Self-Assembly of Measles Virus Nucleocapsid-like Particles: Kinetics and RNA Sequence Dependence

The cryoEM structure of the Hendra henipavirus nucleoprotein reveals insights into paramyxoviral nucleocapsid architectures

We report the first cryoEM structure of the Hendra henipavirus nucleoprotein in complex with RNA, at 3.5 Å resolution, derived from single particle analysis of a double homotetradecameric RNA-bound N protein ring assembly exhibiting D14 symmetry. The structure of the HeV N protein adopts the common bi-lobed paramyxoviral N protein fold; the N-terminal and C-terminal globular domains are bisected by an RNA binding cleft containing six RNA nucleotides and are flanked by the N-terminal and C-terminal arms, respectively. In common with other paramyxoviral nucleocapsids, the lateral interface between adjacent Ni and Ni+1 protomers involves electrostatic and hydrophobic interactions mediated primarily through the N-terminal arm and globular domains with minor contribution from the C-terminal arm. However, the HeV N multimeric assembly uniquely identifies an additional protomer-protomer contact between the Ni+1 N-terminus and Ni-1 C-terminal arm linker. The model presented here broadens the understanding of RNA-bound paramyxoviral nucleocapsid architectures and provides a platform for further insight into the molecular biology of HeV, as well as the development of antiviral interventions.

3D Puzzle at the Nanoscale-How do RNA Viruses Self-Assemble their Capsids into Perfectly Ordered Structures

The phenomenon of RNA virus self-organization, first observed in the mid-20th century in tobacco mosaic virus, is the subject of extensive research. Efforts to comprehend this process intensify due to its potential for producing vaccines or antiviral compounds as well as nanocarriers and nanotemplates. However, direct observation of the self-assembly is hindered by its prevalence within infected host cells. One of the approaches involves in vitro and in silico research using model viruses featuring a ssRNA(+) genome enclosed within a capsid made up of a single type protein. While various pathways are proposed based on these studies, their relevance in vivo remains uncertain. On the other hand, the development of advanced microscopic methods provide insights into the events within living cells, where following viral infection, specialized compartments form to facilitate the creation of nascent virions. Intriguingly, a growing body of evidence indicates that the primary function of packaging signals in viral RNA is to effectively initiate the virion self-assembly. This is in contrast to earlier opinions suggesting a role in marking RNA for encapsidation. Another noteworthy observation is that many viruses undergo self-assembly within membraneless liquid organelles, which are specifically induced by viral proteins.

Cryo-EM structure of influenza helical nucleocapsid reveals NP-NP and NP-RNA interactions as a model for the genome encapsidation

Influenza virus genome encapsidation is essential for the formation of a helical viral ribonucleoprotein (vRNP) complex composed of nucleoproteins (NP), the trimeric polymerase, and the viral genome. Although low-resolution vRNP structures are available, it remains unclear how the viral RNA is encapsidated and how NPs assemble into the helical filament specific of influenza vRNPs. In this study, we established a biological tool, the RNP-like particles assembled from recombinant influenza A virus NP and synthetic RNA, and we present the first subnanometric cryo-electron microscopy structure of the helical NP-RNA complex (8.7 to 5.3 Å). The helical RNP-like structure reveals a parallel double-stranded conformation, allowing the visualization of NP-NP and NP-RNA interactions. The RNA, located at the interface of neighboring NP protomers, interacts with conserved residues previously described as essential for the NP-RNA interaction. The NP undergoes conformational changes to enable RNA binding and helix formation. Together, our findings provide relevant insights for understanding the mechanism for influenza genome encapsidation.

Nucleocapsid proteins from human coronaviruses possess phase separation capabilities and promote FUS pathological aggregation

The nucleocapsid (N) protein is an essential structural component necessary for genomic packaging and replication in various human coronaviruses (HCoVs), such as SARS-CoV-2 and MERS-CoV. Recent studies have revealed that the SARS-CoV-2 N protein exhibits a high capacity for liquid-liquid phase separation (LLPS), which plays multiple roles in viral infection and replication. In this study, we systematically investigate the LLPS capabilities of seven homologous N proteins from different HCoVs using a high-throughput protein phase separation assay. We found that LLPS is a shared intrinsic property among these N proteins. However, the phase separation profiles of the various N protein homologs differ, and they undergo phase separation under distinct in vitro conditions. Moreover, we demonstrate that N protein homologs can co-phase separate with FUS, a SG-containing protein, and accelerate its liquid-to-solid phase transition and amyloid aggregation, which is closely related to amyotrophic lateral sclerosis. Further study shows that N protein homologs can directly bind to the low complexity domain of FUS. Together, our work demonstrates that N proteins of different HCoVs possess phase separation capabilities, which may contribute to promoting pathological aggregation of host proteins and disrupting SG homeostasis during the infection and replication of various HCoVs.

