cryoSPARC: algorithms for rapid unsupervised cryo-EM structure determination

Cryo-Electron Microscopy in the Study of Antiviral Innate Immunity

Cryo-electron microscopy is a powerful methodology in structural biology and has been broadly used in high-resolution structure determination for challenging samples, which are not readily available for traditional techniques. In particular, the strength of super macro-complexes and the lack of a need for crystals for cryo-EM make this technique feasible for the structural study of complexes involved in antiviral innate immunity. This chapter presents detailed information and experimental procedures of Cryo-EM for determining the structures of the complexes using STING as an example. The procedures included a sample quality check, high-resolution data acquisition, and image processing for Cryo-EM 3D structure determination.

Elucidating structural variability in p53 conformers using combinatorial refinement strategies and molecular dynamics

Low molecular weight proteins and protein assemblies can now be investigated using cryo-electron microscopy (EM) as a complement to traditional structural biology techniques. It is important, however, to not lose sight of the dynamic information inherent in macromolecules that give rise to their exquisite functionality. As computational methods continue to advance the field of biomedical imaging, so must strategies to resolve the minute details of disease-related entities. Here, we employed combinatorial modeling approaches to assess flexible properties among low molecular weight proteins (~100 kDa or less). Through a blend of rigid body refinement and simulated annealing, we determined new hidden conformations for wild type p53 monomer and dimer forms. Structures for both states converged to yield new conformers, each revealing good stereochemistry and dynamic information about the protein. Based on these insights, we identified fluid parts of p53 that complement the stable central core of the protein responsible for engaging DNA. Molecular dynamics simulations corroborated the modeling results and helped pinpoint the more flexible residues in wild type p53. Overall, the new computational methods may be used to shed light on other small protein features in a vast ensemble of structural data that cannot be easily delineated by other algorithms.

The structure of inactivated mature tick-borne encephalitis virus at 3.0 Å resolution

Tick-borne encephalitis virus (TBEV) causes a severe disease, tick-borne encephalitis (TBE), that has a substantial epidemiological importance for Northern Eurasia. Between 10,000 and 15,000 TBE cases are registered annually despite the availability of effective formaldehyde-inactivated full-virion vaccines due to insufficient vaccination coverage, as well as sporadic cases of vaccine breakthrough. The development of improved vaccines would benefit from the atomic resolution structure of the antigen. Here we report the refined single-particle cryo-electron microscopy (cryo-EM) structure of the inactivated mature TBEV vaccine strain Sofjin-Chumakov (Far-Eastern subtype) at a resolution of 3.0 Å. The increase of the resolution with respect to the previously published structures of TBEV strains Hypr and Kuutsalo-14 (European subtype) was reached due to improvement of the virus sample quality achieved by the optimized preparation methods. All the surface epitopes of TBEV were structurally conserved in the inactivated virions. ELISA studies with monoclonal antibodies supported the hypothesis of TBEV protein shell cross-linking upon inactivation with formaldehyde.

Structural diversity of tick-borne encephalitis virus particles in the inactivated vaccine based on strain Sofjin

The main approach to preventing tick-borne encephalitis (TBE) is vaccination. Formaldehyde-inactivated TBE vaccines have a proven record of safety and efficiency but have never been characterized structurally with atomic resolution. We report a cryoelectron microscopy (cryo-EM) structure of the formaldehyde-inactivated TBE virus (TBEV) of Sofjin-Chumakov strain representing the Far-Eastern subtype. A 3.8 Å resolution reconstruction reveals the structural integrity of the envelope E proteins, specifically the E protein ectodomains. The comparative study shows a high structural similarity to the previously published structures of the TBEV European subtype strains Hypr and Kuutsalo-14. A fraction of inactivated virions exhibits asymmetric features including the deformations of the membrane profile. We propose that the heterogeneity is caused by inactivation and perform a local variability analysis on the small parts of the envelope protein shell to reveal membrane curvature features possibly induced by the inactivation. The results of this study will have implications for the design of novel vaccines against diseases caused by flaviviruses.

A novel bispecific antibody targeting two overlapping epitopes in RBD improves neutralizing potency and breadth against SARS-CoV-2

SARS-CoV-2 has been evolving into a large number of variants, including the highly pathogenic Delta variant, and the currently prevalent Omicron subvariants with extensive evasion capability, which raises an urgent need to develop new broad-spectrum neutralizing antibodies. Herein, we engineer two IgG-(scFv)2 form bispecific antibodies with overlapping epitopes (bsAb1) or non-overlapping epitopes (bsAb2). Both bsAbs are significantly superior to the parental monoclonal antibodies in terms of their antigen-binding and virus-neutralizing activities against all tested circulating SARS-CoV-2 variants including currently dominant JN.1. The bsAb1 can efficiently neutralize all variants insensitive to parental monoclonal antibodies or the cocktail with IC50 lower than 20 ng/mL, even slightly better than bsAb2. Furthermore, the cryo-EM structures of bsAb1 in complex with the Omicron spike protein revealed that bsAb1 with overlapping epitopes effectively locked the S protein, which accounts for its conserved neutralization against Omicron variants. The bispecific antibody strategy engineered from overlapping epitopes provides a novel solution for dealing with viral immune evasion.

Class B1 GPCRs: insights into multireceptor pharmacology for the treatment of metabolic disease

The secretin-like, class B1 subfamily of seven transmembrane-spanning G protein-coupled receptors (GPCRs) consists of 15 members that coordinate important physiological processes. These receptors bind peptide ligands and use a distinct mechanism of activation that is driven by evolutionarily conserved structural features. For the class B1 receptors, the C-terminus of the cognate ligand is initially recognized by the receptor via an N-terminal extracellular domain that forms a hydrophobic ligand-binding groove. This binding enables the N-terminus of the ligand to engage deep into a large volume, open transmembrane pocket of the receptor. Importantly, the phylogenetic basis of this ligand-receptor activation mechanism has provided opportunities to engineer analogs of several class B1 ligands for therapeutic use. Among the most accepted of these are drugs targeting the glucagon-like peptide-1 (GLP-1) receptor for the treatment of type 2 diabetes and obesity. Recently, multifunctional agonists possessing activity at the GLP-1 receptor and the glucose-dependent insulinotropic polypeptide (GIP) receptor, such as tirzepatide, and others that also contain glucagon receptor activity, have been developed. In this article, we review members of the class B1 GPCR family with focus on receptors for GLP-1, GIP, and glucagon, including their signal transduction and receptor trafficking characteristics. The metabolic importance of these receptors is also highlighted, along with the benefit of polypharmacologic ligands. Furthermore, key structural features and comparative analyses of high-resolution cryogenic electron microscopy structures for these receptors in active-state complexes with either native ligands or multifunctional agonists are provided, supporting the pharmacological basis of such therapeutic agents.

