Engineering stress as a motivation for filamentous virus morphology.

Many viruses are pleomorphic in shape and size, with pleomorphism often thought to correlate with infectivity, pathogenicity, or virus survival. For example, influenza and respiratory syncytial virus particles range in size from small spherical virions to filaments reaching many micrometers in length. We have used a pressure vessel model to investigate how the length and width of spherical and filamentous virions can vary for a given critical stress and fluorescence super-resolution microscopy along with image analysis tools to fit imaged influenza viruses to the model. We have shown that influenza virion dimensions fit within the theoretical limits of the model, suggesting that filament formation may be a way to increase an individual virus's volume without particle rupture. We have also used cryoelectron microscopy to investigate influenza and respiratory syncytial virus dimensions at the extrema of the model and used the pressure vessel model to explain the lack of alternative virus particle geometries. Our approach offers insight into the possible purpose of filamentous virus morphology and is applicable to a wide range of other biological entities, including bacteria and fungi.

Cryopreserved Kidney Epithelial (Vero) Cell Monolayers for Rapid Viral Quantification, Enabled by a Combination of Macromolecular Cryoprotectants.

Plaque assays quantify the amount of active, replicating virus to study and detect infectious diseases by application of samples to monolayers of cultured cells. Due to the time taken in thawing, propagating, plating, counting, and then conducting the assay, the process can take over a week to gather data. Here, we introduce assay-ready cryopreserved Vero monolayers in multiwell plates, which can be used directly from the freezer with no cell culture to accelerate the process of plaque determination. Standard dimethyl sulfoxide cryopreservation resulted in just 25% recovery, but addition of polyampholytes (macromolecular cryoprotectants) increased post-thaw recovery and viability in 12- and 24-well plate formats. Variability between individual wells was reduced by chemically induced ice nucleation to prevent supercooling. Cryopreserved cells were used to determine influenza viral plaques in just 24 h, matching results from nonfrozen controls. This innovation may accelerate viral detection and quantification and facilitate automation by eliminating extensive cell culturing.

High-throughput super-resolution analysis of influenza virus pleomorphism reveals insights into viral spatial organization.

Many viruses form highly pleomorphic particles. In influenza, virion structure is of interest not only in the context of virus assembly, but also because pleomorphic variations may correlate with infectivity and pathogenicity. We have used fluorescence super-resolution microscopy combined with a rapid automated analysis pipeline, a method well-suited to the study of large numbers of pleomorphic structures, to image many thousands of individual influenza virions; gaining information on their size, morphology and the distribution of membrane-embedded and internal proteins. We observed broad phenotypic variability in filament size, and Fourier transform analysis of super-resolution images demonstrated no generalized common spatial frequency patterning of HA or NA on the virion surface, suggesting a model of virus particle assembly where the release of progeny filaments from cells occurs in a stochastic way. We also showed that viral RNP complexes are located preferentially within Archetti bodies when these were observed at filament ends, suggesting that these structures may play a role in virus transmission. Our approach therefore offers exciting new insights into influenza virus morphology and represents a powerful technique that is easily extendable to the study of pleomorphism in other pathogenic viruses.

Single-molecule FRET for virology: 20 years of insight into protein structure and dynamics.

Although viral protein structure and replication mechanisms have been explored extensively with X-ray crystallography, cryo-electron microscopy, and population imaging studies, these methods are often not able to distinguish dynamic conformational changes in real time. Single-molecule fluorescence resonance energy transfer (smFRET) offers unique insights into interactions and states that may be missed in ensemble studies, such as nucleic acid or protein structure, and conformational transitions during folding, receptor-ligand interactions, and fusion. We discuss the application of smFRET to the study of viral protein conformational dynamics, with a particular focus on viral glycoprotein dynamics, viral helicases, proteins involved in HIV reverse transcription, and the influenza RNA polymerase. smFRET experiments have played a crucial role in deciphering conformational changes in these processes, emphasising the importance of smFRET as a tool to help elucidate the life cycle of viral pathogens and identify key anti-viral targets.

Reliability and accuracy of single-molecule FRET studies for characterization of structural dynamics and distances in proteins.

Single-molecule Förster-resonance energy transfer (smFRET) experiments allow the study of biomolecular structure and dynamics in vitro and in vivo. We performed an international blind study involving 19 laboratories to assess the uncertainty of FRET experiments for proteins with respect to the measured FRET efficiency histograms, determination of distances, and the detection and quantification of structural dynamics. Using two protein systems with distinct conformational changes and dynamics, we obtained an uncertainty of the FRET efficiency ≤0.06, corresponding to an interdye distance precision of ≤2 Å and accuracy of ≤5 Å. We further discuss the limits for detecting fluctuations in this distance range and how to identify dye perturbations. Our work demonstrates the ability of smFRET experiments to simultaneously measure distances and avoid the averaging of conformational dynamics for realistic protein systems, highlighting its importance in the expanding toolbox of integrative structural biology.

