Studies on the effect of chymotrypsin on reovirions

Inhibition of HIF-1α accumulation in prostate cancer cells is initiated during early stages of mammalian orthoreovirus infection

Mammalian orthoreovirus (MRV) is a safe and effective cancer killing virus that has completed Phase I-III clinical trials against numerous cancer types. While many patients experience benefit from MRV therapy, pre-defined set points necessary for FDA approval have not been reached. Therefore, additional research into MRV biology and the effect of viral therapy on different tumor genetic subtypes and microenvironments is necessary to identify tumors most amenable to MRV virotherapy. In this work we analyzed the stage of viral infection necessary to inhibit HIF-1α, an aggressive cancer activator induced by hypoxia. We demonstrated that two viral capsid proteins were not necessary and that a step parallel with virus core movement across the endosomal membrane was required for this inhibition. Altogether, this work clarifies the mechanisms of MRV-induced HIF-1α inhibition and provides biological relevance for using MRV to inhibit the devastating effects of tumor hypoxia.

Breast Tumor-Associated Metalloproteases Restrict Reovirus Oncolysis by Cleaving the σ1 Cell Attachment Protein and Can Be Overcome by Mutation of σ1

Reovirus is undergoing clinical testing as an oncolytic therapy for breast cancer. Given that reovirus naturally evolved to thrive in enteric environments, we sought to better understand how breast tumor microenvironments impinge on reovirus infection. Reovirus was treated with extracellular extracts generated from polyomavirus middle T-antigen-derived mouse breast tumors. Unexpectedly, these breast tumor extracellular extracts inactivated reovirus, reducing infectivity of reovirus particles by 100-fold. Mechanistically, inactivation was attributed to proteolytic cleavage of the viral cell attachment protein σ1, which diminished virus binding to sialic acid (SA)-low tumor cells. Among various specific protease class inhibitors and metal ions, EDTA and ZnCl₂ effectively modulated σ1 cleavage, indicating that breast tumor-associated zinc-dependent metalloproteases are responsible for reovirus inactivation. Moreover, media from MCF7, MB468, MD-MB-231, and HS578T breast cancer cell lines recapitulated σ1 cleavage and reovirus inactivation, suggesting that inactivation of reovirus is shared among mouse and human breast cancers and that breast cancer cells by themselves can be a source of reovirus-inactivating proteases. Binding assays and quantification of SA levels on a panel of cancer cells showed that truncated σ1 reduced virus binding to cells with low surface SA. To overcome this restriction, we generated a reovirus mutant with a mutation (T249I) in σ1 that prevents σ1 cleavage and inactivation by breast tumor-associated proteases. The mutant reovirus showed similar replication kinetics in tumorigenic cells, toxicity equivalent to that of wild-type reovirus in a severely compromised mouse model, and increased tumor titers. Overall, the data show that tumor microenvironments have the potential to reduce infectivity of reovirus.IMPORTANCE We demonstrate that metalloproteases in breast tumor microenvironments can inactivate reovirus. Our findings expose that tumor microenvironment proteases could have a negative impact on proteinaceous cancer therapies, such as reovirus, and that modification of such therapies to circumvent inactivation by tumor metalloproteases merits consideration.

Reovirus: Friend and Foe

Purpose of Review

Mammalian orthoreovirus (reovirus) is a powerful tool for studying viral replication and pathogenesis. Most reovirus infections are subclinical, however recent work has catapulted reovirus into the clinical spotlight.

Recent Findings

Owing to its capacity to kill cancer cells more efficiently than normal cells, reovirus is under development as a therapeutic for a variety of cancers. New efforts have focused on genetically engineering reovirus to increase its oncolytic capacity, and determining how reovirus potentiates immunotherapy. Other recent studies highlight a potential role for reovirus in celiac disease (CeD). Using mouse models of CeD, reovirus caused loss of oral tolerance to dietary antigens, opening the possibility that reovirus could trigger CeD in humans.

Summary

We will focus on new developments in reovirus oncolysis and studies suggesting a role for reovirus as a trigger for celiac disease (CeD) that make reovirus a potential friend and foe to human health.

Selection and Characterization of a Reovirus Mutant with Increased Thermostability

The environment represents a significant barrier to infection. Physical stressors (heat) or chemical agents (ethanol) can render virions noninfectious. As such, discrete proteins are necessary to stabilize the dual-layered structure of mammalian orthoreovirus (reovirus). The outer capsid participates in cell entry: (i) σ3 is degraded to generate the infectious subviral particle, and (ii) μ1 facilitates membrane penetration and subsequent core delivery. μ1-σ3 interactions also prevent inactivation; however, this activity is not fully characterized. Using forward and reverse genetic approaches, we identified two mutations (μ1 M258I and σ3 S344P) within heat-resistant strains. σ3 S344P was sufficient to enhance capsid integrity and to reduce protease sensitivity. Moreover, these changes impaired replicative fitness in a reassortant background. This work reveals new details regarding the determinants of reovirus stability.IMPORTANCE Nonenveloped viruses rely on protein-protein interactions to shield their genomes from the environment. The capsid, or protective shell, must also disassemble during cell entry. In this work, we identified a determinant within mammalian orthoreovirus that regulates heat resistance, disassembly kinetics, and replicative fitness. Together, these findings show capsid function is balanced for optimal replication and for spread to a new host.

Components of the Reovirus Capsid Differentially Contribute to Stability

The mammalian orthoreovirus (reovirus) outer capsid is composed of 200 μ1-σ3 heterohexamers and a maximum of 12 σ1 trimers. During cell entry, σ3 is degraded by luminal or intracellular proteases to generate the infectious subviral particle (ISVP). When ISVP formation is prevented, reovirus fails to establish a productive infection, suggesting proteolytic priming is required for entry. ISVPs are then converted to ISVP*s, which is accompanied by μ1 rearrangements. The μ1 and σ3 proteins confer resistance to inactivating agents; however, neither the impact on capsid properties nor the mechanism (or basis) of inactivation is fully understood. Here, we utilized T1L/T3D M2 and T3D/T1L S4 to investigate the determinants of reovirus stability. Both reassortants encode mismatched subunits. When μ1-σ3 were derived from different strains, virions resembled wild-type particles in structure and protease sensitivity. T1L/T3D M2 and T3D/T1L S4 ISVPs were less thermostable than wild-type ISVPs. In contrast, virions were equally susceptible to heating. Virion associated μ1 adopted an ISVP*-like conformation concurrent with inactivation; σ3 preserves infectivity by preventing μ1 rearrangements. Moreover, thermostability was enhanced by a hyperstable variant of μ1. Unlike the outer capsid, the inner capsid (core) was highly resistant to elevated temperatures. The dual layered architecture allowed for differential sensitivity to inactivating agents.IMPORTANCE Nonenveloped and enveloped viruses are exposed to the environment during transmission to a new host. Protein-protein and/or protein-lipid interactions stabilize the particle and protect the viral genome. Mammalian orthoreovirus (reovirus) is composed of two concentric, protein shells. The μ1 and σ3 proteins form the outer capsid; contacts between neighboring subunits are thought to confer resistance to inactivating agents. We further investigated the determinants of reovirus stability. The outer capsid was disrupted concurrent with the loss of infectivity; virion associated μ1 rearranged into an altered conformation. Heat sensitivity was controlled by σ3; however, particle integrity was enhanced by a single μ1 mutation. In contrast, the inner capsid (core) displayed superior resistance to heating. These findings reveal structural components that differentially contribute to reovirus stability.

