In Situ Structures of the Polymerase Complex and RNA Genome Show How Aquareovirus Transcription Machineries Respond to Uncoating

RNA genome packaging and capsid assembly of bluetongue virus visualized in host cells

Unlike those of double-stranded DNA (dsDNA), single-stranded DNA (ssDNA), and ssRNA viruses, the mechanism of genome packaging of dsRNA viruses is poorly understood. Here, we combined the techniques of high-resolution cryoelectron microscopy (cryo-EM), cellular cryoelectron tomography (cryo-ET), and structure-guided mutagenesis to investigate genome packaging and capsid assembly of bluetongue virus (BTV), a member of the Reoviridae family of dsRNA viruses. A total of eleven assembly states of BTV capsid were captured, with resolutions up to 2.8 Å, with most visualized in the host cytoplasm. ATPase VP6 was found underneath the vertices of capsid shell protein VP3 as an RNA-harboring pentamer, facilitating RNA packaging. RNA packaging expands the VP3 shell, which then engages middle- and outer-layer proteins to generate infectious virions. These revealed "duality" characteristics of the BTV assembly mechanism reconcile previous contradictory co-assembly and core-filling models and provide insights into the mysterious RNA packaging and capsid assembly of Reoviridae members and beyond.

The recruitment of TRiC chaperonin in rotavirus viroplasms correlates with virus replication

Rotavirus (RV) replication takes place in the viroplasms, cytosolic inclusions that allow the synthesis of virus genome segments and their encapsidation in the core shell, followed by the addition of the second layer of the virion. The viroplasms are composed of several viral proteins, including NSP5, which serves as the main building block. Microtubules, lipid droplets, and miRNA-7 are among the host components recruited in viroplasms. We investigated the interaction between RV proteins and host components of the viroplasms by performing a pull-down assay of lysates from RV-infected cells expressing NSP5-BiolD2. Subsequent tandem mass spectrometry identified all eight subunits of the tailless complex polypeptide I ring complex (TRiC), a cellular chaperonin responsible for folding at least 10% of the cytosolic proteins. Our confirmed findings reveal that TRiC is brought into viroplasms and wraps around newly formed double-layered particles. Chemical inhibition of TRiC and silencing of its subunits drastically reduced virus progeny production. Through direct RNA sequencing, we show that TRiC is critical for RV replication by controlling dsRNA genome segment synthesis, particularly negative-sense single-stranded RNA. Importantly, cryo-electron microscopy analysis shows that TRiC inhibition results in defective virus particles lacking genome segments and polymerase complex (VP1/VP3). Moreover, TRiC associates with VP2 and NSP5 but not with VP1. Also, VP2 is shown to be essential for recruiting TRiC in viroplasms and preserving their globular morphology. This study highlights the essential role of TRiC in viroplasm formation and in facilitating virion assembly during the RV life cycle.

IMPORTANCE

The replication of rotavirus takes place in cytosolic inclusions termed viroplasms. In these inclusions, the distinct 11 double-stranded RNA genome segments are co-packaged to complete a genome in newly generated virus particles. In this study, we show for the first time that the tailless complex polypeptide I ring complex (TRiC), a cellular chaperonin responsible for the folding of at least 10% of the cytosolic proteins, is a component of viroplasms and is required for the synthesis of the viral negative-sense single-stranded RNA. Specifically, TRiC associates with NSP5 and VP2, the cofactor involved in RNA replication. Our study adds a new component to the current model of rotavirus replication, where TRiC is recruited to viroplasms to assist replication.

