our findings reveal critical evolutionary differences in the role of supercomplexes in mitochondrial biology and provide insight into cytochrome b inhibition, informing future drug discovery.

DEK-nucleosome structure shows DEK modulates H3K27me3 and stem cell fate

DEK is a highly conserved chromatin-associated oncoprotein that has important roles in regulating chromatin dynamics and stem cell fate. Dysregulation of DEK is associated with stem cell dysfunction and cancers, including acute myeloid leukemia. Despite its importance in chromatin regulation, the structural mechanisms underlying DEK's interaction with chromatin and its influence on gene regulation remain poorly understood. Here we combined cryogenic electron microscopy (cryo-EM), biochemical and cellular approaches to investigate the molecular mechanisms and functional importance of DEK's interaction with chromatin. Our cryo-EM structures reveal the structural basis of the DEK-nucleosome interaction. Biochemical and cellular results demonstrate that this interaction is crucial for DEK deposition onto chromatin. Furthermore, our results reveal that DEK safeguards mouse embryonic stem cells from acquiring primitive endoderm fates by modulating the repressive histone mark H3K27me3. Together, our study provides crucial molecular insights into the structure and function of DEK, establishing a framework for understanding its roles in chromatin biology and cell fate determination.

Molecular architecture and inhibition mechanism of human ATR-ATRIP

The ataxia telangiectasia-mutated and Rad3-related (ATR) kinase is a master regulator of DNA damage response and replication stress in humans. Targeting ATR is the focus of oncology drug pipelines with a number of potent, selective ATR inhibitors currently in clinical development. Here, we determined the cryo-EM structures of the human ATR-ATRIP complex in the presence of VE-822 and RP-3500, two ATR inhibitors currently in Phase II clinical trials, achieving an overall resolution of approximately 3 Å. These structures yield a near-complete atomic model of the ATR-ATRIP complex, revealing subunit stoichiometry, intramolecular and intermolecular interactions, and critical regulatory sites including an insertion in the PIKK regulatory domain (PRD). Structural comparison provides insights into the modes of action and selectivity of ATR inhibitors. The divergent binding modes near the solvent side and in the rear pocket area of VE-822 and RP-3500, particularly their disparate binding orientations, lead to varying conformational changes in the active site. Surprisingly, one ATR-ATRIP complex binds four VE-822 molecules, with two in the ATR active site and two at the ATR-ATR dimer interface. The binding and selectivity of RP-3500 depend on two bound water molecules, which may be further enhanced by the substitution of these bound waters. Our study provides a structural framework for understanding ATR regulation and holds promise for assisting future efforts in rational drug design targeting ATR.

Structure and mechanism of human vesicular polyamine transporter

Polyamines play essential roles in gene expression and modulate neuronal transmission in mammals. Vesicular polyamine transporters (VPAT) from the SLC18 family exploit the transmembrane H+ gradient to translocate polyamines into secretory vesicles, enabling the quantal release of polyamine neuromodulators and underpinning learning and memory formation. Here, we report the cryo-electron microscopy structures of human VPAT in complex with spermine, spermidine, H+, or tetrabenazine, elucidating discrete lumen-facing states of the antiporter and pivotal interactions between VPAT and its substrate or inhibitor. Leveraging structure-inspired mutagenesis studies and protein structure prediction, we deduce an unforeseen mechanism whereby the polyamine and H+ compete for multiple acidic protein residues both directly and indirectly, and rationalize how the antidopaminergic therapeutic tetrabenazine impedes vesicular transport of polyamines. This study unravels the mechanism of an H+-coupled polyamine antiporter, reveals mechanistic diversity between VPAT and other SLC18 antiporters, and raises new prospects for combating human disorders of polyamine homeostasis.

Structure and mechanism of the plastid/parasite ATP/ADP translocator

Adenosine triphosphate (ATP) is the principal energy currency of all living cells1,2. Metabolically impaired obligate intracellular parasites, such as the human pathogens Chlamydia trachomatis and Rickettsia prowazekii, can acquire ATP from their host cells through a unique ATP/adenosine diphosphate (ADP) translocator, which mediates the import of ATP into and the export of ADP and phosphate out of the parasite cells, thus allowing the exploitation of the energy reserves of host cells (also known as energy parasitism). This type of ATP/ADP translocator also exists in the obligate intracellular endosymbionts of protists and the plastids of plants and algae and has been implicated to play an important role in endosymbiosis3-31. The plastid/parasite type of ATP/ADP translocator is phylogenetically and functionally distinct from the mitochondrial ATP/ADP translocator, and its structure and transport mechanism are still unknown. Here we report the cryo-electron microscopy structures of two plastid/parasite types of ATP/ADP translocators in the apo and substrate-bound states. The ATP/ADP-binding pocket is located at the interface between the N and C domains of the translocator, and a conserved asparagine residue within the pocket is critical for substrate specificity. The translocator operates through a rocker-switch alternating access mechanism involving the relative rotation of the two domains as rigid bodies. Our results provide critical insights for understanding ATP translocation across membranes in energy parasitism and endosymbiosis and offer a structural basis for developing drugs against obligate intracellular parasites.

Molecular basis of pyruvate transport and inhibition of the human mitochondrial pyruvate carrier

The mitochondrial pyruvate carrier transports pyruvate, produced by glycolysis from sugar molecules, into the mitochondrial matrix, as a crucial transport step in eukaryotic energy metabolism. The carrier is a drug target for the treatment of cancers, diabetes mellitus, neurodegeneration, and metabolic dysfunction-associated steatotic liver disease. We have solved the structure of the human MPC1L/MPC2 heterodimer in the inward- and outward-open states by cryo-electron microscopy, revealing its alternating access rocker-switch mechanism. The carrier has a central binding site for pyruvate, which contains an essential lysine and histidine residue, important for its ΔpH-dependent transport mechanism. We have also determined the binding poses of three chemically distinct inhibitor classes, which exploit the same binding site in the outward-open state by mimicking pyruvate interactions and by using aromatic stacking interactions.

Preparation of oxygen-sensitive proteins for high-resolution cryoEM structure determination using blot-free vitrification

High-quality grid preparation for single-particle cryogenic electron microscopy (cryoEM) remains a bottleneck for routinely obtaining high-resolution structures. The issues that arise from traditional grid preparation workflows are particularly exacerbated for oxygen-sensitive proteins, including metalloproteins, whereby oxygen-induced damage and alteration of oxidation states can result in protein inactivation, denaturation, and/or aggregation. Indeed, 99% of the current structures in the EMBD were prepared aerobically and limited successes for anaerobic cryoEM grid preparation exist. Current practices for anaerobic grid preparation involve a vitrification device located in an anoxic chamber, which presents significant challenges including temperature and humidity control, optimization of freezing conditions, costs for purchase and operation, as well as accessibility. Here, we present a streamlined approach that allows for the vitrification of oxygen-sensitive proteins in reduced states using an automated blot-free grid vitrification device - the SPT Labtech chameleon. This robust workflow allows for high-resolution structure determination of dynamic, oxygen-sensitive proteins, of varying complexity and molecular weight.