Cryo-EM reveals a phosphorylated R-domain envelops the NBD1 catalytic domain in an ABC transporter

Many ATP-binding cassette transporters are regulated by phosphorylation on long and disordered loops which presents a challenge to visualize with structural methods. We have trapped an activated state of the regulatory domain (R-domain) of yeast cadmium factor 1 (Ycf1) by enzymatically enriching the phosphorylated state. A 3.23 Å cryo-EM structure reveals an R-domain structure with four phosphorylated residues and the position for the entire R-domain. The structure reveals key R-domain interactions including a bridging interaction between NBD1 and NBD2 and an interaction with the R-insertion, another regulatory region. We scanned these interactions by systematically replacing segments along the entire R-domain with scrambled combinations of alanine, glycine, and glutamine and probing function under cellular conditions that require the Ycf1 function. We find a close match with these interactions and interacting regions on our R-domain structure that points to the importance of most well-structured segments for function. We propose a model where the R-domain stabilizes a transport-competent state upon phosphorylation by enveloping NBD1 entirely.

Structural roles of Ump1 and β-subunit propeptides in proteasome biogenesis

The yeast pre1-1(β4-S142F) mutant accumulates late 20S proteasome core particle precursor complexes (late-PCs). We report a 2.1 Å cryo-EM structure of this intermediate with full-length Ump1 trapped inside, and Pba1-Pba2 attached to the α-ring surfaces. The structure discloses intimate interactions of Ump1 with β2- and β5-propeptides, which together fill most of the antechambers between the α- and β-rings. The β5-propeptide is unprocessed and separates Ump1 from β6 and β7. The β2-propeptide is disconnected from the subunit by autocatalytic processing and localizes between Ump1 and β3. A comparison of different proteasome maturation states reveals that maturation goes along with global conformational changes in the rings, initiated by structuring of the proteolytic sites and their autocatalytic activation. In the pre1-1 strain, β2 is activated first enabling processing of β1-, β6-, and β7-propeptides. Subsequent maturation of β5 and β1 precedes degradation of Ump1, tightening of the complex, and finally release of Pba1-Pba2.

High-resolution reconstruction of a C. elegans ribosome sheds light on evolutionary dynamics and tissue specificity

Caenorhabditis elegans is an important model organism for human health and disease, with foundational contributions to the understanding of gene expression and tissue patterning in animals. An invaluable tool in modern gene expression research is the presence of a high-resolution ribosome structure, though no such structure exists for C. elegans Here, we present a high-resolution single-particle cryogenic electron microscopy (cryo-EM) reconstruction and molecular model of a C. elegans ribosome, revealing a significantly streamlined animal ribosome. Many facets of ribosome structure are conserved in C. elegans, including overall ribosomal architecture and the mechanism of cycloheximide, whereas other facets, such as expansion segments and eL28, are rapidly evolving. We identify uL5 and uL23 as two instances of tissue-specific ribosomal protein paralog expression conserved in Caenorhabditis, suggesting that C. elegans ribosomes vary across tissues. The C. elegans ribosome structure will provide a basis for future structural, biochemical, and genetic studies of translation in this important animal system.

Structural Basis of Bifunctional CTP/dCTP Synthase

The final step in the de novo synthesis of cytidine 5'-triphosphate (CTP) is catalyzed by CTP synthase (CTPS), which can form cytoophidia in all three domains of life. Recently, we have discovered that CTPS binds to ribonucleotides (NTPs) to form filaments, and have successfully resolved the structures of Drosophila melanogaster CTPS bound with NTPs. Previous biochemical studies have shown that CTPS can bind to deoxyribonucleotides (dNTPs) to produce 2'-deoxycytidine-5'-triphosphate (dCTP). However, the structural basis of CTPS binding to dNTPs is still unclear. In this study, we find that Drosophila CTPS can also form filaments with dNTPs. Using cryo-electron microscopy, we are able to resolve the structure of Drosophila melanogaster CTPS bound to dNTPs with a resolution of up to 2.7 Å. By combining these structural findings with biochemical analysis, we compare the binding and reaction characteristics of NTPs and dNTPs with CTPS. Our results indicate that the same enzyme can act bifunctionally as CTP/dCTP synthase in vitro, and provide a structural basis for these activities.

CryoSeek: A strategy for bioentity discovery using cryoelectron microscopy

Structural biology is experiencing a paradigm shift from targeted structural determination to structure-guided discovery of previously uncharacterized bioentities. We employed cryoelectron microscopy (cryo-EM) to analyze filtered water samples collected from the Tsinghua Lotus Pond. Here, we report the structural determination and characterization of two highly similar helical fibrils, named TLP-1a and TLP-1b, each approximately 8 nm in diameter with a 15-Å wide tunnel. These fibrils are assembled from a similar protein protomer, whose structure was conveniently automodeled in CryoNet. The protomer structure does not match any available experimental structures, but shares the same fold as many predicted structures of unknown functions. The amino-terminal β strand of protomer n + 4 inserts into a cleft in protomer n to complete an immunoglobulin (Ig)-like domain. This packing mechanism, known as donor-strand exchange (DSE), has been observed in several bacterial pilus assemblies, wherein the donor is protomer n + 1. Despite distinct shape and thickness, this reminiscence suggests that TLP-1a/b fibrils may represent uncharacterized bacterial pili. Our study demonstrates an emerging paradigm in structural biology, where high-resolution structural determination precedes and drives the identification and characterization of completely unknown objects.

Structural Basis for the Recognition of GPRC5D by Talquetamab, a Bispecific Antibody for Multiple Myeloma

Multiple myeloma (MM) is a complex hematological malignancy characterized by abnormal antibody production from plasma cells. Despite advances in the treatment, many patients experience disease relapse or become refractory to treatment. G-protein-coupled receptor class C group 5 member D (GPRC5D), an orphan GPCR predominantly expressed in MM cells, is emerging as a promising target for MM immunotherapy. Talquetamab, a Food and Drug Administration-approved T-cell-directing bispecific antibody developed for treatment of MM, targets GPRC5D. Here, we elucidate the structure of GPRC5D complexed with the Fab fragment of talquetamab, using cryo-electron microscopy, providing the basis for recognition of GPRC5D by the bispecific antibody. GPRC5D forms a symmetric homodimer with the interface between transmembrane helix (TM) 4 of one protomer and TM4/5 of the other protomer. A single talquetamab Fab interacts with the GPRC5D dimer with its orientation toward the dimer interface. All six complementarity-determining regions of talquetamab engage with extracellular loops and TM3/5/7. In particular, the side-chain of an arginine residue from the antibody penetrates into a shallow pocket on the extracellular surface of GPRC5D. The structure offers insights for optimizing antibody design against GPRC5D for relapsed or refractory MM therapy.

Conserved N-terminal Regulation of the ACA8 Calcium Pump with Two Calmodulin Binding Sites

The autoinhibited plasma membrane calcium ATPase ACA8 from A. thaliana has an N-terminal autoinhibitory domain. Binding of calcium-loaded calmodulin at two sites located at residues 42-62 and 74-96 relieves autoinhibition of ACA8 activity. Through activity studies and a yeast complementation assay we investigated wild-type (WT) and N-terminally truncated ACA8 constructs (Δ20, Δ30, Δ35, Δ37, Δ40, Δ74 and Δ100) to explore the role of conserved motifs in the N-terminal segment preceding the calmodulin binding sites. Furthermore, we purified WT, Δ20- and Δ100-ACA8, tested activity in vitro and performed structural studies of purified Δ20-ACA8 stabilized in a lipid nanodisc to explore the mechanism of autoinhibition. We show that an N-terminal segment between residues 20 and 35 including conserved Phe32, upstream of the calmodulin binding sites, is important for autoinhibition and the activation by calmodulin. Cryo-EM structure determination at 3.3 Å resolution of a beryllium fluoride inhibited E2 form, and at low resolution for an E1 state combined with AlphaFold prediction provide a model for autoinhibition, consistent with the mutational studies.