Sequences at gene segment termini inclusive of untranslated regions and partial open reading frames play a critical role in mammalian orthoreovirus S gene packaging

Mammalian orthoreovirus (MRV) is a prototypic member of the Spinareoviridae family and has ten double-stranded RNA segments. One copy of each segment must be faithfully packaged into the mature virion, and prior literature suggests that nucleotides (nts) at the terminal ends of each gene likely facilitate their packaging. However, little is known about the precise packaging sequences required or how the packaging process is coordinated. Using a novel approach, we have determined that 200 nts at each terminus, inclusive of untranslated regions (UTR) and parts of the open reading frame (ORF), are sufficient for packaging S gene segments (S1-S4) individually and together into replicating virus. Further, we mapped the minimal sequences required for packaging the S1 gene segment into a replicating virus to 25 5' nts and 50 3' nts. The S1 UTRs, while not sufficient, were necessary for efficient packaging, as mutations of the 5' or 3' UTRs led to a complete loss of virus recovery. Using a second novel assay, we determined that 50 5' nts and 50 3' nts of S1 are sufficient to package a non-viral gene segment into MRV. The 5' and 3' termini of the S1 gene are predicted to form a panhandle structure and specific mutations within the stem of the predicted panhandle region led to a significant decrease in viral recovery. Additionally, mutation of six nts that are conserved across the three major serotypes of MRV that are predicted to form an unpaired loop in the S1 3' UTR, led to a complete loss of viral recovery. Overall, our data provide strong experimental proof that MRV packaging signals lie at the terminal ends of the S gene segments and offer support that the sequence requirements for efficient packaging of the S1 segment include a predicted panhandle structure and specific sequences within an unpaired loop in the 3' UTR.

Sequences at gene segment termini inclusive of untranslated regions and partial open reading frames play a critical role in mammalian orthoreovirus S gene packaging

Mammalian orthoreovirus (MRV) is a prototypic member of the Spinareoviridae family and has ten double-stranded RNA segments. One copy of each segment must be faithfully packaged into the mature virion, and prior literature suggests that nucleotides (nts) at the terminal ends of each gene likely facilitate their packaging. However, little is known about the precise packaging sequences required or how the packaging process is coordinated. Using a novel approach, we have determined that 200 nts at each terminus, inclusive of untranslated regions (UTR) and parts of the open reading frame (ORF), are sufficient for packaging each S gene segment (S1-S4) individually and together into replicating virus. Further, we mapped the minimal sequences required for packaging the S1 gene segment to 25 5' nts and 50 3' nts. The S1 UTRs alone are not sufficient, but are necessary for packaging, as mutations of the 5' or 3' UTRs led to a complete loss of virus recovery. Using a second novel assay, we determined that 50 5'nts and 50 3' nts of S1 are sufficient to package a non-viral gene segment into MRV. The 5' and 3' termini of the S1 gene are predicted to form a panhandle structure and specific mutations within the predicted stem of the panhandle region led to a significant decrease in viral recovery. Additionally, mutation of six nts that are conserved in the three major serotypes of MRV and are predicted to form an unpaired loop in the S1 3'UTR, led to a complete loss of viral recovery. Overall, our data provide strong experimental proof that MRV packaging signals lie at the terminal ends of the S gene segments and offer support that the sequence requirements for efficient packaging of the S1 segment include a predicted panhandle structure and specific sequences within an unpaired loop in the 3' UTR.

The structure of a 12-segmented dsRNA reovirus: New insights into capsid stabilization and organization

Infecting a wide range of hosts, members of Reovirales (formerly Reoviridae) consist of a genome with different numbers of segmented double stranded RNAs (dsRNA) encapsulated by a proteinaceous shell and carry out genome replication and transcription inside the virion. Several cryo-electron microscopy (cryo-EM) structures of reoviruses with 9, 10 or 11 segmented dsRNA genomes have revealed insights into genome arrangement and transcription. However, the structure and genome arrangement of 12-segmented Reovirales members remain poorly understood. Using cryo-EM, we determined the structure of mud crab reovirus (MCRV), a 12-segmented dsRNA virus that is a putative member of Reovirales in the non-turreted Sedoreoviridae family, to near-atomic resolutions with icosahedral symmetry (3.1 Å) and without imposing icosahedral symmetry (3.4 Å). These structures revealed the organization of the major capsid proteins in two layers: an outer T = 13 layer consisting of VP12 trimers and unique VP11 clamps, and an inner T = 1 layer consisting of VP3 dimers. Additionally, ten RNA dependent RNA polymerases (RdRp) were well resolved just below the VP3 layer but were offset from the 5-fold axes and arranged with D5 symmetry, which has not previously been seen in other members of Reovirales. The N-termini of VP3 were shown to adopt four unique conformations; two of which anchor the RdRps, while the other two conformations are likely involved in genome organization and capsid stability. Taken together, these structures provide a new level of understanding for capsid stabilization and genome organization of segmented dsRNA viruses.