Cleavage of the C-Terminal Fragment of Reovirus μ1 Is Required for Optimal Infectivity

The mammalian orthoreovirus (reovirus) outer capsid, which is composed of 200 μ1/σ3 heterohexamers and a maximum of 12 σ1 trimers, contains all of the proteins that are necessary for attaching to and entering host cells. Following attachment, reovirus is internalized by receptor-mediated endocytosis and acid-dependent cathepsin proteases degrade the σ3 protein. This process generates a metastable intermediate, called infectious subviral particle (ISVP), in which the μ1 membrane penetration protein is exposed. ISVPs undergo a second structural rearrangement to deposit the genome-containing core into the host cytoplasm. The conformationally altered particle is called ISVP*. ISVP-to-ISVP* conversion culminates in the release of μ1 N- and C-terminal fragments, μ1N and Φ, respectively. Released μ1N is thought to facilitate core delivery by generating size-selective pores within the endosomal membrane, whereas the precise role of Φ, particularly in the context of viral entry, is undefined. In this report, we characterize a recombinant reovirus that fails to cleave Φ from μ1 in vitro Φ cleavage, which is not required for ISVP-to-ISVP* conversion, enhances the disruption of liposomal membranes and facilitates the recruitment of ISVP*s to the site of pore formation. Moreover, the Φ cleavage-deficient strain initiates infection of host cells less efficiently than the parental strain. These results indicate that μ1N and Φ contribute to reovirus pore forming activity.IMPORTANCE Host membranes represent a physical barrier that prevents infection. To overcome this barrier, viruses utilize diverse strategies, such as membrane fusion or membrane disruption, to access internal components of the cell. These strategies are characterized by discrete protein-protein and protein-lipid interactions. The mammalian orthoreovirus (reovirus) outer capsid undergoes a series of well-defined conformational changes, which conclude with pore formation and delivery of the viral genetic material. In this report, we characterize the role of the small, reovirus-derived Φ peptide in pore formation. Φ cleavage from the outer capsid enhances membrane disruption and facilitates the recruitment of virions to membrane-associated pores. Moreover, Φ cleavage promotes the initiation of infection. Together, these results reveal an additional component of the reovirus pore forming apparatus and highlight a strategy for penetrating host membranes.

Infectious Subviral Particle to Membrane Penetration Active Particle (ISVP-to-ISVP*) Conversion Assay for Mammalian Orthoreovirus

The mammalian orthoreovirus (reovirus) outer capsid undergoes a series of conformational changes prior to or during viral entry. These transitions are necessary for delivering the genome-containing core across host cell membranes. This protocol describes an in vitro assay for monitoring the transition into a membrane penetration-active form (i.e., ISVP*).

Infectious Subviral Particle-induced Hemolysis Assay for Mammalian Orthoreovirus

Mammalian orthoreovirus (reovirus) utilizes pore forming peptides to penetrate host cell membranes. This step is essential for delivering its genome containing core particle during viral entry. This protocol describes an in vitro assay for measuring reovirus-induced pore formation.

The Loop Formed by Residues 340 to 343 of Reovirus μ1 Controls Entry-Related Conformational Changes

Reovirus particles are covered with 200 μ1/σ3 heterohexamers. Following attachment to cell surface receptors, reovirus is internalized by receptor-mediated endocytosis. Within the endosome, particles undergo a series of stepwise disassembly events. First, the σ3 protector protein is degraded by cellular proteases to generate infectious subviral particles (ISVPs). Second, the μ1 protein rearranges into a protease-sensitive conformation to generate ISVP*s and releases two virus-encoded peptides, μ1N and Φ. The released peptides promote delivery of the genome-containing core by perforating the endosomal membrane. Thus, to establish a productive infection, virions must be stable in the environment but flexible to disassemble in response to the appropriate cellular cue. The reovirus outer capsid is stabilized by μ1 intratrimer, intertrimer, and trimer-core interactions. As a consequence of ISVP-to-ISVP* conversion, neighboring μ1 trimers unwind and separate. Located within the μ1 jelly roll β barrel domain, which is a known regulator of ISVP* formation, residues 340 to 343 form a loop and have been proposed to facilitate viral entry. To test this idea, we generated recombinant reoviruses that encoded deletions within this loop (Δ341 and Δ342). Both deletions destabilized the outer capsid. Notably, Δ342 impaired the viral life cycle; however, replicative fitness was restored by an additional change (V403A) within the μ1 jelly roll β barrel domain. In the Δ341 and Δ342 backgrounds, V403A also rescued defects in ISVP-to-ISVP* conversion. Together, these findings reveal a new region that regulates reovirus disassembly and how perturbing a metastable capsid can compromise replicative fitness.IMPORTANCE Capsids of nonenveloped viruses are composed of protein complexes that encapsulate, or form a shell around, nucleic acid. The protein-protein interactions that form this shell must be stable to protect the viral genome but also sufficiently flexible to disassemble during cell entry. Thus, capsids adopt conformations that undergo rapid disassembly in response to a specific cellular cue. In this work, we identify a new region within the mammalian orthoreovirus outer capsid that regulates particle stability. Amino acid deletions that destabilize this region impair the viral replication cycle. Nonetheless, replicative fitness is restored by a compensatory mutation that restores particle stability. Together, this work demonstrates the critical balance between assembling virions that are stable and maintaining conformational flexibility. Any factor that perturbs this balance has the potential to block a productive infection.

