11 Mar 2021

A small molecule compound with an indole moiety inhibits the main protease of SARS-CoV-2 and blocks virus replication (Nature Communications)

Except remdesivir, no specific antivirals for SARS-CoV-2 infection are currently available. Here, H. Mitsuya et. al. characterize two small-molecule-compounds, named GRL-1720 and 5h, containing an indoline and indole moiety, respectively, which target the SARS-CoV-2 main protease (Mpro). They use VeroE6 cell-based assays with RNA-qPCR, cytopathic assays, and immu- nocytochemistry, and show both compounds to block the infectivity of SARS-CoV-2 with EC50 values of 15 ± 4 and 4.2 ± 0.7 μM for GRL-1720 and 5h, respectively. Remdesivir permitted viral breakthrough at high concentrations; however, compound 5h completely blocks SARS- CoV-2 infection in vitro without viral breakthrough or detectable cytotoxicity. Combination of 5h and remdesivir exhibits synergism against SARS-CoV-2. Additional X-ray structural analysis show that 5h forms a covalent bond with Mpro and makes polar interactions with multiple active site amino acid residues. The present data suggest that 5h might serve as a lead Mpro inhibitor for the development of therapeutics for SARS-CoV-2 infection.

19 Feb 2021

Proton-Coupled Conformational Activation of SARS Coronavirus Main Proteases and Opportunity for Designing Small-Molecule Broad-Spectrum Targeted Covalent Inhibitors (JACS)

The SARS coronavirus 2 (SARS-CoV-2) main protease (Mpro) is an attractive broad-spectrum antiviral drug target. Despite the enormous progress in structureelucidation, the Mpro's structure-function relationship remains poorly understood. Recently, a peptidomimetic inhibitor has entered clinical trial; however, small-molecule orally available antiviral drugs have yet to be developed. Intrigued by a long-standing controversy regarding the existence of an inactive state, J. Shen et al. explored the proton-coupled dynamics of the Mpros of SARS-CoV-2 and the closely related SARS-CoV using a newly developed continuous constant pH molecular dynamics (MD) method and microsecond fixed-charge all-atom MD simulations. Their data supports a general base mechanism for Mpro's proteolytic function. The simulations revealed that protonation of His172 alters a conserved interaction network that upholds the oxyanion loop, leading to a partial collapse of the conserved S1 pocket, consistent with the first and controversial crystal structure of SARS-CoV Mpro determined at pH 6. Interestingly, a natural flavonoid binds SARS-CoV-2 Mpro in the close proximity to a conserved cysteine (Cys44), which is hyper-reactive according to the CpHMD titration. This finding offers an exciting new opportunity for small-molecule targeted covalent inhibitor design. Theirwork represents a first step toward the mechanistic understanding of the proton-coupled structure-dynamics-function relationship of CoV Mpros; the proposed strategy of designing small-molecule covalent inhibitors may help accelerate the development of orally available broad-spectrum antiviral drugs to stop the current pandemic and prevent future. 

19 Feb 2021

The SARS-CoV-2 nucleocapsid phosphoprotein forms mutually exclusive condensates with RNA and the membrane-associated M protein (Nature Communications)

The multifunctional nucleocapsid (N) protein in SARS-CoV-2 binds the similar to 30kb viral RNA genome to aid its packaging into the 80-90nm membrane-enveloped virion. The N protein is composed of N-terminal RNA-binding and C-terminal dimerization domains that are flanked by three intrinsically disordered regions. Here D.W. Cleveland, K.D. Corbett et.al.  demonstrate that the N protein's central disordered domain drives phase separation with RNA, and that phosphorylation of an adjacent serine/arginine rich region modulates the physical properties of the resulting condensates. In cells, N forms condensates that recruit the stress granule protein G3BP1, highlighting a potential role for N in G3BP1 sequestration and stress granule inhibition. The SARS-CoV-2 membrane (M) protein independently induces N protein phase separation, and three-component mixtures of N+M+RNA form condensates with mutually exclusive compartments containing N+M or N+RNA, including annular structures in which the M protein coats the outside of an N+RNA condensate. These findings support a model in which phase separation of the SARS-CoV-2 N protein contributes both to suppression of the G3BP1-dependent host immune response and to packaging genomic RNA during virion assembly. The SARS-CoV-2 nucleocapsid (N) protein binds the viral RNA genome and contains two ordered domains flanked by three intrinsically-disordered regions. Here, the authors show that RNA binding induces liquid-liquid phase separation of N, which is driven by its central intrinsically-disordered region and is modulated by phosphorylation. The SARS-CoV-2 Membrane (M) protein also phase-separates with N, and three-component mixtures of N+M+RNA form mutually exclusive compartments containing N+M or N+RNA.

