The TRiC chaperonin controls reovirus replication through outer-capsid folding

Lassa virus Z protein hijacks the autophagy machinery for efficient transportation by interrupting CCT2-mediated cytoskeleton network formation

The Lassa virus (LASV) is a widely recognized virulent pathogen that frequently results in lethal viral hemorrhagic fever (VHF). Earlier research has indicated that macroautophagy/autophagy plays a role in LASV replication, but, the precise mechanism is unknown. In this present study, we show that LASV matrix protein (LASV-Z) is essential for blocking intracellular autophagic flux. LASV-Z hinders actin and tubulin folding by interacting with CCT2, a component of the chaperonin-containing T-complexes (TRiC). When the cytoskeleton is disrupted, lysosomal enzyme transit is hampered. In addition, cytoskeleton disruption inhibits the merge of autophagosomes with lysosomes, resulting in autophagosome accumulation that promotes the budding of LASV virus-like particles (VLPs). Inhibition of LASV-Z-induced autophagosome accumulation blocks the LASV VLP budding process. Furthermore, it is found that glutamine at position 29 and tyrosine at position 48 on LASV-Z are important in interacting with CCT2. When these two sites are mutated, LASV-mut interacts with CCT2 less efficiently and can no longer inhibit the autophagic flux. These findings demonstrate a novel strategy for LASV-Z to hijack the host autophagy machinery to accomplish effective transportation.Abbreviation: 3-MA: 3-methyladenine; ATG5: autophagy related 5; ATG7: autophagy related 7; Baf-A1: bafilomycin A1; CCT2: chaperonin containing TCP1 subunit 2; co-IP: co-immunoprecipitation; CTSD: cathepsin D; DAPI: 4',6-diamidino-2'-phenylindole; DMSO: dimethyl sulfoxide; EGFR: epidermal growth factor receptor; GFP: green fluorescent protein; hpi: hours post-infection; hpt: hours post-transfection; LAMP1: lysosomal-associated membrane protein 1; LASV: lassa virus; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; mCherry: red fluorescent protein; PM: plasma membrane; SQSTM1/p62: sequestosome 1; STX6: syntaxin 6; VLP: virus-like particle; TEM: transmission electron microscopy; TRiC: chaperonin-containing T-complex; WB: western blotting; μm: micrometer; μM: micromole.

Enterovirus-A71 exploits RAB11 to recruit chaperones for virus morphogenesis

BACKGROUND

Enterovirus 71 (EV-A71) causes Hand, Foot and Mouth Disease (HFMD) in children and has been associated with neurological complications. The molecular mechanisms involved in EV-A71 pathogenesis have remained elusive.

METHODS

A siRNA screen in EV-A71 infected-motor neurons was performed targeting 112 genes involved in intracellular membrane trafficking, followed by validation of the top four hits using deconvoluted siRNA. Downstream approaches including viral entry by-pass, intracellular viral genome quantification by qPCR, Western blot analyses, and Luciferase reporter assays allowed determine the stage of the infection cycle the top candidate, RAB11A was involved in. Proximity ligation assay, co-immunoprecipitation and multiplex confocal imaging were employed to study interactions between viral components and RAB11A. Dominant negative and constitutively active RAB11A constructs were used to determine the importance of the protein's GTPase activity during EV-A71 infection. Mass spectrometry and protein interaction analyses were employed for the identification of RAB11A's host interacting partners during infection.

RESULTS

Small GTPase RAB11A was identified as a novel pro-viral host factor during EV-A71 infection. RAB11A and RAB11B isoforms were interchangeably exploited by strains from major EV-A71 genogroups and by Coxsackievirus A16, another major causative agent of HFMD. We showed that RAB11A was not involved in viral entry, IRES-mediated protein translation, viral genome replication, and virus exit. RAB11A co-localized with replication organelles where it interacted with structural and non-structural viral components. Over-expression of dominant negative (S25N; GDP-bound) and constitutively active (Q70L; GTP-bound) RAB11A mutants had no effect on EV-A71 infection outcome, ruling out RAB11A's involvement in intracellular trafficking of viral or host components. Instead, decreased ratio of intracellular mature viral particles to viral RNA copies and increased VP0:VP2 ratio in siRAB11-treated cells supported a role in provirion maturation hallmarked by VP0 cleavage into VP2 and VP4. Finally, chaperones, not trafficking and transporter proteins, were found to be RAB11A's top interacting partners during EV-A71 infection. Among which, CCT8 subunit from the chaperone complex TRiC/CCT was further validated and shown to interact with viral structural proteins specifically, representing yet another novel pro-viral host factor during EV-A71 infection.

CONCLUSIONS

This study describes a novel, unconventional role for RAB11A during viral infection where it participates in the complex process of virus morphogenesis by recruiting essential chaperone proteins.

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.

Chaperonin TRiC/CCT subunit CCT7 is involved in the replication of canine parvovirus in F81 cells

Canine parvovirus (CPV) is one of the most common lethal viruses in canines. The virus disease is prevalent throughout the year, with high morbidity and mortality rate, causing serious harm to dogs and the dog industry. Previously, yeast two hybrid method was used to screen the protein chaperonin containing TCP-1 (CCT7) that interacts with VP2. However, the mechanism of interactions between CCT7 and VP2 on CPV replication remains unclear. In this study, we first verified the interaction between CCT7 and viral VP2 proteins using yeast one-to-one experiment and co-immunoprecipitation (CoIP) experiment. Laser confocal microscopy observation showed that CCT7 and VP2 were able to co-localize and were mostly localized in the cytoplasm. In addition, the study of VP2 truncated mutant found that the interaction region of VP2 with CCT7 was located between amino acids 231 and 320. Cycloheximide (CHX) chase experiments showed that CCT7 can improve the stability of VP2 protein. After further regulation of CCT7 expression in F81 cells, it was found that the expression level of VP2 protein was significantly reduced after knocking down CCT7 expression by RNA interference (RNAi) or HSF1A inhibitor, and increased after overexpressing host CCT7. The study reveals the role of VP2 interacting protein CCT7 in the replication process of CPV, which could provide a potential target for the prevention and control of CPV.