A human monoclonal antibody targeting the monomeric N6 neuraminidase confers protection against avian H5N6 influenza virus infection

The influenza neuraminidase (NA) is a potential target for the development of a next-generation influenza vaccine, but its antigenicity is not well understood. Here, we isolate an anti-N6 human monoclonal antibody, named 18_14D, from an H5N6 avian influenza virus (AIV) infected patient. The antibody weakly inhibits enzymatic activity but confers protection in female mice, mainly via ADCC function. The cryo-EM structure shows that 18_14D binds to a unique epitope on the lateral surface of the N6 tetramer, preventing the formation of tightly closed NA tetramers. These findings contribute to the molecular understanding of protective immune responses to NA of AIVs in humans and open an avenue for the rational design of NA-based vaccines.

Single Acetylation-mimetic Mutation in TDP-43 Nuclear Localization Signal Disrupts Importin α1/β Signaling

Cytoplasmic aggregation of the TAR-DNA binding protein of 43 kDa (TDP-43) is the hallmark of sporadic amyotrophic lateral sclerosis (ALS). Most ALS patients with TDP-43 aggregates in neurons and glia do not have mutations in the TDP-43 gene but contain aberrantly post-translationally modified TDP-43. Here, we found that a single acetylation-mimetic mutation (K82Q) near the TDP-43 minor Nuclear Localization Signal (NLS) box, which mimics a post-translational modification identified in an ALS patient, can lead to TDP-43 mislocalization to the cytoplasm and irreversible aggregation. We demonstrate that the acetylation mimetic disrupts binding to importins, halting nuclear import and preventing importin α1/β anti-aggregation activity. We propose that perturbations near the NLS are an additional mechanism by which a cellular insult other than a genetically inherited mutation leads to TDP-43 aggregation and loss of function. Our findings are relevant to deciphering the molecular etiology of sporadic ALS.

Structural basis for difunctional mechanism of m-AMSA against African swine fever virus pP1192R

The African swine fever virus (ASFV) type II topoisomerase (Topo II), pP1192R, is the only known Topo II expressed by mammalian viruses and is essential for ASFV replication in the host cytoplasm. Herein, we report the structures of pP1192R in various enzymatic stages using both X-ray crystallography and single-particle cryo-electron microscopy. Our data structurally define the pP1192R-modulated DNA topology changes. By presenting the A2+-like metal ion at the pre-cleavage site, the pP1192R-DNA-m-AMSA complex structure provides support for the classical two-metal mechanism in Topo II-mediated DNA cleavage and a better explanation for nucleophile formation. The unique inhibitor selectivity of pP1192R and the difunctional mechanism of pP1192R inhibition by m-AMSA highlight the specificity of viral Topo II in the poison binding site. Altogether, this study provides the information applicable to the development of a pP1192R-targeting anti-ASFV strategy.

HURP binding to the vinca domain of β-tubulin accounts for cancer drug resistance

Vinca alkaloids, a class of tubulin-binding agent, are widely used in treating cancer, yet the emerging resistance compromises their efficacy. Hepatoma up-regulated protein (HURP), a microtubule-associated protein displaying heightened expression across various cancer types, reduces cancer cells' sensitivity to vinca-alkaloid drugs upon overexpression. However, the molecular basis behind this drug resistance remains unknown. Here we discover a tubulin-binding domain within HURP, and establish its role in regulating microtubule growth. Cryo-EM analysis reveals interactions between HURP's tubulin-binding domain and the vinca domain on β-tubulin -- the site targeted by vinca alkaloid drugs. Importantly, HURP competes directly with vinorelbine, a vinca alkaloid-based chemotherapeutic agent, countering microtubule growth defects caused by vinorelbine both in vitro and in vivo. Our findings elucidate a mechanism driving drug resistance in HURP-overexpressing cancer cells and emphasize HURP tubulin-binding domain's role in mitotic spindle assembly. This underscores its potential as a therapeutic target to improve cancer treatment.

Structural basis for the concerted antiphage activity in the SIR2-HerA system

Recently, a novel two-gene bacterial defense system against phages, encoding a SIR2 NADase and a HerA ATPase/helicase, has been identified. However, the molecular mechanism of the bacterial SIR2-HerA immune system remains unclear. Here, we determine the cryo-EM structures of SIR2, HerA and their complex from Paenibacillus sp. 453MF in different functional states. The SIR2 proteins oligomerize into a dodecameric ring-shaped structure consisting of two layers of interlocked hexamers, in which each subunit exhibits an auto-inhibited conformation. Distinct from the canonical AAA+ proteins, HerA hexamer alone in this antiphage system adopts a split spiral arrangement, which is stabilized by a unique C-terminal extension. SIR2 and HerA proteins assemble into a ∼1.1 MDa torch-shaped complex to fight against phage infection. Importantly, disruption of the interactions between SIR2 and HerA largely abolishes the antiphage activity. Interestingly, binding alters the oligomer state of SIR2, switching from a dodecamer to a tetradecamer state. The formation of the SIR2-HerA binary complex activates NADase and nuclease activities in SIR2 and ATPase and helicase activities in HerA. Together, our study not only provides a structural basis for the functional communications between SIR2 and HerA proteins, but also unravels a novel concerted antiviral mechanism through NAD+ degradation, ATP hydrolysis, and DNA cleavage.

Assembly of the bacterial ribosome with circularly permuted rRNA

Co-transcriptional assembly is an integral feature of the formation of RNA-protein complexes that mediate translation. For ribosome synthesis, prior studies have indicated that the strict order of transcription of rRNA domains may not be obligatory during bacterial ribosome biogenesis, since a series of circularly permuted rRNAs are viable. In this work, we report the structural insights into assembly of the bacterial ribosome large subunit (LSU) based on cryo-EM density maps of intermediates that accumulate during in vitro ribosome synthesis using a set of circularly permuted (CiPer) rRNAs. The observed ensemble of 23 resolved ribosome large subunit intermediates reveals conserved assembly routes with an underlying hierarchy among cooperative assembly blocks. There are intricate interdependencies for the formation of key structural rRNA helices revealed from the circular permutation of rRNA. While the order of domain synthesis is not obligatory, the order of domain association does appear to proceed with a particular order, likely due to the strong evolutionary pressure on efficient ribosome synthesis. This work reinforces the robustness of the known assembly hierarchy of the bacterial large ribosomal subunit and offers a coherent view of how efficient assembly of CiPer rRNAs can be understood in that context.

