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Application of cryo-electron tomography and subtomogram averaging to determine the structure of the Ebola virus nucleocapsid within intact viruses and recombinant nucleocapsid-like assemblies. Assembling Ebola's box The Ebola virus nucleocapsid—a protein shell—encloses, or 'encapsidates', the viral genome and acts as a scaffold for virus assembly and as a template for genome replication. John Briggs and colleagues use cryo-electron tomography to solve the structure of the nucleocapsid of the Ebola virus. They use the structures of the Ebola virus nucleocapsid within intact viruses and recombinant assemblies to propose a model for viral RNA encapsidation and accessory protein recruitment. Ebola and Marburg viruses are filoviruses: filamentous, enveloped viruses that cause haemorrhagic fever 1 . Filoviruses are within the order Mononegavirales 2 , which also includes rabies virus, measles virus, and respiratory syncytial virus. Mononegaviruses have non-segmented, single-stranded negative-sense RNA genomes that are encapsidated by nucleoprotein and other viral proteins to form a helical nucleocapsid. The nucleocapsid acts as a scaffold for virus assembly and as a template for genome transcription and replication. Insights into nucleoprotein–nucleoprotein interactions have been derived from structural studies of oligomerized, RNA-encapsidating nucleoprotein 3 , 4 , 5 , 6 , and cryo-electron microscopy of nucleocapsid 7 , 8 , 9 , 10 , 11 , 12 or nucleocapsid-like structures 11 , 12 , 13 . There have been no high-resolution reconstructions of complete mononegavirus nucleocapsids. Here we apply cryo-electron tomography and subtomogram averaging to determine the structure of Ebola virus nucleocapsid within intact viruses and recombinant nucleocapsid-like assemblies. These structures reveal the identity and arrangement of the nucleocapsid components, and suggest that the formation of an extended α-helix from the disordered carboxy-terminal region of nucleoprotein-core links nucleoprotein oligomerization, nucleocapsid condensation, RNA encapsidation, and accessory protein recruitment.

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.

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 N i and N i+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 N i+1 N-terminus and N i−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.

Ebola virus causes haemorrhagic fever with a high fatality rate in humans and non-human primates. It belongs to the family Filoviridae in the order Mononegavirales , which are viruses that contain linear, non-segmented, negative-sense, single-stranded genomic RNA 1 , 2 . The enveloped, filamentous virion contains the nucleocapsid, consisting of the helical nucleoprotein–RNA complex, VP24, VP30, VP35 and viral polymerase 1 , 3 . The nucleoprotein–RNA complex acts as a scaffold for nucleocapsid formation and as a template for RNA replication and transcription by condensing RNA into the virion 4 , 5 . RNA binding and nucleoprotein oligomerization are synergistic and do not readily occur independently 6 . Although recent cryo-electron tomography studies have revealed the overall architecture of the nucleocapsid core 4 , 5 , there has been no high-resolution reconstruction of the nucleocapsid. Here we report the structure of a recombinant Ebola virus nucleoprotein–RNA complex expressed in mammalian cells without chemical fixation, at near-atomic resolution using single-particle cryo-electron microscopy. Our structure reveals how the Ebola virus nucleocapsid core encapsidates its viral genome, its sequence-independent coordination with RNA by nucleoprotein, and the dynamic transition between the RNA-free and RNA-bound states. It provides direct structural evidence for the role of the N terminus of nucleoprotein in subunit oligomerization, and for the hydrophobic and electrostatic interactions that lead to the formation of the helical assembly. The structure is validated as representative of the native biological assembly of the nucleocapsid core by consistent dimensions and symmetry with the full virion 5 . The atomic model provides a detailed mechanistic basis for understanding nucleocapsid assembly and highlights key structural features that may serve as targets for anti-viral drug development. Near-atomic resolution cryo-electron microscopy structures of the Zaire ebolavirus nucleoprotein indicate a complex transition from the RNA-free to RNA-bound forms of the protein, and reveal the mechanism of oligomer formation and helical assembly.

Baculoviruses are large DNA viruses found in nature propagating amongst insects and lepidoptera in particular. They have been studied for decades and are nowadays considered as invaluable biotechnology tools used as biopesticides, recombinant expression systems or delivery vehicle for gene therapy. However, little is known about the baculovirus nucleocapsid assembly at a molecular level. Here, we solve the whole structure of the Autographa californica multiple nucleopolyhedrovirus (AcMNPV) nucleocapsid by applying cryo-electron microscopy (CryoEM) combined with de novo modelling and Alphafold predictions. Our structure completes prior observations and elucidates the intricate architecture of the apical cap, unravelling the organization of a DNA portal featuring intriguing symmetry mismatches between its core and vertex. The core, closing the capsid at the apex, holds two DNA helices of the viral genome tethered to Ac54 proteins. Different symmetry components at the apical cap and basal structure are constituted of the same building block, made of Ac101/Ac144, proving the versatility of this modular pair. The crown forming the portal vertex displays a C21 symmetry and contains, amongst others, the motor-like protein Ac66. Our findings support the viral portal to be involved in DNA packaging, probably in conjunction with other parts of a larger DNA packaging apparatus. Baculoviruses are large DNA virus widely used as biotechnological tools. Here, authors present the complete cryoEM structure of a circulating nucleocapsid, including the DNA packaging and exiting hub termed the apical cap.