Functional benefit of structural disorder for the replication of measles, Nipah and Hendra viruses

Measles, Nipah and Hendra viruses are severe human pathogens within the Paramyxoviridae family. Their non-segmented, single-stranded, negative-sense RNA genome is encapsidated by the nucleoprotein (N) within a helical nucleocapsid that is the substrate used by the viral RNA-dependent-RNA-polymerase (RpRd) for transcription and replication. The RpRd is a complex made of the large protein (L) and of the phosphoprotein (P), the latter serving as an obligate polymerase cofactor and as a chaperon for N. Both the N and P proteins are enriched in intrinsically disordered regions (IDRs), i.e. regions devoid of stable secondary and tertiary structure. N possesses a C-terminal IDR (NTAIL), while P consists of a large, intrinsically disordered N-terminal domain (NTD) and a C-terminal domain (CTD) encompassing alternating disordered and ordered regions. The V and W proteins, two non-structural proteins that are encoded by the P gene via a mechanism of co-transcriptional edition of the P mRNA, are prevalently disordered too, sharing with P the disordered NTD. They are key players in the evasion of the host antiviral response and were shown to phase separate and to form amyloid-like fibrils in vitro. In this review, we summarize the available information on IDRs within the N, P, V and W proteins from these three model paramyxoviruses and describe their molecular partnership. We discuss the functional benefit of disorder to virus replication in light of the critical role of IDRs in affording promiscuity, multifunctionality, fine regulation of interaction strength, scaffolding functions and in promoting liquid-liquid phase separation and fibrillation.

Structure and Dynamics of the Unassembled Nucleoprotein of Rabies Virus in Complex with Its Phosphoprotein Chaperone Module

As for all non-segmented negative RNA viruses, rabies virus has its genome packaged in a linear assembly of nucleoprotein (N), named nucleocapsid. The formation of new nucleocapsids during virus replication in cells requires the production of soluble N protein in complex with its phosphoprotein (P) chaperone. In this study, we reconstituted a soluble heterodimeric complex between an armless N protein of rabies virus (RABV), lacking its N-terminal subdomain (N_NT-ARM), and a peptide encompassing the N⁰ chaperon module of the P protein. We showed that the chaperone module undergoes a disordered-order transition when it assembles with N⁰ and measured an affinity in the low nanomolar range using a competition assay. We solved the crystal structure of the complex at a resolution of 2.3 Å, unveiling the details of the conserved interfaces. MD simulations showed that both the chaperon module of P and RNA-mediated polymerization reduced the ability of the RNA binding cavity to open and close. Finally, by reconstituting a complex with full-length P protein, we demonstrated that each P dimer could independently chaperon two N⁰ molecules.

Structural Basis of Human Parainfluenza Virus 3 Unassembled Nucleoprotein in Complex with Its Viral Chaperone

Human parainfluenza virus 3 (HPIV3) belongs to the Paramyxoviridae, causing annual worldwide epidemics of respiratory diseases, especially in newborns and infants. The core components consist of just three viral proteins: nucleoprotein (N), phosphoprotein (P), and RNA polymerase (L), playing essential roles in replication and transcription of HPIV3 as well as other paramyxoviruses. Viral genome encapsidated by N is as a template and recognized by RNA-dependent RNA polymerase complex composed of L and P. The offspring RNA also needs to assemble with N to form nucleocapsids. The N is one of the most abundant viral proteins in infected cells and chaperoned in the RNA-free form (N0) by P before encapsidation. In this study, we presented the structure of unassembled HPIV3 N0 in complex with the N-terminal portion of the P, revealing the molecular details of the N0 and the conserved N0-P interaction. Combined with biological experiments, we showed that the P binds to the C-terminal domain of N0 mainly by hydrophobic interaction and maintains the unassembled conformation of N by interfering with the formation of N-RNA oligomers, which might be a target for drug development. Based on the complex structure, we developed a method to obtain the monomeric N0. Furthermore, we designed a P-derived fusion peptide with 10-fold higher affinity, which hijacked the N and interfered with the binding of the N to RNA significantly. Finally, we proposed a model of conformational transition of N from the unassembled state to the assembled state, which helped to further understand viral replication. IMPORTANCE Human parainfluenza virus 3 (HPIV3) causes annual epidemics of respiratory diseases, especially in newborns and infants. For the replication of HPIV3 and other paramyxoviruses, only three viral proteins are required: phosphoprotein (P), RNA polymerase (L), and nucleoprotein (N). Here, we report the crystal structure of the complex of N and its chaperone P. We describe in detail how P acts as a chaperone to maintain the unassembled conformation of N. Our analysis indicated that the interaction between P and N is conserved and mediated by hydrophobicity, which can be used as a target for drug development. We obtained a high-affinity P-derived peptide inhibitor, specifically targeted N, and greatly interfered with the binding of the N to RNA, thereby inhibiting viral encapsidation and replication. In summary, our results provide new insights into the paramyxovirus genome replication and nucleocapsid assembly and lay the basis for drug development.

Insights into Paramyxovirus Nucleocapsids from Diverse Assemblies

All paramyxoviruses, which include the mumps virus, measles virus, Nipah virus, Newcastle disease virus, and Sendai virus, have non-segmented single-stranded negative-sense RNA genomes. These RNA genomes are enwrapped throughout the viral life cycle by nucleoproteins, forming helical nucleocapsids. In addition to these helical structures, recombinant paramyxovirus nucleocapsids may occur in other assembly forms such as rings, clam-shaped structures, and double-headed nucleocapsids; the latter two are composed of two single-stranded helices packed in a back-to-back pattern. In all of these assemblies, the neighboring nucleoprotein protomers adopt the same domain-swapping mode via the N-terminal arm, C-terminal arm, and recently disclosed N-hole. An intrinsically disordered region in the C-terminal domain of the nucleoproteins, called the N-tail, plays an unexpected role in regulating the transition among the different assembly forms that occurs with other viral proteins, especially phosphoprotein. These structures, together with the helical nucleocapsids, significantly enrich the structural diversity of the paramyxovirus nucleocapsids and help explain the functions of these diverse assemblies, including RNA genome protection, transcription, and replication, as well as encapsulation.