Filament structures unveil the dynamic organization of human acetyl-CoA carboxylase

Human acetyl-coenzyme A (CoA) carboxylases (ACCs) catalyze the carboxylation of acetyl-CoA, which is the rate-limiting step in fatty acid synthesis. The molecular mechanism underlying the dynamic organization of ACCs is largely unknown. Here, we determined the cryo-electron microscopy (EM) structure of human ACC1 in its inactive state, which forms a unique filament structure and is in complex with acetyl-CoA. We also determined the cryo-EM structure of human ACC1 activated by dephosphorylation and citrate treatment, at a resolution of 2.55 Å. Notably, the covalently linked biotin binds to a site that is distant from the acetyl-CoA binding site when acetyl-CoA is absent, suggesting a potential coordination between biotin binding and acetyl-CoA binding. These findings provide insights into the structural dynamics and regulatory mechanisms of human ACCs.

Structural basis for synthase activation and cellulose modification in the E. coli Type II Bcs secretion system

Bacterial cellulosic polymers constitute a prevalent class of biofilm matrix exopolysaccharides that are synthesized by several types of bacterial cellulose secretion (Bcs) systems, which include conserved cyclic diguanylate (c-di-GMP)-dependent cellulose synthase modules together with diverse accessory subunits. In E. coli, the biogenesis of phosphoethanolamine (pEtN)-modified cellulose relies on the BcsRQABEFG macrocomplex, encompassing inner-membrane and cytosolic subunits, and an outer membrane porin, BcsC. Here, we use cryogenic electron microscopy to shed light on the molecular mechanisms of BcsA-dependent recruitment and stabilization of a trimeric BcsG pEtN-transferase for polymer modification, and a dimeric BcsF-dependent recruitment of an otherwise cytosolic BcsE2R2Q2 regulatory complex. We further demonstrate that BcsE, a secondary c-di-GMP sensor, can remain dinucleotide-bound and retain the essential-for-secretion BcsRQ partners onto the synthase even in the absence of direct c-di-GMP-synthase complexation, likely lowering the threshold for c-di-GMP-dependent synthase activation. Such activation-by-proxy mechanism could allow Bcs secretion system activity even in the absence of substantial intracellular c-di-GMP increase, and is reminiscent of other widespread synthase-dependent polysaccharide secretion systems where dinucleotide sensing and/or synthase stabilization are carried out by key co-polymerase subunits.

Mechanism of nucleosomal H2A K13/15 monoubiquitination and adjacent dual monoubiquitination by RNF168

The DNA damage repair regulatory protein RNF168, a monomeric RING-type E3 ligase, has a crucial role in regulating cell fate and DNA repair by specific and efficient ubiquitination of the adjacent K13 and K15 (K13/15) sites at the H2A N-terminal tail. However, understanding how RNF168 coordinates with its cognate E2 enzyme UbcH5c to site-specifically ubiquitinate H2A K13/15 has long been hampered by the lack of high-resolution structures of RNF168 and UbcH5c~Ub (ubiquitin) in complex with nucleosomes. Here we developed chemical strategies and determined the cryo-electron microscopy structures of the RNF168-UbcH5c~Ub-nucleosome complex captured in transient H2A K13/15 monoubiquitination and adjacent dual monoubiquitination reactions, providing a 'helix-anchoring' mode for monomeric E3 ligase RNF168 on nucleosome in contrast to the 'compass-binding' mode of dimeric E3 ligases. Our work not only provides structural snapshots of H2A K13/15 site-specific monoubiquitination and adjacent dual monoubiquitination but also offers a near-atomic-resolution structural framework for understanding pathogenic amino acid substitutions and physiological modifications of RNF168.

Architectures of photosynthetic RC-LH1 supercomplexes from Rhodobacter blasticus

The reaction center-light-harvesting complex 1 (RC-LH1) plays an essential role in the primary reactions of bacterial photosynthesis. Here, we present high-resolution structures of native monomeric and dimeric RC-LH1 supercomplexes from Rhodobacter (Rba.) blasticus using cryo-electron microscopy. The RC-LH1 monomer is composed of an RC encircled by an open LH1 ring comprising 15 αβ heterodimers and a PufX transmembrane polypeptide. In the RC-LH1 dimer, two crossing PufX polypeptides mediate dimerization. Unlike Rhodabacter sphaeroides counterpart, Rba. blasticus RC-LH1 dimer has a less bent conformation, lacks the PufY subunit near the LH1 opening, and includes two extra LH1 αβ subunits, forming a more enclosed S-shaped LH1 ring. Spectroscopic assays reveal that these unique structural features are accompanied by changes in the kinetics of quinone/quinol trafficking between RC-LH1 and cytochrome bc1. Our findings reveal the assembly principles and structural variability of photosynthetic RC-LH1 supercomplexes, highlighting diverse strategies used by phototrophic bacteria to optimize light-harvesting and electron transfer in competitive environments.

DNA methylation activates retron Ec86 filaments for antiphage defense

Retrons are a class of multigene antiphage defense systems typically consisting of a retron reverse transcriptase, a non-coding RNA, and a cognate effector. Although triggers for several retron systems have been discovered recently, the complete mechanism by which these systems detect invading phages and mediate defense remains unclear. Here, we focus on the retron Ec86 defense system, elucidating its modes of activation and mechanisms of action. We identified a phage-encoded DNA cytosine methyltransferase (Dcm) as a trigger of the Ec86 system and demonstrated that Ec86 is activated upon multicopy single-stranded DNA (msDNA) methylation. We further elucidated the structure of a tripartite retron Ec86-effector filament assembly that is primed for activation by Dcm and capable of hydrolyzing nicotinamide adenine dinucleotide (NAD+). These findings provide insights into the retron Ec86 defense mechanism and underscore an emerging theme of antiphage defense through supramolecular complex assemblies.

Mechanism of degrader-targeted protein ubiquitinability

Small-molecule degraders of disease-driving proteins offer a clinically proven modality with enhanced therapeutic efficacy and potential to tackle previously undrugged targets. Stable and long-lived degrader-mediated ternary complexes drive fast and profound target degradation; however, the mechanisms by which they affect target ubiquitination remain elusive. Here, we show cryo-EM structures of the VHL Cullin 2 RING E3 ligase with the degrader MZ1 directing target protein Brd4BD2 toward UBE2R1-ubiquitin, and Lys456 at optimal positioning for nucleophilic attack. In vitro ubiquitination and mass spectrometry illuminate a patch of favorably ubiquitinable lysines on one face of Brd4BD2, with cellular degradation and ubiquitinomics confirming the importance of Lys456 and nearby Lys368/Lys445, identifying the "ubiquitination zone." Our results demonstrate the proficiency of MZ1 in positioning the substrate for catalysis, the favorability of Brd4BD2 for ubiquitination by UBE2R1, and the flexibility of CRL2 for capturing suboptimal lysines. We propose a model for ubiquitinability of degrader-recruited targets, providing a mechanistic blueprint for further rational drug design.