In situ structures of polymerase complex of mammalian reovirus illuminate RdRp activation and transcription regulation

Mammalian reovirus (reovirus) is a multilayered, turreted member of Reoviridae characterized by transcription of dsRNA genome within the innermost capsid shell. Here, we present high-resolution in situ structures of reovirus transcriptase complex in an intact double-layered virion, and in the uncoated single-layered core particles in the unloaded, reloaded, pre-elongation, and elongation states, respectively, obtained by cryo-electron microscopy and sub-particle reconstructions. At the template entry of RNA-dependent RNA polymerase (RdRp), the RNA-loading region gets flexible after uncoating resulting in the unloading of terminal genomic RNA and inactivity of transcription. However, upon adding transcriptional substrates, the RNA-loading region is recovered leading the RNAs loaded again. The priming loop in RdRp was found to play a critical role in regulating transcription, which hinders the elongation of transcript in virion and triggers the rearrangement of RdRp C-terminal domain (CTD) during elongation, resulting in splitting of template-transcript hybrid and opening of transcript exit. With the integration of these structures, a transcriptional model of reovirus with five states is proposed. Our structures illuminate the RdRp activation and regulation of the multilayered turreted reovirus.

AlphaFold2 and CryoEM: Revisiting CryoEM modeling in near-atomic resolution density maps

With the advent of new artificial intelligence and machine learning algorithms, predictive modeling can, in some cases, produce structures on par with experimental methods. The combination of predictive modeling and experimental structure determination by electron cryomicroscopy (cryoEM) offers a tantalizing approach for producing robust atomic models of macromolecular assemblies. Here, we apply AlphaFold2 to a set of community standard data sets and compare the results with the corresponding reference maps and models. Moreover, we present three unique case studies from previously determined cryoEM density maps of viruses. Our results show that AlphaFold2 can not only produce reasonably accurate models for analysis and additional hypotheses testing, but can also potentially yield incorrect structures if not properly validated with experimental data. Whereas we outline numerous shortcomings and potential pitfalls of predictive modeling, the obvious synergy between predictive modeling and cryoEM will undoubtedly result in new computational modeling tools.

Evidence for a non-fusogenic aquareovirus encoding a transmembrane protein

Fusogenic aquareoviruses can induce host cell-cell fusion, forming syncytia via a fusion-associated transmembrane protein. However, there have been very few reports on non-fusogenic aquareoviruses encoding a membrane-associated protein. Previously, sequence-based analysis has indicated that grass carp reovirus strain 104 (GCRV-104), a non-fusogenic aquareovirus, encodes the proteins VP8 (nt 36-263) and VP15 (nt 400-822) in its genome segment S11. Here, we employed a liquid chromatography-tandem mass spectrometry assay to experimentally annotate small coding genes in the GCRV-104 genome and confirmed that segment S11 indeed functions as bicistronic mRNA. Notably, some additional polypeptides were identified that are encoded upstream of the VP15 open reading frame (ORF), which suggests that the virus uses a novel ORF with a non-AUG initiator codon, tentatively named VP15L (nt 274-822), which is longer than the previous putative VP15 ORF. Furthermore, a transmembrane domain was identified at the N-terminus of VP15L, but its function is unclear. Thus, the aquareovirus GCRV-104 potentially encodes a transmembrane protein, which opens a new perspective on the properties of viral proteins and the pathogenesis of this non-fusogenic reovirus.