Sendai Virus and a Unified Model of Mononegavirus RNA Synthesis

Vesicular stomatitis virus (VSV), the founding member of the mononegavirus order (Mononegavirales), was found to be a negative strand RNA virus in the 1960s, and since then the number of such viruses has continually increased with no end in sight. Sendai virus (SeV) was noted soon afterwards due to an outbreak of newborn pneumonitis in Japan whose putative agent was passed in mice, and nowadays this mouse virus is mainly the bane of animal houses and immunologists. However, SeV was important in the study of this class of viruses because, like flu, it grows to high titers in embryonated chicken eggs, facilitating the biochemical characterization of its infection and that of its nucleocapsid, which is very close to that of measles virus (MeV). This review and opinion piece follow SeV as more is known about how various mononegaviruses express their genetic information and carry out their RNA synthesis, and proposes a unified model based on what all MNV have in common.

The Nucleocapsid of Paramyxoviruses: Structure and Function of an Encapsidated Template

Viruses of the Paramyxoviridae family share a common and complex molecular machinery for transcribing and replicating their genomes. Their non-segmented, negative-strand RNA genome is encased in a tight homopolymer of viral nucleoproteins (N). This ribonucleoprotein complex, termed a nucleocapsid, is the template of the viral polymerase complex made of the large protein (L) and its co-factor, the phosphoprotein (P). This review summarizes the current knowledge on several aspects of paramyxovirus transcription and replication, including structural and functional data on (1) the architecture of the nucleocapsid (structure of the nucleoprotein, interprotomer contacts, interaction with RNA, and organization of the disordered C-terminal tail of N), (2) the encapsidation of the genomic RNAs (structure of the nucleoprotein in complex with its chaperon P and kinetics of RNA encapsidation in vitro), and (3) the use of the nucleocapsid as a template for the polymerase complex (release of the encased RNA and interaction network allowing the progress of the polymerase complex). Finally, this review presents models of paramyxovirus transcription and replication.

Quantitative Description of Intrinsically Disordered Proteins Using Single-Molecule FRET, NMR, and SAXS

Studying the conformational landscape of intrinsically disordered and partially folded proteins is challenging and only accessible to a few solution state techniques, such as nuclear magnetic resonance (NMR), small-angle scattering techniques, and single-molecule Förster resonance energy transfer (smFRET). While each of the techniques is sensitive to different properties of the disordered chain, such as local structural propensities, overall dimension, or intermediate- and long-range contacts, conformational ensembles describing intrinsically disordered proteins (IDPs) accurately should ideally respect all of these properties. Here we develop an integrated approach using a large set of FRET efficiencies and fluorescence lifetimes, NMR chemical shifts, and paramagnetic relaxation enhancements (PREs), as well as small-angle X-ray scattering (SAXS) to derive quantitative conformational ensembles in agreement with all parameters. Our approach is tested using simulated data (five sets of PREs and 15 FRET efficiencies) and validated experimentally on the example of the disordered domain of measles virus phosphoprotein, providing new insights into the conformational landscape of this viral protein that comprises transient structural elements and is more compact than an unfolded chain throughout its length. Rigorous cross-validation using FRET efficiencies, fluorescence lifetimes, and SAXS demonstrates the predictive nature of the calculated conformational ensembles and underlines the potential of this strategy in integrative dynamic structural biology.

Fabrication of Supramolecular Antibacterial Nanofibers with Membrane-Disruptive Mechanism

By studying the principles of self-assembly and combining the structural parameters required for the asymmetric distribution of antimicrobial peptides (AMPs), we newly designed and screened the high-activity and low-toxicity AMP F2I-LL. This peptide can form a supramolecular hydrogel with a nanofiber microstructure in a simulated physiological environment (phosphate buffered saline), which exhibits broad-spectrum antibacterial activity. Compared with monomeric peptides, the introduction of a self-assembly strategy not only improved the bactericidal titer but also enhanced the serum stability of AMPs. Mechanistic studies showed that the positive charge enriched on the surface of the nanofiber was conducive to its rapid binding to the negatively charged part of the outer membrane of bacteria and further entered the inner membrane, increasing its permeability and ultimately leading to cell membrane rupture and death. This work provides insights into the design of nanopeptides with broad-spectrum antibacterial activity and provides new results for the development of biomedicine.

A Close Look at Molecular Self-Assembly with the Transmission Electron Microscope

Molecular self-assembly is pervasive in the formation of living and synthetic materials. Knowledge gained from research into the principles of molecular self-assembly drives innovation in the biological, chemical, and materials sciences. Self-assembly processes span a wide range of temporal and spatial domains and are often unintuitive and complex. Studying such complex processes requires an arsenal of analytical and computational tools. Within this arsenal, the transmission electron microscope stands out for its unique ability to visualize and quantify self-assembly structures and processes. This review describes the contribution that the transmission electron microscope has made to the field of molecular self-assembly. An emphasis is placed on which TEM methods are applicable to different structures and processes and how TEM can be used in combination with other experimental or computational methods. Finally, we provide an outlook on the current challenges to, and opportunities for, increasing the impact that the transmission electron microscope can have on molecular self-assembly.

CryoEM structure of the Nipah virus nucleocapsid assembly

Nipah and its close relative Hendra are highly pathogenic zoonotic viruses, storing their ssRNA genome in a helical nucleocapsid assembly formed by the N protein, a major viral immunogen. Here, we report the first cryoEM structure for a Henipavirus RNA-bound nucleocapsid assembly, at 3.5 Å resolution. The helical assembly is stabilised by previously undefined N- and C-terminal segments, contributing to subunit-subunit interactions. RNA is wrapped around the nucleocapsid protein assembly with a periodicity of six nucleotides per protomer, in the “3-bases-in, 3-bases-out” conformation, with protein plasticity enabling non-sequence specific interactions. The structure reveals commonalities in RNA binding pockets and in the conformation of bound RNA, not only with members of the Paramyxoviridae family, but also with the evolutionarily distant Filoviridae Ebola virus. Significant structural differences with other Paramyxoviridae members are also observed, particularly in the position and length of the exposed α-helix, residues 123–139, which may serve as a valuable epitope for surveillance and diagnostics.