Structural basis of antimicrobial membrane coat assembly by human GBP1

Guanylate-binding proteins (GBPs) are interferon-inducible guanosine triphosphate hydrolases (GTPases) mediating host defense against intracellular pathogens. Their antimicrobial activity hinges on their ability to self-associate and coat pathogen-associated compartments or cytosolic bacteria. Coat formation depends on GTPase activity but how nucleotide binding and hydrolysis prime coat formation remains unclear. Here, we report the cryo-electron microscopy structure of the full-length human GBP1 dimer in its guanine nucleotide-bound state and describe the molecular ultrastructure of the GBP1 coat on liposomes and bacterial lipopolysaccharide membranes. Conformational changes of the middle and GTPase effector domains expose the isoprenylated C terminus for membrane association. The α-helical middle domains form a parallel, crossover arrangement essential for coat formation and position the extended effector domain for intercalation into the lipopolysaccharide layer of gram-negative membranes. Nucleotide binding and hydrolysis create oligomeric scaffolds with contractile abilities that promote membrane extrusion and fragmentation. Our data offer a structural and mechanistic framework for understanding GBP1 effector functions in intracellular immunity.

Structures of the Foamy virus fusion protein reveal an unexpected link with the F protein of paramyxo- and pneumoviruses

Foamy viruses (FVs) constitute a subfamily of retroviruses. Their envelope (Env) glycoprotein drives the merger of viral and cellular membranes during entry into cells. The only available structures of retroviral Envs are those from human and simian immunodeficiency viruses from the subfamily of orthoretroviruses, which are only distantly related to the FVs. We report the cryo-electron microscopy structures of the FV Env ectodomain in the pre- and post-fusion states, which unexpectedly demonstrate structural similarity with the fusion protein (F) of paramyxo- and pneumoviruses, implying an evolutionary link between the viral fusogens. We describe the structural features that are unique to the FV Env and propose a mechanistic model for its conformational change, highlighting how the interplay of its structural elements could drive membrane fusion and viral entry. The structural knowledge on the FV Env now provides a framework for functional investigations, which can benefit the design of FV Env variants with improved features for use as gene therapy vectors.

Structural insights into human zinc transporter ZnT1 mediated Zn2+ efflux

Zinc transporter 1 (ZnT1), the principal carrier of cytosolic zinc to the extracellular milieu, is important for cellular zinc homeostasis and resistance to zinc toxicity. Despite recent advancements in the structural characterization of various zinc transporters, the mechanism by which ZnTs-mediated Zn2+ translocation is coupled with H+ or Ca2+ remains unclear. To visualize the transport dynamics, we determined the cryo-electron microscopy (cryo-EM) structures of human ZnT1 at different functional states. ZnT1 dimerizes via extensive interactions between the cytosolic (CTD), the transmembrane (TMD), and the unique cysteine-rich extracellular (ECD) domains. At pH 7.5, both protomers adopt an outward-facing (OF) conformation, with Zn2+ ions coordinated at the TMD binding site by distinct compositions. At pH 6.0, ZnT1 complexed with Zn2+ exhibits various conformations [OF/OF, OF/IF (inward-facing), and IF/IF]. These conformational snapshots, together with biochemical investigation and molecular dynamic simulations, shed light on the mechanism underlying the proton-dependence of ZnT1 transport.

SAR Analysis of Novel Coxsackie virus A9 Capsid Binders

Enterovirus infections are common in humans, yet there are no approved antiviral treatments. In this study we concentrated on inhibition of one of the Enterovirus B (EV-B), namely Coxsackievirus A9 (CVA9), using a combination of medicinal chemistry, virus inhibition assays, structure determination from cryogenic electron microscopy and molecular modeling, to determine the structure activity relationships for a promising class of novel N-phenylbenzylamines. Of the new 29 compounds synthesized, 10 had half maximal effective concentration (EC50) values between 0.64-10.46 μM, and of these, 7 had 50% cytotoxicity concentration (CC50) values higher than 200 μM. In addition, this new series of compounds showed promising physicochemical properties and act through capsid stabilization, preventing capsid expansion and subsequent release of the genome.

A highly conserved SusCD transporter determines the import and species-specific antagonism of Bacteroides ubiquitin homologues

Efficient interbacterial competitions and diverse defensive strategies employed by various bacteria play a crucial role in acquiring a hold within a dense microbial community. The gut symbiont Bacteroides fragilis secretes an antimicrobial ubiquitin homologue (BfUbb) that targets an essential periplasmic PPIase to drive intraspecies bacterial competition. However, the mechanisms by which BfUbb enters the periplasm and its potential for interspecies antagonism remain poorly understood. Here, we employ transposon mutagenesis and identify a highly conserved TonB-dependent transporter SusCD (designated as ButCD) in B. fragilis as the BfUbb transporter. As a putative protein-related nutrient utilization system, ButCD is widely distributed across diverse Bacteroides species with varying sequence similarity, resulting in distinct import efficiency of Bacteroides ubiquitin homologues (BUbb) and thereby determining the species-specific toxicity of BUbb. Cryo-EM structural and functional investigations of the BfUbb-ButCD complex uncover distinctive structural features of ButC that are crucial for its targeting by BfUbb. Animal studies further demonstrate the specific and efficient elimination of enterotoxigenic B. fragilis (ETBF) in the murine gut by BfUbb, suggesting its potential as a therapeutic against ETBF-associated inflammatory bowel disease and colorectal cancer. Our findings provide a comprehensive elucidation of the species-specific toxicity exhibited by BUbb and explore its potential applications.

Structural basis of μ-opioid receptor targeting by a nanobody antagonist

The μ-opioid receptor (μOR), a prototypical G protein-coupled receptor (GPCR), is the target of opioid analgesics such as morphine and fentanyl. Due to the severe side effects of current opioid drugs, there is considerable interest in developing novel modulators of μOR function. Most GPCR ligands today are small molecules, however biologics, including antibodies and nanobodies, represent alternative therapeutics with clear advantages such as affinity and target selectivity. Here, we describe the nanobody NbE, which selectively binds to the μOR and acts as an antagonist. We functionally characterize NbE as an extracellular and genetically encoded μOR ligand and uncover the molecular basis for μOR antagonism by determining the cryo-EM structure of the NbE-μOR complex. NbE displays a unique ligand binding mode and achieves μOR selectivity by interactions with the orthosteric pocket and extracellular receptor loops. Based on a β-hairpin loop formed by NbE that deeply protrudes into the μOR, we design linear and cyclic peptide analogs that recapitulate NbE's antagonism. The work illustrates the potential of nanobodies to uniquely engage with GPCRs and describes lower molecular weight μOR ligands that can serve as a basis for therapeutic developments.

ERK1/2 interaction with DHPS regulates eIF5A deoxyhypusination independently of ERK kinase activity

This study explores a non-kinase effect of extracellular regulated kinases 1/2 (ERK1/2) on the interaction between deoxyhypusine synthase (DHPS) and its substrate, eukaryotic translation initiation factor 5A (eIF5A). We report that Raf/MEK/ERK activation decreases the DHPS-ERK1/2 interaction while increasing DHPS-eIF5A association in cells. We determined the cryoelectron microscopy (cryo-EM) structure of the DHPS-ERK2 complex at 3.5 Å to show that ERK2 hinders substrate entrance to the DHPS active site, subsequently inhibiting deoxyhypusination in vitro. In cells, impairing the ERK2 activation loop, but not the catalytic site, prolongs the DHPS-ERK2 interaction irrespective of Raf/MEK signaling. The ERK2 Ser-Pro-Ser motif, but not the common docking or F-site recognition sites, also regulates this complex. These data suggest that ERK1/2 dynamically regulate the DHPS-eIF5A interaction in response to Raf/MEK activity, regardless of its kinase function. In contrast, ERK1/2 kinase activity is necessary to regulate the expression of DHPS and eIF5A. These findings highlight an ERK1/2-mediated dual kinase-dependent and -independent regulation of deoxyhypusination.