Protein Quality Control Systems and ER Stress as Key Players in SARS-CoV-2-Induced Neurodegeneration

The COVID-19 pandemic has brought to the forefront the intricate relationship between SARS-CoV-2 and its impact on neurological complications, including potential links to neurodegenerative processes, characterized by a dysfunction of the protein quality control systems and ER stress. This review article explores the role of protein quality control systems, such as the Unfolded Protein Response (UPR), the Endoplasmic Reticulum-Associated Degradation (ERAD), the Ubiquitin-Proteasome System (UPS), autophagy and the molecular chaperones, in SARS-CoV-2 infection. Our hypothesis suggests that SARS-CoV-2 produces ER stress and exploits the protein quality control systems, leading to a disruption in proteostasis that cannot be solved by the host cell. This disruption culminates in cell death and may represent a link between SARS-CoV-2 and neurodegeneration.

Quantitative analysis of acetylation in peste des petits ruminants virus-infected Vero cells

BACKGROUND

Peste des petits ruminants virus (PPRV) is a highly contagious pathogen that strongly influences the productivity of small ruminants worldwide. Acetylation is an important post-translational modification involved in regulation of multiple biological functions. However, the extent and function of acetylation in host cells during PPRV infection remains unknown.

METHODS

Dimethylation-labeling-based quantitative proteomic analysis of the acetylome of PPRV-infected Vero cells was performed.

RESULTS

In total, 1068 proteins with 2641 modification sites were detected in response to PPRV infection, of which 304 differentially acetylated proteins (DAcPs) with 410 acetylated sites were identified (fold change < 0.83 or > 1.2 and P < 0.05), including 109 up-regulated and 195 down-regulated proteins. Gene Ontology (GO) classification indicated that DAcPs were mostly located in the cytoplasm (43%) and participated in cellular and metabolic processes related to binding and catalytic activity. Functional enrichment indicated that the DAcPs were involved in the minichromosome maintenance complex, unfolded protein binding, helicase activity. Only protein processing in endoplasmic reticulum pathway was enriched. A protein-protein interaction (PPI) network of the identified proteins further indicated that a various chaperone and ribosome processes were modulated by acetylation.

CONCLUSIONS

To the best of our knowledge, this is the first study on acetylome in PPRV-infected host cell. Our findings establish an important baseline for future study on the roles of acetylation in the host response to PPRV replication and provide novel insights for understanding the molecular pathological mechanism of PPRV infection.

Elucidating cellular interactome of chikungunya virus identifies host dependency factors

Chikungunya virus (CHIKV) is a re-emerging mosquito-transmitted RNA virus causing joint and muscle pain. To better understand how CHIKV rewires the host cell and usurps host cell functions, we generated a systematic CHIKV-human protein-protein interaction map and revealed several novel connections that will inform further mechanistic studies. One of these novel interactions, between the viral protein E1 and STIP1 homology and U-box containing protein 1 (STUB1), was found to mediate ubiquitination of E1 and degrade E1 through the proteasome. Capsid associated with G3BP1, G3BP2 and AAA+ ​ATPase valosin-containing protein (VCP). Furthermore, VCP inhibitors blocked CHIKV infection, suggesting VCP could serve as a therapeutic target. Further work is required to fully understand the functional consequences of these interactions. Given that CHIKV proteins are conserved across alphaviruses, many virus-host protein-protein interactions identified in this study might also exist in other alphaviruses. Construction of interactome of CHIKV provides the basis for further studying the function of alphavirus biology.

Oncolytic Reoviruses: Can These Emerging Zoonotic Reoviruses Be Tamed and Utilized?

Orthoreovirus is a nonenveloped double-stranded RNA virus under the Reoviridae family. This group of viruses, especially mammalian orthoreovirus (MRV), are reported with great therapeutic values due to their oncolytic effects. In this review, the life cycle and oncolytic effect of MRV and a few emerging reoviruses were summarized. This article also highlights the challenges and strategies of utilizing MRV and the emerging reoviruses, avian orthoreovirus (ARV) and pteropine orthoreovirus (PRV), as oncolytic viruses (OVs). Besides, the emergence of potential ARV and PRV as OVs were discussed in comparison to MRV. Finally, the risk of reovirus as zoonosis or reverse zoonosis (zooanthroponosis) were debated, and concerns were raised in this article, which warrant continue surveillance of reovirus (MRV, ARV, and PRV) in animals, humans, and the environment.

Chaperonin TRiC/CCT Participates in Mammarenavirus Multiplication in Human Cells via Interaction with the Viral Nucleoprotein

The eukaryotic chaperonin containing tailless complex polypeptide 1 ring complex (CCT, also known as TCP-1 Ring Complex, TRiC/CCT) participates in the folding of 5% to 10% of the cellular proteome and has been involved in the life cycle of several viruses, including dengue, Zika, and influenza viruses, but the mechanisms by which the TRiC/CCT complex contributes to virus multiplication remain poorly understood. Here, we document that the nucleoprotein (NP) of the mammarenavirus lymphocytic choriomeningitis virus (LCMV) is a substrate of the human TRiC/CCT complex, and that pharmacological inhibition of TRiC/CCT complex function, or RNAi-mediated knockdown of TRiC/CCT complex subunits, inhibited LCMV multiplication in human cells. We obtained evidence that the TRiC/CCT complex is required for the production of NP-containing virus-like particles (VLPs), and the activity of the virus ribonucleoprotein (vRNP) responsible for directing replication and transcription of the viral genome. Pharmacological inhibition of the TRIC/CCT complex also restricted multiplication of the live-attenuated vaccine candidates Candid#1 and ML29 of the hemorrhagic fever causing Junin (JUNV) and Lassa (LASV) mammarenaviruses, respectively. Our findings indicate that the TRiC/CCT complex is required for mammarenavirus multiplication and is an attractive candidate for the development of host directed antivirals against human-pathogenic mammarenaviruses. IMPORTANCE Host-directed antivirals have gained great interest as an antiviral strategy to counteract the rapid emergence of drug-resistant viruses. The chaperonin TRiC/CCT complex has been involved in the life cycle of several viruses, including dengue, Zika, and influenza viruses. Here, we have provided evidence that the chaperonin TRiC/CCT complex participates in mammarenavirus infection via its interaction with the viral NP. Importantly, pharmacological inhibition of TRiC/CCT function significantly inhibited multiplication of LCMV and the distantly related mammarenavirus JUNV in human cells. Our findings support that the TRiC/CCT complex is required for multiplication of mammarenaviruses and that the TRiC/CCT complex is an attractive host target for the development of antivirals against human-pathogenic mammarenaviruses.

