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Accurate segregation of chromosomes is required to maintain genome integrity during cell division. This feat is accomplished by the microtubule-based spindle. To build a spindle rapidly and with high fidelity, cells take advantage of branching microtubule nucleation, which exponentially amplifies microtubules during cell division. Branching microtubule nucleation relies on the hetero-octameric augmin complex, but understanding how augmin promotes branching has been hindered by a lack of structural information about the complex. Here, we report an integrated model of vertebrate augmin, combining cryo-electron microscopy, advanced protein structural prediction, and the visualization of fused bulky tags via negative stain electron microscopy. This strategy allowed us to identify the location and orientation of each subunit within the structure. Evolutionary analysis of augmin’s structure reveals that it is highly conserved across diverse eukaryotes, and that augmin contains a previously-unidentified microtubule binding site. Moreover, we identify homology with the kinetochore-localized NDC80 complex. This new model of the augmin complex provides insight towards the mechanism and evolution of branching microtubule nucleation.

In motile cilia, a mechanoregulatory network is responsible for converting the action of thousands of dynein motors bound to doublet microtubules into a single propulsive waveform. Here, we use two complementary cryo-EM strategies to determine structures of the major mechanoregulators that bind ciliary doublet microtubules in Chlamydomonas reinhardtii. We determine structures of isolated radial spoke RS1 and the microtubule-bound RS1, RS2 and the nexin-dynein regulatory complex (N-DRC). From these structures, we identify and build atomic models for 30 proteins, including 23 radial-spoke subunits. We reveal how mechanoregulatory complexes dock to doublet microtubules with regular 96-nm periodicity and communicate with one another. Additionally, we observe a direct and dynamically coupled association between RS2 and the dynein motor inner dynein arm subform c (IDAc), providing a molecular basis for the control of motor activity by mechanical signals. These structures advance our understanding of the role of mechanoregulation in defining the ciliary waveform.

In living cells, microtubules (MTs) play pleiotropic roles, which require very different mechanical properties. Unlike the dynamic MTs found in the cytoplasm of metazoan cells, the specialized cortical MTs from Toxoplasma gondii, a prevalent human pathogen, are extraordinarily stable and resistant to detergent and cold treatments. Using single-particle cryo-EM, we determine their ex vivo structure and identify three proteins (TrxL1, TrxL2 and SPM1) as bona fide microtubule inner proteins (MIPs). These three MIPs form a mesh on the luminal surface and simultaneously stabilize the tubulin lattice in both longitudinal and lateral directions. Consistent with previous observations, deletion of the identified MIPs compromises MT stability and integrity under challenges by chemical treatments. We also visualize a small molecule like density at the Taxol-binding site of β-tubulin. Our results provide the structural basis to understand the stability of cortical MTs and suggest an evolutionarily conserved mechanism of MT stabilization from the inside., Cortical microtubules (MTs) in Apicomplexa are specialized MTs involved in maintaining the parasite’s shape and are, as such, unusually stable. Here, cryo-EM analysis of cortical MTs from Toxoplasma gondii offers insight into the mechanism of their stabilization by three bona fide microtubule inner proteins.

SUMMARYThe Coronavirus Disease 2019 pandemic has made deployment of an effective vaccine a global health priority. We evaluated the protective activity of a chimpanzee adenovirus-vectored vaccine encoding a pre-fusion stabilized spike protein (ChAd-SARS-CoV-2-S) in challenge studies with Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and mice expressing the human angiotensin-converting enzyme 2 receptor. Intramuscular dosing of ChAd-SARS-CoV-2-S induces robust systemic humoral and cell-mediated immune responses and protects against lung infection, inflammation, and pathology but does not confer sterilizing immunity, as evidenced by detection of viral RNA and induction of anti-nucleoprotein antibodies after SARS-CoV-2 challenge. In contrast, a single intranasal dose of ChAd-SARS-CoV-2-S induces high levels of systemic and mucosal IgA and T cell responses, completely prevents SARS-CoV-2 infection in the upper and lower respiratory tracts, and likely confers sterilizing immunity in most animals. Intranasal administration of ChAd-SARS-CoV-2-S is a candidate for preventing SARS-CoV-2 infection and transmission, and curtailing pandemic spread.