African Swine Fever Virus NP868R Capping Enzyme Promotes Reovirus Rescue during Reverse Genetics by Promoting Reovirus Protein Expression, Virion Assembly, and RNA Incorporation into Infectious Virions

Reoviruses, like many eukaryotic viruses, contain an inverted 7-methylguanosine (m7G) cap linked to the 5' nucleotide of mRNA. The traditional functions of capping are to promote mRNA stability, protein translation, and concealment from cellular proteins that recognize foreign RNA. To address the role of mRNA capping during reovirus replication, we assessed the benefits of adding the African swine fever virus NP868R capping enzyme during reovirus rescue. C3P3, a fusion protein containing T7 RNA polymerase and NP868R, was found to increase protein expression 5- to 10-fold compared to T7 RNA polymerase alone while enhancing reovirus rescue from the current reverse genetics system by 100-fold. Surprisingly, RNA stability was not increased by C3P3, suggesting a direct effect on protein translation. A time course analysis revealed that C3P3 increased protein synthesis within the first 2 days of a reverse genetics transfection. This analysis also revealed that C3P3 enhanced processing of outer capsid μ1 protein to μ1C, a previously described hallmark of reovirus assembly. Finally, to determine the rate of infectious-RNA incorporation into new virions, we developed a new recombinant reovirus S1 gene that expressed the fluorescent protein UnaG. Following transfection of cells with UnaG and infection with wild-type virus, passage of UnaG through progeny was significantly enhanced by C3P3. These data suggest that capping provides nontraditional functions to reovirus, such as promoting assembly and infectious-RNA incorporation.IMPORTANCE Our findings expand our understanding of how viruses utilize capping, suggesting that capping provides nontraditional functions to reovirus, such as promoting assembly and infectious-RNA incorporation, in addition to enhancing protein translation. Beyond providing mechanistic insight into reovirus replication, our findings also show that reovirus reverse genetics rescue is enhanced 100-fold by the NP868R capping enzyme. Since reovirus shows promise as a cancer therapy, efficient reovirus reverse genetics rescue will accelerate production of recombinant reoviruses as candidates to enhance therapeutic potency. NP868R-assisted reovirus rescue will also expedite production of recombinant reovirus for mechanistic insights into reovirus protein function and structure.

Lipids Cooperate with the Reovirus Membrane Penetration Peptide to Facilitate Particle Uncoating

Virus-host interactions play a role in many stages of the viral lifecycle, including entry. Reovirus, a model system for studying the entry mechanisms of nonenveloped viruses, undergoes a series of regulated structural transitions that culminate in delivery of the viral genetic material. Lipids can trigger one of these conformational changes, infectious subviral particle (ISVP)-to-ISVP* conversion. ISVP* formation releases two virally encoded peptides, myristoylated μ1N (myr-μ1N) and Φ. Among these, myr-μ1N is sufficient to form pores within membranes. Released myr-μ1N can also promote ISVP* formation in trans Using thermal inactivation as a readout for ISVP-to-ISVP* conversion, we demonstrate that lipids render ISVPs less thermostable in a virus concentration-dependent manner. Under conditions in which neither lipids alone nor myr-μ1N alone promotes ISVP-to-ISVP* conversion, myr-μ1N induces particle uncoating when lipids are present. These data suggest that the pore-forming activity and the ISVP*-promoting activity of myr-μ1N are linked. Lipid-associated myr-μ1N interacts with ISVPs and triggers efficient ISVP* formation. The cooperativity between a reovirus component and lipids reveals a distinct virus-host interaction in which membranes can facilitate nonenveloped virus entry.

Lipid Membranes Facilitate Conformational Changes Required for Reovirus Cell Entry

Cellular entry of nonenveloped and enveloped viruses is often accompanied by dramatic conformational changes within viral structural proteins. These rearrangements are triggered by a variety of mechanisms, such as low pH, virus-receptor interactions, and virus-host chaperone interactions. Reoviruses, a model system for entry of nonenveloped viruses, undergo a series of disassembly steps within the host endosome. One of these steps, infectious subviral particle (ISVP)-to-ISVP* conversion, is necessary for delivering the genome-containing viral core into host cells, but the physiological trigger that mediates ISVP-to-ISVP* conversion during cell entry is unknown. Structural studies of the reovirus membrane penetration protein, μ1, predict that interactions between μ1 and negatively charged lipid head groups may promote ISVP* formation; however, experimental evidence for this idea is lacking. Here, we show that the presence of polyanions (SO4(2-) and HPO4(2-)) or lipids in the form of liposomes facilitates ISVP-to-ISVP* conversion. The requirement for charged lipids appears to be selective, since phosphatidylcholine and phosphatidylethanolamine promoted ISVP* formation, whereas other lipids, such as sphingomyelin and sulfatide, either did not affect ISVP* formation or prevented ISVP* formation. Thus, our work provides evidence that interactions with membranes can function as a trigger for a nonenveloped virus to gain entry into host cells.

IMPORTANCE

Cell entry, a critical stage in the virus life cycle, concludes with the delivery of the viral genetic material across host membranes. Regulated structural transitions within nonenveloped and enveloped viruses are necessary for accomplishing this step; these conformational changes are predominantly triggered by low pH and/or interactions with host proteins. In this work, we describe a previously unknown trigger, interactions with lipid membranes, which can induce the structural rearrangements required for cell entry. This mechanism operates during entry of mammalian orthoreoviruses. We show that interactions between reovirus entry intermediates and lipid membranes devoid of host proteins promote conformational changes within the viral outer capsid that lead to membrane penetration. Thus, this work illustrates a novel strategy that nonenveloped viruses can use to gain access into cells and how viruses usurp disparate host factors to initiate infection.

Reovirus uses multiple endocytic pathways for cell entry

Entry of reovirus virions has been well studied in several tissue culture systems. After attachment to junctional adhesion molecule A (JAM-A), virions undergo clathrin-mediated endocytosis followed by proteolytic disassembly of the capsid and penetration to the cytoplasm. However, during in vivo infection of the intestinal tract, and likely in the tumor microenvironment, capsid proteolysis (uncoating) is initiated extracellularly. We used multiple approaches to determine if uncoated reovirus particles, called intermediate subviral particles (ISVPs), enter cells by directly penetrating the limiting membrane or if they take advantage of endocytic pathways to establish productive infection. We found that entry and infection by reovirus ISVPs was inhibited by dynasore, an inhibitor of dynamin-dependent endocytosis, as well as by genistein and dominant-negative caveolin-1, which block caveolar endocytosis. Inhibition of caveolar endocytosis also reduced infection by reovirus virions. Extraction of membrane cholesterol with methyl-β-cyclodextrin inhibited infection by virions but had no effect when infection was initiated with ISVPs. We found this pathway to be independent of both clathrin and caveolin. Together, these data suggest that reovirus virions can use both dynamin-dependent and dynamin-independent endocytic pathways during cell entry, and they reveal that reovirus ISVPs can take advantage of caveolar endocytosis to establish productive infection.