14th International dsRNA Virus Symposium, Banff, Alberta, Canada, 10-14 October 2022

This triennial International dsRNA Virus Symposium covered original data which have accrued during the most recent five years. In detail, the genomic diversity of these viruses continued to be explored; various structure-function studies were carried out using reverse genetics and biophysical techniques; intestinal organoids proved to be very suitable for special pathogenesis studies; and the potential of next generation rotavirus vaccines including use of rotavirus recombinants as vectored vaccine candidates was explored. 'Non-lytic release of enteric viruses in cloaked vesicles' was the topic of the keynote lecture by Nihal Altan-Bonnet, NIH, Bethesda, USA. The Jean Cohen lecturer of this meeting was Polly Roy, London School of Hygiene and Tropical Medicine, who spoke on aspects of the replication cycle of bluetongue viruses, and how some of the data are similar to details of rotavirus replication.

Mechanistic insights into protein folding by the eukaryotic chaperonin complex CCT

The cytosolic chaperonin CCT is indispensable to eukaryotic life, folding the cytoskeletal proteins actin and tubulin along with an estimated 10% of the remaining proteome. However, it also participates in human diseases such as cancer and viral infections, rendering it valuable as a potential therapeutic target. CCT consists of two stacked rings, each comprised of eight homologous but distinct subunits, that assists the folding of a remarkable substrate clientele that exhibits both broad diversity and specificity. Much of the work in recent years has been aimed at understanding the mechanisms of CCT substrate recognition and folding. These studies have revealed new binding sites and mechanisms by which CCT uses its distinctive subunit arrangement to fold structurally unrelated substrates. Here, we review recent structural insights into CCT-substrate interactions and place them into the broader context of CCT function and its implications for human health.

Reovirus Activated Cell Death Pathways

Mammalian orthoreoviruses (ReoV) are non-enveloped viruses with segmented double-stranded RNA genomes. In humans, ReoV are generally considered non-pathogenic, although members of this family have been proven to cause mild gastroenteritis in young children and may contribute to the development of inflammatory conditions, including Celiac disease. Because of its low pathogenic potential and its ability to efficiently infect and kill transformed cells, the ReoV strain Type 3 Dearing (T3D) is clinical trials as an oncolytic agent. ReoV manifests its oncolytic effects in large part by infecting tumor cells and activating programmed cell death pathways (PCDs). It was previously believed that apoptosis was the dominant PCD pathway triggered by ReoV infection. However, new studies suggest that ReoV also activates other PCD pathways, such as autophagy, pyroptosis, and necroptosis. Necroptosis is a caspase-independent form of PCD reliant on receptor-interacting serine/threonine-protein kinase 3 (RIPK3) and its substrate, the pseudokinase mixed-lineage kinase domain-like protein (MLKL). As necroptosis is highly inflammatory, ReoV-induced necroptosis may contribute to the oncolytic potential of this virus, not only by promoting necrotic lysis of the infected cell, but also by inflaming the surrounding tumor microenvironment and provoking beneficial anti-tumor immune responses. In this review, we summarize our current understanding of the ReoV replication cycle, the known and potential mechanisms by which ReoV induces PCD, and discuss the consequences of non-apoptotic cell death—particularly necroptosis—to ReoV pathogenesis and oncolysis.

The Reovirus σ3 Protein Inhibits NF-κB-Dependent Antiviral Signaling

Viral antagonism of innate immune pathways is a common mechanism by which viruses evade immune surveillance. Infection of host cells with reovirus leads to the blockade of NF-κB, a key transcriptional regulator of the hosts' innate immune response. One mechanism by which reovirus infection results in inhibition of NF-κB is through a diminishment in levels of upstream activators, IKKβ and NEMO. Here, we demonstrate a second, distinct mechanism by which reovirus blocks NF-κB. We report that expression of a single viral protein, σ3, is sufficient to inhibit expression of NF-κB target genes. Further, σ3-mediated blockade of NF-κB occurs without changes to IκB kinase (IKK) levels or activity. Among NF-κB targets, the expression of type I interferon is significantly diminished by σ3 expression. Expression of NF-κB target genes varies following infection with closely related reovirus strains. Our genetic analysis identifies that these differences are controlled by polymorphisms in the amino acid sequence of σ3. This work identifies a new role for reovirus σ3 as a viral antagonist of NF-κB-dependent antiviral pathways. IMPORTANCE Host cells mount a response to curb virus replication in infected cells and prevent spread of virus to neighboring, as yet uninfected, cells. The NF-κB family of proteins is important for the cell to mediate this response. In this study, we show that a single protein, σ3, produced by mammalian reovirus, impairs the function of NF-κB. We demonstrate that by blocking NF-κB, σ3 diminishes the hosts' response to infection to promote viral replication. This work identifies a second, previously unknown, mechanism by which reovirus blocks this aspect of the host cell response.

Snapshots of actin and tubulin folding inside the TRiC chaperonin

The integrity of a cell's proteome depends on correct folding of polypeptides by chaperonins. The chaperonin TCP-1 ring complex (TRiC) acts as obligate folder for >10% of cytosolic proteins, including he cytoskeletal proteins actin and tubulin. Although its architecture and how it recognizes folding substrates are emerging from structural studies, the subsequent fate of substrates inside the TRiC chamber is not defined. We trapped endogenous human TRiC with substrates (actin, tubulin) and cochaperone (PhLP2A) at different folding stages, for structure determination by cryo-EM. The already-folded regions of client proteins are anchored at the chamber wall, positioning unstructured regions toward the central space to achieve their native fold. Substrates engage with different sections of the chamber during the folding cycle, coupled to TRiC open-and-close transitions. Further, the cochaperone PhLP2A modulates folding, acting as a molecular strut between substrate and TRiC chamber. Our structural snapshots piece together an emerging model of client protein folding within TRiC.

A CRISPR-Cas9 screen reveals a role for WD repeat-containing protein 81 (WDR81) in the entry of late penetrating viruses

Successful initiation of infection by many different viruses requires their uptake into the endosomal compartment. While some viruses exit this compartment early, others must reach the degradative, acidic environment of the late endosome. Mammalian orthoreovirus (reovirus) is one such late penetrating virus. To identify host factors that are important for reovirus infection, we performed a CRISPR-Cas9 knockout (KO) screen that targets over 20,000 genes in fibroblasts derived from the embryos of C57/BL6 mice. We identified seven genes (WDR81, WDR91, RAB7, CCZ1, CTSL, GNPTAB, and SLC35A1) that were required for the induction of cell death by reovirus. Notably, CRISPR-mediated KO of WD repeat-containing protein 81 (WDR81) rendered cells resistant to reovirus infection. Susceptibility to reovirus infection was restored by complementing KO cells with human WDR81. Although the absence of WDR81 did not affect viral attachment efficiency or uptake into the endosomal compartments for initial disassembly, it reduced viral gene expression and diminished infectious virus production. Consistent with the role of WDR81 in impacting the maturation of endosomes, WDR81-deficiency led to the accumulation of reovirus particles in dead-end compartments. Though WDR81 was dispensable for infection by VSV (vesicular stomatitis virus), which exits the endosomal system at an early stage, it was required for VSV-EBO GP (VSV that expresses the Ebolavirus glycoprotein), which must reach the late endosome to initiate infection. These results reveal a previously unappreciated role for WDR81 in promoting the replication of viruses that transit through late endosomes.