Virus Detection and Identification in Minutes Using Single-Particle Imaging and Deep Learning.

The increasing frequency and magnitude of viral outbreaks in recent decades, epitomized by the COVID-19 pandemic, has resulted in an urgent need for rapid and sensitive diagnostic methods. Here, we present a methodology for virus detection and identification that uses a convolutional neural network to distinguish between microscopy images of fluorescently labeled intact particles of different viruses. Our assay achieves labeling, imaging, and virus identification in less than 5 min and does not require any lysis, purification, or amplification steps. The trained neural network was able to differentiate SARS-CoV-2 from negative clinical samples, as well as from other common respiratory pathogens such as influenza and seasonal human coronaviruses. We were also able to differentiate closely related strains of influenza, as well as SARS-CoV-2 variants. Additional and novel pathogens can easily be incorporated into the test through software updates, offering the potential to rapidly utilize the technology in future infectious disease outbreaks or pandemics. Single-particle imaging combined with deep learning therefore offers a promising alternative to traditional viral diagnostic and genomic sequencing methods and has the potential for significant impact.

Virus morphology: Insights from super-resolution fluorescence microscopy.

As epitomised by the COVID-19 pandemic, diseases caused by viruses are one of the greatest health and economic burdens to human society. Viruses are 'nanostructures', and their small size (typically less than 200 nm in diameter) can make it challenging to obtain images of their morphology and structure. Recent advances in fluorescence microscopy have given rise to super-resolution techniques, which have enabled the structure of viruses to be visualised directly at a resolution in the order of 20 nm. This mini-review discusses how recent state-of-the-art super-resolution imaging technologies are providing new nanoscale insights into virus structure.

Viral detection and identification in 20 min by rapid single-particle fluorescence in-situ hybridization of viral RNA.

The increasing risk from viral outbreaks such as the ongoing COVID-19 pandemic exacerbates the need for rapid, affordable and sensitive methods for virus detection, identification and quantification; however, existing methods for detecting virus particles in biological samples usually depend on multistep protocols that take considerable time to yield a result. Here, we introduce a rapid fluorescence in situ hybridization (FISH) protocol capable of detecting influenza virus, avian infectious bronchitis virus and SARS-CoV-2 specifically and quantitatively in approximately 20 min, in virus cultures, combined nasal and throat swabs with added virus and likely patient samples without previous purification. This fast and facile workflow can be adapted both as a lab technique and a future diagnostic tool in enveloped viruses with an accessible genome.

FRET-based dynamic structural biology: Challenges, perspectives and an appeal for open-science practices.

Single-molecule FRET (smFRET) has become a mainstream technique for studying biomolecular structural dynamics. The rapid and wide adoption of smFRET experiments by an ever-increasing number of groups has generated significant progress in sample preparation, measurement procedures, data analysis, algorithms and documentation. Several labs that employ smFRET approaches have joined forces to inform the smFRET community about streamlining how to perform experiments and analyze results for obtaining quantitative information on biomolecular structure and dynamics. The recent efforts include blind tests to assess the accuracy and the precision of smFRET experiments among different labs using various procedures. These multi-lab studies have led to the development of smFRET procedures and documentation, which are important when submitting entries into the archiving system for integrative structure models, PDB-Dev. This position paper describes the current 'state of the art' from different perspectives, points to unresolved methodological issues for quantitative structural studies, provides a set of 'soft recommendations' about which an emerging consensus exists, and lists openly available resources for newcomers and seasoned practitioners. To make further progress, we strongly encourage 'open science' practices.

Reinfection with SARS-CoV-2: Discrete SIR (Susceptible, Infected, Recovered) Modeling Using Empirical Infection Data.

BACKGROUND: The novel coronavirus SARS-CoV-2, which causes the COVID-19 disease, has resulted in a global pandemic. Since its emergence in December 2019, the virus has infected millions of people, caused the deaths of hundreds of thousands, and resulted in incalculable social and economic damage. Understanding the infectivity and transmission dynamics of the virus is essential to determine how best to reduce mortality while ensuring minimal social restrictions on the lives of the general population. Anecdotal evidence is available, but detailed studies have not yet revealed whether infection with the virus results in immunity. OBJECTIVE: The objective of this study was to use mathematical modeling to investigate the reinfection frequency of COVID-19. METHODS: We have used the SIR (Susceptible, Infected, Recovered) framework and random processing based on empirical SARS-CoV-2 infection and fatality data from different regions to calculate the number of reinfections that would be expected to occur if no immunity to the disease occurred. RESULTS: Our model predicts that cases of reinfection should have been observed by now if primary SARS-CoV-2 infection did not protect individuals from subsequent exposure in the short term; however, no such cases have been documented. CONCLUSIONS: This work concludes that infection with SARS-CoV-2 provides short-term immunity to reinfection and therefore offers useful insight for serological testing strategies, lockdown easing, and vaccine development.

