13 Jan 2021

Papain-like protease regulates SARS-CoV-2 viral spread and innate immunity (Nature)

The papain-like protease PLpro is an essential coronavirus enzyme that is required for processing viral polyproteins to generate a functional replicase complex and enable viral spread. PLpro is also implicated in cleaving proteinaceous post-translational modifications on host proteins as an evasion mechanism against host antiviral immune responses. Here I. Dikic et. al.  perform biochemical, structural and functional characterization of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) PLpro (SCoV2-PLpro) and outline differences with SARS-CoV PLpro (SCoV-PLpro) in regulation of host interferon and NF-κB pathways. SCoV2-PLpro and SCoV-PLpro share 83% sequence identity but exhibit different host substrate preferences; SCoV2-PLpro preferentially cleaves the ubiquitin-like interferon-stimulated gene 15 protein (ISG15), whereas SCoV-PLpro predominantly targets ubiquitin chains. The crystal structure of SCoV2-PLpro in complex with ISG15 reveals distinctive interactions with the amino-terminal ubiquitin-like domain of ISG15, highlighting the high affinity and specificity of these interactions. Furthermore, upon infection, SCoV2-PLpro contributes to the cleavage of ISG15 from interferon responsive factor 3 (IRF3) and attenuates type I interferon responses. Notably, inhibition of SCoV2-PLpro with GRL-0617 impairs the virus-induced cytopathogenic effect, maintains the antiviral interferon pathway and reduces viral replication in infected cells. These results highlight a potential dual therapeutic strategy in which targeting of SCoV2-PLpro can suppress SARS-CoV-2 infection and promote antiviral immunity.

13 Jan 2021

An ultrapotent synthetic nanobody neutralizes SARS-CoV-2 by stabilizing inactive Spike (Science)

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus enters host cells via an interaction between its Spike protein and the host cell receptor angiotensin-converting enzyme 2 (ACE2). By screening a yeast surface-displayed library of synthetic nanobody sequences, QCRG Structural Biology Consortiumdeveloped nanobodies that disrupt the interaction between Spike and ACE2. Cryo–electron microscopy (cryo-EM) revealed that one nanobody, Nb6, binds Spike in a fully inactive conformation with its receptor binding domains locked into their inaccessible down state, incapable of binding ACE2. Affinity maturation and structure-guided design of multivalency yielded a trivalent nanobody, mNb6-tri, with femtomolar affinity for Spike and picomolar neutralization of SARS-CoV-2 infection. mNb6-tri retains function after aerosolization, lyophilization, and heat treatment, which enables aerosol-mediated delivery of this potent neutralizer directly to the airway epithelia.

13 Jan 2021

Comparative host-coronavirus protein interaction networks reveal pan-viral disease mechanisms (Science)

The COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is a grave threat to public health and the global economy. SARS-CoV-2 is closely related to the more lethal but less transmissible coronaviruses SARS-CoV-1 and Middle East respiratory syndrome coronavirus (MERS-CoV). Here, the authors have carried out comparative viral-human protein-protein interaction and viral protein localization analyses for all three viruses. Subsequent functional genetic screening identified host factors that functionally impinge on coronavirus proliferation, including Tom70, a mitochondrial chaperone protein that interacts with both SARS-CoV-1 and SARS- CoV-2 ORF9b, an interaction was structurally characterized using cryo–electron microscopy. Combining genetically validated host factors with both COVID-19 patient genetic data and medical billing records identified molecular mechanisms and potential drug treatments that merit further molecular and clinical study.

1 Feb 2021

Structure of mycobacterial ATP synthase bound to the tuberculosis drug bedaquiline (Nature)

Tuberculosis—the world’s leading cause of death by infectious disease—is increasingly resistant to current first-line antibiotics. The bacterium Mycobacterium tuberculosis (which causes tuberculosis) can survive low-energy conditions, allowing infections to remain dormant and decreasing their susceptibility to many antibiotics. Bedaquiline was developed in 2005 from a lead compound identified in a phenotypic screen against Mycobacterium smegmatis. This drug can sterilize even latent M. tuberculosis infections and has become a cornerstone of treatment for multidrug-resistant and extensively drug-resistant tuberculosis.Bedaquiline targets the mycobacterial ATP synthase, which is an essential enzyme in the obligate aerobic Mycobacterium genus, but how it binds the intact enzyme is unknown. Here John L. Rubinstein et.al.determined cryo-electron microscopy structures of M. smegmatis ATP synthase alone and in complex with bedaquiline. The drug-free structure suggests that hook-like extensions from the α-subunits prevent the enzyme from running in reverse, inhibiting ATP hydrolysis and preserving energy in hypoxic conditions. Bedaquiline binding induces large conformational changes in the ATP synthase, creating tight binding pockets at the interface of subunits a and c that explain the potency of this drug as an antibiotic for tuberculosis.