Viral Capsid and Polymerase in Reoviridae

The members of the family Reoviridae (reoviruses) consist of 9-12 discrete double-stranded RNA (dsRNA) segments enclosed by single, double, or triple capsid layers. The outer capsid proteins of reoviruses exhibit the highest diversity in both sequence and structural organization. By contrast, the conserved RNA-dependent RNA polymerase (RdRp) structure in the conserved innermost shell in all reoviruses suggests that they share common transcriptional regulatory mechanisms. After reoviruses are delivered into the cytoplasm of a host cell, their inner capsid particles (ICPs) remain intact and serve as a stable nanoscale machine for RNA transcription and capping performed using enzymes in ICPs. Advances in cryo-electron microscopy have enabled the reconstruction at near-atomic resolution of not only the icosahedral capsid, including capping enzymes, but also the nonicosahedrally distributed complexes of RdRps within the capsid at different transcriptional stages. These near-atomic resolution structures allow us to visualize highly coordinated structural changes in the related enzymes, genomic RNA, and capsid protein during reovirus transcription. In addition, reoviruses encode their own enzymes for nascent RNA capping before RNA releasing from their ICPs.

Asymmetric reconstruction of mammalian reovirus reveals interactions among RNA, transcriptional factor µ2 and capsid proteins

Mammalian reovirus (MRV) is the prototypical member of genus Orthoreovirus of family Reoviridae. However, lacking high-resolution structures of its RNA polymerase cofactor μ2 and infectious particle, limits understanding of molecular interactions among proteins and RNA, and their contributions to virion assembly and RNA transcription. Here, we report the 3.3 Å-resolution asymmetric reconstruction of transcribing MRV and in situ atomic models of its capsid proteins, the asymmetrically attached RNA-dependent RNA polymerase (RdRp) λ3, and RdRp-bound nucleoside triphosphatase μ2 with a unique RNA-binding domain. We reveal molecular interactions among virion proteins and genomic and messenger RNA. Polymerase complexes in three Spinoreovirinae subfamily members are organized with different pseudo-D_3d symmetries to engage their highly diversified genomes. The above interactions and those between symmetry-mismatched receptor-binding σ1 trimers and RNA-capping λ2 pentamers balance competing needs of capsid assembly, external protein removal, and allosteric triggering of endogenous RNA transcription, before, during and after infection, respectively.

Mammalian reovirus (MRV) is a double-stranded RNA (dsRNA) virus that affects the gastrointestinal and respiratory tracts. Here, the authors present the 3.3 Å cryo-EM asymmetric reconstruction of transcribing MRV that reveals the organization of the dsRNA genome, RNA interaction with the polymerase complex, and how the polymerase interacts extensively with its co-factor, µ2, to form a transcription enzyme complex, which engages and regulates RNA transcription.

Atomic Structure of the Trichomonas vaginalis Double-Stranded RNA Virus 2

Trichomonas vaginalis viruses (TVVs) are double-stranded RNA (dsRNA) viruses that cohabitate in Trichomonas vaginalis, the causative pathogen of trichomoniasis, the most common nonviral sexually transmitted disease worldwide. Featuring an unsegmented dsRNA genome encoding a single capsid shell protein (CSP), TVVs contrast with multisegmented dsRNA viruses, such as the diarrhea-causing rotavirus, whose larger genome is split into 10 dsRNA segments encoding 5 unique capsid proteins.