4 May 2021

Single-molecule FRET imaging of GPCR dimers in living cells (Nature Methods)

Class C G protein-coupled receptors (GPCRs) are known to form stable homodimers or heterodimers critical for function, but the oligomeric status of class A and B receptors, which constitute >90% of all GPCRs, remains hotly debated. Single-molecule fluorescence resonance energy transfer (smFRET) is a powerful approach with the potential to reveal valuable insights into GPCR organization but has rarely been used in living cells to study protein systems. Here, J. A. Javitch et. al. report generally applicable methods for using smFRET to detect and track transmembrane proteins diffusing within the plasma membrane of mammalian cells. They leverage this in-cell smFRET approach to show agonist-induced structural dynamics within individual metabotropic glutamate receptor dimers. They apply these methods to representative class A, B and C receptors, finding evidence for receptor monomers, density-dependent dimers and constitutive dimers, respectively.

26 Apr 2021

Structures of human dual oxidase 1 complex in low-calcium and high-calcium states (Nature Communications)

Dual oxidases (DUOXs) produce hydrogen peroxide by transferring electrons from intra- cellular NADPH to extracellular oxygen. They are involved in many crucial biological processes and human diseases, especially in thyroid diseases. DUOXs are protein complexes co-assembled from the catalytic DUOX subunits and the auxiliary DUOXA subunits and their activities are regulated by intracellular calcium concentrations. Here, L. Chen et. al. report the cryo-EM structures of human DUOX1-DUOXA1 complex in both high-calcium and low-calcium states.These structures reveal the DUOX1 complex is a symmetric 2:2 hetero-tetramer stabilized by extensive inter-subunit interactions. Substrate NADPH and cofactor FAD are sandwiched between transmembrane domain and the cytosolic dehydrogenase domain of DUOX. In the presence of calcium ions, intracellular EF-hand modules might enhance the catalytic activity of DUOX by stabilizing the dehydrogenase domain in a conformation that allows electron transfer.

26 Apr 2021

Structural analysis of RIG-I-like receptors reveals ancient rules of engagement between diverse RNA helicases and TRIM ubiquitin ligases (Mol. Cell)

RNA helicases and E3 ubiquitin ligases mediate many critical functions in cells, but their actions have largely been studied in distinct biological contexts. Here, S. Hur et. al. uncover evolutionarily conserved rules of engagement between RNA helicases and tripartite motif (TRIM) E3 ligases that lead to their functional coordination in vertebrate innate immunity. Using cryoelectron microscopy and biochemistry, they show that RIG-I-like receptors (RLRs), viral RNA receptors with helicase domains, interact with their cognate TRIM/TRIM-like E3 ligases through similar epitopes in the helicase domains. Their interactions are avidity driven, restricting the actions of TRIM/TRIM-like proteins and consequent immune activation to RLR multimers. Mass spectrometry and phylogeny-guided biochemical analyses further reveal that similar rules of engagement may apply to diverse RNA helicases and TRIM/TRIM-like proteins. Their analyses suggest not only conserved substrates for TRIM proteins but also, unexpectedly, deep evolutionary connections between TRIM proteins and RNA helicases, linking ubiquitin and RNA biology throughout animal evolution.

13 Apr 2021

The story behind COVID-19 vaccines (Science Editorial)

Two activities predate the successful COVID-19 vaccines: the utilization of highly adaptable vaccine platforms such as RNA (among others) and the adaptation of structural biology tools to design agents (immunogens) that powerfully stimulate the immune system. Anthony S. Fauci recollects in this Science Editorial history of RNA vaccines developments and presents reasons why SARS-CoV-2 vaccines based on the new immunogen rapidly moved to clinical trials.