One-step nanoscale expansion microscopy reveals individual protein shapes

The attainable resolution of fluorescence microscopy has reached the subnanometer range, but this technique still fails to image the morphology of single proteins or small molecular complexes. Here, we expand the specimens at least tenfold, label them with conventional fluorophores and image them with conventional light microscopes, acquiring videos in which we analyze fluorescence fluctuations. One-step nanoscale expansion (ONE) microscopy enables the visualization of the shapes of individual membrane and soluble proteins, achieving around 1-nm resolution. We show that conformational changes are readily observable, such as those undergone by the ~17-kDa protein calmodulin upon Ca2+ binding. ONE is also applied to clinical samples, analyzing the morphology of protein aggregates in cerebrospinal fluid from persons with Parkinson disease, potentially aiding disease diagnosis. This technology bridges the gap between high-resolution structural biology techniques and light microscopy, providing new avenues for discoveries in biology and medicine.

Lineage-specific pathogenicity, immune evasion, and virological features of SARS-CoV-2 BA.2.86/JN.1 and EG.5.1/HK.3

SARS-CoV-2 JN.1 with an additional L455S mutation on spike when compared with its parental variant BA.2.86 has outcompeted all earlier variants to become the dominant circulating variant. Recent studies investigated the immune resistance of SARS-CoV-2 JN.1 but additional factors are speculated to contribute to its global dominance, which remain elusive until today. Here, we find that SARS-CoV-2 JN.1 has a higher infectivity than BA.2.86 in differentiated primary human nasal epithelial cells (hNECs). Mechanistically, we demonstrate that the gained infectivity of SARS-CoV-2 JN.1 over BA.2.86 associates with increased entry efficiency conferred by L455S and better spike cleavage in hNECs. Structurally, S455 altered the mode of binding of JN.1 spike protein to ACE2 when compared to BA.2.86 spike at ACE2H34, and modified the internal structure of JN.1 spike protein by increasing the number of hydrogen bonds with neighboring residues. These findings indicate that a single mutation (L455S) enhances virus entry in hNECs and increases immune evasiveness, which contribute to the robust transmissibility of SARS-CoV-2 JN.1. We further evaluate the in vitro and in vivo virological characteristics between SARS-CoV-2 BA.2.86/JN.1 and EG.5.1/HK.3, and identify key lineage-specific features of the two Omicron sublineages that contribute to our understanding on Omicron antigenicity, transmissibility, and pathogenicity.

Structural and functional insights into the T-even type bacteriophage topoisomerase II

T-even type bacteriophages are virulent phages commonly used as model organisms, playing a crucial role in understanding various biological processes. One such process involves the regulation of DNA topology during phage replication upon host infection, governed by type IIA DNA topoisomerases. In spite of various studies on prokaryotic and eukaryotic counterparts, viral topoisomerase II remains insufficiently understood, especially the unique domain composition of T4 phage. In this study, we determine the cryo-EM structures of topoisomerase II from T4 and T6 phages, including full-length structures of both apo and DNA-binding states which have never been determined before. Together with other conformational states, these structures provide an explicit blueprint of mechanisms of phage topoisomerase II. Particularly, the asymmetric dimeric interactions observed in cryo-EM structures of T6 phage topoisomerase II ATPase domain and central domain bound with DNA shed light on the asynchronous ATP usage and asynchronous cleavage of the G-segment DNA, respectively. The elucidation of phage topoisomerase II's structures and functions not only enhances our understanding of mechanisms and evolutionary parallels with prokaryotic and eukaryotic homologs but also highlights its potential as a model for developing type IIA topoisomerase inhibitors.

Targeting RSV-neutralizing B cell receptors with anti-idiotypic antibodies

Respiratory syncytial virus (RSV) causes lower respiratory tract infections with significant morbidity and mortality at the extremes of age. Vaccines based on the viral fusion protein are approved for adults over 60, but infant protection relies on passive immunity via antibody transfer or maternal vaccination. An infant vaccine that rapidly elicits protective antibodies would fulfill a critical unmet need. Antibodies arising from the VH3-21/VL1-40 gene pairing can neutralize RSV without the need for affinity maturation, making them attractive to target through vaccination. Here, we develop an anti-idiotypic monoclonal antibody (ai-mAb) immunogen that is specific for unmutated VH3-21/VL1-40 B cell receptors (BCRs). The ai-mAb efficiently engages B cells with bona fide target BCRs and does not activate off-target non-neutralizing B cells, unlike recombinant pre-fusion (preF) protein used in current RSV vaccines. These results establish proof of concept for using an ai-mAb-derived vaccine to target B cells hardwired to produce RSV-neutralizing antibodies.

Structure of TnsABCD transpososome reveals mechanisms of targeted DNA transposition

Tn7-like transposons are characterized by their ability to insert specifically into host chromosomes. Recognition of the attachment (att) site by TnsD recruits the TnsABC proteins to form the transpososome and facilitate transposition. Although this pathway is well established, atomic-level structural insights of this process remain largely elusive. Here, we present the cryo-electron microscopy (cryo-EM) structures of the TnsC-TnsD-att DNA complex and the TnsABCD transpososome from the Tn7-like transposon in Peltigera membranacea cyanobiont 210A, a type I-B CRISPR-associated transposon. Our structures reveal a striking bending of the att DNA, featured by the intercalation of an arginine side chain of TnsD into a CC/GG dinucleotide step. The TnsABCD transpososome structure reveals TnsA-TnsB interactions and demonstrates that TnsC not only recruits TnsAB but also directly participates in the transpososome assembly. These findings provide mechanistic insights into targeted DNA insertion by Tn7-like transposons, with implications for improving the precision and efficiency of their genome-editing applications.

Cryo-EM structure of orf virus scaffolding protein orfv075

Capsid-like poxvirus scaffold proteins self-assemble into semi-regular lattice that govern the formation of spherical immature virus particles. The scaffolding is a critical step in virus morphogenesis as exemplified by the drug rifampicin that impairs the recruitment of scaffold onto the viral membrane in vaccinia virus (VACV). Here we report cryo-electron microscopy structure of scaffolding protein Orfv075 of orf virus (ORFV) that causes smallpox-like diseases in sheep, goats and occasionally humans via zoonotic infection. We demonstrate that the regions that are involved in intertrimeric interactions for scaffold assembly are largely conserved in comparison to its VACV orthologue protein D13 whose intermediate assembly structures have been previously characterized. By contrast, less conserved regions are located away from these interfaces, indicating both viruses share similar assembly mechanisms. We also show that the phenylalanine-rich binding site of rifampicin in D13 is conserved in Orfv075, and molecular docking simulation confirms similar binding modes. Our study provides structural basis of scaffolding protein as a target for anti-poxvirus treatment across wide range of poxvirus genera.