G-quadruplex-stalled eukaryotic replisome structure reveals helical inchworm DNA translocation

DNA G-quadruplexes (G4s) are non-B-form DNA secondary structures that threaten genome stability by impeding DNA replication. To elucidate how G4s induce replication fork arrest, we characterized fork collisions with preformed G4s in the parental DNA using reconstituted yeast and human replisomes. We demonstrate that a single G4 in the leading strand template is sufficient to stall replisomes by arresting the CMG helicase. Cryo-electron microscopy structures of stalled yeast and human CMG complexes reveal that the folded G4 is lodged inside the central CMG channel, arresting translocation. The G4 stabilizes the CMG at distinct translocation intermediates, suggesting an unprecedented helical inchworm mechanism for DNA translocation. These findings illuminate the eukaryotic replication fork mechanism under normal and perturbed conditions.

Structural basis of ClC-3 inhibition by TMEM9 and PI(3,5)P2

The trafficking and activity of endosomes relies on the exchange of chloride ions and protons by members of the CLC family of chloride channels and transporters, whose mutations are associated with numerous diseases. Despite their critical roles, the mechanisms by which CLC transporters are regulated are poorly understood. Here, we show that two related accessory β-subunits, TMEM9 and TMEM9B, directly interact with ClC-3, -4 and -5. Cryo-EM structures reveal that TMEM9 inhibits ClC-3 by sealing the cytosolic entrance to the Cl- ion pathway. Unexpectedly, we find that PI(3,5)P2 stabilizes the interaction between TMEM9 and ClC-3 and is required for proper regulation of ClC-3 by TMEM9. Collectively, our findings reveal that TMEM9 and PI(3,5)P2 collaborate to regulate endosomal ion homeostasis by modulating the activity of ClC-3.

Structure of the yeast ceramide synthase

Ceramides are essential lipids involved in forming complex sphingolipids and acting as signaling molecules. They result from the N-acylation of a sphingoid base and a CoA-activated fatty acid, a reaction catalyzed by the ceramide synthase (CerS) family of enzymes. Yet, the precise structural details and catalytic mechanisms of CerSs have remained elusive. Here we used cryo-electron microscopy single-particle analysis to unravel the structure of the yeast CerS complex in both an active and a fumonisin B1-inhibited state. Our results reveal the complex's architecture as a dimer of Lip1 subunits bound to the catalytic subunits Lag1 and Lac1. Each catalytic subunit forms a hydrophobic crevice connecting the cytosolic site with the intermembrane space. The active site, located centrally in the tunnel, was resolved in a substrate preloaded state, representing one intermediate in ceramide synthesis. Our data provide evidence for competitive binding of fumonisin B1 to the acyl-CoA-binding tunnel.

Advances in cryo-electron microscopy (cryoEM) for structure-based drug discovery

INTRODUCTION

Macromolecular X-ray crystallography (XRC), nuclear magnetic resonance (NMR), and cryo-electron microscopy (cryoEM) are the primary techniques for determining atomic-level, three-dimensional structures of macromolecules essential for drug discovery. With advancements in artificial intelligence (AI) and cryoEM, the Protein Data Bank (PDB) is solidifying its role as a key resource for 3D macromolecular structures. These developments underscore the growing need for enhanced quality metrics and robust validation standards for experimental structures.

AREAS COVERED

This review examines recent advancements in cryoEM for drug discovery, analyzing structure quality metrics, resolution improvements, metal-ligand and water molecule identification, and refinement software. It compares cryoEM with other techniques like XRC and NMR, emphasizing the global expansion of cryoEM facilities and its increasing significance in drug discovery.

EXPERT OPINION

CryoEM is revolutionizing structural biology and drug discovery, particularly for large, complex structures in induced proximity and antibody-antigen interactions. It supports vaccine design, CAR T-cell optimization, gene editing, and gene therapy. Combined with AI, cryoEM enhances particle identification and 3D structure determination. With recent breakthroughs, cryoEM is emerging as a crucial tool in drug discovery, driving the development of new, effective therapies.

Structural development of the HIV-1 apex-directed PGT145-PGDM1400 antibody lineage

Broadly neutralizing antibodies (bNAbs) targeting the apex of the HIV-1-envelope (Env) trimer comprise the most potent category of HIV-1 bNAbs and have emerged as promising therapeutics. Here, we investigate the development of the HIV-1 apex-directed PGT145-PGDM1400 antibody lineage and report cryo-EM structures at 3.4 Å resolution of PGDM1400 and of an improved PGT145 variant (PGT145-R100aS), each bound to the BG505 Env trimer. Cross-species-based engineering improves PGT145 IC80 breadth to near that of PGDM1400. Despite similar breadth and potency, the two antibodies differ in their residue-level interactions with important apex features, including N160 glycans and apex cavity, with residue 100i of PGT145 (sulfated tyrosine) penetrating ∼7 Å farther than residue 100i of PGDM1400 (aspartic acid). While apex-directed bNAbs from other donors use maturation pathways that often converge on analogous residue-level recognition, our results demonstrate that divergent residue-level recognition can occur within the same lineage, thereby enabling improved coverage of escape variants.

Autoinhibition of dimeric NINJ1 prevents plasma membrane rupture

Lytic cell death culminates in plasma membrane rupture, which releases large intracellular molecules to augment the inflammatory response. Plasma membrane rupture is mediated by the effector membrane protein ninjurin-1 (NINJ1)1, which polymerizes and ruptures the membrane via its hydrophilic face1-4. How NINJ1 is restrained under steady-state conditions to ensure cell survival remains unknown. Here we describe the molecular underpinnings of NINJ1 inhibition. Using cryogenic electron microscopy, we determined the structure of inactive-state mouse NINJ1 bound to the newly developed nanobody Nb538. Inactive NINJ1 forms a face-to-face homodimer by adopting a three-helix conformation with unkinked transmembrane helix 1 (TM1), in contrast to the four-helix TM1-kinked active conformation2-4. Accordingly, endogenous NINJ1 from primary macrophages is a dimer under steady-state conditions. Inactive dimers sequester the membrane rupture-inducing hydrophilic face of NINJ1 and occlude the binding site for kinked TM1 from neighbouring activated NINJ1 molecules. Mutagenesis studies in cells show that destabilization of inactive face-to-face dimers leads to NINJ1-mediated cell death, whereas stabilization of face-to-face dimers inhibits NINJ1 activity. Moreover, destabilizing mutations prompt spontaneous TM1 kink formation, a hallmark of NINJ1 activation. Collectively, our data demonstrate that dimeric NINJ1 is autoinhibited in trans to prevent unprovoked plasma membrane rupture and cell death.