Viruses are obligate intracellular parasites that require the contributions of numerous host factors to complete the viral life cycle. Thus, the host-pathogen interaction can regulate cell death signaling and virus entry, replication, assembly, and egress. Functional genetic screens are useful tools to identify host factors that are important for establishing infection. Such information can also be used to understand cell biology. Notably, genome-scale CRISPR-Cas9 knockout screens are robust due to their specificity and the loss of host gene expression. Mammalian orthoreovirus (reovirus) is a tractable model system to investigate the pathogenesis of neurotropic and cardiotropic viruses. Using a CRISPR-Cas9 screen, we identified WD repeat-containing protein 81 (WDR81) as a host factor required for efficient reovirus infection of murine cells. Ablation of WDR81 blocked a late step in the viral entry pathway. Further, our work indicates that WDR81 is required for the entry of vesicular stomatitis virus that expresses the Ebolavirus glycoprotein.

p17-Modulated Hsp90/Cdc37 Complex Governs Oncolytic Avian Reovirus Replication by Chaperoning p17, Which Promotes Viral Protein Synthesis and Accumulation of Viral Proteins σC and σA in Viral Factories

In this work we have determined that heat shock protein 90 (Hsp90) is essential for avian reovirus (ARV) replication by chaperoning the ARV p17 protein. p17 modulates the formation of the Hsp90/Cdc37 complex by phosphorylation of Cdc37, and this chaperone machinery protects p17 from ubiquitin-proteasome degradation. Inhibition of the Hsp90/Cdc37 complex by inhibitors (17-N-allylamino-17-demethoxygeldanamycin 17-AGG, and celastrol) or short hairpin RNAs (shRNAs) significantly reduced expression levels of viral proteins and virus yield, suggesting that the Hsp90/Cdc37 chaperone complex functions in virus replication. The expression levels of p17 were decreased at the examined time points (2 to 7 h and 7 to 16 h) in 17-AAG-treated cells in a dose-dependent manner while the expression levels of viral proteins σA, σC, and σNS were decreased at the examined time point (7 to 16 h). Interestingly, the expression levels of σC, σA, and σNS proteins increased along with coexpression of p17 protein. p17 together with the Hsp90/Cdc37 complex does not increase viral genome replication but enhances viral protein stability, maturation, and virus production. Virus factories of ARV are composed of nonstructural proteins σNS and μNS. We found that the Hsp90/Cdc37 chaperone complex plays an important role in accumulation of the outer-capsid protein σC, inner core protein σA, and nonstructural protein σNS of ARV in viral factories. Depletion of Hsp90 inhibited σA, σC, and p17 proteins colocalized with σNS in viral factories. This study provides novel insights into p17-modulated formation of the Hsp90/Cdc37 chaperone complex governing virus replication via stabilization and maturation of viral proteins and accumulation of viral proteins in viral factories for virus assembly. IMPORTANCE Molecular mechanisms that control stabilization of ARV proteins and the intermolecular interactions among inclusion components remain largely unknown. Here, we show that the ARV p17 is an Hsp90 client protein. The Hsp90/Cdc37 chaperone complex is essential for ARV replication by protecting p17 chaperone from ubiquitin-proteasome degradation. p17 modulates the formation of Hsp90/Cdc37 complex by phosphorylation of Cdc37, and this chaperone machinery protects p17 from ubiquitin-proteasome degradation, suggesting a feedback loop between p17 and the Hsp90/Cdc37 chaperone complex. p17 together with the Hsp90/Cdc37 complex does not increase viral genome replication but enhances viral protein stability and virus production. Depletion of Hsp90 prevented viral proteins σA, σC, and p17 from colocalizing with σNS in viral factories. Our findings elucidate that the Hsp90/Cdc37 complex chaperones p17, which, in turn, promotes the synthesis of viral proteins σA, σC, and σNS and facilitates accumulation of the outer-capsid protein σC and inner core protein σA in viral factories for virus assembly.

Molecular chaperone TRiC governs avian reovirus replication by protecting outer-capsid protein σC and inner core protein σA and non-structural protein σNS from ubiquitin- proteasome degradation

Avian reoviruses (ARVs) are important pathogens that cause considerable economic losses in poultry farming. To date, host factors that control stabilization of ARV proteins remain largely unknown. In this work we determined that the eukaryotic chaperonin T-complex protein-1 (TCP-1) ring complex (TRiC) is essential for avian reovirus (ARV) replication by stabilizing outer-capsid protein σC, inner core protein σA, and the non-structural protein σNS of ARV. TriC serves as a chaperone of viral proteins and prevent their degradation via the ubiquitin-proteasome pathway. Furthermore, reciprocal co-immunoprecipitation assays confirmed the association of viral proteins (σA, σC, and σNS) with TRiC. Immunofluorescence staining indicated that the TRiC chaperonins (CCT2 and CCT5) are colocalized with viral proteins σC, σA, and σNS of ARV. In this study, inhibition of TRiC chaperonins (CCT2 and CCT5) by the inhibitor HSF1A or shRNAs significantly reduced expression levels of the σC, σA, and σNS proteins of ARV as well as virus yield, suggesting that the TRiC complex functions in stabilization of viral proteins and virus replication. This study provides novel insights into TRiC chaperonin governing virus replication via stabilization of outer-capsid protein σC, inner core protein σA, and the non-structural protein σNS of ARV.

Sorbitol and PKC1 overexpression alleviate temperature sensitivity in chaperonin mutants of Saccharomyces cerevisiae

CCT (Chaperonin containing TCP-1) is a constitutively expressed eukaryotic chaperonin complex involved in the proper folding of proteins like actin and tubulin. Temperature sensitive mutants of CCT complex have been employed in various genetic screens, acting as models to study human CCT, the defects of which are implicated in disease conditions like neurodegeneration. Mutants of CCT complex are sensitive to cell wall stress agents. In this study, we have tested the effects of sorbitol and protein kinase C overexpression on the temperature sensitivity of cct mutants. We report that both the factors alleviated temperature sensitivity of cct mutants, indicating the possible role of CCT in maintaining cell wall integrity in S. cerevisiae.