The discovery and structural basis of two distinct state-dependent inhibitors of BamA

BamA is the central component of the essential β-barrel assembly machine (BAM), a conserved multi-subunit complex that dynamically inserts and folds β-barrel proteins into the outer membrane of Gram-negative bacteria. Despite recent advances in our mechanistic and structural understanding of BamA, there are few potent and selective tool molecules that can bind to and modulate BamA activity. Here, we explored in vitro selection methods and different BamA/BAM protein formulations to discover peptide macrocycles that kill Escherichia coli by targeting extreme conformational states of BamA. Our studies show that Peptide Targeting BamA-1 (PTB1) targets an extracellular divalent cation-dependent binding site and locks BamA into a closed lateral gate conformation. By contrast, PTB2 targets a luminal binding site and traps BamA into an open lateral gate conformation. Our results will inform future antibiotic discovery efforts targeting BamA and provide a template to prospectively discover modulators of other dynamic integral membrane proteins.

Ribosomes hibernate on mitochondria during cellular stress

Cell survival under nutrient-deprived conditions relies on cells' ability to adapt their organelles and rewire their metabolic pathways. In yeast, glucose depletion induces a stress response mediated by mitochondrial fragmentation and sequestration of cytosolic ribosomes on mitochondria. This cellular adaptation promotes survival under harsh environmental conditions; however, the underlying mechanism of this response remains unknown. Here, we demonstrate that upon glucose depletion protein synthesis is halted. Cryo-electron microscopy structure of the ribosomes show that they are devoid of both tRNA and mRNA, and a subset of the particles depicted a conformational change in rRNA H69 that could prevent tRNA binding. Our in situ structural analyses reveal that the hibernating ribosomes tether to fragmented mitochondria and establish eukaryotic-specific, higher-order storage structures by assembling into oligomeric arrays on the mitochondrial surface. Notably, we show that hibernating ribosomes exclusively bind to the outer mitochondrial membrane via the small ribosomal subunit during cellular stress. We identify the ribosomal protein Cpc2/RACK1 as the molecule mediating ribosomal tethering to mitochondria. This study unveils the molecular mechanism connecting mitochondrial stress with the shutdown of protein synthesis and broadens our understanding of cellular responses to nutrient scarcity and cell quiescence.

Structural basis for Vipp1 membrane binding: from loose coats and carpets to ring and rod assemblies

Vesicle-inducing protein in plastids 1 (Vipp1) is critical for thylakoid membrane biogenesis and maintenance. Although Vipp1 has recently been identified as a member of the endosomal sorting complexes required for transport III superfamily, it is still unknown how Vipp1 remodels membranes. Here, we present cryo-electron microscopy structures of Synechocystis Vipp1 interacting with membranes: seven structures of helical and stacked-ring assemblies at 5-7-Å resolution engulfing membranes and three carpet structures covering lipid vesicles at ~20-Å resolution using subtomogram averaging. By analyzing ten structures of N-terminally truncated Vipp1, we show that helix α0 is essential for membrane tubulation and forms the membrane-anchoring domain of Vipp1. Lastly, using a conformation-restrained Vipp1 mutant, we reduced the structural plasticity of Vipp1 and determined two structures of Vipp1 at 3.0-Å resolution, resolving the molecular details of membrane-anchoring and intersubunit contacts of helix α0. Our data reveal membrane curvature-dependent structural transitions from carpets to rings and rods, some of which are capable of inducing and/or stabilizing high local membrane curvature triggering membrane fusion.

Structure of a Rhs effector clade domain provides mechanistic insights into type VI secretion system toxin delivery

The type VI secretion system (T6SS) is a molecular machine utilised by many Gram-negative bacteria to deliver antibacterial toxins into adjacent cells. Here we present the structure of Tse15, a T6SS Rhs effector from the nosocomial pathogen Acinetobacter baumannii. Tse15 forms a triple layered β-cocoon Rhs domain with an N-terminal α-helical clade domain and an unfolded C-terminal toxin domain inside the Rhs cage. Tse15 is cleaved into three domains, through independent auto-cleavage events involving aspartyl protease activity for toxin self-cleavage and a nucleophilic glutamic acid for N-terminal clade cleavage. Proteomic analyses identified that significantly more peptides from the N-terminal clade and toxin domains were secreted than from the Rhs cage, suggesting toxin delivery often occurs without the cage. We propose the clade domain acts as an internal chaperone to mediate toxin tethering to the T6SS machinery. Conservation of the clade domain in other Gram-negative bacteria suggests this may be a common mechanism for delivery.

Cryo-EM structure of the human subcortical maternal complex and the associated discovery of infertility-associated variants

The functionally conserved subcortical maternal complex (SCMC) is essential for early embryonic development in mammals. Reproductive disorders caused by pathogenic variants in NLRP5, TLE6 and OOEP, three core components of the SCMC, have attracted much attention over the past several years. Evaluating the pathogenicity of a missense variant in the SCMC is limited by the lack of information on its structure, although we recently solved the structure of the mouse SCMC and proposed that reproductive disorders caused by pathogenic variants are related to the destabilization of the SCMC core complex. Here we report the cryogenic electron microscopy structure of the human SCMC and uncover that the pyrin domain of NLRP5 is essential for the stability of SCMC. By combining prediction of SCMC stability and in vitro reconstitution, we provide a method for identifying deleterious variants, and we successfully identify a new pathogenic variant of TLE6 (p.A396T). Thus, on the basis of the structure of the human SCMC, we offer a strategy for the diagnosis of reproductive disorders and the discovery of new infertility-associated variants.

Capturing a methanogenic carbon monoxide dehydrogenase/acetyl-CoA synthase complex via cryogenic electron microscopy

Approximately two-thirds of the estimated one-billion metric tons of methane produced annually by methanogens is derived from the cleavage of acetate. Acetate is broken down by a Ni-Fe-S-containing A-cluster within the enzyme acetyl-CoA synthase (ACS) to carbon monoxide (CO) and a methyl group (CH3+). The methyl group ultimately forms the greenhouse gas methane, whereas CO is converted to the greenhouse gas carbon dioxide (CO2) by a Ni-Fe-S-containing C-cluster within the enzyme carbon monoxide dehydrogenase (CODH). Although structures have been solved of CODH/ACS from acetogens, which use these enzymes to make acetate from CO2, no structure of a CODH/ACS from a methanogen has been reported. In this work, we use cryo-electron microscopy to reveal the structure of a methanogenic CODH and CODH/ACS from Methanosarcina thermophila (MetCODH/ACS). We find that the N-terminal domain of acetogenic ACS, which is missing in all methanogens, is replaced by a domain of CODH. This CODH domain provides a channel for CO to travel between the two catalytic Ni-Fe-S clusters. It generates the binding surface for ACS and creates a remarkably similar CO alcove above the A-cluster using residues from CODH rather than ACS. Comparison of our MetCODH/ACS structure with our MetCODH structure reveals a molecular mechanism to restrict gas flow from the CO channel when ACS departs, preventing CO escape into the cell. Overall, these long-awaited structures of a methanogenic CODH/ACS reveal striking functional similarities to their acetogenic counterparts despite a substantial difference in domain organization.

