Conformal Partial Wave Expansions for N=4 Chiral Four Point Functions

The conformal partial wave analysis of four point functions of half BPS operators belonging to the SU(4) [0,p,0] representation is undertaken for p=2,3,4. Using the results of N=4 superconformal Ward identities the contributions from protected short and semi-short multiplets are identified in terms of the free field theory. In the large N limit contributions corresponding to long multiplets with twist up to 2p-2 are absent. The anomalous dimensions for twist two singlet multiplets are found to order g^4 and agree with other perturbative calculations. Results for twist four and six are also found.Comment: 53 pages, uses harvmac, includes 1 figure, version 2 some corrections and minor extensions, version 3 some further corrections, version 4 as to be publishe

Superconformal Symmetry, Correlation Functions and the Operator Product Expansion

Superconformal transformations are derived for the $\N=2,4 supermultiplets corresponding to the simplest chiral primary operators. These are applied to two, three and four point correlation functions. When$\N=4$, results are obtained for the three point function of various descendant operators, including the energy momentum tensor and SU(4) current. For both$\N=2$ or 4 superconformal identities are derived for the functions of the two conformal invariants appearing in the four point function for the chiral primary operator. These are solved in terms of a single arbitrary function of the two conformal invariants and one or three single variable functions. The results are applied to the operator product expansion using the exact formula for the contribution of an operator in the operator product expansion in four dimensions to a scalar four point function. Explicit expressions representing exactly the contribution of both long and possible short supermultiplets to the chiral primary four point function are obtained. These are applied to give the leading perturbative and large N corrections to the scale dimensions of long supermultiplets.Comment: 75 pages, plain TeX file using harvmac; revised version, minor corrections and extra referenc

The pre-fusion structure of Herpes simplex virus glycoprotein B

Cell entry of enveloped viruses requires specialized viral proteins which mediate fusion with the host membrane by substantial structural rearrangements from a metastable pre- to a stable postfusion conformation. This metastability renders the Herpes simplex virus (HSV-1) fusion glycoprotein B (gB) highly unstable such that it readily converts into the post-fusion form, thereby precluding structural elucidation of the pharmacologically relevant pre-fusion conformation. By identification of conserved sequence signatures and molecular dynamics simulations, we devised a mutation that stabilized this form. Functionally locking gB, allowed the structural determination of its membrane-embedded pre-fusion conformation at sub-nanometer resolution and enabled the unambiguous fit of all ectodomains. The resulting pseudo-atomic model reveals a striking conservation of conformational domain rearrangements during fusion between HSV-1 gB and the Vesicular Stomatitis Virus glycoprotein G (VSV-G) despite their very distant phylogeny. In combination with our comparative sequence-structure analysis, these findings suggest common fusogenic domain rearrangements in all class III viral fusion proteins. Rey, M. Topf, K

Multimeric single-domain antibody complexes protect against bunyavirus infections

The World Health Organization has included three bunyaviruses posing an increasing threat to human health on the Blueprint list of viruses likely to cause major epidemics and for which no, or insufficient countermeasures exist. Here, we describe a broadly applicable strategy, based on llama-derived single-domain antibodies (VHHs), for the development of bunyavirus biotherapeutics. The method was validated using the zoonotic Rift Valley fever virus (RVFV) and Schmallenberg virus (SBV), an emerging pathogen of ruminants, as model pathogens. VHH building blocks were assembled into highly potent neutralizing complexes using bacterial superglue technology. The multimeric complexes were shown to reduce and prevent virus-induced morbidity and mortality in mice upon prophylactic administration. Bispecific molecules engineered to present two different VHHs fused to an Fc domain were further shown to be effective upon therapeutic administration. The presented VHH-based technology holds great promise for the development of bunyavirus antiviral therapies

Risk profiles and one-year outcomes of patients with newly diagnosed atrial fibrillation in India: Insights from the GARFIELD-AF Registry.