Antibodies disrupt bacterial adhesion by ligand mimicry and allosteric interference

A critical step in infections is the attachment of many microorganisms to host cells using lectins that bind surface glycans, making lectins promising antimicrobial targets. Upon binding mannosylated glycans, FimH, the most studied lectin adhesin of type 1 fimbriae in E. coli, undergoes an allosteric transition from an inactive to an active conformation that can act as a catch-bond. Monoclonal antibodies that alter FimH glycan binding in various ways are available, but the mechanisms of these antibodies remain unclear. Here, we use cryoEM, mass spectrometry, binding assays, and molecular dynamics simulations to determine the structure-function relationships underlying antibody-FimH binding. Our study reveals four distinct antibody mechanisms of action: ligand mimicry by an N-linked, high-mannose glycan; stabilization of the ligand pocket in the inactive state; conformational trapping of the active and inactive states; and locking of the ligand pocket through long-range allosteric effects. These structures reveal multiple mechanisms of antibody responses to an allosteric protein and provide blueprints for new antimicrobial that target adhesins.

Bacterial Rps3 counters oxidative and UV stress by recognizing and processing AP-sites on mRNA via a novel mechanism

Lesions and stable secondary structures in mRNA severely impact the translation efficiency, causing ribosome stalling and collisions. Prokaryotic ribosomal proteins Rps3, Rps4 and Rps5, located in the mRNA entry tunnel, form the mRNA helicase center and unwind stable mRNA secondary structures during translation. However, the mechanism underlying the detection of lesions on translating mRNA is unclear. We used Cryo-EM, biochemical assays, and knockdown experiments to investigate the apurinic/apyrimidinic (AP) endoribonuclease activity of bacterial ribosomes on AP-site containing mRNA. Our biochemical assays show that Rps3, specifically the 130RR131 motif, is important for recognizing and performing the AP-endoribonuclease activity. Furthermore, structural analysis revealed cleaved mRNA product in the 30S ribosome entry tunnel. Additionally, knockdown studies in Mycobacterium tuberculosis confirmed the protective role of Rps3 against oxidative and UV stress. Overall, our results show that prokaryotic Rps3 recognizes and processes AP-sites on mRNA via a novel mechanism that is distinct from eukaryotes.

Two-stage binding of mitochondrial ferredoxin-2 to the core iron-sulfur cluster assembly complex

Iron-sulfur (FeS) protein biogenesis in eukaryotes begins with the de novo assembly of [2Fe-2S] clusters by the mitochondrial core iron-sulfur cluster assembly (ISC) complex. This complex comprises the scaffold protein ISCU2, the cysteine desulfurase subcomplex NFS1-ISD11-ACP1, the allosteric activator frataxin (FXN) and the electron donor ferredoxin-2 (FDX2). The structural interaction of FDX2 with the complex remains unclear. Here, we present cryo-EM structures of the human FDX2-bound core ISC complex showing that FDX2 and FXN compete for overlapping binding sites. FDX2 binds in either a 'distal' conformation, where its helix F interacts electrostatically with an arginine patch of NFS1, or a 'proximal' conformation, where this interaction tightens and the FDX2-specific C terminus binds to NFS1, facilitating the movement of the [2Fe-2S] cluster of FDX2 closer to the ISCU2 FeS cluster assembly site for rapid electron transfer. Structure-based mutational studies verify the contact areas of FDX2 within the core ISC complex.

Gliocidin is a nicotinamide-mimetic prodrug that targets glioblastoma

Glioblastoma is incurable and in urgent need of improved therapeutics1. Here we identify a small compound, gliocidin, that kills glioblastoma cells while sparing non-tumour replicative cells. Gliocidin activity targets a de novo purine synthesis vulnerability in glioblastoma through indirect inhibition of inosine monophosphate dehydrogenase 2 (IMPDH2). IMPDH2 blockade reduces intracellular guanine nucleotide levels, causing nucleotide imbalance, replication stress and tumour cell death2. Gliocidin is a prodrug that is anabolized into its tumoricidal metabolite, gliocidin-adenine dinucleotide (GAD), by the enzyme nicotinamide nucleotide adenylyltransferase 1 (NMNAT1) of the NAD+ salvage pathway. The cryo-electron microscopy structure of GAD together with IMPDH2 demonstrates its entry, deformation and blockade of the NAD+ pocket3. In vivo, gliocidin penetrates the blood-brain barrier and extends the survival of mice with orthotopic glioblastoma. The DNA alkylating agent temozolomide induces Nmnat1 expression, causing synergistic tumour cell killing and additional survival benefit in orthotopic patient-derived xenograft models. This study brings gliocidin to light as a prodrug with the potential to improve the survival of patients with glioblastoma.

Cryo-EM structure of a natural prion: chronic wasting disease fibrils from deer

Chronic wasting disease (CWD) is a widely distributed prion disease of cervids with implications for wildlife conservation and also for human and livestock health. The structures of infectious prions that cause CWD and other natural prion diseases of mammalian hosts have been poorly understood. Here we report a 2.8 Å resolution cryogenic electron microscopy-based structure of CWD prion fibrils from the brain of a naturally infected white-tailed deer expressing the most common wild-type PrP sequence. Like recently solved rodent-adapted scrapie prion fibrils, our atomic model of CWD fibrils contains single stacks of PrP molecules forming parallel in-register intermolecular β-sheets and intervening loops comprising major N- and C-terminal lobes within the fibril cross-section. However, CWD fibrils from a natural cervid host differ markedly from the rodent structures in many other features, including a ~ 180° twist in the relative orientation of the lobes. This CWD structure suggests mechanisms underlying the apparent CWD transmission barrier to humans and should facilitate more rational approaches to the development of CWD vaccines and therapeutics.

Nucleation limited assembly and polarized growth of a de novo-designed allosterically modulatable protein filament

The design of inducibly assembling protein nanomaterials is an outstanding challenge. Here, we describe the computational design of a protein filament formed from a monomeric subunit which binds a peptide ligand. The cryoEM structure of the micron scale fibers is very close to the computational design model. The ligand acts as a tunable allosteric modulator: while not part of the fiber subunit-subunit interfaces, the assembly of the filament is dependent on ligand addition, with longer peptides having more extensive interaction surfaces with the monomer promoting more rapid growth. Seeded growth and capping experiments reveal that the filaments grow primarily from one end. Oligomers containing 12 copies of the peptide ligand nucleate fiber assembly from monomeric subunit and peptide mixtures at concentrations where assembly occurs very slowly, likely by generating critical local concentrations of monomer in the assembly competent conformation. Following filament assembly, the peptide ligand can be exchanged with free peptide in solution, and it can be readily fused to any functional protein of interest, opening the door to a wide variety of tunable engineered materials.