Interaction with the CCT chaperonin complex limits APOBEC3A cytidine deaminase cytotoxicity

The APOBEC3 cytidine deaminases are implicated as the cause of a prevalent somatic mutation pattern found in cancer genomes. The APOBEC3 enzymes act as viral restriction factors by mutating viral genomes. Mutation of the cellular genome is presumed to be an off-target activity of the enzymes, although the regulatory measures for APOBEC3 expression and activity remain undefined. It is therefore difficult to predict circumstances that enable APOBEC3 interaction with cellular DNA that leads to mutagenesis. The APOBEC3A (A3A) enzyme is the most potent deaminase of the family. Using proteomics, we evaluate protein interactors of A3A to identify potential regulators. We find that A3A interacts with the chaperonin-containing TCP-1 (CCT) complex, a cellular machine that assists in protein folding and function. Importantly, depletion of CCT results in A3A-induced DNA damage and cytotoxicity. Evaluation of cancer genomes demonstrates an enrichment of A3A mutational signatures in cancers with silencing mutations in CCT subunit genes. Together, these data suggest that the CCT complex interacts with A3A, and that disruption of CCT function results in increased A3A mutational activity.

The multi-functional reovirus σ3 protein is a virulence factor that suppresses stress granule formation and is associated with myocardial injury

The mammalian orthoreovirus double-stranded (ds) RNA-binding protein σ3 is a multifunctional protein that promotes viral protein synthesis and facilitates viral entry and assembly. The dsRNA-binding capacity of σ3 correlates with its capacity to prevent dsRNA-mediated activation of protein kinase R (PKR). However, the effect of σ3 binding to dsRNA during viral infection is largely unknown. To identify functions of σ3 dsRNA-binding activity during reovirus infection, we engineered a panel of thirteen σ3 mutants and screened them for the capacity to bind dsRNA. Six mutants were defective in dsRNA binding, and mutations in these constructs cluster in a putative dsRNA-binding region on the surface of σ3. Two recombinant viruses expressing these σ3 dsRNA-binding mutants, K287T and R296T, display strikingly different phenotypes. In a cell-type dependent manner, K287T, but not R296T, replicates less efficiently than wild-type (WT) virus. In cells in which K287T virus demonstrates a replication deficit, PKR activation occurs and abundant stress granules (SGs) are formed at late times post-infection. In contrast, the R296T virus retains the capacity to suppress activation of PKR and does not mediate formation of SGs at late times post-infection. These findings indicate that σ3 inhibits PKR independently of its capacity to bind dsRNA. In infected mice, K287T produces lower viral titers in the spleen, liver, lungs, and heart relative to WT or R296T. Moreover, mice inoculated with WT or R296T viruses develop myocarditis, whereas those inoculated with K287T do not. Overall, our results indicate that σ3 functions to suppress PKR activation and subsequent SG formation during viral infection and that these functions correlate with virulence in mice.

The σ3 protein of mammalian orthoreoviruses is a double-stranded RNA binding protein that has classically been thought to function by scavenging dsRNA within infected cells and thus prevents activation of cellular sensors of dsRNA such as the kinase PKR. Here we used mutagenesis to identify the region of σ3 responsible for binding dsRNA. Characterization of mutant viruses expressing σ3 proteins incapable of binding dsRNA show that contrary to expectation, dsRNA binding is not required for σ3-mediated inhibition of PKR. We show that one mutant virus (R296T) despite being deficient in dsRNA-binding can inhibit PKR and replicates similar to WT virus. In contrast, another mutant virus (K287T) that bears a σ3 protein that cannot prevent dsRNA-mediated activation of PKR induces stress granules in infected cells and replicates less efficiently than WT virus. In vivo, the K287T mutant is attenuated in its replication and unlike WT virus and the R296T mutant virus does not cause heart disease (myocarditis).

Reovirus Nonstructural Protein σNS Recruits Viral RNA to Replication Organelles

The function of the mammalian orthoreovirus (reovirus) σNS nonstructural protein is enigmatic. σNS is an RNA-binding protein that forms oligomers and enhances the stability of bound RNAs, but the mechanisms by which it contributes to reovirus replication are unknown. To determine the function of σNS-RNA binding in reovirus replication, we engineered σNS mutants deficient in RNA-binding capacity. We found that alanine substitutions of positively charged residues in a predicted RNA-binding domain decrease RNA-dependent oligomerization. To define steps in reovirus replication facilitated by the RNA-binding property of σNS, we established a complementation system in which wild-type or mutant forms of σNS could be tested for the capacity to overcome inhibition of σNS expression. Mutations in σNS that disrupt RNA binding also diminish viral replication and σNS distribution to viral factories. Moreover, viral mRNAs only incorporate into viral factories or factory-like structures (formed following expression of nonstructural protein μNS) when σNS is present and capable of binding RNA. Collectively, these findings indicate that σNS requires positively charged residues in a putative RNA-binding domain to recruit viral mRNAs to sites of viral replication and establish a function for σNS in reovirus replication.

Human Papillomavirus infection requires the CCT Chaperonin Complex

Human papillomavirus (HPV) infection is a multi-step process that implies complex interactions of the viral particles with cellular proteins. The HPV capsid includes the two structural proteins L1 and L2, that play crucial roles on infectious viral entry. L2 is particularly relevant for the intracellular trafficking of the viral DNA towards the nucleus. Here, using proteomic studies we identified CCT proteins as novel interaction partners of HPV-16 L2. The CCT multimeric complex is an essential chaperonin which interacts with a large number of protein targets. We analysed the binding of different components of the CCT complex to L2. We confirmed the interaction of this structural viral protein with the CCT subunit 3 (CCT3) and we found that this interaction requires the N-terminal region of L2. Defects in HPV-16 pseudoviral particle (PsVs) infection were revealed by siRNA-mediated knockdown of some CCT subunits. While a substantial drop in the viral infection was associated with the ablation of CCT component 2, even more pronounced effects on infectivity were observed upon depletion of CCT component 3. Using confocal immunofluorescence assays, CCT3 co-localised with HPV PsVs at early times after infection, with L2 being required for this to occur. Further analysis showed the colocalization of several other subunits of CCT with the PsVs. Moreover, we observed a defect in capsid uncoating and a change in PsVs intracellular normal processing when ablating CCT3. Taken together, these studies demonstrate the importance of CCT chaperonin during HPV infectious entry.ImportanceSeveral of the mechanisms that function during the infection of target cells by HPV particles have been previously described. However, many aspects of this process remain unknown. In particular, the role of cellular proteins functioning as molecular chaperones during HPV infections has been only partially investigated. To the best of our knowledge, we describe here for the first time, a requirement of the CCT chaperonin for HPV infection. The role of this cellular complex seems to be determined by the binding of its component 3 to the viral structural protein L2. However, CCT's effect on HPV infection most probably comprises the whole chaperonin complex. Altogether, these studies define an important role for the CCT chaperonin in the processing and intracellular trafficking of HPV particles and in subsequent viral infectious entry.