Rapid functionalisation and detection of viruses via a novel Ca2+-mediated virus-DNA interaction.

Current virus detection methods often take significant time or can be limited in sensitivity and specificity. The increasing frequency and magnitude of viral outbreaks in recent decades has resulted in an urgent need for diagnostic methods that are facile, sensitive, rapid and inexpensive. Here, we describe and characterise a novel, calcium-mediated interaction of the surface of enveloped viruses with DNA, that can be used for the functionalisation of intact virus particles via chemical groups attached to the DNA. Using DNA modified with fluorophores, we have demonstrated the rapid and sensitive labelling and detection of influenza and other viruses using single-particle tracking and particle-size determination. With this method, we have detected clinical isolates of influenza in just one minute, significantly faster than existing rapid diagnostic tests. This powerful technique is easily extendable to a wide range of other enveloped pathogenic viruses and holds significant promise as a future diagnostic tool.

Real-time analysis of single influenza virus replication complexes reveals large promoter-dependent differences in initiation dynamics.

The viral RNA (vRNA) genome of influenza viruses is replicated by the RNA-dependent RNA polymerase (RNAP) via a complementary RNA (cRNA) intermediate. The vRNA promoter can adopt multiple conformations when bound by the RNAP. However, the dynamics, determinants, and biological role of these conformations are unknown; further, little is known about cRNA promoter conformations. To probe the RNA conformations adopted during initial replication, we monitored single, surface-immobilized vRNA and cRNA initiation complexes in real-time. Our results show that, while the 3' terminus of the vRNA promoter exists in dynamic equilibrium between pre-initiation and initiation conformations, the cRNA promoter exhibited very limited dynamics. Two residues in the proximal 3' region of the cRNA promoter (residues absent in the vRNA promoter) allowed the cRNA template strand to reach further into the active site, limiting promoter dynamics. Our results highlight promoter-dependent differences in influenza initiation mechanisms, and advance our understanding of virus replication.

Publisher Correction: Precision and accuracy of single-molecule FRET measurements-a multi-laboratory benchmark study.

This paper was originally published under standard Springer Nature copyright. As of the date of this correction, the Analysis is available online as an open-access paper with a CC-BY license. No other part of the paper has been changed.

Precision and accuracy of single-molecule FRET measurements-a multi-laboratory benchmark study.

Single-molecule Förster resonance energy transfer (smFRET) is increasingly being used to determine distances, structures, and dynamics of biomolecules in vitro and in vivo. However, generalized protocols and FRET standards to ensure the reproducibility and accuracy of measurements of FRET efficiencies are currently lacking. Here we report the results of a comparative blind study in which 20 labs determined the FRET efficiencies (E) of several dye-labeled DNA duplexes. Using a unified, straightforward method, we obtained FRET efficiencies with s.d. between ±0.02 and ±0.05. We suggest experimental and computational procedures for converting FRET efficiencies into accurate distances, and discuss potential uncertainties in the experiment and the modeling. Our quantitative assessment of the reproducibility of intensity-based smFRET measurements and a unified correction procedure represents an important step toward the validation of distance networks, with the ultimate aim of achieving reliable structural models of biomolecular systems by smFRET-based hybrid methods.

Conformational heterogeneity and bubble dynamics in single bacterial transcription initiation complexes.

Transcription initiation is a major step in gene regulation for all organisms. In bacteria, the promoter DNA is first recognized by RNA polymerase (RNAP) to yield an initial closed complex. This complex subsequently undergoes conformational changes resulting in DNA strand separation to form a transcription bubble and an RNAP-promoter open complex; however, the series and sequence of conformational changes, and the factors that influence them are unclear. To address the conformational landscape and transitions in transcription initiation, we applied single-molecule Förster resonance energy transfer (smFRET) on immobilized Escherichia coli transcription open complexes. Our results revealed the existence of two stable states within RNAP-DNA complexes in which the promoter DNA appears to adopt closed and partially open conformations, and we observed large-scale transitions in which the transcription bubble fluctuated between open and closed states; these transitions, which occur roughly on the 0.1 s timescale, are distinct from the millisecond-timescale dynamics previously observed within diffusing open complexes. Mutational studies indicated that the σ70 region 3.2 of the RNAP significantly affected the bubble dynamics. Our results have implications for many steps of transcription initiation, and support a bend-load-open model for the sequence of transitions leading to bubble opening during open complex formation.

Single-molecule FRET reveals the pre-initiation and initiation conformations of influenza virus promoter RNA.