Summary The Coronavirus Disease 2019 pandemic has made deployment of an effective vaccine a global health priority. We evaluated the protective activity of a chimpanzee adenovirus-vectored vaccine encoding a pre-fusion stabilized spike protein (ChAd-SARS-CoV-2-S) in challenge studies with Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) and mice expressing the human angiotensin-converting enzyme 2 receptor. Intramuscular dosing of ChAd-SARS-CoV-2-S induces robust systemic humoral and cell-mediated immune responses and protects against lung infection, inflammation, and pathology but does not confer sterilizing immunity, as evidenced by detection of viral RNA and induction of anti-nucleoprotein antibodies after SARS-CoV-2 challenge. In contrast, a single intranasal dose of ChAd-SARS-CoV-2-S induces high levels of neutralizing antibodies, promotes systemic and mucosal IgA and T cell responses, and virtually completely prevents SARS-CoV-2 infection in both the upper and lower respiratory tracts. Intranasal administration of ChAd-SARS-CoV-2-S is a candidate for preventing SARS-CoV-2 infection and transmission, and curtailing pandemic spread., Highlights • Chimpanzee adenoviral vaccines encoding stabilized S induce neutralizing Abs • Chimpanzee adenoviral vaccines protect against SARS-CoV-2 infection and pneumonia • Intranasal vaccine delivery generates robust mucosal B and T cell responses • Intranasal ChAd-SARS-CoV-2 prevents upper and low respiratory tract infection, Intranasal or intramuscular immunization of ChAd-SARS-CoV-2, a chimpanzee adenoviral vaccine encoding stabilized spike intramuscular prevents SARS-CoV-2 lung infection and pneumonia in mice. In particular, intranasally delivered ChAd-SARS-CoV-2 uniquely prevents both upper and lower respiratory tract infections, potentially protecting against SARS-CoV-2 infection and transmission.

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has necessitated the rapid development of antibody-based therapies and vaccines as countermeasures. Here, we use cryoelectron microscopy (cryo-EM) to characterize two protective anti-SARS-CoV-2 murine monoclonal antibodies (mAbs) in complex with the spike protein, revealing similarities between epitopes targeted by human and murine B cells. The more neutralizing mAb, 2B04, binds the receptor-binding motif (RBM) of the receptor-binding domain (RBD) and competes with angiotensin-converting enzyme 2 (ACE2). By contrast, 2H04 binds adjacent to the RBM and does not compete for ACE2 binding. Naturally occurring sequence variants of SARS-CoV-2 and corresponding neutralization escape variants selected in vitro map to our structurally defined epitopes, suggesting that SARS-CoV-2 might evade therapeutic antibodies with a limited set of mutations, underscoring the importance of combination mAb therapeutics. Finally, we show that 2B04 neutralizes SARS-CoV-2 infection by preventing ACE2 engagement, whereas 2H04 reduces host cell attachment without directly disrupting ACE2-RBM interactions, providing distinct inhibitory mechanisms used by RBD-specific mAbs., Graphical abstract, Errico et al. use cryoelectron microscopy to solve the structure of two murine-derived neutralizing antibodies against the SARS-CoV-2 receptor-binding domain, showing that they target discrete epitopes and utilize distinct mechanisms of neutralization.

Phosphatidylinositol 3-phosphate (PI3P) plays essential roles in vesicular trafficking, organelle biogenesis and autophagy. Two class III phosphatidylinositol 3-kinase (PI3KC3) complexes have been identified in mammals, the ATG14L complex (PI3KC3-C1) and the UVRAG complex (PI3KC3-C2). PI3KC3-C1 is crucial for autophagosome biogenesis, and PI3KC3-C2 is involved in various membrane trafficking events. Here we report the cryo-EM structures of human PI3KC3-C1 and PI3KC3-C2 at sub-nanometer resolution. The two structures share a common L-shaped overall architecture with distinct features. EM examination revealed that PI3KC3-C1 “stands up” on lipid monolayers, with the ATG14L BATs domain and the VPS34 C-terminal domain (CTD) directly contacting the membrane. Biochemical dissection indicated that the ATG14L BATs domain is responsible for membrane anchoring, whereas the CTD of VPS34 determines the orientation. Furthermore, PI3KC3-C2 binds much more weakly than PI3KC3-C1 to both PI-containing liposomes and purified endoplasmic reticulum (ER) vesicles, a property that is specifically determined by the ATG14L BATs domain. The in vivo ER localization analysis indicated that the BATs domain was required for ER localization of PI3KC3. We propose that the different lipid binding capacity is the key factor that differentiates the functions of PI3KC3-C1 and PI3KC3-C2 in autophagy.