Structural and functional dissection of reovirus capsid folding and assembly by the prefoldin-TRiC/CCT chaperone network

Intracellular protein homeostasis is maintained by a network of chaperones that function to fold proteins into their native conformation. The eukaryotic TRiC chaperonin (TCP1-ring complex, also called CCT for cytosolic chaperonin containing TCP1) facilitates folding of a subset of proteins with folding constraints such as complex topologies. To better understand the mechanism of TRiC folding, we investigated the biogenesis of an obligate TRiC substrate, the reovirus σ3 capsid protein. We discovered that the σ3 protein interacts with a network of chaperones, including TRiC and prefoldin. Using a combination of cryoelectron microscopy, cross-linking mass spectrometry, and biochemical approaches, we establish functions for TRiC and prefoldin in folding σ3 and promoting its assembly into higher-order oligomers. These studies illuminate the molecular dynamics of σ3 folding and establish a biological function for TRiC in virus assembly. In addition, our findings provide structural and functional insight into the mechanism by which TRiC and prefoldin participate in the assembly of protein complexes.

The Role of Molecular Chaperones in Virus Infection and Implications for Understanding and Treating COVID-19

The COVID-19 pandemic made imperative the search for means to end it, which requires a knowledge of the mechanisms underpinning the multiplication and spread of its cause, the coronavirus SARS-CoV-2. Many viruses use members of the hosts’ chaperoning system to infect the target cells, replicate, and spread, and here we present illustrative examples. Unfortunately, the role of chaperones in the SARS-CoV-2 cycle is still poorly understood. In this review, we examine the interactions of various coronaviruses during their infectious cycle with chaperones in search of information useful for future research on SARS-CoV-2. We also call attention to the possible role of molecular mimicry in the development of autoimmunity and its widespread pathogenic impact in COVID-19 patients. Viral proteins share highly antigenic epitopes with human chaperones, eliciting anti-viral antibodies that crossreact with the chaperones. Both, the critical functions of chaperones in the infectious cycle of viruses and the possible role of these molecules in COVID-19 autoimmune phenomena, make clear that molecular chaperones are promising candidates for the development of antiviral strategies. These could consist of inhibiting-blocking those chaperones that are necessary for the infectious viral cycle, or those that act as autoantigens in the autoimmune reactions causing generalized destructive effects on human tissues.

Mammalian orthoreovirus core protein μ2 reorganizes host microtubule-organizing center components

Filamentous mammalian orthoreovirus (MRV) viral factories (VFs) are membrane-less cytosolic inclusions in which virus transcription, replication of dsRNA genome segments, and packaging of virus progeny into newly synthesized virus cores take place. In infected cells, the MRV μ2 protein forms punctae in the enlarged region of the filamentous VFs that are co-localized with γ-tubulin and resistant to nocodazole treatment, and permitted microtubule (MT)-extension, features common to MT-organizing centers (MTOCs). Using a previously established reconstituted VF model, we addressed the functions of MT-components and MTOCs concerning their roles in the formation of filamentous VFs. Indeed, the MTOC markers γ-tubulin and centrin were redistributed within the VF-like structures (VFLS) in a μ2-dependent manner. Moreover, the MT-nucleation centers significantly increased in numbers, and γ-tubulin was pulled-down in a binding assay when co-expressed with histidine-tagged-μ2 and μNS. Thus, μ2, by interaction with γ-tubulin, can modulate MTOCs localization and function according to viral needs.

TRiC/CCT Complex, a Binding Partner of NS1 Protein, Supports the Replication of Zika Virus in Both Mammalians and Mosquitoes

Mosquito-borne Zika virus (ZIKV) can cause congenital microcephaly and Guillain–Barré syndrome, among other symptoms. Specific treatments and vaccines for ZIKV are not currently available. To further understand the host factors that support ZIKV replication, we used mass spectrometry to characterize mammalian proteins that associate with the ZIKV NS1 protein and identified the TRiC/CCT complex as an interacting partner. Furthermore, the suppression of CCT2, one of the critical components of the TRiC/CCT complex, inhibited ZIKV replication in both mammalian cells and mosquitoes. These results highlight an important role for the TRiC/CCT complex in ZIKV infection, suggesting that the TRiC/CCT complex may be a promising therapeutic target.

Proteostasis in Viral Infection: Unfolding the Complex Virus-Chaperone Interplay

Viruses are obligate intracellular parasites that rely on their hosts for protein synthesis, genome replication, and viral particle production. As such, they have evolved mechanisms to divert host resources, including molecular chaperones, facilitate folding and assembly of viral proteins, stabilize complex structures under constant mutational pressure, and modulate signaling pathways to dampen antiviral responses and prevent premature host death. Biogenesis of viral proteins often presents unique challenges to the proteostasis network, as it requires the rapid and orchestrated production of high levels of a limited number of multifunctional, multidomain, and aggregation-prone proteins. To overcome such challenges, viruses interact with the folding machinery not only as clients but also as regulators of chaperone expression, function, and subcellular localization. In this review, we summarize the main types of interactions between viral proteins and chaperones during infection, examine evolutionary aspects of this relationship, and discuss the potential of using chaperone inhibitors as broad-spectrum antivirals.

How Many Mammalian Reovirus Proteins are involved in the Control of the Interferon Response?

As with most viruses, mammalian reovirus can be recognized and attacked by the host-cell interferon response network. Similarly, many viruses have developed resistance mechanisms to counteract the host-cell response at different points of this response. Reflecting the complexity of the interferon signaling pathways as well as the resulting antiviral response, viruses can-and often have-evolved many determinants to interfere with this innate immune response and allow viral replication. In the last few years, it has been evidenced that mammalian reovirus encodes many different determinants that are involved in regulating the induction of the interferon response or in interfering with the action of interferon-stimulated gene products. In this brief review, we present our current understanding of the different reovirus proteins known to be involved, introduce their postulated modes of action, and raise current questions that may lead to further investigations.