BACKGROUND: The Global Anticoagulant Registry in the FIELD-Atrial Fibrillation (GARFIELD-AF) is an ongoing prospective noninterventional registry, which is providing important information on the baseline characteristics, treatment patterns, and 1-year outcomes in patients with newly diagnosed non-valvular atrial fibrillation (NVAF). This report describes data from Indian patients recruited in this registry. METHODS AND RESULTS: A total of 52,014 patients with newly diagnosed AF were enrolled globally; of these, 1388 patients were recruited from 26 sites within India (2012-2016). In India, the mean age was 65.8 years at diagnosis of NVAF. Hypertension was the most prevalent risk factor for AF, present in 68.5% of patients from India and in 76.3% of patients globally (P < 0.001). Diabetes and coronary artery disease (CAD) were prevalent in 36.2% and 28.1% of patients as compared with global prevalence of 22.2% and 21.6%, respectively (P < 0.001 for both). Antiplatelet therapy was the most common antithrombotic treatment in India. With increasing stroke risk, however, patients were more likely to receive oral anticoagulant therapy [mainly vitamin K antagonist (VKA)], but average international normalized ratio (INR) was lower among Indian patients [median INR value 1.6 (interquartile range {IQR}: 1.3-2.3) versus 2.3 (IQR 1.8-2.8) (P < 0.001)]. Compared with other countries, patients from India had markedly higher rates of all-cause mortality [7.68 per 100 person-years (95% confidence interval 6.32-9.35) vs 4.34 (4.16-4.53), P < 0.0001], while rates of stroke/systemic embolism and major bleeding were lower after 1 year of follow-up. CONCLUSION: Compared to previously published registries from India, the GARFIELD-AF registry describes clinical profiles and outcomes in Indian patients with AF of a different etiology. The registry data show that compared to the rest of the world, Indian AF patients are younger in age and have more diabetes and CAD. Patients with a higher stroke risk are more likely to receive anticoagulation therapy with VKA but are underdosed compared with the global average in the GARFIELD-AF. CLINICAL TRIAL REGISTRATION-URL: http://www.clinicaltrials.gov. Unique identifier: NCT01090362

Week 96 efficacy and safety results of the phase 3, randomized EMERALD trial to evaluate switching from boosted-protease inhibitors plus emtricitabine/tenofovir disoproxil fumarate regimens to the once daily, single-tablet regimen of darunavir/cobicistat/emtricitabine/tenofovir alafenamide (D/C/F/TAF) in treatment-experienced, virologically-suppressed adults living with HIV-1

Darunavir/cobicistat/emtricitabine/tenofovir alafenamide (D/C/F/TAF) 800/150/200/10 mg was investigated through 96 weeks in EMERALD (NCT02269917). Virologically-suppressed, HIV-1-positive treatment-experienced adults (previous non-darunavir virologic failure [VF] allowed) were randomized (2:1) to D/C/F/TAF or boosted protease inhibitor (PI) plus emtricitabine/tenofovir-disoproxil-fumarate (F/TDF) over 48 weeks. At week 52 participants in the boosted PI arm were offered switch to D/C/F/TAF (late-switch, 44 weeks D/C/F/TAF exposure). All participants were followed on D/C/F/TAF until week 96. Efficacy endpoints were percentage cumulative protocol-defined virologic rebound (PDVR; confirmed viral load [VL] ≥50 copies/mL) and VL < 50 copies/mL (virologic suppression) and ≥50 copies/mL (VF) (FDA-snapshot analysis). Of 1141 randomized patients, 1080 continued in the extension phase. Few patients had PDVR (D/C/F/TAF: 3.1%, 24/763 cumulative through week 96; late-switch: 2.3%, 8/352 week 52–96). Week 96 virologic suppression was 90.7% (692/763) (D/C/F/TAF) and 93.8% (330/352) (late-switch). VF was 1.2% and 1.7%, respectively. No darunavir, primary PI, tenofovir or emtricitabine resistance-associated mutations were observed post-baseline. No patients discontinued for efficacy-related reasons. Few discontinued due to adverse events (2% D/C/F/TAF arm). Improved renal and bone parameters were maintained in the D/C/F/TAF arm and observed in the late-switch arm, with small increases in total cholesterol/high-density-lipoprotein-cholesterol ratio. A study limitation was the lack of a control arm in the week 96 analysis. Through 96 weeks, D/C/F/TAF resulted in low PDVR rates, high virologic suppression rates, very few VFs, and no resistance development. Late-switch results were consistent with D/C/F/TAF week 48 results. EMERALD week 96 results confirm the efficacy, high genetic barrier to resistance and safety benefits of D/C/F/TAF