Reovirus variants with mutations in genome segments S1 and L2 exhibit enhanced virion infectivity and superior oncolysis

Reovirus preferentially replicates in transformed cells and is being explored as a cancer therapy. Immunological and physical barriers to virotherapy inspired a quest for reovirus variants with enhanced oncolytic potency. Using a classical genetics approach, we isolated two reovirus variants (T3v1 and T3v2) with superior replication relative to wild-type reovirus serotype 3 Dearing (T3wt) on various human and mouse tumorigenic cell lines. Unique mutations in reovirus λ2 vertex protein and σ1 cell attachment protein were associated with the large plaque-forming phenotype of T3v1 and T3v2, respectively. Both T3v1 and T3v2 exhibited higher infectivity (i.e., a higher PFU-to-particle ratio) than T3wt. A detailed analysis of virus replication revealed that virus cell binding and uncoating were equivalent for variant and wild-type reoviruses. However, T3v1 and T3v2 were significantly more efficient than T3wt in initiating productive infection. Thus, when cells were infected with equivalent input virus particles, T3v1 and T3v2 produced significantly higher levels of early viral RNAs relative to T3wt. Subsequent steps of virus replication (viral RNA and protein synthesis, virus assembly, and cell death) were equivalent for all three viruses. In a syngeneic mouse model of melanoma, both T3v1 and T3v2 prolonged mouse survival compared to wild-type reovirus. Our studies reveal that oncolytic potency of reovirus can be improved through distinct mutations that increase the infectivity of reovirus particles.

Cell entry-associated conformational changes in reovirus particles are controlled by host protease activity

Membrane penetration by reovirus requires successive formation of two cell entry intermediates, infectious subvirion particles (ISVPs) and ISVP*s. In vitro incubation of reovirus virions with high concentration of chymotrypsin (CHT) results in partial digestion of the viral outer capsid to form ISVPs. When virions are instead digested with low concentrations of chymotrypsin, the outer capsid is completely proteolyzed to form cores. We investigated the basis for the inverse relationship between CHT activity and protease susceptibility of the reovirus outer capsid. We report that core formation following low-concentration CHT digestion proceeds via formation of particles that contain a protease-sensitive form of the μ1C protein, a characteristic of ISVP*s. In addition, we found that both biochemical features and viral genetic requirements for ISVP* formation and core formation following low-concentration CHT digestion are identical, suggesting that core formation proceeds via a particle resembling ISVP*s. Furthermore, we determined that intermediates generated following low-concentration CHT digestion are distinct from ISVPs and convert to ISVP*-like particles much more readily than ISVPs. These results suggest that the activity of host proteases used to generate ISVPs can influence the efficiency with which the next step in reovirus cell entry, namely, ISVP-to-ISVP* conversion, occurs.

A positive-feedback mechanism promotes reovirus particle conversion to the intermediate associated with membrane penetration

Membrane penetration by reovirus is associated with conversion of a metastable intermediate, the ISVP, to a further-disassembled particle, the ISVP*. Factors that promote this conversion in cells are poorly understood. Here, we report the in vitro characterization of a positive-feedback mechanism for promoting ISVP* conversion. At high particle concentration, conversion approximated second-order kinetics, and products of the reaction operated in trans to promote the conversion of target ISVPs. Pore-forming peptide mu1N, which is released from particles during conversion, was sufficient for promoting activity. A mutant that does not undergo mu1N release failed to exhibit second-order conversion kinetics and also failed to promote conversion of wild-type target ISVPs. Susceptibility of target ISVPs to promotion in trans was temperature dependent and correlated with target stability, suggesting that capsid dynamics are required to expose the interacting epitope. A positive-feedback mechanism of promoting escape from the metastable intermediate has not been reported for other viruses but represents a generalizable device for sensing a confined volume, such as that encountered during cell entry.

Formation of the factory matrix is an important, though not a sufficient function of nonstructural protein mu NS during reovirus infection

Genome replication of mammalian orthoreovirus (MRV) occurs in cytoplasmic inclusion bodies called viral factories. Nonstructural protein microNS, encoded by genome segment M3, is a major constituent of these structures. When expressed without other viral proteins, microNS forms cytoplasmic inclusions morphologically similar to factories, suggesting a role for microNS as the factory framework or matrix. In addition, most other MRV proteins, including all five core proteins (lambda1, lambda2, lambda3, micro2, and sigma2) and nonstructural protein sigmaNS, can associate with microNS in these structures. In the current study, small interfering RNA targeting M3 was transfected in association with MRV infection and shown to cause a substantial reduction in microNS expression as well as, among other effects, a reduction in infectious yields by as much as 4 log(10) values. By also transfecting in vitro-transcribed M3 plus-strand RNA containing silent mutations that render it resistant to the small interfering RNA, we were able to complement microNS expression and to rescue infectious yields by ~100-fold. We next used microNS mutants specifically defective at forming factory-matrix structures to show that this function of microNS is important for MRV growth; point mutations in a C-proximal, putative zinc-hook motif as well as small deletions at the extreme C terminus of microNS prevented rescue of viral growth while causing microNS to be diffusely distributed in cells. We furthermore confirmed that an N-terminally truncated form of microNS, designed to represent microNSC and still able to form factory-matrix structures, is unable to rescue MRV growth, localizing one or more other important functions to an N-terminal region of microNS known to be involved in both micro2 and sigmaNS association. Thus, factory-matrix formation is an important, though not a sufficient function of microNS during MRV infection; microNS is multifunctional in the course of viral growth.

A role for molecular chaperone Hsc70 in reovirus outer capsid disassembly

After crossing the cellular membrane barrier during cell entry, most animal viruses must undergo further disassembly before initiating viral gene expression. In many cases, these disassembly mechanisms remain poorly defined. For this report, we examined a final step in disassembly of the mammalian reovirus outer capsid: cytoplasmic release of the central, delta fragment of membrane penetration protein mu1 to yield the transcriptionally active viral core particle. An in vitro assay with reticulocyte lysate recapitulated the release of intact delta molecules. Requirements for activity in this system were shown to include a protein factor, ATP, and Mg(2+) and K(+) ions, consistent with involvement of a molecular chaperone such as Hsc70. Immunodepletion of Hsc70 and Hsp70 impaired delta release, which was then rescued by addition of purified Hsc70. Hsc70 was associated with released delta molecules not only in the lysate but also during cell entry. We conclude that Hsc70 plays a defined role in reovirus outer capsid disassembly, during or soon after membrane penetration, to prepare the entering particle for gene expression and replication.