DomainFit: Identification of protein domains in cryo-EM maps at intermediate resolution using AlphaFold2-predicted models

Cryoelectron microscopy (cryo-EM) has revolutionized the structural determination of macromolecular complexes. With the paradigm shift to structure determination of highly complex endogenous macromolecular complexes ex vivo and in situ structural biology, there are an increasing number of structures of native complexes. These complexes often contain unidentified proteins, related to different cellular states or processes. Identifying proteins at resolutions lower than 4 Å remains challenging because side chains cannot be visualized reliably. Here, we present DomainFit, a program for semi-automated domain-level protein identification from cryo-EM maps, particularly at resolutions lower than 4 Å. By fitting domains from AlphaFold2-predicted models into cryo-EM maps, the program performs statistical analyses and attempts to identify the domains and protein candidates forming the density. Using DomainFit, we identified two microtubule inner proteins, one of which contains a CCDC81 domain and is exclusively localized in the proximal region of the doublet microtubule in Tetrahymena thermophila.

Preparation of oxygen-sensitive proteins for high-resolution cryoEM structure determination using (an)aerobic blot-free vitrification

High-quality grid preparation for single-particle cryogenic electron microscopy (cryoEM) remains a bottleneck for routinely obtaining high-resolution structures. The issues that arise from traditional grid preparation workflows are particularly exacerbated for oxygen-sensitive proteins, including metalloproteins, whereby oxygen-induced damage and alteration of oxidation states can result in protein inactivation, denaturation, and/or aggregation. Indeed, 99% of the current structures in the EMBD were prepared aerobically and limited successes for anaerobic cryoEM grid preparation exist. Current practices for anaerobic grid preparation involve a vitrification device located in an anoxic chamber, which presents significant challenges including temperature and humidity control, optimization of freezing conditions, costs for purchase and operation, as well as accessibility. Here, we present a streamlined approach that allows for the (an)aerobic vitrification of oxygen-sensitive proteins using an automated aerobic blot-free grid vitrification device - the SPT Labtech chameleon. This robust workflow allows for high-resolution structure determination of dynamic, oxygen-sensitive proteins, of varying complexity and molecular weight.

De novo design of pH-responsive self-assembling helical protein filaments

Biological evolution has led to precise and dynamic nanostructures that reconfigure in response to pH and other environmental conditions. However, designing micrometre-scale protein nanostructures that are environmentally responsive remains a challenge. Here we describe the de novo design of pH-responsive protein filaments built from subunits containing six or nine buried histidine residues that assemble into micrometre-scale, well-ordered fibres at neutral pH. The cryogenic electron microscopy structure of an optimized design is nearly identical to the computational design model for both the subunit internal geometry and the subunit packing into the fibre. Electron, fluorescent and atomic force microscopy characterization reveal a sharp and reversible transition from assembled to disassembled fibres over 0.3 pH units, and rapid fibre disassembly in less than 1 s following a drop in pH. The midpoint of the transition can be tuned by modulating buried histidine-containing hydrogen bond networks. Computational protein design thus provides a route to creating unbound nanomaterials that rapidly respond to small pH changes.

Multi-modal cryo-EM reveals trimers of protein A10 to form the palisade layer in poxvirus cores

Poxviruses are among the largest double-stranded DNA viruses, with members such as variola virus, monkeypox virus and the vaccination strain vaccinia virus (VACV). Knowledge about the structural proteins that form the viral core has remained sparse. While major core proteins have been annotated via indirect experimental evidence, their structures have remained elusive and they could not be assigned to individual core features. Hence, which proteins constitute which layers of the core, such as the palisade layer and the inner core wall, has remained enigmatic. Here we show, using a multi-modal cryo-electron microscopy (cryo-EM) approach in combination with AlphaFold molecular modeling, that trimers formed by the cleavage product of VACV protein A10 are the key component of the palisade layer. This allows us to place previously obtained descriptions of protein interactions within the core wall into perspective and to provide a detailed model of poxvirus core architecture. Importantly, we show that interactions within A10 trimers are likely generalizable over members of orthopox- and parapoxviruses.

A neutralizing antibody prevents postfusion transition of measles virus fusion protein

Measles virus (MeV) presents a public health threat that is escalating as vaccine coverage in the general population declines and as populations of immunocompromised individuals, who cannot be vaccinated, increase. There are no approved therapeutics for MeV. Neutralizing antibodies targeting viral fusion are one potential therapeutic approach but have not yet been structurally characterized or advanced to clinical use. We present cryo-electron microscopy (cryo-EM) structures of prefusion F alone [2.1-angstrom (Å) resolution], F complexed with a fusion-inhibitory peptide (2.3-Å resolution), F complexed with the neutralizing and protective monoclonal antibody (mAb) 77 (2.6-Å resolution), and an additional structure of postfusion F (2.7-Å resolution). In vitro assays and examination of additional EM classes show that mAb 77 binds prefusion F, arrests F in an intermediate state, and prevents transition to the postfusion conformation. These structures shed light on antibody-mediated neutralization that involves arrest of fusion proteins in an intermediate state.

Program VUE: analysing distributions of cryo-EM projections using uniform spherical grids

Three-dimensional cryo electron microscopy reconstructions are obtained by extracting information from a large number of projections of the object. These projections correspond to different 'views' or 'orientations', i.e. directions in which these projections show the reconstructed object. Uneven distribution of these views and the presence of dominating preferred orientations may distort the reconstructed spatial images. This work describes the program VUE (views on uniform grids for cryo electron microscopy), designed to study such distributions. Its algorithms, based on uniform virtual grids on a sphere, allow an easy calculation and accurate quantitative analysis of the frequency distribution of the views. The key computational element is the Lambert azimuthal equal-area projection of a spherical uniform grid onto a disc. This projection keeps the surface area constant and represents the frequency distribution with no visual bias. Since it has multiple tunable parameters, the program is easily adaptable to individual needs, and to the features of a particular project or of the figure to be produced. It can help identify problems related to an uneven distribution of views. Optionally, it can modify the list of projections, distributing the views more uniformly. The program can also be used as a teaching tool.

Plasmid targeting and destruction by the DdmDE bacterial defence system

Although eukaryotic Argonautes have a pivotal role in post-transcriptional gene regulation through nucleic acid cleavage, some short prokaryotic Argonaute variants (pAgos) rely on auxiliary nuclease factors for efficient foreign DNA degradation1. Here we reveal the activation pathway of the DNA defence module DdmDE system, which rapidly eliminates small, multicopy plasmids from the Vibrio cholerae seventh pandemic strain (7PET)2. Through a combination of cryo-electron microscopy, biochemistry and in vivo plasmid clearance assays, we demonstrate that DdmE is a catalytically inactive, DNA-guided, DNA-targeting pAgo with a distinctive insertion domain. We observe that the helicase-nuclease DdmD transitions from an autoinhibited, dimeric complex to a monomeric state upon loading of single-stranded DNA targets. Furthermore, the complete structure of the DdmDE-guide-target handover complex provides a comprehensive view into how DNA recognition triggers processive plasmid destruction. Our work establishes a mechanistic foundation for how pAgos utilize ancillary factors to achieve plasmid clearance, and provides insights into anti-plasmid immunity in bacteria.