Macromolecular assemblies

The approaches used by structural biologists evolve as they tackle the study of increasingly large, complex, and flexible assemblies. Such studies require synergistic combinations of methods over different length and timescales to provide a functional picture of the macromolecular machine under scrutiny. This themed issue on Macromolecular Assemblies comprises seven reviews covering recent conceptual and methodological developments to study complex structures. Two of the reviews analyze biological advances — obtained by X-ray crystallography — in understanding the viral molecular machines that are used for the invasion of animal cells. The other reviews focus on assemblies that have been approached by complementary methodologies. One of the first steps during the entry of a virus into an animal cell is specific recognition of a cellular receptor. In this issue, Stehle and Casasnovas review several examples of crystal structures of viral envelope proteins or whole virus particles in complex with cellular recepors, bringing forth important common features that govern this specific recognition. Virus–receptor interactions are indeed important in defining key aspects of virus pathogenicity and understanding receptor switching — which appears in many cases to be distressingly facile — is a major issue in contemporary virology discussed in this review. A key step in the infection of a cell by enveloped viruses is the fusion of the viral and cellular membrane, and the review by Backovic and Jardetzky covers the recently identified viral membrane fusion proteins belonging to structural class III. The membrane fusogenic glycoproteins of totally unrelated RNA and DNA viruses were found to share a common 3D fold, indicating homology in spite of the absence of sequence conservation. This may have arisen by horizontal gene transfer between viruses, or by the independent acquisition of the gene from the cellular host at different times during evolution. Like the previously characterized class I and II fusion proteins, class III proteins change conformation to induce fusion of the target membrane with the virus envelope. These conformational changes can be mapped onto the structural homologs, providing a striking example of the use of structural biology to identify links between otherwise totally unrelated organisms. The viral proteins analyzed in this issue were studied by X-ray crystallography, and additional studies by other techniques will certainly contribute in the understanding of their function. This is the case for the poorly understood, higher order complexes in membrane fusion. Indeed, the study of most complex macromolecular assemblies relies increasingly on hybrid methods, combining different methodologies such as electron microscopy, NMR spectroscopy, X-ray crystallography, solution studies such as small angle X-ray or neutron scattering (SAXS and SANS), computational modeling and mass spectrometry/proteomics. One example is the tumor suppressor molecule p53, which can adopt a number of different conformations because of its intrinsic flexibility, which relates to its high degree of promiscuity of binding partners. The study of such proteins involves X-ray crystallographic and/or NMR analyses of the individual domains, and then fitting the pieces together with the aid of other techniques, like electron microscopy and solution scattering. Other examples include the proteasome, as well as large virus particles that cannot be crystallized as a whole. The structure of the nuclear pore complex and of cellular organelles are other challenging examples. Such studies are the topic of the remaining reviews in this issue. The structure of p53 has been studied by X-ray crystallography, NMR, SAXS, and EM, but remains controversial. A prominent model of the human p53 tetramer is based on X-ray and NMR structures of the so-called tetramerization domain, which forms tetramers when this domain is expressed in isolation. In this model, four of these domains form a tetramer unit from which the other domains extend. However, a recent cryo-EM structure of the murine p53 tetramer shows a different, more compact organization, in which the tetramer is formed by pairwise contacts between N-terminal and C-terminal regions. Some properties of p53 implicate a coordinated action of N-terminal and C-terminal domains and are better explained by the recent cryo-EM structure. This topic is covered in the review by Okorokov and Orlova. Regulated proteolysis plays a key role in cellular function, and two of the reviews in this volume are concerned with the mechanisms of action of large proteolytic