Trichomonas vaginalis, the causative pathogen for the most common nonviral sexually transmitted infection worldwide, is itself frequently infected with one or more of the four types of small double-stranded RNA (dsRNA) Trichomonas vaginalis viruses (TVV1 to 4, genus Trichomonasvirus, family Totiviridae). Each TVV encloses a nonsegmented genome within a single-layered capsid and replicates entirely intracellularly, like many dsRNA viruses, and unlike those in the Reoviridae family. Here, we have determined the structure of TVV2 by cryo-electron microscopy (cryoEM) at 3.6 Å resolution and derived an atomic model of its capsid. TVV2 has an icosahedral, T = 2*, capsid comprised of 60 copies of the icosahedral asymmetric unit (a dimer of the two capsid shell protein [CSP] conformers, CSP-A and CSP-B), typical of icosahedral dsRNA virus capsids. However, unlike the robust CSP-interlocking interactions such as the use of auxiliary “clamping” proteins among Reoviridae, only lateral CSP interactions are observed in TVV2, consistent with an assembly strategy optimized for TVVs’ intracellular-only replication cycles within their protozoan host. The atomic model reveals both a mostly negatively charged capsid interior, which is conducive to movement of the loosely packed genome, and channels at the 5-fold vertices, which we suggest as routes of mRNA release during transcription. Structural comparison of TVV2 to the Saccharomyces cerevisiae L-A virus reveals a conserved helix-rich fold within the CSP and putative guanylyltransferase domain along the capsid exterior, suggesting conserved mRNA maintenance strategies among Totiviridae. This first atomic structure of a TVV provides a framework to guide future biochemical investigations into the interplay between Trichomonas vaginalis and its viruses.

Assembly intermediates of orthoreovirus captured in the cell

Traditionally, molecular assembly pathways for viruses are inferred from high resolution structures of purified stable intermediates, low resolution images of cell sections and genetic approaches. Here, we directly visualise an unsuspected 'single shelled' intermediate for a mammalian orthoreovirus in cryo-preserved infected cells, by cryo-electron tomography of cellular lamellae. Particle classification and averaging yields structures to 5.6 Å resolution, sufficient to identify secondary structural elements and produce an atomic model of the intermediate, comprising 120 copies each of protein λ1 and σ2. This λ1 shell is 'collapsed' compared to the mature virions, with molecules pushed inwards at the icosahedral fivefolds by ~100 Å, reminiscent of the first assembly intermediate of certain prokaryotic dsRNA viruses. This supports the supposition that these viruses share a common ancestor, and suggests mechanisms for the assembly of viruses of the Reoviridae. Such methodology holds promise for dissecting the replication cycle of many viruses.

Aquareovirus NS31 protein serves as a specific inducer for host heat shock 70-kDa protein

Elevation of heat-shock protein expression, known as cellular heat-shock responses, occurs during infection of many viruses, which is involved in viral replication through various mechanisms. Herein, transcriptome analysis revealed that over-expression of non-structural protein NS31 of grass carp reovirus (GCRV) in grass carp Ctenopharyngodon idellus kidney (CIK) cells specifically induced expression of heat-shock response (HSR) genes HSP30 and HSP70. We further found that, among the HSR genes, only HSP70 protein were shown to be efficiently induced in fish cells following NS31 over-expression or GCRV infection. Employing a luciferase assay, we were able to show that the promoter of the HSP70 gene can be specifically activated by NS31. In addition, over-expressing HSP70 in grass carp CIK cells resulted in enhanced replication efficiency of GCRV, and an inhibitor for HSP70 resulted in the inhibition of GCRV replication, indicating that HSP70 should serve as a pro-viral factor. We also found that NS31 could activate HSP70 expression in cells of other vertebrate animals. Similar inducing effect on HSP70 expression was demonstrated for NS31-homologue proteins of other aquareoviruses including American grass carp reovirus (AGCRV) and GRCV (green river chinook virus). All these results indicated NS31 proteins in the Aquareovirus genus should play a key role for up-regulating specific HSP70 gene during viral replication.

A Perfect Ten-Decoy Maps Uncover Polymerase Complexes within Reoviridae Virion

In this issue of Structure, Kaelber et al. (2020) use cryo-EM and synthetic decoy maps to reveal the patterning of 10 polymerase complexes within FAKV, a Reoviridae family member containing 9 genome segments. Their findings support a model for FAKV assembly that has implications for the entire Reoviridae family.