Thermostabilizing mutations in reovirus outer-capsid protein mu1 selected by heat inactivation of infectious subvirion particles

The 76-kDa mu1 protein of nonfusogenic mammalian reovirus is a major component of the virion outer capsid, which contains 200 mu1 trimers arranged in an incomplete T=13 lattice. In virions, mu1 is largely covered by a second major outer-capsid protein, sigma3, which limits mu1 conformational mobility. In infectious subvirion particles, from which sigma3 has been removed, mu1 is broadly exposed on the surface and can be promoted to rearrange into a protease-sensitive and hydrophobic conformer, leading to membrane perforation or penetration. In this study, mutants that resisted loss of infectivity upon heat inactivation (heat-resistant mutants) were selected from infectious subvirion particles of reovirus strains Type 1 Lang and Type 3 Dearing. All of the mutants were found to have mutations in mu1, and the heat-resistance phenotype was mapped to mu1 by both recoating and reassortant genetics. Heat-resistant mutants were also resistant to rearrangement to the protease-sensitive conformer of mu1, suggesting that heat inactivation is associated with mu1 rearrangement, consistent with published results. Rate constants of heat inactivation were determined, and the dependence of inactivation rate on temperature was consistent with the Arrhenius relationship. The Gibbs free energy of activation was calculated with reference to transition-state theory and was found to be correlated with the degree of heat resistance in each of the analyzed mutants. The mutations are located in upper portions of the mu1 trimer, near intersubunit contacts either within or between trimers in the viral outer capsid. We propose that the mutants stabilize the outer capsid by interfering with unwinding of the mu1 trimer.

Mammalian reovirus, a nonfusogenic nonenveloped virus, forms size-selective pores in a model membrane

During cell entry, reovirus particles with a diameter of 70-80 nm must penetrate the cellular membrane to access the cytoplasm. The mechanism of penetration, without benefit of membrane fusion, is not well characterized for any such nonenveloped animal virus. Lysis of RBCs is an in vitro assay for the membrane perforation activity of reovirus; however, the mechanism of lysis has been unknown. In this report, osmotic-protection experiments using PEGs of different sizes revealed that reovirus-induced lysis of RBCs occurs osmotically, after formation of small size-selective lesions or "pores." Consistent results were obtained by monitoring leakage of fluorophore-tagged dextrans from the interior of resealed RBC ghosts. Gradient fractionations showed that whole virus particles, as well as the myristoylated fragment mu1N that is released from particles, are recruited to RBC membranes in association with pore formation. We propose that formation of small pores is a discrete, intermediate step in the reovirus membrane-penetration pathway, which may be shared by other nonenveloped animal viruses.

High-resolution mass spectrometric mapping of reovirus digestion

Reovirus is an enteric virus built from eight structural proteins that form a double-layered capsid. During virus entry into cells the reovirus outermost capsid layer (composed of proteins sigma3 and mu1C) is proteolytically processed to generate first an infectious subviral particle (ISVP), then the transcriptionally active core particle. Previous studies have demonstrated that protein sigma3, the outermost protein in the viral capsid, is removed from virus particles extremely rapidly. Other studies, using the detergent tetradecyl sulfate (14SO4) in combination with the protease chymotrypsin, have shown that mu1C cleavage is not necessary for infectious viral processing. We have recently used mass spectrometry to characterize the cascade of sigma3 proteolysis in intact reovirus serotype 1 Lang (T1L) virions (Mendez et al., Virology 2003; 311: 289-304). In the present study, we use high-resolution mass spectrometry to characterize the cascade of outer capsid digestion of both T1L and the other commonly used reovirus strain (serotype 3 Dearing [T3D]), with the protease trypsin, both in the presence and absence of 14SO4. These studies indicate that digestion kinetics and specificities are determined both by virus type and by presence or absence of detergent. Presence of detergent accelerated digestion of both outer capsid proteins. In contrast to chymotrypsin digestion, which segregated sigma3 digestion from mu1 digestion, both proteins were rapidly digested by trypsin in the presence of detergent.

Putative autocleavage of reovirus mu1 protein in concert with outer-capsid disassembly and activation for membrane permeabilization

Capsid proteins of several different families of non-enveloped animal viruses with single-stranded RNA genomes undergo autocatalytic cleavage (autocleavage) as a maturation step in assembly. Similarly, the 76 kDa major outer-capsid protein mu1 of mammalian orthoreoviruses (reoviruses), which are non-enveloped and have double-stranded RNA genomes, undergoes putative autocleavage between residues 42 and 43, yielding N-terminal N-myristoylated fragment mu1N and C-terminal fragment mu1C. Cleavage at this site allows release of mu1N, which is thought to be critical for penetration of the host-cell membrane during cell entry. Most previous studies have suggested that cleavage at the mu1N/mu1C junction precedes addition to the outer capsid during virion assembly, such that only a small number of the mu1 subunits in mature virions remain uncleaved at that site (approximately 5%). In this study, we varied the conditions for disruption of virions before running the proteins on denaturing gels and in several circumstances recovered much higher levels of uncleaved mu1 (up to approximately 60%). Elements of the disruption conditions that allowed greater recovery of uncleaved protein were increased pH, absence of reducing agent, and decreased temperature. These same elements allowed comparably higher levels of the mu1delta protein, in which cleavage at the mu1N/delta junction has not occurred, to be recovered from particle uncoating intermediates in which mu1 had been previously cleaved by chymotrypsin in a distinct protease-sensitive region near residue 580. The capacity to recover higher levels of mu1delta following disruption of these particles for electrophoresis was lost, however, in concert with a series of structural changes that activate the particles for membrane permeabilization, suggesting that the putative autocleavage is itself one of these changes.

Putative autocleavage of outer capsid protein micro1, allowing release of myristoylated peptide micro1N during particle uncoating, is critical for cell entry by reovirus

Several nonenveloped animal viruses possess an autolytic capsid protein that is cleaved as a maturation step during assembly to yield infectious virions. The 76-kDa major outer capsid protein micro1 of mammalian orthoreoviruses (reoviruses) is also thought to be autocatalytically cleaved, yielding the virion-associated fragments micro1N (4 kDa; myristoylated) and micro1C (72 kDa). In this study, we found that micro1 cleavage to yield micro1N and micro1C was not required for outer capsid assembly but contributed greatly to the infectivity of the assembled particles. Recoated particles containing mutant, cleavage-defective micro1 (asparagine --> alanine substitution at amino acid 42) were competent for attachment; processing by exogenous proteases; structural changes in the outer capsid, including micro1 conformational change and sigma1 release; and transcriptase activation but failed to mediate membrane permeabilization either in vitro (no hemolysis) or in vivo (no coentry of the ribonucleotoxin alpha-sarcin). In addition, after these particles were allowed to enter cells, the delta region of micro1 continued to colocalize with viral core proteins in punctate structures, indicating that both elements remained bound together in particles and/or trapped within the same subcellular compartments, consistent with a defect in membrane penetration. If membrane penetration activity was supplied in trans by a coinfecting genome-deficient particle, the recoated particles with cleavage-defective micro1 displayed much higher levels of infectivity. These findings led us to propose a new uncoating intermediate, at which particles are trapped in the absence of micro1N/micro1C cleavage. We additionally showed that this cleavage allowed the myristoylated, N-terminal micro1N fragment to be released from reovirus particles during entry-related uncoating, analogous to the myristoylated, N-terminal VP4 fragment of picornavirus capsid proteins. The results thus suggest that hydrophobic peptide release following capsid protein autocleavage is part of a general mechanism of membrane penetration shared by several diverse nonenveloped animal viruses.