12 Jan 2021

Structure of human RNA polymerase III (Nature Communication)

In eukaryotes, RNA Polymerase (Pol) III is specialized for the transcription of tRNAs and other short, untranslated RNAs. Pol III is a determinant of cellular growth and lifespan across eukaryotes. Upregulation of Pol III transcription is observed in cancer and causative Pol III mutations have been described in neurodevelopmental disorders and hypersensitivity to viral infection. Here, C. Engel, A. Vannini et. al report a cryo-EM reconstruction at 4.0 angstrom of human Pol III, allowing mapping and rationalization of reported genetic mutations. Mutations causing neurodevelopmental defects cluster in hotspots affecting Pol III stability and/or biogenesis, whereas mutations affecting viral sensing are located in proximity to DNA binding regions, suggesting an impairment of Pol III cytosolic viral DNA-sensing. Integrating x-ray crystallography and SAXS, they also describe the structure of the higher eukaryote specific RPC5 C-terminal extension. Surprisingly, experiments in living cells highlight a role for this module in the assembly and stability of human Pol III. The eukaryotic RNA Polymerase III transcribes tRNAs, some ribosomal and spliceosomal RNAs. Here, the authors resolve a cryo-EM structure of human RNA Polymerase III in its apo form and complemented it with crystal structures and SAXS analysis of RPC5, revealing insights into the molecular mechanisms of Pol III transcription.

12 Jan 2021

Small-molecule inhibitors of human mitochondrial DNA transcription (Nature)

Altered expression of mitochondrial DNA (mtDNA) occurs in ageing and a range of human pathologies (for example, inborn errors of metabolism, neurodegeneration and cancer). Here Claes M. Gustafsson, Nils-Göran Larsson et. al. describe first-in-class specific inhibitors of mitochondrial transcription (IMTs) that target the human mitochondrial RNA polymerase (POLRMT), which is essential for biogenesis of the oxidative phosphorylation (OXPHOS) system. The IMTs efficiently impair mtDNA transcription in a reconstituted recombinant system and cause a dose-dependent inhibition of mtDNA expression and OXPHOS in cell lines. To verify the cellular target, they performed exome sequencing of mutagenized cells and identified a cluster of amino acid substitutions in POLRMT that cause resistance to IMTs. They obtained a cryo-electron microscopy (cryo-EM) structure of POLRMT bound to an IMT, which further defined the allosteric binding site near the active centre cleft of POLRMT. The growth of cancer cells and the persistence of therapy-resistant cancer stem cells has previously been reported to depend on OXPHOS7-17, and they therefore investigated whether IMTs have anti-tumour effects. Four weeks of oral treatment with an IMT is well-tolerated in mice and does not cause OXPHOS dysfunction or toxicity in normal tissues, despite inducing a strong anti-tumour response in xenografts of human cancer cells. In summary, IMTs provide a potent and specific chemical biology tool to study the role of mtDNA expression in physiology and disease.

12 Jan 2021

Structural Basis of WLS/Evi-Mediated Wnt Transport and Secretion (Cell)

Wnts are evolutionarily conserved ligands that signal at short range to regulate morphogenesis, cell fate, and stem cell renewal. The first and essential steps in Wnt secretion are their O-palmitoleation and subsequent loading onto the dedicated transporter Wntless/evenness interrupted (WLS/Evi). D.M. Wirshup, F. Marcia et.al.  report the 3.2A resolution cryogenic electron microscopy (cryo-EM) structure of palmitoleated human WNT8A in complex with WLS. This is accompanied by biochemical experiments to probe the physiological implications of the observed association. The WLS membrane domain has close structural homology to G protein-coupled receptors (GPCRs). A Wnt hairpin inserts into a conserved hydrophobic cavity in the GPCR-like domain, and the palmitoleate protrudes between two helices into the bilayer. A conformational switch of highly conserved residues on a separate Wnt hairpin might contribute to its transfer to receiving cells. This work provides molecular-level insights into a central mechanism in animal body plan development and stem cell biology.