Arrangement of the Polymerase Complexes inside a Nine-Segmented dsRNA Virus

Members of the family Reoviridae package several copies of the viral polymerase complex into their capsid to carry out replication and transcription within viral particles. Classical single-particle reconstruction encounters difficulties resolving structures such as the intraparticle polymerase complex because refinement can converge to an incorrect map and because the map could depict a nonrepresentative subset of particles or an average of heterogeneous particles. Using the nine-segmented Fako virus, we tested hypotheses for the arrangement and number of polymerase complexes within the virion by measuring how well each hypothesis describes the set of cryoelectron microscopy images of individual viral particles. We find that the polymerase complex in Fako virus binds at ten possible sites despite having only nine genome segments. A single asymmetric configuration describes the arrangement of these complexes in both virions and genome-free capsids. Similarities between the arrangements of Reoviridae with 9, 10, and 11 segments indicate the generalizability of this architecture.

Polymorphisms in the Most Oncolytic Reovirus Strain Confer Enhanced Cell Attachment, Transcription, and Single-Step Replication Kinetics

Reovirus serotype 3 Dearing (T3D) replicates preferentially in transformed cells and is in clinical trials as a cancer therapy. Laboratory strains of T3D, however, exhibit differences in plaque size on cancer cells and differences in oncolytic activity in vivo This study aimed to determine why the most oncolytic T3D reovirus lab strain, the Patrick Lee laboratory strain (T3D^PL), replicates more efficiently in cancer cells than other commonly used laboratory strains, the Kevin Coombs laboratory strain (T3D^KC) and Terence Dermody laboratory (T3D^TD) strain. In single-step growth curves, T3D^PL titers increased at higher rates and produced ∼9-fold higher burst size. Furthermore, the number of reovirus antigen-positive cells increased more rapidly for T3D^PL than for T3D^TD In conclusion, the most oncolytic T3D^PL possesses replication advantages in a single round of infection. Two specific mechanisms for enhanced infection by T3D^PL were identified. First, T3D^PL exhibited higher cell attachment, which was attributed to a higher proportion of virus particles with insufficient (≤3) σ1 cell attachment proteins. Second, T3D^PL transcribed RNA at rates superior to those of the less oncolytic T3D strains, which is attributed to polymorphisms in M1-encoding μ2 protein, as confirmed in an in vitro transcription assay, and which thus demonstrates that T3D^PL has an inherent transcription advantage that is cell type independent. Accordingly, T3D^PL established rapid onset of viral RNA and protein synthesis, leading to more rapid kinetics of progeny virus production, larger virus burst size, and higher levels of cell death. Together, these results emphasize the importance of paying close attention to genomic divergence between virus laboratory strains and, mechanistically, reveal the importance of the rapid onset of infection for reovirus oncolysis.IMPORTANCE Reovirus serotype 3 Dearing (T3D) is in clinical trials for cancer therapy. Recently, it was discovered that highly related laboratory strains of T3D exhibit large differences in their abilities to replicate in cancer cells in vitro, which correlates with oncolytic activity in a murine model of melanoma. The current study reveals two mechanisms for the enhanced efficiency of T3D^PL in cancer cells. Due to polymorphisms in two viral genes, within the first round of reovirus infection, T3D^PL binds to cells more efficiency and more rapidly produces viral RNAs; this increased rate of infection relative to that of the less oncolytic strains gives T3D^PL a strong inherent advantage that culminates in higher virus production, more cell death, and higher virus spread.

Conservative transcription in three steps visualized in a double-stranded RNA virus

Endogenous RNA transcription characterizes double-stranded RNA (dsRNA) viruses in the Reoviridae, a family that is exemplified by its simple, single-shelled member cytoplasmic polyhedrosis virus (CPV). Because of the lack of in situ structures of the intermediate stages of RNA-dependent RNA polymerase (RdRp) during transcription, it is poorly understood how RdRp detects environmental cues and internal transcriptional states to initiate and coordinate repeated cycles of transcript production inside the capsid. Here, we captured five high-resolution (2.8-3.5 Å) RdRp-RNA in situ structures-representing quiescent, initiation, early elongation, elongation and abortive states-under seven experimental conditions of CPV. We observed the 'Y'-form initial RNA fork in the initiation state and the complete transcription bubble in the elongation state. These structures reveal that de novo RNA transcription involves three major conformational changes during state transitions. Our results support an ouroboros model for endogenous conservative transcription in dsRNA viruses.