Phosphatidylinositol 3-phosphate (PI3P) plays essential roles in vesicular trafficking, organelle biogenesis and autophagy. Two class III phosphatidylinositol 3-kinase (PI3KC3) complexes have been identified in mammals, the ATG14L complex (PI3KC3-C1) and the UVRAG complex (PI3KC3-C2). PI3KC3-C1 is crucial for autophagosome biogenesis, and PI3KC3-C2 is involved in various membrane trafficking events. Here we report the cryo-EM structures of human PI3KC3-C1 and PI3KC3-C2 at sub-nanometer resolution. The two structures share a common L-shaped overall architecture with distinct features. EM examination revealed that PI3KC3-C1 "stands up" on lipid monolayers, with the ATG14L BATs domain and the VPS34 C-terminal domain (CTD) directly contacting the membrane. Biochemical dissection indicated that the ATG14L BATs domain is responsible for membrane anchoring, whereas the CTD of VPS34 determines the orientation. Furthermore, PI3KC3-C2 binds much more weakly than PI3KC3-C1 to both PI-containing liposomes and purified endoplasmic reticulum (ER) vesicles, a property that is specifically determined by the ATG14L BATs domain. The in vivo ER localization analysis indicated that the BATs domain was required for ER localization of PI3KC3. We propose that the different lipid binding capacity is the key factor that differentiates the functions of PI3KC3-C1 and PI3KC3-C2 in autophagy.

The axoneme of motile cilia is the largest macromolecular machine of eukaryotic cells. In humans, impaired axoneme function causes a range of ciliopathies. Axoneme assembly, structure, and motility requires a radially arranged set of doublet microtubules, each decorated in repeating patterns with non-tubulin components. We use single-particle cryo-electron microscopy to visualize and build an atomic model of the repeating structure of a native axonemal doublet microtubule, which reveals the identities, positions, repeat lengths, and interactions of 38 associated proteins including 33 microtubule inner proteins (MIPs). The structure demonstrates how these proteins establish the unique architecture of doublet microtubules, maintain coherent periodicities along the axoneme, and stabilize the microtubules against the repeated mechanical stress induced by ciliary motility. Our work elucidates the architectural principles that underpin the assembly of this large, repetitive eukaryotic structure, and provides a molecular basis for understanding the etiology of human ciliopathies.

SUMMARY Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused millions of human infections and an effective vaccine is critical to mitigate coronavirus-induced disease 2019 (COVID-19). Previously, we developed a replication-competent vesicular stomatitis virus (VSV) expressing a modified form of the SARS-CoV-2 spike gene in place of the native glycoprotein gene (VSV-eGFP-SARS-CoV-2). Here, we show that vaccination with VSV-eGFP-SARS-CoV-2 generates neutralizing immune responses and protects mice from SARS-CoV-2. Immunization of mice with VSV-eGFP-SARS-CoV-2 elicits high antibody titers that neutralize SARS-CoV-2 and target the receptor binding domain that engages human angiotensin converting enzyme-2 (ACE2). Upon challenge with a human isolate of SARS-CoV-2, mice expressing human ACE2 and immunized with VSV-eGFP-SARS-CoV-2 show profoundly reduced viral infection and inflammation in the lung, indicating protection against pneumonia. Passive transfer of sera from VSV-eGFP-SARS-CoV-2-immunized animals also protects naïve mice from SARS-CoV-2 challenge. These data support development of VSV-eGFP-SARS-CoV-2 as an attenuated, replication-competent vaccine against SARS-CoV-2., Graphical Abstract, Highlights • A replicating VSV-SARS-CoV-2 vaccine induces high-titer neutralizing antibodies • Infectious SARS-CoV-2 is undetectable in the lung of vaccinated mice post-challenge • SARS-CoV-2-induced lung inflammation and pathology is decreased in vaccinated mice • Transfer of vaccine-derived immune sera to naïve mice protects against SARS-CoV-2, Case, Rothlauf et al. report the efficacy of a replicating VSV-based SARS-CoV-2 vaccine. Immunized hACE2-expressing mice challenged with SARS-CoV-2 are protected against lung infection, inflammation, and pneumonia. Neutralizing antibodies are a correlate of this protection, as passive transfer of vaccine-derived immune sera protects naïve mice from subsequent SARS-CoV-2 challenge.