Influenza viruses have a segmented viral RNA (vRNA) genome, which is replicated by the viral RNA-dependent RNA polymerase (RNAP). Replication initiates on the vRNA 3' terminus, producing a complementary RNA (cRNA) intermediate, which serves as a template for the synthesis of new vRNA. RNAP structures show the 3' terminus of the vRNA template in a pre-initiation state, bound on the surface of the RNAP rather than in the active site; no information is available on 3' cRNA binding. Here, we have used single-molecule Förster resonance energy transfer (smFRET) to probe the viral RNA conformations that occur during RNAP binding and initial replication. We show that even in the absence of nucleotides, the RNAP-bound 3' termini of both vRNA and cRNA exist in two conformations, corresponding to the pre-initiation state and an initiation conformation in which the 3' terminus of the viral RNA is in the RNAP active site. Nucleotide addition stabilises the 3' vRNA in the active site and results in unwinding of the duplexed region of the promoter. Our data provide insights into the dynamic motions of RNA that occur during initial influenza replication and has implications for our understanding of the replication mechanisms of similar pathogenic viruses.

The role of the priming loop in influenza A virus RNA synthesis.

RNA-dependent RNA polymerases (RdRps) are used by RNA viruses to replicate and transcribe their RNA genomes(1). They adopt a closed, right-handed fold with conserved subdomains called palm, fingers and thumb(1,2). Conserved RdRp motifs A-F coordinate the viral RNA template, NTPs and magnesium ions to facilitate nucleotide condensation(1). For the initiation of RNA synthesis, most RdRps use either a primer-dependent or de novo mechanism(3). The influenza A virus RdRp, in contrast, uses a capped RNA oligonucleotide to initiate transcription, and a combination of terminal and internal de novo initiation for replication(4). To understand how the influenza A virus RdRp coordinates these processes, we analysed the function of a thumb subdomain ß-hairpin using initiation, elongation and single-molecule Förster resonance energy transfer (sm-FRET) assays. Our data indicate that this ß-hairpin is essential for terminal initiation during replication, but not necessary for internal initiation and transcription. Analysis of individual residues in the tip of the ß-hairpin shows that PB1 proline 651 is critical for efficient RNA synthesis in vitro and in cell culture. Overall, this work advances our understanding of influenza A virus RNA synthesis and identifies the initiation platform of viral replication.

The role of the priming loop in Influenza A virus RNA synthesis.

RNA-dependent RNA polymerases (RdRps) are used by RNA viruses to replicate and transcribe their RNA genomes1. They adopt a closed, right-handed fold with conserved subdomains called palm, fingers, and thumb1,2. Conserved RdRp motifs A-F coordinate the viral RNA template, NTPs, and magnesium ions to facilitate nucleotide condensation1. For the initiation of RNA synthesis, most RdRps use either a primer-dependent or de novo mechanism3. The Influenza A virus RdRp in contrast, uses a capped RNA oligonucleotide to initiate transcription, and a combination of terminal and internal de novo initiation for replication4. To understand how the Influenza A virus RdRp coordinates these processes, we analysed the function of a thumb subdomain ß-hairpin using initiation, elongation, and single-molecule FRET assays. Our data shows that this ß-hairpin is essential for terminal initiation during replication, but auxiliary for internal initiation and transcription. Analysis of individual residues in the tip of the ß-hairpin shows that PB1 proline 651 is critical for efficient RNA synthesis in vitro and in cell culture. Overall, this work advances our understanding of Influenza A virus RNA synthesis and identifies the initiation platform of viral replication.

Single-molecule FRET reveals a corkscrew RNA structure for the polymerase-bound influenza virus promoter.

The influenza virus is a major human and animal pathogen responsible for seasonal epidemics and occasional pandemics. The genome of the influenza A virus comprises eight segments of single-stranded, negative-sense RNA with highly conserved 5' and 3' termini. These termini interact to form a double-stranded promoter structure that is recognized and bound by the viral RNA-dependent RNA polymerase (RNAP); however, no 3D structural information for the influenza polymerase-bound promoter exists. Functional studies have led to the proposal of several 2D models for the secondary structure of the bound promoter, including a corkscrew model in which the 5' and 3' termini form short hairpins. We have taken advantage of an insect-cell system to prepare large amounts of active recombinant influenza virus RNAP, and used this to develop a highly sensitive single-molecule FRET assay to measure distances between fluorescent dyes located on the promoter and map its structure both with and without the polymerase bound. These advances enabled the direct analysis of the influenza promoter structure in complex with the viral RNAP, and provided 3D structural information that is in agreement with the corkscrew model for the influenza virus promoter RNA. Our data provide insights into the mechanisms of promoter binding by the influenza RNAP and have implications for the understanding of the regulatory mechanisms involved in the transcription of viral genes and replication of the viral RNA genome. In addition, the simplicity of this system should translate readily to the study of any virus polymerase-promoter interaction.