In Vitro Double-Stranded RNA Synthesis by Rotavirus Polymerase Mutants with Lesions at Core Shell Contact Sites

The rotavirus polymerase VP1 mediates all stages of viral RNA synthesis within the confines of subviral particles and while associated with the core shell protein VP2. Transcription (positive-strand RNA [+RNA] synthesis) by VP1 occurs within double-layered particles (DLPs), while genome replication (double-stranded RNA [dsRNA] synthesis) by VP1 occurs within assembly intermediates. VP2 is critical for VP1 enzymatic activity; yet, the mechanism by which the core shell protein triggers polymerase function remains poorly understood. Structural analyses of transcriptionally competent DLPs show that VP1 is located beneath the VP2 core shell and sits slightly off-center from each of the icosahedral 5-fold axes. In this position, the polymerase is contacted by the core shell at 5 distinct surface-exposed sites, comprising VP1 residues 264 to 267, 547 to 550, 614 to 620, 968 to 980, and 1022 to 1025. Here, we sought to test the functional significance of these VP2 contact sites on VP1 with regard to polymerase activity. We engineered 19 recombinant VP1 (rVP1) proteins that contained single- or multipoint alanine mutations within each individual contact site and assayed them for the capacity to synthesize dsRNA in vitro in the presence of rVP2. Three rVP1 mutants (E265A/L267A, R614A, and D971A/S978A/I980A) exhibited diminished in vitro dsRNA synthesis. Despite their loss-of-function phenotypes, the mutants did not show major structural changes in silico, and they maintained their overall capacity to bind rVP2 in vitro via their nonmutated contact sites. These results move us toward a mechanistic understanding of rotavirus replication and identify precise VP2-binding sites on the polymerase surface that are critical for its enzymatic activation.IMPORTANCE Rotaviruses are important pathogens that cause severe gastroenteritis in the young of many animals. The viral polymerase VP1 mediates all stages of viral RNA synthesis, and it requires the core shell protein VP2 for its enzymatic activity. Yet, there are several gaps in knowledge about how VP2 engages and activates VP1. Here, we probed the functional significance of 5 distinct VP2 contact sites on VP1 that were revealed through previous structural studies. Specifically, we engineered alanine amino acid substitutions within each of the 5 VP1 regions and assayed the mutant polymerases for the capacity to synthesize RNA in the presence of VP2 in a test tube. Our results identified residues within 3 of the VP2 contact sites that are critical for robust polymerase activity. These results are important because they enhance the understanding of a key step of the rotavirus replication cycle.

Transferrin receptor binds virus capsid with dynamic motion

Canine parvovirus (CPV) is an important pathogen causing severe diseases in dogs, including acute hemorrhagic enteritis, myocarditis, and cerebellar disease. Cross-species transmission of CPV occurs as a result of mutations on the viral capsid surface that alter the species-specific binding to the host receptor, transferrin receptor type-1 (TfR). The interaction between CPV and TfR has been extensively studied, and previous analyses have suggested that the CPV-TfR complex is asymmetric. To enhance the understanding of the underlying molecular mechanisms, we determined the CPV-TfR interaction using cryo-electron microscopy to solve the icosahedral (3.0-Å resolution) and asymmetric (5.0-Å resolution) complex structures. Structural analyses revealed conformational variations of the TfR molecules relative to the binding site, which translated into dynamic molecular interactions between CPV and TfR. The precise footprint of the receptor on the virus capsid was identified, along with the identity of the amino acid residues in the virus-receptor interface. Our "rock-and-roll" model provides an explanation for previous findings and gives insights into species jumping and the variation in host ranges associated with new pandemics in dogs.