A fundamental question regarding autophagosome formation is how the shape of the double-membrane autophagosomal vesicle is generated. Here we show that in mammalian cells assembly of an actin scaffold inside the isolation membrane (the autophagosomal precursor) is essential for autophagosomal membrane shaping. Actin filaments are depolymerized shortly after starvation and actin is assembled into a network within the isolation membrane. When formation of actin puncta is disrupted by an actin polymerization inhibitor or by knocking down the actin-capping protein CapZβ, isolation membranes and omegasomes collapse into mixed-membrane bundles. Formation of actin puncta is PtdIns(3)P dependent, and inhibition of PtdIns(3)P formation by treating cells with the PI(3)K inhibitor 3-MA, or by knocking down Beclin-1, abolishes the formation of actin puncta. Binding of CapZ to PtdIns(3)P, which is enriched in omegasomes, stimulates actin polymerization. Our findings illuminate the mechanism underlying autophagosomal membrane shaping and provide key insights into how autophagosomes are formed.

Interferon regulatory factors (IRFs) are well-known to be crucial for modulating the innate immune responses to viral infections. In the present study, the IRF-1 gene of grass carp (Ctenopharyngodon idella) (termed CiIRF-1) was cloned and characterized. The complete genomic sequence of CiIRF-1 was 3150 bp in length and comprised 9 exons and 8 introns. The CiIRF-1 promoter sequence was 558 bp in length. The largest open reading frame (ORF) of the full CiIRF-1 cDNA sequence was 870 bp, and encoded a polypeptide of 289 amino acids. The putative CiIRF-1 was characterized by a conserved N-terminal DBD (113 aa), and included a signature of five conserved tryptophan residues. Phylogenetic relationship analysis revealed that CiIRF-1 was highly homologous to the counterparts of other teleosts and mammalians. CiIRF-1 was expressed at a low constitutive level but was significantly up-regulated following stimulation with either Poly I:C or recombinant grass carp (C. idella) IFN (rCiIFN) in all 6 tested tissues, especially in spleen and gill. The recombinant CiIRF-1 was expressed in BL21 Escherichia coli, and the expressed protein was purified by affinity chromatography with the Ni-NTA His-Bind Resin. Three different fragments of promoter sequences from grass carp type I IFN (CiIFN) gene (GU139255) were amplified. These fragments included the proximal region (CiIFNP2), the distal region (CiIFNP6), and the full length of CiIFN promoter sequences (CiIFNP7). Gel mobility shift assays were employed to analyze the interaction between CiIRF-1 and CiIFN promoter sequences. The results revealed that CiIRF-1 could bind to CiIFN promoter with high affinity in vitro. Subsequently, the recombinant plasmid of pGL3-CiIFNPs and pcDNA3.1-CiIRF-1 were constructed and transiently co-transfected into C. idella kidney (CIK) cells. The impact of CiIRF-1 on CiIFN promoter sequences were measured by luciferase assays. These results demonstrated that CiIRF-1 acts as a positive regulator in the transcription of grass carp IFN gene.

Acetylation and Autophagy Autophagy allows cells to digest their own components when necessary to survive stressful conditions. Lin et al. (p. 477) and Yi et al. (p. 474) describe signaling mechanisms in mammalian cells and yeast, respectively, by which autophagy is regulated by protein acetylation. In mammalian cells deprived of serum, the acetyltransferase TIP60 was activated by phosphorylation by the protein kinase GSK3 (glycogen synthase kinase 3). TIP60's target appeared to be a protein kinase central to autophagy regulation, ULK1. This activating pathway was required for autophagy in the absence of serum, but was not needed for autophagy in cells deprived of glucose. In the yeast Saccharomyces cerevisiae starved of nitrogen, another acetylation mechanism was uncovered. Starvation led to activation of the histone acetyltransferase Esa1, which acetylated the protein Atg3, a key component of the autophagy machinery, thus increasing its interaction with another autophagy protein, Atg8.

The Beclin 1 gene is a haplo-insufficient tumor suppressor and plays an essential role in autophagy. However, the molecular mechanism by which Beclin 1 functions remains largely unknown. Here we report the crystal structure of the evolutionarily conserved domain (ECD) of Beclin 1 at 1.6 A resolution. Beclin 1 ECD exhibits a previously unreported fold, with three structural repeats arranged symmetrically around a central axis. Beclin 1 ECD defines a novel class of membrane-binding domain, with a strong preference for lipid membrane enriched with cardiolipin. The tip of a surface loop in Beclin 1 ECD, comprising three aromatic amino acids, acts as a hydrophobic finger to associate with lipid membrane, consequently resulting in the deformation of membrane and liposomes. Mutation of these aromatic residues rendered Beclin 1 unable to stably associate with lipid membrane in vitro and unable to fully rescue autophagy in Beclin 1-knockdown cells in vivo. These observations form an important framework for deciphering the biological functions of Beclin 1.