Negative strand RNA viruses (NSVs) include many important human pathogens, such as influenza virus, Ebola virus, and rabies virus. One of the unique characteristics that NSVs share is the assembly of the nucleocapsid and its role in viral RNA synthesis. In NSVs, the single strand RNA genome is encapsidated in the linear nucleocapsid throughout the viral replication cycle. Subunits of the nucleocapsid protein are parallelly aligned along the RNA genome that is sandwiched between two domains composed of conserved helix motifs. The viral RNA-dependent-RNA polymerase (vRdRp) must recognize the protein–RNA complex of the nucleocapsid and unveil the protected genomic RNA in order to initiate viral RNA synthesis. In addition, vRdRp must continuously translocate along the protein–RNA complex during elongation in viral RNA synthesis. This unique mechanism of viral RNA synthesis suggests that the nucleocapsid may play a regulatory role during NSV replication.

The Ebola virus, a member of the Filoviridae family, causes severe hemorrhagic fever in humans. Filamentous virions contain a helical nucleocapsid responsible for genome transcription, replication, and packaging into progeny virions. The nucleocapsid consists of a helical nucleoprotein (NP)–viral genomic RNA complex forming the core structure, to which VP24 and VP35 bind externally. Two NPs, each paired with a VP24 molecule, constitute a repeating unit. However, the detailed nucleocapsid structure remains unclear. Here, we determine the nucleocapsid-like structure within virus-like particles at 4.6 Å resolution using single-particle cryo-electron microscopy. Mutational analysis identifies specific interactions between the two NPs and two VP24s and demonstrates that each of the two VP24s in different orientations distinctively regulates nucleocapsid assembly, viral RNA synthesis, intracellular transport of the nucleocapsid, and infectious virion production. Our findings highlight the sophisticated mechanisms underlying the assembly and functional regulation of the nucleocapsid and provide insights into antiviral development. Here, the authors use cryo-electron microscopy to structurally characterise the Ebola virus nucleocapsid within virus-like particles, uncovering how specific interactions regulate viral genome synthesis, assembly, and infectious particle production.

Marburg virus (MARV) causes lethal hemorrhagic fever in humans, posing a threat to global health. We determined by cryogenic electron microscopy (cryo-EM) the MARV helical ribonucleoprotein (RNP) complex structure in single-layered conformation, which differs from the previously reported structure of a double-layered helix. Our findings illuminate novel RNP interactions and expand knowledge on MARV genome packaging and nucleocapsid assembly, both processes representing attractive targets for the development of antiviral therapeutics against MARV disease. Zinzula et al. reconstituted the helical ribonucleoprotein complex of Marburg virus in vitro, and determined its structure by cryo-electron microscopy in a single-layer conformation that recapitulates the assembly of authentic filovirus particles.

Ebola virus is a highly pathogenic filovirus causing severe hemorrhagic fever with high mortality rates. It assembles heterogenous, filamentous, enveloped virus particles containing a negative-sense, single-stranded RNA genome packaged within a helical nucleocapsid (NC). We have used cryo-electron microscopy and tomography to visualize Ebola virus particles, as well as Ebola virus-like particles, in three dimensions in a near-native state. The NC within the virion forms a left-handed helix with an inner nucleoprotein layer decorated with protruding arms composed of VP24 and VP35. A comparison with the closely related Marburg virus shows that the N-terminal region of nucleoprotein defines the inner diameter of the Ebola virus NC, whereas the RNA genome defines its length. Binding of the nucleoprotein to RNA can assemble a loosely coiled NC-like structure; the loose coil can be condensed by binding of the viral matrix protein VP40 to the terminus of the nucleoprotein, and rigidified by binding of VP24 and VP35 to alternate copies of the nucleoprotein. Four proteins (NP, VP24, VP35, and VP40) are necessary and sufficient to mediate assembly of an NC with structure, symmetry, variability, and flexibility indistinguishable from that in Ebola virus particles released from infected cells. Together these data provide a structural and architectural description of Ebola virus and define the roles of viral proteins in its structure and assembly.

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.