Effects of transforming growth factor-alpha (TGF-alpha) in vitro and in vivo on reovirus replication

We have utilized growth factors in in vitro and in vivo systems to examine the role of cellular proliferation in reovirus replication. In vitro, proliferating RIE-1 cells can be infected with whole reovirus virions, but are relatively resistant to infection once confluent (Go arrest). It has been shown that TGF-alpha, which signals through the EGF-receptor (EGF-R), is capable of dramatically increasing the number of RIE-1 cells entering the S-phase in the presence of additional serum factors. Stimulation of the EGF-R without serum results in minimal increases in cells entering the S-phase with a restriction in reovirus replication. Therefore, other factors in serum are essential for fully permissive infection. In vivo, we used metallothionein (MT) promoter/enhancer-TGF-alpha transgenic mice to study the effect of cytokine activation on reovirus type 1 infection. Virus replication decreased following oral infection in these transgenic mice at 1 month of age, concordant with increased mucin production. Titers of reovirus obtained from the livers of 1 year old transgenic mice were approximately 10-fold higher than titers obtained in control mice. Taken together, these data indicate that while growth factor activation ultimately leads to an increase in virus infectivity, other factors may be necessary for reovirus replication.

Mapping the site of guanylylation on VP1, the protein primer for infectious pancreatic necrosis virus RNA synthesis

VP1, the putative virion-associated RNA-dependent RNA polymerase (RdRp) of infectious pancreatic necrosis virus (IPNV) can be guanylylated in vitro whereupon it becomes a primer for in vitro RNA synthesis [Virology 208 (1995) 19]. The role of a template or other virion polypeptides in the reaction is unknown. To shed light on this question, his-tagged recombinant VP(1) (rVP1) was expressed both in Escherichia coli and insect cells and used in the guanylylation reaction. Unlike other viral VPg polypeptides, the purified rVP1 alone could guanylylate itself in vitro in a template-independent manner. Chemical and enzymatic cleavage in combination with site-directed mutagenesis mapped the site of guanylylation to serine 163. The purified rVP1 functioned as a primer as well as an RdRp in vitro, producing labeled dsRNA in the presence of [alpha(32)P] NTP and synthetically produced viral ss + RNA as a template. Only a single cycle of replication was observed and labeled VPg could be recovered from the dsRNA by RNase V(1) digestion. Denaturation of the dsRNA yielded genome-length labeled ssRNA, indicating that RNA synthesis was not initiated by 3'-end snap-back self-priming. Mutating serine 163 to alanine of rVP1 abolished both its self-guanylylating and polymerizing activity.

The delta region of outer-capsid protein micro 1 undergoes conformational change and release from reovirus particles during cell entry

Cell entry by reoviruses requires a large, transcriptionally active subvirion particle to gain access to the cytoplasm. The features of this particle have been the subject of debate, but three primary candidates-the infectious subvirion particle (ISVP), ISVP*, and core particle forms-that differ in whether putative membrane penetration protein micro 1 and adhesin sigma1 remain particle bound have been identified. Experiments with antibody reagents in this study yielded new information about the steps in particle disassembly during cell entry. Monoclonal antibodies specific for the delta region of micro 1 provided evidence for a conformational change in micro 1 and for release of the delta proteolytic fragment from entering particles. Antiserum raised against cores provided evidence for entry-related changes in particle structure and identified entering particles that largely lack the delta fragment inside cells. Antibodies specific for sigma1 showed that it is also largely shed from entering particles. Limited coimmunostaining with markers for late endosomes and lysosomes indicated the particles lacking delta and sigma1 did not localize to those subcellular compartments, and other observations suggested that both the particles and free delta were released into the cytoplasm. Essentially equivalent findings were obtained with native ISVPs and highly infectious recoated particles containing wild-type proteins. Poorly infectious recoated particles containing a hyperstable mutant form of micro 1, however, showed no evidence for the in vitro and intracellular changes in particle structure normally detected by antibodies, and these particles instead accumulated in late endosomes or lysosomes. Recoated particles with hyperstable micro 1 were also ineffective at mediating erythrocyte lysis in vitro and promoting alpha-sarcin coentry and intoxication of cells in cultures. Based on these and other findings, we propose that ISVP* is a transient intermediate in cell entry which mediates membrane penetration and is then further uncoated in the cytoplasm to yield particles, resembling cores, that largely lack the delta fragment of micro 1.

Cathepsin S supports acid-independent infection by some reoviruses

In murine fibroblasts, efficient proteolysis of reovirus outer capsid protein sigma3 during cell entry by virions requires the acid-dependent lysosomal cysteine protease cathepsin L. The importance of cathepsin L for infection of other cell types is unknown. Here we report that the acid-independent lysosomal cysteine protease cathepsin S mediates outer capsid processing in macrophage-like P388D cells. P388D cells supported infection by virions of strain Lang, but not strain c43. Genetic studies revealed that this difference is determined by S4, the viral gene segment that encodes sigma3. c43-derived subvirion particles that lack sigma3 replicated normally in P388D cells, suggesting that the difference in infectivity of Lang and c43 virions is at the level of sigma3 processing. Infection of P388D cells with Lang virions was inhibited by the broad spectrum cysteine protease inhibitor trans-epoxysuccinyl-l-leucylamido-(4-guanidino)butane but not by NH(4)Cl, which raises the endocytic pH and thereby inhibits acid-dependent proteases such as cathepsins L and B. Outer capsid processing and infection of P388D cells with Lang virions were also inhibited by a cathepsin S-specific inhibitor. Furthermore, in the presence of NH(4)Cl, cell lines engineered to express cathepsin S supported infection by Lang, but not c43, virions. Our results thus indicate that differences in susceptibility to cathepsin S-mediated sigma3 processing are responsible for strain differences in reovirus infection of macrophage-like P388D cells and other cathepsin S-expressing cells. Additionally, our data suggest that the acid dependence of reovirus infections of most other cell types may reflect the low pH requirement for the activities of most other lysosomal proteases rather, than some other acid-dependent aspect of cell entry.