Nipah virus is a highly pathogenic RNA virus which, along with the closely related Hendra virus, emerged relatively recently. Due to ~40% mortality rate and evidence of animal-to-human as well as human-to-human transmission, development of antivirals against the Nipah and henipaviral disease is particularly urgent. In common with other single-stranded RNA viruses, including Ebola and coronaviruses, the nucleocapsid assembly of the Nipah virus safeguards the viral genome, protecting it from degradation and facilitating its encapsidation and storage inside the virion. Here, we used cryo-electron microscopy to determine accurate three-dimensional structure for several different assemblies of the Nipah virus nucleocapsid protein, in particular a detailed structure for the complex of this protein with RNA. This structural information is important for understanding detailed molecular interactions driving and stabilizing the nucleocapsid assembly formation that are of fundamental importance for understanding similar processes in a large group of ssRNA viruses. Apart from highlighting structural similarities and differences with nucleocapsid proteins of other viruses of the Paramyxoviridae family, these data will inform the development of new antiviral approaches for the henipaviruses.

Cryo-EM structure of the cetacean morbillivirus nucleoprotein-RNA complex

Cetacean morbillivirus (CeMV) is an emerging and highly infectious paramyxovirus that causes outbreaks in cetaceans and occasionally in pinnipeds, representing a major threat to biodiversity and conservation of endangered marine mammal populations in both hemispheres. As for all non-segmented, negative-sense, single-stranded RNA (ssRNA) viruses, the morbilliviral genome is enwrapped by thousands of nucleoprotein (N) protomers. Each bound to six ribonucleotides, N protomers assemble to form a helical ribonucleoprotein (RNP) complex that serves as scaffold for nucleocapsid formation and as template for viral replication and transcription. While the molecular details on RNP complexes elucidated in human measles virus (MeV) served as paradigm model for these processes in all members of the Morbillivirus genus, no structural information has been obtained from other morbilliviruses, nor has any CeMV structure been solved so far. We report the structure of the CeMV RNP complex, reconstituted in vitro upon binding of recombinant CeMV N to poly-adenine ssRNA hexamers and solved to 4.0 Å resolution by cryo-electron microscopy. In spite of the amino acid sequence similarity and consequently similar folding of the N protomer, the CeMV RNP complex exhibits different helical parameters as compared to previously reported MeV orthologs. The CeMV structure reveals exclusive interactions leading to more extensive protomer-RNA and protomer-protomer interfaces. We identified twelve residues, among those varying between CeMV strains, as putatively important for the stabilization of the RNP complex, which highlights the need to study the potential of CeMV N mutations that modulate nucleocapsid assembly to also affect viral phenotype and host adaptation.

Structural Insights into the Respiratory Syncytial Virus RNA Synthesis Complexes

RNA synthesis in respiratory syncytial virus (RSV), a negative-sense (-) nonsegmented RNA virus, consists of viral gene transcription and genome replication. Gene transcription includes the positive-sense (+) viral mRNA synthesis, 5'-RNA capping and methylation, and 3' end polyadenylation. Genome replication includes (+) RNA antigenome and (-) RNA genome synthesis. RSV executes the viral RNA synthesis using an RNA synthesis ribonucleoprotein (RNP) complex, comprising four proteins, the nucleoprotein (N), the large protein (L), the phosphoprotein (P), and the M2-1 protein. We provide an overview of the RSV RNA synthesis and the structural insights into the RSV gene transcription and genome replication process. We propose a model of how the essential four proteins coordinate their activities in different RNA synthesis processes.

SARS-CoV-2 nucleocapsid protein phase-separates with RNA and with human hnRNPs

Tightly packed complexes of nucleocapsid protein and genomic RNA form the core of viruses and assemble within viral factories, dynamic compartments formed within the host cells associated with human stress granules. Here, we test the possibility that the multivalent RNA-binding nucleocapsid protein (N) from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) condenses with RNA via liquid-liquid phase separation (LLPS) and that N protein can be recruited in phase-separated forms of human RNA-binding proteins associated with SG formation. Robust LLPS with RNA requires two intrinsically disordered regions (IDRs), the N-terminal IDR and central-linker IDR, as well as the folded C-terminal oligomerization domain, while the folded N-terminal domain and the C-terminal IDR are not required. N protein phase separation is induced by addition of non-specific RNA. In addition, N partitions in vitro into phase-separated forms of full-length human hnRNPs (TDP-43, FUS, hnRNPA2) and their low-complexity domains (LCs). These results provide a potential mechanism for the role of N in SARS-CoV-2 viral genome packing and in host-protein co-opting necessary for viral replication and infectivity.

