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34 Janelia Publications

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    02/01/20 | In situ structure determination at nanometer resolution using TYGRESS.
    Song K, Shang Z, Fu X, Lou X, Grigorieff N, Nicastro D
    Nature Methods. 2020 Feb 01;17(2):201-08. doi: 10.1038/s41592-019-0651-0

    The resolution of subtomogram averages calculated from cryo-electron tomograms (cryo-ET) of crowded cellular environments is often limited owing to signal loss in, and misalignment of, the subtomograms. By contrast, single-particle cryo-electron microscopy (SP-cryo-EM) routinely reaches near-atomic resolution of isolated complexes. We report a method called 'tomography-guided 3D reconstruction of subcellular structures' (TYGRESS) that is a hybrid of cryo-ET and SP-cryo-EM, and is able to achieve close-to-nanometer resolution of complexes inside crowded cellular environments. TYGRESS combines the advantages of SP-cryo-EM (images with good signal-to-noise ratio and contrast, as well as minimal radiation damage) and subtomogram averaging (three-dimensional alignment of macromolecules in a complex sample). Using TYGRESS, we determined the structure of the intact ciliary axoneme with up to resolution of 12 Å. These results reveal many structural details that were not visible by cryo-ET alone. TYGRESS is generally applicable to cellular complexes that are amenable to subtomogram averaging.

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    01/27/20 | Structure and assembly of calcium homeostasis modulator proteins.
    Syrjanen JL, Michalski K, Chou T, Grant T, Rao S, Simorowski N, Tucker SJ, Grigorieff N, Furukawa H
    Nature Structural and Molecular Biology. 2020 Jan 27;27(2):150-9. doi: 10.1038/s41594-019-0369-9

    The biological membranes of many cell types contain large-pore channels through which a wide variety of ions and metabolites permeate. Examples include connexin, innexin and pannexin, which form gap junctions and/or bona fide cell surface channels. The most recently identified large-pore channels are the calcium homeostasis modulators (CALHMs), through which ions and ATP permeate in a voltage-dependent manner to control neuronal excitability, taste signaling and pathologies of depression and Alzheimer's disease. Despite such critical biological roles, the structures and patterns of their oligomeric assembly remain unclear. Here, we reveal the structures of two CALHMs, chicken CALHM1 and human CALHM2, by single-particle cryo-electron microscopy (cryo-EM), which show novel assembly of the four transmembrane helices into channels of octamers and undecamers, respectively. Furthermore, molecular dynamics simulations suggest that lipids can favorably assemble into a bilayer within the larger CALHM2 pore, but not within CALHM1, demonstrating the potential correlation between pore size, lipid accommodation and channel activity.

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    01/07/20 | Structure of the vesicular stomatitis virus L protein in complex with Its phosphoprotein cofactor.
    Jenni S, Bloyet L, Diaz-Avalos R, Liang B, Whelan SP, Grigorieff N, Harrison SC
    Cell Reports. 2020 Jan 07;30(1):53-60.e5. doi: 10.1016/j.celrep.2019.12.024

    The large (L) proteins of non-segmented, negative-strand RNA viruses are multifunctional enzymes that produce capped, methylated, and polyadenylated mRNA and replicate the viral genome. A phosphoprotein (P), required for efficient RNA-dependent RNA polymerization from the viral ribonucleoprotein (RNP) template, regulates the function and conformation of the L protein. We report the structure of vesicular stomatitis virus L in complex with its P cofactor determined by electron cryomicroscopy at 3.0 Å resolution, enabling us to visualize bound segments of P. The contacts of three P segments with multiple L domains show how P induces a closed, compact, initiation-competent conformation. Binding of P to L positions its N-terminal domain adjacent to a putative RNA exit channel for efficient encapsidation of newly synthesized genomes with the nucleoprotein and orients its C-terminal domain to interact with an RNP template. The model shows that a conserved tryptophan in the priming loop can support the initiating 5' nucleotide.