Digestion pattern of reovirus outer capsid protein sigma3 determined by mass spectrometry

Reovirus is an enteric virus comprising eight structural proteins that form a double-layered capsid. During reovirus entry into cells, the outermost capsid layer (composed of proteins sigma3 and mu1C) is proteolytically processed to generate an infectious subviral particle (ISVP) that is subsequently uncoated to produce the transcriptionally active core particle. Kinetic studies suggest that protein sigma3 is rapidly removed from virus particles and then protein mu1C is cleaved. Initial cleavage of mu1C has been well described and generates an amino (N)-terminal delta peptide and a carboxyl (C)-terminal phi peptide. However, cleavage and removal of sigma3 is an extremely rapid event that has not been well defined. We have treated purified reovirus serotype 1 Lang virions with a variety of endoproteases. Time-course digestions with chymotrypsin, Glu-C, pepsin, and trypsin resulted in the initial generation of two peptides that were resolved in SDS-PAGE and analyzed by in-gel tryptic digestion and MALDI-Qq-TOFMS. Most tested proteases cut sigma3 within a "hypersensitive" region between amino acids 217 and 238. In addition, to gain a better understanding of the sequence of subsequent proteolytic events that result in generation of reovirus subviral particles, time-course digestions of purified particles were performed under physiologic salt conditions and released peptide fragments ranging from 500 to 5000 Da were directly analyzed by MALDI-Qq-TOFMS. Trypsin digestion initially released a peptide that corresponded to the C-terminus of mu1C, followed by a peptide that corresponded to amino acids 214-236 of the sigma3 protein. Other regions of mu1C were not observed until protein sigma3 was completely digested. Similar experiments with Glu-C indicated the hypersensitive region of sigma3 was cut first when virions were treated at pH values of 4.5 or 7.4, but treatment of virions with pepsin at pH 3.0 released different sigma3 peptides, suggesting acid-induced conformational changes in this outer capsid protein. These studies also revealed that the N-terminus of sigma3 is acetylated.

Strategy for nonenveloped virus entry: a hydrophobic conformer of the reovirus membrane penetration protein micro 1 mediates membrane disruption

The mechanisms employed by nonenveloped animal viruses to penetrate the membranes of their host cells remain enigmatic. Membrane penetration by the nonenveloped mammalian reoviruses is believed to deliver a partially uncoated, but still large ( approximately 70-nm), particle with active transcriptases for viral mRNA synthesis directly into the cytoplasm. This process is likely initiated by a particle form that resembles infectious subvirion particles (ISVPs), disassembly intermediates produced from virions by proteolytic uncoating. Consistent with that idea, ISVPs, but not virions, can induce disruption of membranes in vitro. Both activities ascribed to ISVP-like particles, membrane disruption in vitro and membrane penetration within cells, are linked to N-myristoylated outer-capsid protein micro 1, present in 600 copies at the surfaces of ISVPs. To understand how micro 1 fulfills its role as the reovirus penetration protein, we monitored changes in ISVPs during the permeabilization of red blood cells induced by these particles. Hemolysis was preceded by a major structural transition in ISVPs, characterized by conformational change in micro 1 and elution of fibrous attachment protein sigma 1. The altered conformer of micro 1 was required for hemolysis and was markedly hydrophobic. The structural transition in ISVPs was further accompanied by derepression of genome-dependent mRNA synthesis by the particle-associated transcriptases. We propose a model for reovirus entry in which (i) primed and triggered conformational changes, analogous to those in enveloped-virus fusion proteins, generate a hydrophobic micro 1 conformer capable of inserting into and disrupting cell membranes and (ii) activation of the viral particles for membrane interaction and mRNA synthesis are concurrent events. Reoviruses provide an opportune system for defining the molecular details of membrane penetration by a large nonenveloped animal virus.

Sites and determinants of early cleavages in the proteolytic processing pathway of reovirus surface protein sigma3

Entry of mammalian reovirus virions into target cells requires proteolytic processing of surface protein sigma3. In the virion, sigma3 mostly covers the membrane-penetration protein mu1, appearing to keep it in an inactive form and to prevent it from interacting with the cellular membrane until the proper time in infection. The molecular mechanism by which sigma3 maintains mu1 in this inactive state and the structural changes that accompany sigma3 processing and mu1 activation, however, are not well understood. In this study we characterized the early steps in sigma3 processing and determined their effects on mu1 function and particle infectivity. We identified two regions of high protease sensitivity, "hypersensitive" regions located at residues 208 to 214 and 238 to 244, within which all proteases tested selectively cleaved sigma3 as an early step in processing. Further processing of sigma3 was required for infection, consistent with the fact that the fragments resulting from these early cleavages remained bound to the particles. Reovirus type 1 Lang (T1L), type 3 Dearing (T3D), and T1L x T3D reassortant virions differed in the sites of early sigma3 cleavage, with T1L sigma3 being cleaved mainly at residues 238 to 244 and T3D sigma3 being cleaved mainly at residues 208 to 214. These virions also differed in the rates at which the early cleavages occurred, with cleavage of T1L sigma3 occurring faster than cleavage of T3D sigma3. Analyses using chimeric and site-directed mutants of recombinant sigma3 identified carboxy-proximal residues 344, 347, and 353 as the primary determinants of these strain differences. The spatial relationships between these more carboxy-proximal residues and the hypersensitive regions were discerned from the sigma3 crystal structure. The results indicate that proteolytic processing of sigma3 during reovirus disassembly is a multistep pathway with a number of molecular determinants.

Complete in vitro assembly of the reovirus outer capsid produces highly infectious particles suitable for genetic studies of the receptor-binding protein

Mammalian reoviruses, prototype members of the Reoviridae family of nonenveloped double-stranded RNA viruses, use at least three proteins--sigma1, mu1, and sigma3--to enter host cells. sigma1, a major determinant of cell tropism, mediates viral attachment to cellular receptors. Studies of sigma1 functions in reovirus entry have been restricted by the lack of methodologies to produce infectious virions containing engineered mutations in viral proteins. To mitigate this problem, we produced virion-like particles by "recoating" genome-containing core particles that lacked sigma1, mu1, and sigma3 with recombinant forms of these proteins in vitro. Image reconstructions from cryoelectron micrographs of the recoated particles revealed that they closely resembled native virions in three-dimensional structure, including features attributable to sigma1. The recoated particles bound to and infected cultured cells in a sigma1-dependent manner and were approximately 1 million times as infectious as cores and 0.5 times as infectious as native virions. Experiments with recoated particles containing recombinant sigma1 from either of two different reovirus strains confirmed that differences in cell attachment and infectivity previously observed between those strains are determined by the sigma1 protein. Additional experiments showed that recoated particles containing sigma1 proteins with engineered mutations can be used to analyze the effects of such mutations on the roles of particle-bound sigma1 in infection. The results demonstrate a powerful new system for molecular genetic dissections of sigma1 with respect to its structure, assembly into particles, and roles in entry.