Structural insights into drug transport by an aquaglyceroporin

Pentamidine and melarsoprol are primary drugs used to treat the lethal human sleeping sickness caused by the parasite Trypanosoma brucei. Cross-resistance to these two drugs has recently been linked to aquaglyceroporin 2 of the trypanosome (TbAQP2). TbAQP2 is the first member of the aquaporin family described as capable of drug transport; however, the underlying mechanism remains unclear. Here, we present cryo-electron microscopy structures of TbAQP2 bound to pentamidine or melarsoprol. Our structural studies, together with the molecular dynamic simulations, reveal the mechanisms shaping substrate specificity and drug permeation. Multiple amino acids in TbAQP2, near the extracellular entrance and inside the pore, create an expanded conducting tunnel, sterically and energetically allowing the permeation of pentamidine and melarsoprol. Our study elucidates the mechanism of drug transport by TbAQP2, providing valuable insights to inform the design of drugs against trypanosomiasis.

Cryo-EM structure of wheat ribosome reveals unique features of the plant ribosomes

Plants being sessile organisms exhibit unique features in ribosomes, which might aid in rapid gene expression and regulation in response to varying environmental conditions. Here, we present high-resolution structures of the 60S and 80S ribosomes from wheat, a monocot staple crop plant (Triticum aestivum). While plant ribosomes have unique plant-specific rRNA modification (Cm1847) in the peptide exit tunnel (PET), the zinc-finger motif in eL34 is absent, and uL4 is extended, making an exclusive interaction network. We note differences in the eL15-helix 11 (25S) interaction, eL6-ES7 assembly, and certain rRNA chemical modifications between monocot and dicot ribosomes. In eukaryotes, we observe highly conserved rRNA modification (Gm75) in 5.8S rRNA and a flipped base (G1506) in PET. These features are likely involved in sensing or stabilizing nascent chain. Finally, we discuss the importance of the universal conservation of three consecutive rRNA modifications in all ribosomes for their interaction with A-site aminoacyl-tRNA.

Structural basis of lipid head group entry to the Kennedy pathway by FLVCR1

Phosphatidylcholine and phosphatidylethanolamine, the two most abundant phospholipids in mammalian cells, are synthesized de novo by the Kennedy pathway from choline and ethanolamine, respectively1-6. Despite the essential roles of these lipids, the mechanisms that enable the cellular uptake of choline and ethanolamine remain unknown. Here we show that the protein encoded by FLVCR1, whose mutation leads to the neurodegenerative syndrome posterior column ataxia and retinitis pigmentosa7-9, transports extracellular choline and ethanolamine into cells for phosphorylation by downstream kinases to initiate the Kennedy pathway. Structures of FLVCR1 in the presence of choline and ethanolamine reveal that both metabolites bind to a common binding site comprising aromatic and polar residues. Despite binding to a common site, FLVCR1 interacts in different ways with the larger quaternary amine of choline in and with the primary amine of ethanolamine. Structure-guided mutagenesis identified residues that are crucial for the transport of ethanolamine, but dispensable for choline transport, enabling functional separation of the entry points into the two branches of the Kennedy pathway. Altogether, these studies reveal how FLVCR1 is a high-affinity metabolite transporter that serves as the common origin for phospholipid biosynthesis by two branches of the Kennedy pathway.

Structural insights into the GTP-driven monomerization and activation of a bacterial LRRK2 homolog using allosteric nanobodies

Roco proteins entered the limelight after mutations in human LRRK2 were identified as a major cause of familial Parkinson's disease. LRRK2 is a large and complex protein combining a GTPase and protein kinase activity, and disease mutations increase the kinase activity, while presumably decreasing the GTPase activity. Although a cross-communication between both catalytic activities has been suggested, the underlying mechanisms and the regulatory role of the GTPase domain remain unknown. Several structures of LRRK2 have been reported, but structures of Roco proteins in their activated GTP-bound state are lacking. Here, we use single-particle cryo-electron microscopy to solve the structure of a bacterial Roco protein (CtRoco) in its GTP-bound state, aided by two conformation-specific nanobodies: NbRoco1 and NbRoco2. This structure presents CtRoco in an active monomeric state, featuring a very large GTP-induced conformational change using the LRR-Roc linker as a hinge. Furthermore, this structure shows how NbRoco1 and NbRoco2 collaborate to activate CtRoco in an allosteric way. Altogether, our data provide important new insights into the activation mechanism of Roco proteins, with relevance to LRRK2 regulation, and suggest new routes for the allosteric modulation of their GTPase activity.

Machine learning approaches to cryoEM density modification differentially affect biomacromolecule and ligand density quality

The application of machine learning to cryogenic electron microscopy (cryoEM) data analysis has added a valuable set of tools to the cryoEM data processing pipeline. As these tools become more accessible and widely available, the implications of their use should be assessed. We noticed that machine learning map modification tools can have differential effects on cryoEM densities. In this perspective, we evaluate these effects to show that machine learning tools generally improve densities for biomacromolecules while generating unpredictable results for ligands. This unpredictable behavior manifests both in quantitative metrics of map quality and in qualitative investigations of modified maps. The results presented here highlight the power and potential of machine learning tools in cryoEM, while also illustrating some of the risks of their unexamined use.

Mechanism and structural dynamics of sulfur transfer during de novo [2Fe-2S] cluster assembly on ISCU2

Maturation of iron-sulfur proteins in eukaryotes is initiated in mitochondria by the core iron-sulfur cluster assembly (ISC) complex, consisting of the cysteine desulfurase sub-complex NFS1-ISD11-ACP1, the scaffold protein ISCU2, the electron donor ferredoxin FDX2, and frataxin, a protein dysfunctional in Friedreich's ataxia. The core ISC complex synthesizes [2Fe-2S] clusters de novo from Fe and a persulfide (SSH) bound at conserved cluster assembly site residues. Here, we elucidate the poorly understood Fe-dependent mechanism of persulfide transfer from cysteine desulfurase NFS1 to ISCU2. High-resolution cryo-EM structures obtained from anaerobically prepared samples provide snapshots that both visualize different stages of persulfide transfer from Cys381NFS1 to Cys138ISCU2 and clarify the molecular role of frataxin in optimally positioning assembly site residues for fast sulfur transfer. Biochemical analyses assign ISCU2 residues essential for sulfur transfer, and reveal that Cys138ISCU2 rapidly receives the persulfide without a detectable intermediate. Mössbauer spectroscopy assessing the Fe coordination of various sulfur transfer intermediates shows a dynamic equilibrium between pre- and post-sulfur-transfer states shifted by frataxin. Collectively, our study defines crucial mechanistic stages of physiological [2Fe-2S] cluster assembly and clarifies frataxin's molecular role in this fundamental process.