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    06/01/17 | Ensemble cryo-EM elucidates the mechanism of translation fidelity.
    Loveland AB, Demo G, Grigorieff N, Korostelev AA
    Nature. 2017 Jun 01;546(7656):113-117. doi: 10.1038/nature22397

    Gene translation depends on accurate decoding of mRNA, the structural mechanism of which remains poorly understood. Ribosomes decode mRNA codons by selecting cognate aminoacyl-tRNAs delivered by elongation factor Tu (EF-Tu). Here we present high-resolution structural ensembles of ribosomes with cognate or near-cognate aminoacyl-tRNAs delivered by EF-Tu. Both cognate and near-cognate tRNA anticodons explore the aminoacyl-tRNA-binding site (A site) of an open 30S subunit, while inactive EF-Tu is separated from the 50S subunit. A transient conformation of decoding-centre nucleotide G530 stabilizes the cognate codon-anticodon helix, initiating step-wise 'latching' of the decoding centre. The resulting closure of the 30S subunit docks EF-Tu at the sarcin-ricin loop of the 50S subunit, activating EF-Tu for GTP hydrolysis and enabling accommodation of the aminoacyl-tRNA. By contrast, near-cognate complexes fail to induce the G530 latch, thus favouring open 30S pre-accommodation intermediates with inactive EF-Tu. This work reveals long-sought structural differences between the pre-accommodation of cognate and near-cognate tRNAs that elucidate the mechanism of accurate decoding.

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    06/25/19 | High-resolution cryo-EM structures of outbreak strain human norovirus shells reveal size variations.
    Jung J, Grant T, Thomas DR, Diehnelt CW, Grigorieff N, Leemor J
    Proceedings of the National Academy of Sciences of the United States of America. 2019 Jun 25;116(26):12828-32. doi: 10.1073/pnas.1903562116

    Noroviruses are a leading cause of foodborne illnesses worldwide. Although GII.4 strains have been responsible for most norovirus outbreaks, the assembled virus shell structures have been available in detail for only a single strain (GI.1). We present high-resolution (2.6- to 4.1-Å) cryoelectron microscopy (cryo-EM) structures of GII.4, GII.2, GI.7, and GI.1 human norovirus outbreak strain virus-like particles (VLPs). Although norovirus VLPs have been thought to exist in a single-sized assembly, our structures reveal polymorphism between and within genogroups, with small, medium, and large particle sizes observed. Using asymmetric reconstruction, we were able to resolve a Zn2+ metal ion adjacent to the coreceptor binding site, which affected the structural stability of the shell. Our structures serve as valuable templates for facilitating vaccine formulations.

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    05/21/19 | In situ structure of rotavirus VP1 RNA-dependent RNA polymerase.
    Jenni S, Salgado EN, Herrmann T, Li Z, Grant T, Grigorieff N, Trapani S, Estrozi LF, Harrison SC
    Journal of Molecular Biology. 2019 Jun 21;431(17):3124-38. doi: 10.1016/j.jmb.2019.06.016

    Rotaviruses, like other non-enveloped, double-strand RNA (dsRNA) viruses, package an RNA-dependent RNA polymerase (RdRp) with each duplex of their segmented genomes. Rotavirus cell entry results in loss of an outer protein layer and delivery into the cytosol of an intact, inner capsid particle (the “double-layer particle” or DLP). The RdRp, designated VP1, is active inside the DLP; each VP1 achieves many rounds of mRNA transcription from its associated genome segment. Previous work has shown that one VP1 molecule lies close to each fivefold axis of the icosahedrally symmetric DLP, just beneath the inner surface of its protein shell, embedded in tightly packed RNA. We have determined a high-resolution structure for the rotavirus VP1 RdRp in situ, by local reconstruction of density around individual fivefold positions. We have analyzed intact virions (“triple-layer particles” or TLPs), non-transcribing DLPs and transcribing DLPs. Outer layer dissociation enables the DLP to synthesize RNA, in vitro as well as in vivo, but appears not to induce any detectable structural change in the RdRp. Addition of NTPs, Mg2+, and S-adenosyl methionine, which allows active transcription, results in conformational rearrangements, in both VP1 and the DLP capsid shell protein, that allow a transcript to exit the polymerase and the particle. The position of VP1 (among the five symmetrically related alternatives) at one vertex does not correlate with its position at other vertices. This stochastic distribution of site occupancies limits long-range order in the 11-segment, dsRNA genome.