Identification of RUVBL1 and RUVBL2 as Novel Cellular Interactors of the Ebola Virus Nucleoprotein

Ebola virus (EBOV) is a filovirus that has become a global public health threat in recent years. EBOV is the causative agent of a severe, often fatal hemorrhagic fever. A productive viral infection relies on the successful recruitment of host factors for various stages of the viral life cycle. To date, several investigations have discovered specific host-pathogen interactions for various EBOV proteins. However, relatively little is known about the EBOV nucleoprotein (NP) with regard to host interactions. In the present study, we aimed to elucidate NP-host protein-protein interactions (PPIs). Affinity purification-mass spectrometry (AP-MS) was used to identify candidate NP cellular interactors. Candidate interactors RUVBL1 and RUVBL2, partner proteins belonging to the AAA+ (ATPases Associated with various cellular Activities) superfamily, were confirmed to interact with NP in co-immunoprecipitation (co-IP) and immunofluorescence (IF) experiments. Functional studies using a minigenome system revealed that the siRNA-mediated knockdown of RUVBL1 but not RUVBL2 moderately decreased EBOV minigenome activity. Super resolution structured illumination microscopy (SIM) was used to identify an association between NP and components of the R2TP complex, which includes RUVBL1, RUVBL2, RPAP3, and PIH1D1, suggesting a potential role for the R2TP complex in capsid formation. Moreover, the siRNA-mediated knockdown of RPAP3 and subsequent downregulation of PIH1D1 was shown to have no effect on minigenome activity, further suggesting a role in capsid formation. Overall, we identify RUVBL1 and RUVBL2 as novel interactors of EBOV NP and for the first time report EBOV NP recruitment of the R2TP complex, which may provide novel targets for broad-acting anti-EBOV therapeutics.

The ATP-powered gymnastics of TRiC/CCT: an asymmetric protein folding machine with a symmetric origin story

The eukaryotic chaperonin TRiC/CCT is a large hetero-oligomeric complex that plays an essential role assisting cellular protein folding and suppressing protein aggregation. It consists of two rings, and each composed of eight different subunits; non-native polypeptides bind and fold in an ATP-dependent manner within their central chamber. Here, we review recent advances in our understanding of TRiC structure and mechanism enabled by application of hybrid structural methods including the integration of cryo-electron microscopy with distance constraints from crosslinking mass spectrometry. These new insights are revealing how the different TRiC/CCT subunits create asymmetry in its ATP-driven conformational cycle and its interaction with non-native polypeptides, which ultimately underlie its unique ability to fold proteins that cannot be folded by other chaperones.

Function, Architecture, and Biogenesis of Reovirus Replication Neoorganelles

Most viruses that replicate in the cytoplasm of host cells form neoorganelles that serve as sites of viral genome replication and particle assembly. These highly specialized structures concentrate viral proteins and nucleic acids, prevent the activation of cell-intrinsic defenses, and coordinate the release of progeny particles. Reoviruses are common pathogens of mammals that have been linked to celiac disease and show promise for oncolytic applications. These viruses form nonenveloped, double-shelled virions that contain ten segments of double-stranded RNA. Replication organelles in reovirus-infected cells are nucleated by viral nonstructural proteins µNS and σNS. Both proteins partition the endoplasmic reticulum to form the matrix of these structures. The resultant membranous webs likely serve to anchor viral RNA⁻protein complexes for the replication of the reovirus genome and the assembly of progeny virions. Ongoing studies of reovirus replication organelles will advance our knowledge about the strategies used by viruses to commandeer host biosynthetic pathways and may expose new targets for therapeutic intervention against diverse families of pathogenic viruses.

The 13th International Double-Stranded RNA Virus Symposium, Houffalize, Belgium, 24 to 28 September 2018

The triennial International Double-Stranded RNA Virus Symposium, this year organized by J. Matthijnssens, J. S. L. Parker, P. Danthi, and P. Van Damme in Belgium, gathered over 200 scientists to discuss novel observations and hypotheses in the field. The keynote lecture on functional interactions of bacteria and viruses in the gut microbiome was presented by Julie Pfeiffer. Workshops were held on viral diversity, molecular epidemiology, molecular virology, immunity and pathogenesis, virus structure, the viral use and abuse of cellular pathways, and applied double-stranded RNA (dsRNA) virology. The establishment of a plasmid only-based reverse genetics system for rotaviruses by several Japanese research groups in 2017 has now been reproduced by various other research groups and was discussed in detail. The visualization of dsRNA virus replication steps in living cells received much attention. Mechanisms of the cellular innate immune response to virus infection and of viral pathogenesis were explored. Knowledge of the gut microbiome's influence on specific immune responses has increased rapidly, also due to the availability of relevant animal models of virus infection. The method of cryo-electron microscopic (cryo-EM) tomography has elucidated various asymmetric structures in viral particles. The use of orthoreoviruses for oncolytic virotherapy was critically assessed. The application of llama-derived single chain nanobodies for passive immunotherapy was considered attractive. In a satellite symposium the introduction, impact and further developments of rotavirus vaccines were reviewed. The Jean Cohen Lecturer of this meeting was Harry B. Greenberg, who presented aspects of his research on rotaviruses over a period of more than 40 years. He was also interviewed at the meeting by Vincent Racaniello for the 513th session of This Week in Virology.

Requirement of Cellular Protein CCT7 for the Replication of Fowl Adenovirus Serotype 4 (FAdV-4) in Leghorn Male Hepatocellular Cells Via Interaction with the Viral Hexon Protein

Fowl adenovirus serotype 4 (FAdV-4) causes hepatitis-hydropericardium syndrome (HHS), leading to severe economic losses in the poultry industry. Although the pathogenesis of FAdV-4 infection has caused much attention, the underlying molecular mechanisms remain poorly understood. Here, we identified chaperonin containing TCP-1 subunit eta (CCT7) as an interacting partner of the FAdV-4 capsid protein hexon. We found that ectopic expression of CCT7 in leghorn male hepatocellular (LMH) cells enhanced hexon expression in pRK5-flag-hexon transfected cells. On the contrary, knockdown of cellular CCT7 by RNAi markedly reduced hexon expression in FAdV-4-infected cells and suppressed viral replication. These data suggest that CCT7 is required for FAdV-4 replication and may serve as a potential target for controlling FAdV-4 infection.