Engineering substrate channeling in a bifunctional terpene synthase

Fusicoccadiene synthase from Phomopsis amygdala (PaFS) is a bifunctional terpene synthase. It contains a prenyltransferase (PT) domain that generates geranylgeranyl diphosphate (GGPP) from dimethylallyl diphosphate and three equivalents of isopentenyl diphosphate, and a cyclase domain that converts GGPP into fusicoccadiene, a precursor of the diterpene glycoside Fusicoccin A. The two catalytic domains are connected by a flexible 69-residue linker. The PT domain mediates oligomerization to form predominantly octamers, with cyclase domains randomly splayed out around the PT core. Surprisingly, despite the random positioning of cyclase domains, substrate channeling is operative in catalysis since most of the GGPP generated by the PT remains on the enzyme for cyclization. Here, we demonstrate that covalent linkage of the PT and cyclase domains is not required for GGPP channeling, although covalent linkage may improve channeling efficiency. Moreover, GGPP competition experiments with other diterpene cyclases indicate that the PaFS PT and cyclase domains are preferential partners regardless of whether they are covalently linked or not. The cryoelectron microscopy structure of the 600-kD "linkerless" construct, in which the 69-residue linker is spliced out and replaced with the tripeptide PTQ, reveals that cyclase pairs associate with all four sides of the PT octamer and exhibit fascinating quaternary structural flexibility. These results suggest that optimal substrate channeling is achieved when a cyclase domain associates with the side of the PT octamer, regardless of whether the two domains are covalently linked and regardless of whether this interaction is transient or locked in place.

Structural basis for receptor-binding domain mobility of the spike in SARS-CoV-2 BA.2.86 and JN.1

Since 2019, SARS-CoV-2 has undergone mutations, resulting in pandemic and epidemic waves. The SARS-CoV-2 spike protein, crucial for cellular entry, binds to the ACE2 receptor exclusively when its receptor-binding domain (RBD) adopts the up-conformation. However, whether ACE2 also interacts with the RBD in the down-conformation to facilitate the conformational shift to RBD-up remains unclear. Herein, we present the structures of the BA.2.86 and the JN.1 spike proteins bound to ACE2. Notably, we successfully observed the ACE2-bound down-RBD, indicating an intermediate structure before the RBD-up conformation. The wider and mobile angle of RBDs in the up-state provides space for ACE2 to interact with the down-RBD, facilitating the transition to the RBD-up state. The K356T, but not N354-linked glycan, contributes to both of infectivity and neutralizing-antibody evasion in BA.2.86. These structural insights the spike-protein dynamics would help understand the mechanisms underlying SARS-CoV-2 infection and its neutralization.

Recognition of phylogenetically diverse pathogens through enzymatically amplified recruitment of RNF213

Innate immunity senses microbial ligands known as pathogen-associated molecular patterns (PAMPs). Except for nucleic acids, PAMPs are exceedingly taxa-specific, thus enabling pattern recognition receptors to detect cognate pathogens while ignoring others. How the E3 ubiquitin ligase RNF213 can respond to phylogenetically distant pathogens, including Gram-negative Salmonella, Gram-positive Listeria, and eukaryotic Toxoplasma, remains unknown. Here we report that the evolutionary history of RNF213 is indicative of repeated adaptation to diverse pathogen target structures, especially in and around its newly identified CBM20 carbohydrate-binding domain, which we have resolved by cryo-EM. We find that RNF213 forms coats on phylogenetically distant pathogens. ATP hydrolysis by RNF213's dynein-like domain is essential for coat formation on all three pathogens studied as is RZ finger-mediated E3 ligase activity for bacteria. Coat formation is not diffusion-limited but instead relies on rate-limiting initiation events and subsequent cooperative incorporation of further RNF213 molecules. We conclude that RNF213 responds to evolutionarily distant pathogens through enzymatically amplified cooperative recruitment.

Structure of the Nipah virus polymerase phosphoprotein complex

The Nipah virus (NiV), a member of the Paramyxoviridae family, is notorious for its high fatality rate in humans. The RNA polymerase machinery of NiV, comprising the large protein L and the phosphoprotein P, is essential for viral replication. This study presents the 2.9-Å cryo-electron microscopy structure of the NiV L-P complex, shedding light on its assembly and functionality. The structure not only demonstrates the molecular details of the conserved N-terminal domain, RNA-dependent RNA polymerase (RdRp), and GDP polyribonucleotidyltransferase of the L protein, but also the intact central oligomerization domain and the C-terminal X domain of the P protein. The P protein interacts extensively with the L protein, forming an antiparallel β-sheet among the P protomers and with the fingers subdomain of RdRp. The flexible linker domain of one P promoter extends its contact with the fingers subdomain to reach near the nascent RNA exit, highlighting the distinct characteristic of the NiV L-P interface. This distinctive tetrameric organization of the P protein and its interaction with the L protein provide crucial molecular insights into the replication and transcription mechanisms of NiV polymerase, ultimately contributing to the development of effective treatments and preventive measures against this Paramyxoviridae family deadly pathogen.

Cryo-EM investigation of ryanodine receptor type 3

Ryanodine Receptor isoform 3 (RyR3) is a large ion channel found in the endoplasmic reticulum membrane of many different cell types. Within the hippocampal region of the brain, it is found in dendritic spines and regulates synaptic plasticity. It controls myogenic tone in arteries and is upregulated in skeletal muscle in early development. RyR3 has a unique functional profile with a very high sensitivity to activating ligands, enabling high gain in Ca2+-induced Ca2+ release. Here we solve high-resolution cryo-EM structures of RyR3 in non-activating and activating conditions, revealing structural transitions that occur during channel opening. Addition of activating ligands yields only open channels, indicating an intrinsically high open probability under these conditions. RyR3 has reduced binding affinity to the auxiliary protein FKBP12.6 due to several sequence variations in the binding interface. We map disease-associated sequence variants and binding sites for known pharmacological agents. The N-terminal region contains ligand binding sites for a putative chloride anion and ATP, both of which are targeted by sequence variants linked to epileptic encephalopathy.

Platelet integrin αIIbβ3 plays a key role in a venous thrombogenesis mouse model

Venous thrombosis (VT) is a common vascular disease associated with reduced survival and a high recurrence rate. VT is initiated by the accumulation of platelets and neutrophils at sites of endothelial cell activation. A role for platelet αIIbβ3 in VT is not established, a task complicated by the increased bleeding risk caused by partial agonists such as tirofiban. Here, we show that m-tirofiban, a modified version of tirofiban, does not agonize αIIbβ3 based on lack of neoepitope expression and the cryo-EM structure of m-tirofiban/full-length αIIbβ3 complex. m-tirofiban abolishes agonist-induced platelet aggregation while preserving clot retraction ex vivo and, unlike tirofiban, it suppresses venous thrombogenesis in a mouse model without increasing bleeding. These findings establish a key role for αIIbβ3 in VT initiation and suggest that m-tirofiban and compounds with a similar structurally-defined mechanism of action merit consideration as potential thromboprophylaxis agents in patients at high risk for VT and hemorrhage.