13 Apr 2021

Structural interpretation of cryo-EM image reconstructions (Prog. Biophys. Mol. Biol.)

The productivity of single-particle cryo-EM as a structure determination method has rapidly increased as many novel biological structures are being elucidated. The ultimate result of the cryo-EM experiment is an atomic model that should faithfully represent the computed image reconstruction. Although the principal approach of atomic model building and refinement from maps resembles that of the X-ray crystallographic methods, there are important differences due to the unique properties resulting from the 3D image reconstructions. In this review, C. Sachse et. al. discuss the practiced work-flow from the cryo-EM image reconstruction to the atomic model. They give an overview of (i) resolution determination methods in cryo-EM including local and directional resolution variation, (ii) cryo-EM map contrast optimization including complementary map types that can help in identifying ambiguous density features, (iii) atomic model building and (iv) refinement in various resolution regimes including (v) their validation and (vi) discuss differences between X-ray and cryo-EM maps. Based on the methods originally developed for X-ray crystallography, the path from 3D image reconstruction to atomic coordinates has become an integral and important part of the cryo-EM structure determination work-flow that routinely delivers atomic models.

13 Apr 2021

Towards high-throughput in situ structural biology using electron cryotomography (Prog. Biophys. Mol. Biol.)

Electron cryotomography is a rapidly evolving method for imaging macromolecules directly within the native environment of cells and tissues. Combined with sub-tomogram averaging, it allows structural and cell biologists to obtain sub-nanometre resolution structures in situ. However, low throughput in cryo-ET sample preparation and data acquisition, as well as difficulties in target localisation and subtomogram averaging image processing, limit its widespread usability. In this review, Bharat, TAM et. al. discuss new advances in the field that address these throughput and technical problems. They focus on recent efforts made to resolve issues in sample thinning, improvement in data collection speed at the microscope, strategies for localisation of macromolecules using correlated light and electron microscopy and advancements made to improve resolution in sub-tomogram averaging. These advances will considerably decrease the amount of time and effort required for cryo-ET and sub-tomogram averaging, ushering in a new era of structural biology where in situ macromolecular structure determination will be routine. 

13 Apr 2021

NMR spectroscopy captures the essential role of dynamics in regulating biomolecular function (Cell)

Biomolecules are in constant motion. To understand how they function, and why malfunctions can cause disease, it is necessary to describe their three-dimensional structures in terms of dynamic conformational ensembles. Here, L. Kay and T.R. Alderson  demonstrate how nuclearmagnetic resonance (NMR) spectroscopy provides an essential, dynamic view of structural biology that captures biomolecular motions at atomic resolution. They focus on examples that emphasize the diversity of biomolecules and biochemical applications that are amenable to NMR, such as elucidating functional dynamics in large molecular machines, characterizing transient conformations implicated in the onset of disease, and obtaining atomic-level descriptions of intrinsically disordered regions that make weak interactions involved in liquid-liquid phase separation. Finally, theydiscuss the pivotal role that NMR has played in driving forward our understanding of the biomolecular dynamics-function paradigm.

29 Mar 2021

A 'Build and Retrieve' methodology to simultaneously solve cryo-EM structures of membrane proteins (Nat. Methods)

Single-particle cryo-electron microscopy (cryo-EM) has become a powerful technique in the field of structural biology. However, the inability to reliably produce pure, homogeneous membrane protein samples hampers the progress of their structural determination. Here, E. W. Yu et. al. develop a bottom-up iterative method, Build and Retrieve (BaR), that enables the identification and determination of cryo-EM structures of a variety of inner and outer membrane proteins, including membrane protein complexes of different sizes and dimensions, from a heterogeneous, impure protein sample. They also use the BaR methodology to elucidate structural information from Escherichia coli K12 crude membrane and raw lysate. The findings demonstrate that it is possible to solve high-resolution structures of a number of relatively small (<100 kDa) and less abundant (<10%) unidentified membrane proteins within a single, heterogeneous sample. Importantly, these results highlight the potential of cryo-EM for systems structural proteomics.