18 Dec 2020

Nearest-neighbor NMR spectroscopy: categorizing spectral peaks by their adjacent nuclei (Nature Communications)

Methyl-NMR enables atomic-resolution studies of structure and dynamics of large proteins in solution. However, resonance assignment remains challenging. The problem is to combine existing structural informational with sparse distance restraints and search for the most compatible assignment among the permutations. Prior classification of peaks as either from isoleucine, leucine, or valine reduces the search space by many orders of magnitude. However, this is hindered by overlapped leucine and valine frequencies. In contrast, the nearest-neighbor nuclei, coupled to the methyl carbons, resonate in distinct frequency bands. Here, Coote, P.,  Arthanari, H. et. al. develop a framework to imprint additional information about passively coupled resonances onto the observed peaks. This depends on simultaneously orchestrating closely spaced bands of resonances along different magnetization trajectories, using principles from control theory. For methyl-NMR, the method is implemented as a modification to the standard fingerprint spectrum (the 2D-HMQC). The amino acid type is immediately apparent in the fingerprint spectrum. There is no additional relaxation loss or an increase in experimental time. The method is validated on biologically relevant proteins. The idea of generating new spectral information using passive, adjacent resonances is applicable to other contexts in NMR spectroscopy. The structure and dynamics of large proteins and complexes can be studied by methyl-NMR but resonance assignment is still challenging. Here, the authors present a NMR method that leverages optimal control pulse design to unambiguously distinguish between Leu and Val using a simple 2D HMQC experiment and they apply it to several proteins including Cas9, interleukin, and human translation initiation factor eIF4a.

18 Dec 2020

Structural and dynamics analysis of intrinsically disordered proteins by high-speed atomic force microscopy (Nature Nanotechnology)

Intrinsically disordered proteins (IDPs) are ubiquitous proteins that are disordered entirely or partly and play important roles in diverse biologicalphenomena. Their structure dynamically samples a multitude of conformational states, thus rendering their structural analysis very difficult. Here S. Longhi, T. Ando. Et al. explore the potential of high-speed atomic force microscopy (HS-AFM) for characterizing the structure and dynamics of IDPs. Successive HS-AFM images of an IDP molecule can not only identify constantly folded and constantly disordered regions in the molecule, but can also document disorder-to-order transitions. Moreover, the number of amino acids contained in these disordered regions can be roughly estimated, enabling a semiquantitative, realistic description of the dynamic structure of IDPs.

18 Dec 2020

Small-molecule-induced polymerization triggers degradation of BCL6 (Nature)

Effective and sustained inhibition of non-enzymatic oncogenic driver proteins is a major pharmacological challenge. The clinical success of thalidomide analogues demonstrates the therapeutic efficacy of drug-induced degradation of transcription factors and other cancer targets(1-3), but a substantial subset of proteins are resistant to targeted degradation using existing approaches(4,5). B. Elber, S. Fischer et.al. report an alternative mechanism of targeted protein degradation, in which a small molecule induces the highly specific, reversible polymerization of a target protein, followed by its sequestration into cellular foci and subsequent degradation. BI-3802 is a small molecule that binds to the Broad-complex, Tramtrack and Bric-a-brac (BTB) domain of the oncogenic transcription factor B cell lymphoma 6 (BCL6) and leads to the proteasomal degradation of BCL6(6). They use cryo-electron microscopy to reveal how the solvent-exposed moiety of a BCL6-binding molecule contributes to a composite ligand-protein surface that engages BCL6 homodimers to form a supramolecular structure. Drug-induced formation of BCL6 filaments facilitates ubiquitination by the SIAH1 E3 ubiquitin ligase. Our findings demonstrate that a small molecule such as BI-3802 can induce polymerization coupled to highly specific protein degradation, which in the case of BCL6 leads to increased pharmacological activity compared to the effects induced by other BCL6 inhibitors. These findings open new avenues for the development of therapeutic agents and synthetic biology.

8 Dec 2020

Visualizing a protonated RNA state that modulates microRNA-21 maturation (Nature Chemical Biology)

MicroRNAs are evolutionarily conserved small, noncoding RNAs that regulate diverse biological processes. Due to their essential regulatory roles, microRNA biogenesis is tightly regulated, where protein factors are often found to interact with specific primary and precursor microRNAs for regulation. Here, using NMR relaxation dispersion spectroscopy and mutagenesis, Qi Zhang et. al. reveal that the precursor of oncogenic microRNA-21 exists as a pH-dependent ensemble that spontaneously reshuffles the secondary structure of the entire apical stem-loop region, including the Dicer cleavage site. They show that the alternative excited conformation transiently sequesters the bulged adenine into a noncanonical protonated A⁺-G mismatch, conferring a substantial enhancement in Dicer processing over its ground conformational state. These results indicate that microRNA maturation efficiency may be encoded in the intrinsic dynamic ensemble of primary and precursor microRNAs, providing a potential means of regulating microRNA biogenesis in response to environmental and cellular stimuli.