Synthesis and Translation of Viral mRNA in Reovirus-Infected Cells: Progress and Remaining Questions

At the end of my doctoral studies, in 1988, I published a review article on the major steps of transcription and translation during the mammalian reovirus multiplication cycle, a topic that still fascinates me 30 years later. It is in the nature of scientific research to generate further questioning as new knowledge emerges. Our understanding of these fascinating viruses thus remains incomplete but it seemed appropriate at this moment to look back and reflect on our progress and most important questions that still puzzle us. It is also essential of being careful about concepts that seem so well established, but could still be better validated using new approaches. I hope that the few reflections presented here will stimulate discussions and maybe attract new investigators into the field of reovirus research. Many other aspects of the viral multiplication cycle would merit our attention. However, I will essentially limit my discussion to these central aspects of the viral cycle that are transcription of viral genes and their phenotypic expression through the host cell translational machinery. The objective here is not to review every aspect but to put more emphasis on important progress and challenges in the field.

Protein Mismatches Caused by Reassortment Influence Functions of the Reovirus Capsid

Following attachment to host receptors via σ1, reovirus particles are endocytosed and disassembled to generate infectious subvirion particles (ISVPs). ISVPs undergo conformational changes to form ISVP*, releasing σ1 and membrane-targeting peptides from the viral μ1 protein. ISVP* formation is required for delivery of the viral core into the cytoplasm for replication. We characterized the properties of T3D^F/T3D^CS1, an S1 gene monoreassortant between two laboratory isolates of prototype reovirus strain T3D: T3D^F and T3D^C T3D^F/T3D^CS1 is poorly infectious. This deficiency is a consequence of inefficient encapsidation of S1-encoded σ1 on T3D^F/T3D^CS1 virions. Additionally, compared to T3D^F, T3D^F/T3D^CS1 undergoes ISVP-to-ISVP* conversion more readily, revealing an unexpected role for σ1 in regulating ISVP* formation. The σ1 protein is held within turrets formed by the λ2 protein. To test if the altered properties of T3D^F/T3D^CS1 are due to a mismatch between σ1 and λ2 proteins from T3D^F and T3D^C, properties of T3D^F/T3D^CL2 and T3D^F/T3D^CS1L2, which express a T3D^C-derived λ2, were compared. The presence of T3D^C λ2 allowed more efficient σ1 incorporation, producing particles that exhibit T3D^F-like infectivity. Compared to T3D^F, T3D^F/T3D^CL2 prematurely converts to ISVP*, uncovering a role for λ2 in regulating ISVP* formation. Importantly, a virus with matching σ1 and λ2 displayed a more regulated conversion to ISVP* than either T3D^F/T3D^CS1 or T3D^F/T3D^CL2. In addition to identifying new regulators of ISVP* formation, our results highlight that protein mismatches produced by reassortment can alter virus assembly and thereby influence subsequent functions of the virus capsid.IMPORTANCE Cells coinfected with viruses that possess a multipartite or segmented genome reassort to produce progeny viruses that contain a combination of gene segments from each parent. Reassortment places new pairs of genes together, generating viruses in which mismatched proteins must function together. To test if such forced pairing of proteins that form the virus shell or capsid alters the function of the particle, we investigated properties of reovirus variants in which the σ1 attachment protein and the λ2 protein that anchors σ1 on the particle are mismatched. Our studies demonstrate that a σ1-λ2 mismatch produces particles with lower levels of encapsidated σ1, consequently decreasing virus attachment and infectivity. The mismatch between σ1 and λ2 also altered the capacity of the viral capsid to undergo conformational changes required for cell entry. These studies reveal new functions of reovirus capsid proteins and illuminate both predictable and novel implications of reassortment.

Murine cytomegalovirus M72 promotes acute virus replication in vivo and is a substrate of the TRiC/CCT complex

Betaherpesvirus dUTPase homologs are core herpesvirus proteins, but little is known about their role during infection. Human cytomegalovirus (HCMV) UL72 and murine cytomegalovirus (MCMV) M72 have been designated dUTPase homologs, and previous studies indicate UL72 is dispensable for replication and enzymatically inactive. Here, we report the initial characterization of MCMV M72. M72 does not possess dUTPase activity, and is expressed as a leaky-late gene product with multiple protein isoforms. Importantly, M72 augments MCMV replication in vitro and during the early stage of acute infection in vivo. We identify and confirm interaction of M72 with the eukaryotic chaperonin tailless complex protein -1 (TCP-1) ring complex (TRiC) or chaperonin containing tailless complex polypeptide 1 (CCT). Accumulating biochemical evidence indicates M72 forms homo-oligomers and is a substrate of TRiC/CCT. Taken together, we provide the first evidence of M72's contribution to viral pathogenesis, and identify a novel interaction with the TRiC/CCT complex.

Reovirus Nonstructural Protein σNS Acts as an RNA Stability Factor Promoting Viral Genome Replication

Viral nonstructural proteins, which are not packaged into virions, are essential for the replication of most viruses. Reovirus, a nonenveloped, double-stranded RNA (dsRNA) virus, encodes three nonstructural proteins that are required for viral replication and dissemination in the host. The reovirus nonstructural protein σNS is a single-stranded RNA (ssRNA)-binding protein that must be expressed in infected cells for production of viral progeny. However, the activities of σNS during individual steps of the reovirus replication cycle are poorly understood. We explored the function of σNS by disrupting its expression during infection using cells expressing a small interfering RNA (siRNA) targeting the σNS-encoding S3 gene and found that σNS is required for viral genome replication. Using complementary biochemical assays, we determined that σNS forms complexes with viral and nonviral RNAs. We also discovered, using in vitro and cell-based RNA degradation experiments, that σNS increases the RNA half-life. Cryo-electron microscopy revealed that σNS and ssRNAs organize into long, filamentous structures. Collectively, our findings indicate that σNS functions as an RNA-binding protein that increases the viral RNA half-life. These results suggest that σNS forms RNA-protein complexes in preparation for genome replication.IMPORTANCE Following infection, viruses synthesize nonstructural proteins that mediate viral replication and promote dissemination. Viruses from the family Reoviridae encode nonstructural proteins that are required for the formation of progeny viruses. Although nonstructural proteins of different viruses in the family Reoviridae diverge in primary sequence, they are functionally homologous and appear to facilitate conserved mechanisms of dsRNA virus replication. Using in vitro and cell culture approaches, we found that the mammalian reovirus nonstructural protein σNS binds and stabilizes viral RNA and is required for genome synthesis. This work contributes new knowledge about basic mechanisms of dsRNA virus replication and provides a foundation for future studies to determine how viruses in the family Reoviridae assort and replicate their genomes.