Liquid-Liquid Phase Separation by Intrinsically Disordered Protein Regions of Viruses: Roles in Viral Life Cycle and Control of Virus-Host Interactions

Intrinsically disordered proteins (IDPs) are unable to adopt a unique 3D structure under physiological conditions and thus exist as highly dynamic conformational ensembles. IDPs are ubiquitous and widely spread in the protein realm. In the last decade, compelling experimental evidence has been gathered, pointing to the ability of IDPs and intrinsically disordered regions (IDRs) to undergo liquid-liquid phase separation (LLPS), a phenomenon driving the formation of membrane-less organelles (MLOs). These biological condensates play a critical role in the spatio-temporal organization of the cell, where they exert a multitude of key biological functions, ranging from transcriptional regulation and silencing to control of signal transduction networks. After introducing IDPs and LLPS, we herein survey available data on LLPS by IDPs/IDRs of viral origin and discuss their functional implications. We distinguish LLPS associated with viral replication and trafficking of viral components, from the LLPS-mediated interference of viruses with host cell functions. We discuss emerging evidence on the ability of plant virus proteins to interfere with the regulation of MLOs of the host and propose that bacteriophages can interfere with bacterial LLPS, as well. We conclude by discussing how LLPS could be targeted to treat phase separation-associated diseases, including viral infections.

Minimal Elements Required for the Formation of Respiratory Syncytial Virus Cytoplasmic Inclusion Bodies In Vivo and In Vitro

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract illness in infants, elderly, and immunocompromised people. No vaccine or efficient antiviral treatment is available against this virus. The replication and transcription steps of the viral genome are appealing mechanisms to target for the development of new antiviral strategies. These activities take place within cytoplasmic inclusion bodies (IBs) that assemble during infection. Although expression of both the viral nucleoprotein (N) and phosphoprotein (P) allows induction of the formation of these IBs, the mechanism sustaining their assembly remains poorly characterized. Here, we identified key elements of N and P required for the scaffolding of IBs and managed for the first time to reconstitute RSV pseudo-IBs in vitro by coincubating recombinant N and P proteins. Our results provide strong evidence that the biogenesis of RSV IBs occurs through liquid-liquid phase transition mediated by N-P interactions.

Infection of host cells by the respiratory syncytial virus (RSV) is characterized by the formation of spherical cytoplasmic inclusion bodies (IBs). These structures, which concentrate all the proteins of the polymerase complex as well as some cellular proteins, were initially considered aggresomes formed by viral dead-end products. However, recent studies revealed that IBs are viral factories where viral RNA synthesis, i.e., replication and transcription, occurs. The analysis of IBs by electron microscopy revealed that they are membrane-less structures, and accumulated data on their structure, organization, and kinetics of formation revealed that IBs share the characteristics of cellular organelles, such as P-bodies or stress granules, suggesting that their morphogenesis depends on a liquid-liquid phase separation mechanism. It was previously shown that expression of the RSV nucleoprotein N and phosphoprotein P of the polymerase complex is sufficient to induce the formation of pseudo-IBs. Here, using a series of truncated P proteins, we identified the domains of P required for IB formation and show that the oligomeric state of N, provided it can interact with RNA, is critical for their morphogenesis. We also show that pseudo-IBs can form in vitro when recombinant N and P proteins are mixed. Finally, using fluorescence recovery after photobleaching approaches, we reveal that in cellula and in vitro IBs are liquid organelles. Our results strongly support the liquid-liquid phase separation nature of IBs and pave the way for further characterization of their dynamics.

SARS-CoV-2 nucleocapsid protein undergoes liquid-liquid phase separation stimulated by RNA and partitions into phases of human ribonucleoproteins

Tightly packed complexes of nucleocapsid protein and genomic RNA form the core of viruses and may assemble within viral factories, dynamic compartments formed within the host cells. Here, we examine the possibility that the multivalent RNA-binding nucleocapsid protein (N) from the severe acute respiratory syndrome coronavirus (SARS-CoV-2) compacts RNA via protein-RNA liquid-liquid phase separation (LLPS) and that N interactions with host RNA-binding proteins are mediated by phase separation. To this end, we created a construct expressing recombinant N fused to a N-terminal maltose binding protein tag which helps keep the oligomeric N soluble for purification. Using in vitro phase separation assays, we find that N is assembly-prone and phase separates avidly. Phase separation is modulated by addition of RNA and changes in pH and is disfavored at high concentrations of salt. Furthermore, N enters into in vitro phase separated condensates of full-length human hnRNPs (TDP-43, FUS, and hnRNPA2) and their low complexity domains (LCs). However, N partitioning into the LC of FUS, but not TDP-43 or hnRNPA2, requires cleavage of the solubilizing MBP fusion. Hence, LLPS may be an essential mechanism used for SARS-CoV-2 and other RNA viral genome packing and host protein co-opting, functions necessary for viral replication and hence infectivity.

Structural Description of the Nipah Virus Phosphoprotein and Its Interaction with STAT1

The structural characterization of modular proteins containing long intrinsically disordered regions intercalated with folded domains is complicated by their conformational diversity and flexibility and requires the integration of multiple experimental approaches. Nipah virus (NiV) phosphoprotein, an essential component of the viral RNA transcription/replication machine and a component of the viral arsenal that hijacks cellular components and counteracts host immune responses, is a prototypical model for such modular proteins. Curiously, the phosphoprotein of NiV is significantly longer than the corresponding protein of other paramyxoviruses. Here, we combine multiple biophysical methods, including x-ray crystallography, NMR spectroscopy, and small angle x-ray scattering, to characterize the structure of this protein and provide an atomistic representation of the full-length protein in the form of a conformational ensemble. We show that full-length NiV phosphoprotein is tetrameric, and we solve the crystal structure of its tetramerization domain. Using NMR spectroscopy and small angle x-ray scattering, we show that the long N-terminal intrinsically disordered region and the linker connecting the tetramerization domain to the C-terminal X domain exchange between multiple conformations while containing short regions of residual secondary structure. Some of these transient helices are known to interact with partners, whereas others represent putative binding sites for yet unidentified proteins. Finally, using NMR spectroscopy and isothermal titration calorimetry, we map a region of the phosphoprotein, comprising residues between 110 and 140 and common to the V and W proteins, that binds with weak affinity to STAT1 and confirm the involvement of key amino acids of the viral protein in this interaction. This provides new, to our knowledge, insights into how the phosphoprotein and the nonstructural V and W proteins of NiV perform their multiple functions.