Structural basis of human Nav1.5 gating mechanisms

Voltage-gated Nav1.5 channels are central to the generation and propagation of cardiac action potentials1. Aberrations in their function are associated with a wide spectrum of cardiac diseases including arrhythmias and heart failure2-5. Despite decades of progress in Nav1.5 biology6-8, the lack of structural insights into intracellular regions has hampered our understanding of its gating mechanisms. Here we present three cryo-EM structures of human Nav1.5 in previously unanticipated open states, revealing sequential conformational changes in gating charges of the voltage-sensing domains (VSDs) and several intracellular regions. Despite the channel being in the open state, these structures show the IFM motif repositioned in the receptor site but not dislodged. In particular, our structural findings highlight a dynamic C-terminal domain (CTD) and III-IV linker interaction, which regulates the conformation of VSDs and pore opening. Electrophysiological studies confirm that disrupting this interaction results in the fast inactivation of Nav1.5. Together, our structure-function studies establish a foundation for understanding the gating mechanisms of Nav1.5 and the mechanisms underlying CTD-related channelopathies.

Removal of Pseudomonas type IV pili by a small RNA virus

The retractile type IV pilus (T4P) is important for virulence of the opportunistic human pathogen Pseudomonas aeruginosa. The single-stranded RNA (ssRNA) phage PP7 binds to T4P and is brought to the cell surface through pilus retraction. Using fluorescence microscopy, we discovered that PP7 detaches T4P, which impairs cell motility and restricts the pathogen's virulence. Using cryo-electron microscopy, mutagenesis, optical trapping, and Langevin dynamics simulation, we resolved the structure of PP7, T4P, and the PP7/T4P complex and showed that T4P detachment is driven by the affinity between the phage maturation protein and its bound pilin, plus the pilus retraction force and speed, and pilus bending. Pilus detachment may be widespread among other ssRNA phages and their retractile pilus systems and offers new prospects for antibacterial prophylaxis and therapeutics.

Cryo-EM Map Anisotropy Can Be Attenuated by Map Post-Processing and a New Method for Its Estimation

One of the most important challenges in cryogenic electron microscopy (cryo-EM) is the substantial number of samples that exhibit preferred orientations, which leads to an uneven coverage of the projection sphere. As a result, the overall quality of the reconstructed maps can be severely affected, as manifested by the presence of anisotropy in the map resolution. Several methods have been proposed to measure the directional resolution of maps in tandem with experimental protocols to address the problem of preferential orientations in cryo-EM. Following these works, in this manuscript we identified one potential limitation that may affect most of the existing methods and we proposed an alternative approach to evaluate the presence of preferential orientations in cryo-EM reconstructions. In addition, we also showed that some of the most recently proposed cryo-EM map post-processing algorithms can attenuate map anisotropy, thus offering alternative visualization opportunities for cases affected by moderate levels of preferential orientations.

Structural basis of DNA recognition of the Campylobacter jejuni CosR regulator

Campylobacter jejuni is a foodborne pathogen commonly found in the intestinal tracts of animals. This pathogen is a leading cause of gastroenteritis in humans. Besides its highly infectious nature, C. jejuni is increasingly resistant to a number of clinically administrated antibiotics. As a consequence, the Centers for Disease Control and Prevention has designated antibiotic-resistant Campylobacter as a serious antibiotic resistance threat in the United States. The C. jejuni CosR regulator is essential to the viability of this bacterium and is responsible for regulating the expression of a number of oxidative stress defense enzymes. Importantly, it also modulates the expression of the CmeABC multidrug efflux system, the most predominant and clinically important system in C. jejuni that mediates resistance to multiple antimicrobials. Here, we report structures of apo-CosR and CosR bound with a 21 bp DNA sequence located at the cmeABC promotor region using both single-particle cryo-electron microscopy and X-ray crystallography. These structures allow us to propose a novel mechanism for CosR regulation that involves a long-distance conformational coupling and rearrangement of the secondary structural elements of the regulator to bind target DNA.

IMPORTANCE

Campylobacter jejuni has emerged as an antibiotic-resistant threat worldwide. CosR is an essential regulator for this bacterium and is important for Campylobacter adaptation to various stresses. Here, we describe the structural basis of CosR binding to target DNA as determined by cryo-electron microscopy and X-ray crystallography. Since CosR is a potential target for intervention, our studies may facilitate the development of novel therapeutics to combat C. jejuni infection.

Structural mechanisms of α7 nicotinic receptor allosteric modulation and activation

The α7 nicotinic acetylcholine receptor is a pentameric ligand-gated ion channel that plays an important role in cholinergic signaling throughout the nervous system. Its unique physiological characteristics and implications in neurological disorders and inflammation make it a promising but challenging therapeutic target. Positive allosteric modulators overcome limitations of traditional α7 agonists, but their potentiation mechanisms remain unclear. Here, we present high-resolution structures of α7-modulator complexes, revealing partially overlapping binding sites but varying conformational states. Structure-guided functional and computational tests suggest that differences in modulator activity arise from the stable rotation of a channel gating residue out of the pore. We extend the study using a time-resolved cryoelectron microscopy (cryo-EM) approach to reveal asymmetric state transitions for this homomeric channel and also find that a modulator with allosteric agonist activity exploits a distinct channel-gating mechanism. These results define mechanisms of α7 allosteric modulation and activation with implications across the pentameric receptor superfamily.

Toxoplasma gondii actin filaments are tuned for rapid disassembly and turnover

The cytoskeletal protein actin plays a critical role in the pathogenicity of the intracellular parasite, Toxoplasma gondii, mediating invasion and egress, cargo transport, and organelle inheritance. Advances in live cell imaging have revealed extensive filamentous actin networks in the Apicomplexan parasite, but there are conflicting data regarding the biochemical and biophysical properties of Toxoplasma actin. Here, we imaged the in vitro assembly of individual Toxoplasma actin filaments in real time, showing that native, unstabilized filaments grow tens of microns in length. Unlike skeletal muscle actin, Toxoplasma filaments intrinsically undergo rapid treadmilling due to a high critical concentration, fast monomer dissociation, and rapid nucleotide exchange. Cryo-EM structures of jasplakinolide-stabilized and native (i.e. unstabilized) filaments show an architecture like skeletal actin, with differences in assembly contacts in the D-loop that explain the dynamic nature of the filament, likely a conserved feature of Apicomplexan actin. This work demonstrates that evolutionary changes at assembly interfaces can tune the dynamic properties of actin filaments without disrupting their conserved structure.