machines. Cheng is concerned with the structure and function of the 26S proteasome, the large machine responsible for regulated protein degradation in eukaryotic cells. The barrel-shaped core structure has been characterized by X-ray crystallography, but targeting and access to the proteolytic chamber are determined by elaborate regulatory complexes at the ends of the barrel. This is the case of the 19S complex containing the components involved in recognizing and processing ubiquitinated substrates, as well as the ATPases required for unfolding of these substrates so that they can be threaded into the proteolytic chamber. Cryo EM is being used to determine the structure of the full 26S complex including core and 19S complexes. In a related review, Striebel et al. discuss the ATPases of the AAA+ family, which use the energy of ATP hydrolysis to unfold proteins that are introduced into the proteolytic chamber of the proteasome. Their review focuses on their specificity, mechanisms of action, and regulation by cofactors to recognize and unfold substrates in the proteasome as well as in bacterial ATP-dependent proteases. For a full understanding of molecular machineries in the cell, it is necessary to relate atomic structures to larger scale information. The resolution of cryo-EM maps has been steadily improving, with the best single particle structures of icosahedral viruses around 4 Å, and increasing numbers of intermediate resolution maps in the 4–10 Å range. In order to extract the biological information from such maps, efforts must be put into interpretation of the densities. Given the large number of atomic structures available, it is often possible to dock atomic models of components of a large assembly into EM maps. In other cases, it is necessary to create de novo models and use the cryo-EM map to filter the models obtained. Atomistic models can be fitted into maps as rigid bodies or by flexible docking of domains or secondary structure elements. Such hybrid approaches involve computer modeling and density correlation, and ideally make use of all available structural and biochemical information in deriving structural models. The current methodologies that are being developed — and in particular the interpretation of electron density maps in the 4–10 Å resolution range — is covered in the review by Lindert et al. An example of a multiscale approach to a huge cellular assembly, the 100 MDa nuclear pore complex, is presented in the review by Elad et al. This large ring structure spanning the two nuclear membranes forms the portal for selective transport of cargo between nucleus and cytoplasm. A difficultly in defining the full structure of the complex is that the pore is lined with a large mass of natively unstructured chains, the FG repeats, which are directly implicated in the transport process. Owing to the size and complexity of this assembly, a wide variety of approaches are being combined to study its structure and function, including cryo-electron tomography, AFM, and bioinformatics to incorporate the full range of structural and biochemical information. Such a broad combination of approaches to understand the structural dynamics of macromolecular processes in the cell is also the topic of a review appearing in a parallel themed issue on ‘Cell Structure and Dynamics’ in Current Opinion in Cell Biology by Sali and colleagues [1]. Ultimately one would like to relate all this information to the in vivo context and extend the hybrid approaches to even larger scales. Methods for determining the 3D density of whole organelles or cell structures are now rapidly developing [2]. Cryo-electron tomography of vitrified samples has the capacity to reveal the 3D structure of cell and tissue sections or organelles in their native, hydrated state, in the 3–5 nm resolution range. In favorable cases, it is possible to relate molecular and cellular information, such as the ATP synthase complexes on mitochondrial membranes, which can be discerned in tomograms of isolated mitochondria (Figure 1a). The resolution of mitochondrial features in vitrified sections is lower, owing to the greater technical difficulty of imaging vitrified sections, but the membrane topology is clearly seen (Figure 1b). The 3D structure of mitochondria has been recently reviewed [3]. In a study of vitrified sections of skin desmosomes, it has been possible to interpret tomogram density in terms of the molecular packing of the cadherin proteins forming the inter-membrane junction [4]. Electron tomography has also been applied to the native 3D organization of bacterial polysomes, revealing a pseudo-helical arrangement of the ribosomes [5]