Measles virus nucleo- and phosphoproteins form liquid-like phase-separated compartments that promote nucleocapsid assembly

Many viruses are known to form cellular compartments, also called viral factories. Paramyxoviruses, including measles virus, colocalize their proteomic and genomic material in puncta in infected cells. We demonstrate that purified nucleoproteins (N) and phosphoproteins (P) of measles virus form liquid-like membraneless organelles upon mixing in vitro. We identify weak interactions involving intrinsically disordered domains of N and P that are implicated in this process, one of which is essential for phase separation. Fluorescence allows us to follow the modulation of the dynamics of N and P upon droplet formation, while NMR is used to investigate the thermodynamics of this process. RNA colocalizes to droplets, where it triggers assembly of N protomers into nucleocapsid-like particles that encapsidate the RNA. The rate of encapsidation within droplets is enhanced compared to the dilute phase, revealing one of the roles of liquid-liquid phase separation in measles virus replication.

In vitro trackable assembly of RNA-specific nucleocapsids of the respiratory syncytial virus

The templates for transcription and replication by respiratory syncytial virus (RSV) polymerase are helical nucleocapsids (NCs), formed by viral RNAs that are encapsidated by the nucleoprotein (N). Proper NC assembly is vital for RSV polymerase to engage the RNA template for RNA synthesis. Previous studies of NCs or nucleocapsid-like particles (NCLPs) from RSV and other nonsegmented negative-sense RNA viruses have provided insights into the overall NC architecture. However, in these studies, the RNAs were either random cellular RNAs or average viral genomic RNAs. An in-depth mechanistic understanding of NCs has been hampered by lack of an in vitro assay that can track NC or NCLP assembly. Here we established a protocol to obtain RNA-free N protein (N⁰) and successfully demonstrated the utility of a new assay for tracking assembly of N with RNA oligonucleotides into NCLPs. We discovered that the efficiency of the NCLP (N–RNA) assembly depends on the length and sequence of the RNA incorporated into NCLPs. This work provides a framework to generate purified N⁰ and incorporate it with RNA into NCLPs in a controllable manner. We anticipate that our assay for in vitro trackable assembly of RSV-specific nucleocapsids may enable in-depth mechanistic analyses of this process.

The Nucleoprotein and Phosphoprotein of Measles Virus

Measles virus is a negative strand virus and the genomic and antigenomic RNA binds to the nucleoprotein (N), assembling into a helical nucleocapsid. The polymerase complex comprises two proteins, the Large protein (L), that both polymerizes RNA and caps the mRNA, and the phosphoprotein (P) that co-localizes with L on the nucleocapsid. This review presents recent results about N and P, in particular concerning their intrinsically disordered domains. N is a protein of 525 residues with a 120 amino acid disordered C-terminal domain, N_tail. The first 50 residues of N_tail extricate the disordered chain from the nucleocapsid, thereby loosening the otherwise rigid structure, and the C-terminus contains a linear motif that binds P. Recent results show how the 5′ end of the viral RNA binds to N within the nucleocapsid and also show that the bases at the 3′ end of the RNA are rather accessible to the viral polymerase. P is a tetramer and most of the protein is disordered; comprising 507 residues of which around 380 are disordered. The first 37 residues of P bind N, chaperoning against non-specific interaction with cellular RNA, while a second interaction site, around residue 200 also binds N. In addition, there is another interaction between C-terminal domain of P (XD) and N_tail. These results allow us to propose a new model of how the polymerase binds to the nucleocapsid and suggests a mechanism for initiation of transcription.

Constraints of Viral RNA Synthesis on Codon Usage of Negative-Strand RNA Virus

Negative-strand RNA viruses (NSVs) include some of the most pathogenic human viruses known. NSVs completely rely on the host cell for protein translation, but their codon usage bias is often different from that of the host. This discrepancy may have originated from the unique mechanism of NSV RNA synthesis in that the genomic RNA sequestered in the nucleocapsid serves as the template. The stability of the genomic RNA in the nucleocapsid appears to regulate its accessibility to the viral RNA polymerase, thus placing constraints on codon usage to balance viral RNA synthesis. By in situ analyses of vesicular stomatitis virus RNA synthesis, specific activities of viral RNA synthesis were correlated with the genomic RNA sequence. It was found that by simply altering the sequence and not the amino acid that it encoded, a significant reduction, up to an ∼750-fold reduction, in viral RNA transcripts occurred. Through subsequent sequence analysis and thermal shift assays, it was found that the purine/pyrimidine content modulates the overall stability of the polymerase complex, resulting in alteration of the activity of viral RNA synthesis. The codon usage is therefore constrained by the obligation of the NSV genome for viral RNA synthesis.IMPORTANCE Negative-strand RNA viruses (NSVs) include the most pathogenic viruses known. New methods to monitor their evolutionary trends are urgently needed for the development of antivirals and vaccines. The protein translation machinery of the host cell is currently recognized as a main genomic regulator of RNA virus evolution, which works especially well for positive-strand RNA viruses. However, this approach fails for NSVs because it does not consider the unique mechanism of their viral RNA synthesis. For NSVs, the viral RNA-dependent RNA polymerase (vRdRp) must gain access to the genome sequestered in the nucleocapsid. Our work suggests a paradigm shift that the interactions between the RNA genome and the nucleocapsid protein regulate the activity of vRdRp, which selects codon usage.