Molecular details of ruthenium red pore block in TRPV channels

Transient receptor potential vanilloid (TRPV) channels play a critical role in calcium homeostasis, pain sensation, immunological response, and cancer progression. TRPV channels are blocked by ruthenium red (RR), a universal pore blocker for a wide array of cation channels. Here we use cryo-electron microscopy to reveal the molecular details of RR block in TRPV2 and TRPV5, members of the two TRPV subfamilies. In TRPV2 activated by 2-aminoethoxydiphenyl borate, RR is tightly coordinated in the open selectivity filter, blocking ion flow and preventing channel inactivation. In TRPV5 activated by phosphatidylinositol 4,5-bisphosphate, RR blocks the selectivity filter and closes the lower gate through an interaction with polar residues in the pore vestibule. Together, our results provide a detailed understanding of TRPV subfamily pore block, the dynamic nature of the selectivity filter and allosteric communication between the selectivity filter and lower gate.

SlyB encapsulates outer membrane proteins in stress-induced lipid nanodomains

The outer membrane in Gram-negative bacteria consists of an asymmetric phospholipid-lipopolysaccharide bilayer that is densely packed with outer-membrane β-barrel proteins (OMPs) and lipoproteins1. The architecture and composition of this bilayer is closely monitored and is essential to cell integrity and survival2-4. Here we find that SlyB, a lipoprotein in the PhoPQ stress regulon, forms stable stress-induced complexes with the outer-membrane proteome. SlyB comprises a 10 kDa periplasmic β-sandwich domain and a glycine zipper domain that forms a transmembrane α-helical hairpin with discrete phospholipid- and lipopolysaccharide-binding sites. After loss in lipid asymmetry, SlyB oligomerizes into ring-shaped transmembrane complexes that encapsulate β-barrel proteins into lipid nanodomains of variable size. We find that the formation of SlyB nanodomains is essential during lipopolysaccharide destabilization by antimicrobial peptides or acute cation shortage, conditions that result in a loss of OMPs and compromised outer-membrane barrier function in the absence of a functional SlyB. Our data reveal that SlyB is a compartmentalizing transmembrane guard protein that is involved in cell-envelope proteostasis and integrity, and suggest that SlyB represents a larger family of broadly conserved lipoproteins with 2TM glycine zipper domains with the ability to form lipid nanodomains.

Discovery of lipid binding sites in a ligand-gated ion channel by integrating simulations and cryo-EM

Ligand-gated ion channels transduce electrochemical signals in neurons and other excitable cells. Aside from canonical ligands, phospholipids are thought to bind specifically to the transmembrane domain of several ion channels. However, structural details of such lipid contacts remain elusive, partly due to limited resolution of these regions in experimental structures. Here, we discovered multiple lipid interactions in the channel GLIC by integrating cryo-electron microscopy and large-scale molecular simulations. We identified 25 bound lipids in the GLIC closed state, a conformation where none, to our knowledge, were previously known. Three lipids were associated with each subunit in the inner leaflet, including a buried interaction disrupted in mutant simulations. In the outer leaflet, two intrasubunit sites were evident in both closed and open states, while a putative intersubunit site was preferred in open-state simulations. This work offers molecular details of GLIC-lipid contacts particularly in the ill-characterized closed state, testable hypotheses for state-dependent binding, and a multidisciplinary strategy for modeling protein-lipid interactions.

Tracing the substrate translocation mechanism in P-glycoprotein

P-glycoprotein (Pgp) is a prototypical ATP-binding cassette (ABC) transporter of great biological and clinical significance.Pgp confers cancer multidrug resistance and mediates the bioavailability and pharmacokinetics of many drugs (Juliano and Ling, 1976; Ueda et al., 1986; Sharom, 2011). Decades of structural and biochemical studies have provided insights into how Pgp binds diverse compounds (Loo and Clarke, 2000; Loo et al., 2009; Aller et al., 2009; Alam et al., 2019; Nosol et al., 2020; Chufan et al., 2015), but how they are translocated through the membrane has remained elusive. Here, we covalently attached a cyclic substrate to discrete sites of Pgp and determined multiple complex structures in inward- and outward-facing states by cryoEM. In conjunction with molecular dynamics simulations, our structures trace the substrate passage across the membrane and identify conformational changes in transmembrane helix 1 (TM1) as regulators of substrate transport. In mid-transport conformations, TM1 breaks at glycine 72. Mutation of this residue significantly impairs drug transport of Pgp in vivo, corroborating the importance of its regulatory role. Importantly, our data suggest that the cyclic substrate can exit Pgp without the requirement of a wide-open outward-facing conformation, diverting from the common efflux model for Pgp and other ABC exporters. The substrate transport mechanism of Pgp revealed here pinpoints critical targets for future drug discovery studies of this medically relevant system.

Structural basis of branching during RNA splicing

Branching is a critical step in RNA splicing that is essential for 5' splice site selection. Recent spliceosome structures have led to competing models for the recognition of the invariant adenosine at the branch point. However, there are no structures of any splicing complex with the adenosine nucleophile docked in the active site and positioned to attack the 5' splice site. Thus we lack a mechanistic understanding of adenosine selection and splice site recognition during RNA splicing. Here we present a cryo-electron microscopy structure of a group II intron that reveals that active site dynamics are coupled to the formation of a base triple within the branch-site helix that positions the 2'-OH of the adenosine for nucleophilic attack on the 5' scissile phosphate. This structure, complemented with biochemistry and comparative analyses to splicing complexes, supports a base triple model of adenosine recognition for branching within group II introns and the evolutionarily related spliceosome.

A unified Watson-Crick geometry drives transcription of six-letter expanded DNA alphabets by E. coli RNA polymerase

Artificially Expanded Genetic Information Systems (AEGIS) add independently replicable unnatural nucleotide pairs to the natural G:C and A:T/U pairs found in native DNA, joining the unnatural pairs through alternative modes of hydrogen bonding. Whether and how AEGIS pairs are recognized and processed by multi-subunit cellular RNA polymerases (RNAPs) remains unknown. Here, we show that E. coli RNAP selectively recognizes unnatural nucleobases in a six-letter expanded genetic system. High-resolution cryo-EM structures of three RNAP elongation complexes containing template-substrate UBPs reveal the shared principles behind the recognition of AEGIS and natural base pairs. In these structures, RNAPs are captured in an active state, poised to perform the chemistry step. At this point, the unnatural base pair adopts a Watson-Crick geometry, and the trigger loop is folded into an active conformation, indicating that the mechanistic principles underlying recognition and incorporation of natural base pairs also apply to AEGIS unnatural base pairs. These data validate the design philosophy of AEGIS unnatural basepairs. Further, we provide structural evidence supporting a long-standing hypothesis that pair mismatch during transcription occurs via tautomerization. Together, our work highlights the importance of Watson-Crick complementarity underlying the design principles of AEGIS base pair recognition.