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    03/07/19 | Cryo-EM fibril structures from systemic AA amyloidosis reveal the species complementarity of pathological amyloids.
    Liberta F, Loerch S, Rennegarbe M, Schierhorn A, Westermark P, Westermark GT, Hazenberg BP, Grigorieff N, Fändrich M, Schmidt M
    Nature Communications. 2019 Mar 07;10(1):1104. doi: 10.1038/s41467-019-09033-z

    Systemic AA amyloidosis is a worldwide occurring protein misfolding disease of humans and animals. It arises from the formation of amyloid fibrils from the acute phase protein serum amyloid A. Here, we report the purification and electron cryo-microscopy analysis of amyloid fibrils from a mouse and a human patient with systemic AA amyloidosis. The obtained resolutions are 3.0 Å and 2.7 Å for the murine and human fibril, respectively. The two fibrils differ in fundamental properties, such as presence of right-hand or left-hand twisted cross-β sheets and overall fold of the fibril proteins. Yet, both proteins adopt highly similar β-arch conformations within the N-terminal ~21 residues. Our data demonstrate the importance of the fibril protein N-terminus for the stability of the analyzed amyloid fibril morphologies and suggest strategies of combating this disease by interfering with specific fibril polymorphs.

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    11/29/18 | Analysis of discrete local variability and structural covariance in macromolecular assemblies using Cryo-EM and focused classification.
    Zhang C, Cantara W, Jeon Y, Musier-Forsyth K, Grigorieff N, Lyumkis D
    Ultramicroscopy. 2018 Nov 29;203:170. doi: 10.1016/j.ultramic.2018.11.016

    Single-particle electron cryo-microscopy and computational image classification can be used to analyze structural variability in macromolecules and their assemblies. In some cases, a particle may contain different regions that each display a range of distinct conformations. We have developed strategies, implemented within the Frealign and cisTEM image processing packages, to focus classify on specific regions of a particle and detect potential covariance. The strategies are based on masking the region of interest using either a 2-D mask applied to reference projections and particle images, or a 3-D mask applied to the 3-D volume. We show that focused classification approaches can be used to study structural covariance, a concept that is likely to gain more importance as datasets grow in size, allowing the distinction of more structural states and smaller differences between states. Finally, we apply the approaches to an experimental dataset containing the HIV-1 Transactivation Response (TAR) element RNA fused into the large bacterial ribosomal subunit to deconvolve structural mobility within localized regions of interest, and to a dataset containing assembly intermediates of the large subunit to measure structural covariance.

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    04/30/18 | Atomic resolution cryo-EM structure of β-galactosidase.
    Bartesaghi A, Aguerrebere C, Falconieri V, Banerjee S, Earl LA, Zhu X, Grigorieff N, Milne JL, Sapiro G, Wu X, Subramaniam S
    Structure (London, England : 1993). 2018 Apr 30;26(6):848. doi: 10.1016/j.str.2018.04.004

    The advent of direct electron detectors has enabled the routine use of single-particle cryo-electron microscopy (EM) approaches to determine structures of a variety of protein complexes at near-atomic resolution. Here, we report the development of methods to account for local variations in defocus and beam-induced drift, and the implementation of a data-driven dose compensation scheme that significantly improves the extraction of high-resolution information recorded during exposure of the specimen to the electron beam. These advances enable determination of a cryo-EM density map for β-galactosidase bound to the inhibitor phenylethyl β-D-thiogalactopyranoside where the ordered regions are resolved at a level of detail seen in X-ray maps at ∼ 1.5 Å resolution. Using this density map in conjunction with constrained molecular dynamics simulations provides a measure of the local flexibility of the non-covalently bound inhibitor and offers further opportunities for structure-guided inhibitor design.

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    04/10/18 | Structural mechanism of functional modulation by gene splicing in NMDA receptors.
    Regan MC, Grant T, McDaniel MJ, Karakas E, Zhang J, Traynelis SF, Grigorieff N, Furukawa H
    Neuron. 2018 Apr 10;98(3):521-9. doi: 10.1016/j.neuron.2018.03.034

    Alternative gene splicing gives rise to N-methyl-D-aspartate (NMDA) receptor ion channels with defined functional properties and unique contributions to calcium signaling in a given chemical environment in the mammalian brain. Splice variants possessing the exon-5-encoded motif at the amino-terminal domain (ATD) of the GluN1 subunit are known to display robustly altered deactivation rates and pH sensitivity, but the underlying mechanism for this functional modification is largely unknown. Here, we show through cryoelectron microscopy (cryo-EM) that the presence of the exon 5 motif in GluN1 alters the local architecture of heterotetrameric GluN1-GluN2 NMDA receptors and creates contacts with the ligand-binding domains (LBDs) of the GluN1 and GluN2 subunits, which are absent in NMDA receptors lacking the exon 5 motif. The unique interactions established by the exon 5 motif are essential to the stability of the ATD/LBD and LBD/LBD interfaces that are critically involved in controlling proton sensitivity and deactivation.

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