In situ structures of RNA-dependent RNA polymerase inside bluetongue virus before and after uncoating

Bluetongue virus (BTV), a major threat to livestock, is a multilayered, nonturreted member of the Reoviridae, a family of segmented dsRNA viruses characterized by endogenous RNA transcription through an RNA-dependent RNA polymerase (RdRp). To date, the structure of BTV RdRp has been unknown, limiting our mechanistic understanding of BTV transcription and hindering rational drug design effort targeting this essential enzyme. Here, we report the in situ structures of BTV RdRp VP1 in both the triple-layered virion and double-layered core, as determined by cryo-electron microscopy (cryoEM) and subparticle reconstruction. BTV RdRp has 2 unique motifs not found in other viral RdRps: a fingernail, attached to the conserved fingers subdomain, and a bundle of 3 helices: 1 from the palm subdomain and 2 from the N-terminal domain. BTV RdRp VP1 is anchored to the inner surface of the capsid shell via 5 asymmetrically arranged N termini of the inner capsid shell protein VP3A around the 5-fold axis. The structural changes of RdRp VP1 and associated capsid shell proteins between BTV virions and cores suggest that the detachment of the outer capsid proteins VP2 and VP5 during viral entry induces both global movements of the inner capsid shell and local conformational changes of the N-terminal latch helix (residues 34 to 51) of 1 inner capsid shell protein VP3A, priming RdRp VP1 within the capsid for transcription. Understanding this mechanism in BTV also provides general insights into RdRp activation and regulation during viral entry of other multilayered, nonturreted dsRNA viruses.

In situ Structure of Rotavirus VP1 RNA-Dependent RNA Polymerase

Rotaviruses, like other non-enveloped, double-strand RNA viruses, package an RNA-dependent RNA polymerase (RdRp) with each duplex of their segmented genomes. Rotavirus cell entry results in loss of an outer protein layer and delivery into the cytosol of an intact, inner capsid particle (the "double-layer particle," or DLP). The RdRp, designated VP1, is active inside the DLP; each VP1 achieves many rounds of mRNA transcription from its associated genome segment. Previous work has shown that one VP1 molecule lies close to each 5-fold axis of the icosahedrally symmetric DLP, just beneath the inner surface of its protein shell, embedded in tightly packed RNA. We have determined a high-resolution structure for the rotavirus VP1 RdRp in situ, by local reconstruction of density around individual 5-fold positions. We have analyzed intact virions ("triple-layer particles"), non-transcribing DLPs and transcribing DLPs. Outer layer dissociation enables the DLP to synthesize RNA, in vitro as well as in vivo, but appears not to induce any detectable structural change in the RdRp. Addition of NTPs, Mg²⁺, and S-adenosylmethionine, which allows active transcription, results in conformational rearrangements, in both VP1 and the DLP capsid shell protein, that allow a transcript to exit the polymerase and the particle. The position of VP1 (among the five symmetrically related alternatives) at one vertex does not correlate with its position at other vertices. This stochastic distribution of site occupancies limits long-range order in the 11-segment, double-strand RNA genome.

In situ structures of rotavirus polymerase in action and mechanism of mRNA transcription and release

Transcribing and replicating a double-stranded genome require protein modules to unwind, transcribe/replicate nucleic acid substrates, and release products. Here we present in situ cryo-electron microscopy structures of rotavirus dsRNA-dependent RNA polymerase (RdRp) in two states pertaining to transcription. In addition to the previously discovered universal "hand-shaped" polymerase core domain shared by DNA polymerases and telomerases, our results show the function of N- and C-terminal domains of RdRp: the former opens the genome duplex to isolate the template strand; the latter splits the emerging template-transcript hybrid, guides genome reannealing to form a transcription bubble, and opens a capsid shell protein (CSP) to release the transcript. These two "helicase" domains also extensively interact with CSP, which has a switchable N-terminal helix that, like cellular transcriptional factors, either inhibits or promotes RdRp activity. The in situ structures of RdRp, CSP, and RNA in action inform mechanisms of not only transcription, but also replication.