DomainFit: Identification of Protein Domains in cryo-EM maps at Intermediate Resolution using AlphaFold2-predicted Models

Cryo-electron microscopy (cryo-EM) has revolutionized our understanding of macromolecular complexes, enabling high-resolution structure determination. With the paradigm shift to in situ structural biology recently driven by the ground-breaking development of cryo-focused ion beam milling and cryo-electron tomography, there are an increasing number of structures at sub-nanometer resolution of complexes solved directly within their cellular environment. These cellular complexes often contain unidentified proteins, related to different cellular states or processes. Identifying proteins at resolutions lower than 4 Å remains challenging because the side chains cannot be visualized reliably. Here, we present DomainFit, a program for automated domain-level protein identification from cryo-EM maps at resolutions lower than 4 Å. By fitting domains from artificial intelligence-predicted models such as AlphaFold2-predicted models into cryo-EM maps, the program performs statistical analyses and attempts to identify the proteins forming the density. Using DomainFit, we identified two microtubule inner proteins, one of them, a CCDC81 domain-containing protein, is exclusively localized in the proximal region of the doublet microtubule from the ciliate Tetrahymena thermophila. The flexibility and capability of DomainFit makes it a valuable tool for analyzing in situ structures.

Structural basis for stabilisation of the RAD51 nucleoprotein filament by BRCA2

The BRCA2 tumour suppressor protein preserves genomic integrity via interactions with the DNA-strand exchange RAD51 protein in homology-directed repair. The RAD51-binding TR2 motif at the BRCA2 C-terminus is essential for protection and restart of stalled replication forks. Biochemical evidence shows that TR2 recognises filamentous RAD51, but existing models of TR2 binding to RAD51 lack a structural basis. Here we used cryo-electron microscopy and structure-guided mutagenesis to elucidate the mechanism of TR2 binding to nucleoprotein filaments of human RAD51. We find that TR2 binds across the protomer interface in the filament, acting as a brace for adjacent RAD51 molecules. TR2 targets an acidic-patch motif on human RAD51 that serves as a recruitment hub in fission yeast Rad51 for recombination mediators Rad52 and Rad55-Rad57. Our findings provide a structural rationale for RAD51 filament stabilisation by BRCA2 and reveal a common recruitment mechanism of recombination mediators to the RAD51 filament.

Conformational coupling of redox-driven Na+-translocation in Vibrio cholerae NADH:quinone oxidoreductase

In the respiratory chain, NADH oxidation is coupled to ion translocation across the membrane to build up an electrochemical gradient. In the human pathogen Vibrio cholerae, the sodium-pumping NADH:quinone oxidoreductase (Na+-NQR) generates a sodium gradient by a so far unknown mechanism. Here we show that ion pumping in Na+-NQR is driven by large conformational changes coupling electron transfer to ion translocation. We have determined a series of cryo-EM and X-ray structures of the Na+-NQR that represent snapshots of the catalytic cycle. The six subunits NqrA, B, C, D, E, and F of Na+-NQR harbor a unique set of cofactors that shuttle the electrons from NADH twice across the membrane to quinone. The redox state of a unique intramembranous [2Fe-2S] cluster orchestrates the movements of subunit NqrC, which acts as an electron transfer switch. We propose that this switching movement controls the release of Na+ from a binding site localized in subunit NqrB.

Structural titration reveals Ca2+-dependent conformational landscape of the IP3 receptor

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are endoplasmic reticulum Ca2+ channels whose biphasic dependence on cytosolic Ca2+ gives rise to Ca2+ oscillations that regulate fertilization, cell division and cell death. Despite the critical roles of IP3R-mediated Ca2+ responses, the structural underpinnings of the biphasic Ca2+ dependence that underlies Ca2+ oscillations are incompletely understood. Here, we collect cryo-EM images of an IP3R with Ca2+ concentrations spanning five orders of magnitude. Unbiased image analysis reveals that Ca2+ binding does not explicitly induce conformational changes but rather biases a complex conformational landscape consisting of resting, preactivated, activated, and inhibited states. Using particle counts as a proxy for relative conformational free energy, we demonstrate that Ca2+ binding at a high-affinity site allows IP3Rs to activate by escaping a low-energy resting state through an ensemble of preactivated states. At high Ca2+ concentrations, IP3Rs preferentially enter an inhibited state stabilized by a second, low-affinity Ca2+ binding site. Together, these studies provide a mechanistic basis for the biphasic Ca2+-dependence of IP3R channel activity.

Identification of a carbonic anhydrase-Rubisco complex within the alpha-carboxysome

Carboxysomes are proteinaceous organelles that encapsulate key enzymes of CO2 fixation-Rubisco and carbonic anhydrase-and are the centerpiece of the bacterial CO2 concentrating mechanism (CCM). In the CCM, actively accumulated cytosolic bicarbonate diffuses into the carboxysome and is converted to CO2 by carbonic anhydrase, producing a high CO2 concentration near Rubisco and ensuring efficient carboxylation. Self-assembly of the α-carboxysome is orchestrated by the intrinsically disordered scaffolding protein, CsoS2, which interacts with both Rubisco and carboxysomal shell proteins, but it is unknown how the carbonic anhydrase, CsoSCA, is incorporated into the α-carboxysome. Here, we present the structural basis of carbonic anhydrase encapsulation into α-carboxysomes from Halothiobacillus neapolitanus. We find that CsoSCA interacts directly with Rubisco via an intrinsically disordered N-terminal domain. A 1.98 Å single-particle cryoelectron microscopy structure of Rubisco in complex with this peptide reveals that CsoSCA binding is predominantly mediated by a network of hydrogen bonds. CsoSCA's binding site overlaps with that of CsoS2, but the two proteins utilize substantially different motifs and modes of binding, revealing a plasticity of the Rubisco binding site. Our results advance the understanding of carboxysome biogenesis and highlight the importance of Rubisco, not only as an enzyme but also as a central hub for mediating assembly through protein interactions.

Structure of the transcription open complex of distinct σI factors

Bacterial σI factors of the σ70-family are widespread in Bacilli and Clostridia and are involved in the heat shock response, iron metabolism, virulence, and carbohydrate sensing. A multiplicity of σI paralogues in some cellulolytic bacteria have been shown to be responsible for the regulation of the cellulosome, a multienzyme complex that mediates efficient cellulose degradation. Here, we report two structures at 3.0 Å and 3.3 Å of two transcription open complexes formed by two σI factors, SigI1 and SigI6, respectively, from the thermophilic, cellulolytic bacterium, Clostridium thermocellum. These structures reveal a unique, hitherto-unknown recognition mode of bacterial transcriptional promoters, both with respect to domain organization and binding to promoter DNA. The key characteristics that determine the specificities of the σI paralogues were further revealed by comparison of the two structures. Consequently, the σI factors represent a distinct set of the σ70-family σ factors, thus highlighting the diversity of bacterial transcription.