Assembly and cryo-EM structures of RNA-specific measles virus nucleocapsids provide mechanistic insight into paramyxoviral replication

Assembly of paramyxoviral nucleocapsids on the RNA genome is an essential step in the viral cycle. The structural basis of this process has remained obscure due to the inability to control encapsidation. We used a recently developed approach to assemble measles virus nucleocapsid-like particles on specific sequences of RNA hexamers (poly-Adenine and viral genomic 5') in vitro, and determined their cryoelectron microscopy maps to 3.3-Å resolution. The structures unambiguously determine 5' and 3' binding sites and thereby the binding-register of viral genomic RNA within nucleocapsids. This observation reveals that the 3' end of the genome is largely exposed in fully assembled measles nucleocapsids. In particular, the final three nucleotides of the genome are rendered accessible to the RNA-dependent RNA polymerase complex, possibly enabling efficient RNA processing. The structures also reveal local and global conformational changes in the nucleoprotein upon assembly, in particular involving helix α6 and helix α13 that form edges of the RNA binding groove. Disorder is observed in the bound RNA, localized at one of the two backbone conformational switch sites. The high-resolution structure allowed us to identify putative nucleobase interaction sites in the RNA-binding groove, whose impact on assembly kinetics was measured using real-time NMR. Mutation of one of these sites, R195, whose sidechain stabilizes both backbone and base of a bound nucleic acid, is thereby shown to be essential for nucleocapsid-like particle assembly.

Controlling the Self-Assembly of Biomolecules into Functional Nanomaterials through Internal Interactions and External Stimulations: A Review

Biomolecular self-assembly provides a facile way to synthesize functional nanomaterials. Due to the unique structure and functions of biomolecules, the created biological nanomaterials via biomolecular self-assembly have a wide range of applications, from materials science to biomedical engineering, tissue engineering, nanotechnology, and analytical science. In this review, we present recent advances in the synthesis of biological nanomaterials by controlling the biomolecular self-assembly from adjusting internal interactions and external stimulations. The self-assembly mechanisms of biomolecules (DNA, protein, peptide, virus, enzyme, metabolites, lipid, cholesterol, and others) related to various internal interactions, including hydrogen bonds, electrostatic interactions, hydrophobic interactions, π⁻π stacking, DNA base pairing, and ligand⁻receptor binding, are discussed by analyzing some recent studies. In addition, some strategies for promoting biomolecular self-assembly via external stimulations, such as adjusting the solution conditions (pH, temperature, ionic strength), adding organics, nanoparticles, or enzymes, and applying external light stimulation to the self-assembly systems, are demonstrated. We hope that this overview will be helpful for readers to understand the self-assembly mechanisms and strategies of biomolecules and to design and develop new biological nanostructures or nanomaterials for desired applications.

The structure of the nucleoprotein of Influenza D shows that all Orthomyxoviridae nucleoproteins have a similar NPCORE, with or without a NPTAIL for nuclear transport

This paper focuses on the nucleoprotein (NP) of the newly identified member of the Orthomyxoviridae family, Influenza D virus. To date several X-ray structures of NP of Influenza A (A/NP) and B (B/NP) viruses and of infectious salmon anemia (ISA/NP) virus have been solved. Here we purified, characterized and solved the X-ray structure of the tetrameric D/NP at 2.4 Å resolution. The crystal structure of its core is similar to NP of other Influenza viruses. However, unlike A/NP and B/NP which possess a flexible amino-terminal tail containing nuclear localization signals (NLS) for their nuclear import, D/NP possesses a carboxy-terminal tail (D/NP_TAIL). We show that D/NP_TAIL harbors a bipartite NLS and designed C-terminal truncated mutants to demonstrate the role of D/NP_TAIL for nuclear transport.

Stability of local secondary structure determines selectivity of viral RNA chaperones

To maintain genome integrity, segmented double-stranded RNA viruses of the Reoviridae family must accurately select and package a complete set of up to a dozen distinct genomic RNAs. It is thought that the high fidelity segmented genome assembly involves multiple sequence-specific RNA-RNA interactions between single-stranded RNA segment precursors. These are mediated by virus-encoded non-structural proteins with RNA chaperone-like activities, such as rotavirus (RV) NSP2 and avian reovirus σNS. Here, we compared the abilities of NSP2 and σNS to mediate sequence-specific interactions between RV genomic segment precursors. Despite their similar activities, NSP2 successfully promotes inter-segment association, while σNS fails to do so. To understand the mechanisms underlying such selectivity in promoting inter-molecular duplex formation, we compared RNA-binding and helix-unwinding activities of both proteins. We demonstrate that octameric NSP2 binds structured RNAs with high affinity, resulting in efficient intramolecular RNA helix disruption. Hexameric σNS oligomerizes into an octamer that binds two RNAs, yet it exhibits only limited RNA-unwinding activity compared to NSP2. Thus, the formation of intersegment RNA-RNA interactions is governed by both helix-unwinding capacity of the chaperones and stability of RNA structure. We propose that this protein-mediated RNA selection mechanism may underpin the high fidelity assembly of multi-segmented RNA genomes in Reoviridae.