Type I interferons and interferon regulatory factor 7 (IRF7), which are crucial for innate immunity against viral infection, have been identified in many teleost fishes in recent years. In this study, the complete genomic sequence of grass carp (Ctenopharyngodon idella) type I interferon (termed CiIFN) (GU139255) and the full-length IRF7 cDNA sequence of grass carp (termed CiIRF7) (GQ141741) were cloned and characterized. CiIFN consists of 3368 bp, retaining the characteristic 5-exon/4-intron gene organization in fish type I IFNs. The CiIFN spans 5 exons and encodes a polypeptide of 180 amino acids, with the first 22 amino acids representing a putative signal peptide. The CiIFN promoter sequence was found to be 760 bp, which can be divided into a proximal region (from -1 to -140 bp) and a distal region (from -400 to -700 bp). The cDNA of CiIRF7 was found to be 1808 bp in full length, with an ORF of 1293 bp that encodes a putative protein of 430 amino acids. The putative amino acid sequence of CiIRF7 possesses a DNA-binding domain (DBD) in the N-terminal region. Real-time PCR analysis revealed that CiIFN displayed a low constitutive expression in all the tissues tested. After stimulation by polyinosinic:polycytidylic acid (Poly I:C), the expression of CiIFN was significantly up-regulated in most tissues of grass carp, with a relatively strong expression in spleen, kidney and intestine. The recombinant polypeptides of CiIRF7 and CiIRF7-nDBD were analyzed in gel mobility shift assays, along with the PCR amplification products of the proximal region (CiIFNP2), the distal region (CiIFNP6) and the full-length (CiIFNP7) of CiIFN promoter sequence. The results revealed that CiIRF7 could bind to the distal region as well as to the proximal region of CiIFN promoter sequence in vitro. Subsequently, the CiIFNPs (CiIFNP7/2/6) were cloned into pGL3-Basic vectors and CiIRF7 was subcloned into pcDNA3.1 vectors, then pGL3-CiIFNPs were separately transiently transfected or co-transfected with pcDNA3.1-CiIRF7 into the mouse myeloma cell lines (MMCL) SP2/0 and the grass carp kidney cell lines (CIK), and the impact of CiIRF7 on CiIFN promoter activity was measured by luciferase assays in the transfected cells. These results demonstrated that CiIRF7 acted as a positive regulator on the transcription of CiIFN.

The grass carp (Ctenopharyngodon idella) collectin gene was cloned from mixed liver and kidney cDNA library. The full length sequence of grass carp collectin was 1128 bp, contained a 5' untranslated region of 229 bp and a 3' untranslated region of 104 bp. The open reading frame of grass carp collectin was 795 bp which could code a 264 amino acids polypeptide, including a terminal codon. Phylogenetic analyses showed that grass carp collectin shared the highest homology with that of zebrafish (Danio rerio). To understand the function of grass carp collectin, we expressed and purified the recombinant protein (P(CRD)) that comprised carbohydrate recognition domain (CRD). Agglutination of Aeromonas hydrophila and Staphylococcus aureus etc. and sugars inhibition experiments showed that: galactose, glucose, mannose and maltose could inhibit the agglutination of Aeromonas hydrophila. Maltose could lower the agglutination of Staphylococcus aureus, whereas peptidoglycan and glucose inhibited it well. In addition, the activity of grass carp collectin could not dependent on Ca(2+).

1. It has been suggested that the toxic effect of toxoflavin (TXF) produced by Pseudomonas cocovenenas is mainly due to the impairment of electron transfer of the mitochondrial respiratory chain. However, the cardiovascular effect of TXF is unknown. In the present study, the effect of TXF on the isometric contraction of rat isolated tail artery strips and the underlying mechanisms were investigated. 2. The basal force of the tissues was not affected by the toxin. However, the application of TXF before or during KCl (60 mM) stimulation potentiated KCl-induced vasocontraction, specifically the tonic phase of the contraction. 3. When the vessel strips were precontracted with phenylephrine (Phe), TXF further enhanced the tonic contraction of the tissue. Pretreatment of tissues with TXF also potentiated subsequent vasocontraction induced by Phe. The vasocontractor effects of TXF and Phe, however, were not additive. 4. The vascular effect of TXF was not mediated by oxygen-derived free radicals since catalase and SOD did not affect TXF-enhanced vasocontraction. In contrast, the vasocontractor effect of TXF was dependent on extracellular Ca2+ and abolished by nifedipine (a Ca2+ antagonist). TXF also had no effect on caffeine- or U46619-induced vasocontraction. 5. It is suggested that TXF may potentially contract blood vessels via its effect on Ca2+ channels. This effect of TXF depends on the contractile status of the vascular tissues.