An ultraweak interaction in the intrinsically disordered replication machinery is essential for measles virus function

Measles virus genome encapsidation is essential for viral replication and is controlled by the intrinsically disordered phosphoprotein (P) maintaining the nucleoprotein in a monomeric form (N) before nucleocapsid assembly. All paramyxoviruses harbor highly disordered amino-terminal domains (PNTD) that are hundreds of amino acids in length and whose function remains unknown. Using nuclear magnetic resonance (NMR) spectroscopy, we describe the structure and dynamics of the 90-kDa N0PNTD complex, comprising 450 disordered amino acids, at atomic resolution. NMR relaxation dispersion reveals the existence of an ultraweak N-interaction motif, hidden within the highly disordered PNTD, that allows PNTD to rapidly associate and dissociate from a specific site on N while tightly bound at the amino terminus, thereby hindering access to the surface of N. Mutation of this linear motif quenches the long-range dynamic coupling between the two interaction sites and completely abolishes viral transcription/replication in cell-based minigenome assays comprising integral viral replication machinery. This description transforms our understanding of intrinsic conformational disorder in paramyxoviral replication. The essential mechanism appears to be conserved across Paramyxoviridae, opening unique new perspectives for drug development against this family of pathogens.

The control of paramyxovirus genome hexamer length and mRNA editing

The unusual ability of a human parainfluenza virus type 2 (hPIV2) nucleoprotein point mutation (NP^Q202A) to strongly enhance minigenome replication was found to depend on the absence of a functional, internal element of the bipartite replication promoter (CRII). This point mutation allows relatively robust CRII-minus minigenome replication in a CRII-independent manner, under conditions in which NP^wt is essentially inactive. The nature of the amino acid at position 202 apparently controls whether viral RNA-dependent RNA polymerase (vRdRp) can, or cannot, initiate RNA synthesis in a CRII-independent manner. By repressing genome synthesis when vRdRp cannot correctly interact with CRII, gln²⁰² of N, the only residue of the RNA-binding groove that contacts a nucleotide base in the N-RNA, acts as a gatekeeper for wild-type (CRII-dependent) RNA synthesis. This ensures that only hexamer-length genomes are replicated, and that the critical hexamer phase of the cis-acting mRNA editing sequence is maintained.

How order and disorder within paramyxoviral nucleoproteins and phosphoproteins orchestrate the molecular interplay of transcription and replication

In this review, we summarize computational and experimental data gathered so far showing that structural disorder is abundant within paramyxoviral nucleoproteins (N) and phosphoproteins (P). In particular, we focus on measles, Nipah, and Hendra viruses and highlight both commonalities and differences with respect to the closely related Sendai virus. The molecular mechanisms that control the disorder-to-order transition undergone by the intrinsically disordered C-terminal domain (NTAIL) of their N proteins upon binding to the C-terminal X domain (XD) of the homologous P proteins are described in detail. By having a significant residual disorder, NTAIL-XD complexes are illustrative examples of "fuzziness", whose possible functional significance is discussed. Finally, the relevance of N-P interactions as promising targets for innovative antiviral approaches is underscored, and the functional advantages of structural disorder for paramyxoviruses are pinpointed.

Modulation of Re-initiation of Measles Virus Transcription at Intergenic Regions by PXD to NTAIL Binding Strength

Measles virus (MeV) and all Paramyxoviridae members rely on a complex polymerase machinery to ensure viral transcription and replication. Their polymerase associates the phosphoprotein (P) and the L protein that is endowed with all necessary enzymatic activities. To be processive, the polymerase uses as template a nucleocapsid made of genomic RNA entirely wrapped into a continuous oligomer of the nucleoprotein (N). The polymerase enters the nucleocapsid at the 3'end of the genome where are located the promoters for transcription and replication. Transcription of the six genes occurs sequentially. This implies ending and re-initiating mRNA synthesis at each intergenic region (IGR). We explored here to which extent the binding of the X domain of P (XD) to the C-terminal region of the N protein (NTAIL) is involved in maintaining the P/L complex anchored to the nucleocapsid template during the sequential transcription. Amino acid substitutions introduced in the XD-binding site on NTAIL resulted in a wide range of binding affinities as determined by combining protein complementation assays in E. coli and human cells and isothermal titration calorimetry. Molecular dynamics simulations revealed that XD binding to NTAIL involves a complex network of hydrogen bonds, the disruption of which by two individual amino acid substitutions markedly reduced the binding affinity. Using a newly designed, highly sensitive dual-luciferase reporter minigenome assay, the efficiency of re-initiation through the five measles virus IGRs was found to correlate with NTAIL/XD KD. Correlatively, P transcript accumulation rate and F/N transcript ratios from recombinant viruses expressing N variants were also found to correlate with the NTAIL to XD binding strength. Altogether, our data support a key role for XD binding to NTAIL in maintaining proper anchor of the P/L complex thereby ensuring transcription re-initiation at each intergenic region.

Self-Assembly of Measles Virus Nucleocapsid-like Particles: Kinetics and RNA Sequence Dependence

Measles virus RNA genomes are packaged into helical nucleocapsids (NCs), comprising thousands of nucleo‐proteins (N) that bind the entire genome. N‐RNA provides the template for replication and transcription by the viral polymerase and is a promising target for viral inhibition. Elucidation of mechanisms regulating this process has been severely hampered by the inability to controllably assemble NCs. Here, we demonstrate self‐organization of N into NC‐like particles in vitro upon addition of RNA, providing a simple and versatile tool for investigating assembly. Real‐time NMR and fluorescence spectroscopy reveals biphasic assembly kinetics. Remarkably, assembly depends strongly on the RNA‐sequence, with the genomic 5′ end and poly‐Adenine sequences assembling efficiently, while sequences such as poly‐Uracil are incompetent for NC formation. This observation has important consequences for understanding the assembly process.