Structure of the reovirus outer capsid and dsRNA-binding protein sigma3 at 1.8 A resolution

The crystallographically determined structure of the reovirus outer capsid protein sigma3 reveals a two-lobed structure organized around a long central helix. The smaller of the two lobes includes a CCHC zinc-binding site. Residues that vary between strains and serotypes lie mainly on one surface of the protein; residues on the opposite surface are conserved. From a fit of this model to a reconstruction of the whole virion from electron cryomicroscopy, we propose that each sigma3 subunit is positioned with the small lobe anchoring it to the protein mu1 on the surface of the virion, and the large lobe, the site of initial cleavages during entry-related proteolytic disassembly, protruding outwards. The surface containing variable residues faces solvent. The crystallographic asymmetric unit contains two sigma3 subunits, tightly associated as a dimer. One broad surface of the dimer has a positively charged surface patch, which extends across the dyad. In infected cells, sigma3 binds dsRNA and inhibits the interferon response. The location and extent of the positively charged surface patch suggest that the dimer is the RNA-binding form of sigma3.

Transcriptional activities of reovirus RNA polymerase in recoated cores. Initiation and elongation are regulated by separate mechanisms

The particle-associated reovirus polymerase synthesizes mRNA within only certain viral particle types. Reovirus cores, subviral particles lacking outer capsid proteins mu1, sigma3, and sigma1, produce mRNA and abortive transcripts. Reovirus virions, which contain complete outer capsids, cannot produce mRNA and produce few abortive transcripts. Recoated cores are virion-like particles generated by the addition of recombinant outer capsid proteins to cores. We used recoated cores to analyze transcriptional regulation by reovirus outer capsid proteins. Partially recoated particles, containing less than virion amounts of mu1 and sigma3, synthesized mRNA at levels inversely proportional to outer capsid protein levels. Fully recoated cores exhibited undetectable mRNA synthesis levels, as did virions. However, recoated cores produced high levels of abortive transcripts. Recoated core abortive transcripts remained particle-associated and appeared to inhibit further abortive transcript production. Proteolysis of recoated cores removing mu1 and sigma3 released accumulated abortive transcripts and relieved inhibition of mRNA and abortive transcript synthesis. These results suggest transcriptional elongation, but not initiation, is blocked by virion-like amounts of mu1 and sigma3. Particle-associated abortive transcripts may down-regulate transcriptional initiation. Minor outer capsid protein sigma1 had no demonstrable effect on transcriptional activities. Transcriptional regulation may ensure progeny virions do not compete with transcribing particles for ribonucleoside triphosphates.

Trypsin-induced structural transformation in aquareovirus

Aquareovirus, a member of the family Reoviridae, is a large virus with multiple capsid layers surrounding a genome composed of 11 segments of double-stranded RNA. Biochemical studies have shown that treatment with the proteolytic agent trypsin significantly alters the infectivity of the virus. The most infectious stage of the virus is produced by a 5-min treatment with trypsin. However, prolonged trypsin treatment almost completely abolishes the infectivity. We have used three-dimensional electron cryomicroscopy to gain insight into the structural basis of protease-induced alterations in infectivity by examining the structural changes in the virion at various time intervals of trypsin treatment. Our data show that after 5 min of trypsinization, projection-like spikes made of VP7 (35 kDa), associated with the underlying trimeric subunits, are completely removed. Concurrent with the removal of VP7, conformational changes are observed in the trimeric subunit composed of putative VP5 (71 kDa). The removal of VP7 and the accompanied structural changes may expose regions in the putative VP5 important for cell entry processes. Prolonged trypsinization not only entirely removes the outer capsid layer, producing the poorly infectious core particle, but also causes significant conformational changes in the turret protein. These changes result in shortening of the turret and narrowing of its central channel. The turret, as in orthoreoviruses, is likely to play a major role in the capping and translocation of mRNA during transcription, and the observed conformational flexibility in the turret protein may have implications in rendering the particle transcriptionally active or inactive.

Reovirus nonstructural protein muNS binds to core particles but does not inhibit their transcription and capping activities

Previous studies provided evidence that nonstructural protein muNS of mammalian reoviruses is present in particle assembly intermediates isolated from infected cells. Morgan and Zweerink (Virology 68:455-466, 1975) showed that a subset of these intermediates, which can synthesize the viral plus strand RNA transcripts in vitro, comprise core-like particles plus large amounts of muNS. Given the possible role of muNS in particle assembly and/or transcription implied by those findings, we tested whether recombinant muNS can bind to cores in vitro. The muNS protein bound to cores, but not to two particle forms, virions and intermediate subvirion particles, that contain additional outer-capsid proteins. Incubating cores with increasing amounts of muNS resulted in particle complexes of progressively decreasing buoyant density, approaching the density of protein alone when very large amounts of muNS were bound. Thus, the muNS-core interaction did not exhibit saturation or a defined stoichiometry. Negative-stain electron microscopy of the muNS-bound cores revealed that the cores were intact and linked together in large complexes by an amorphous density, which we ascribe to muNS. The muNS-core complexes retained the capacity to synthesize the viral plus strand transcripts as well as the capacity to add methylated caps to the 5' ends of the transcripts. In vitro competition assays showed that mixing muNS with cores greatly reduced the formation of recoated cores by stoichiometric binding of outer-capsid proteins mu1 and sigma3. These findings are consistent with the presence of muNS in transcriptase particles as described previously and suggest that, by binding to cores in the infected cell, muNS may block or delay outer-capsid assembly and allow continued transcription by these particles.

Adding the third dimension to virus life cycles: three-dimensional reconstruction of icosahedral viruses from cryo-electron micrographs

Viruses are cellular parasites. The linkage between viral and host functions makes the study of a viral life cycle an important key to cellular functions. A deeper understanding of many aspects of viral life cycles has emerged from coordinated molecular and structural studies carried out with a wide range of viral pathogens. Structural studies of viruses by means of cryo-electron microscopy and three-dimensional image reconstruction methods have grown explosively in the last decade. Here we review the use of cryo-electron microscopy for the determination of the structures of a number of icosahedral viruses. These studies span more than 20 virus families. Representative examples illustrate the use of moderate- to low-resolution (7- to 35-A) structural analyses to illuminate functional aspects of viral life cycles including host recognition, viral attachment, entry, genome release, viral transcription, translation, proassembly, maturation, release, and transmission, as well as mechanisms of host defense. The success of cryo-electron microscopy in combination with three-dimensional image reconstruction for icosahedral viruses provides a firm foundation for future explorations of more-complex viral pathogens, including the vast number that are nonspherical or nonsymmetrical.

In vitro recoating of reovirus cores with baculovirus-expressed outer-capsid proteins mu1 and sigma3

Reovirus outer-capsid proteins mu1, sigma3, and sigma1 are thought to be assembled onto nascent core-like particles within infected cells, leading to the production of progeny virions. Consistent with this model, we report the in vitro assembly of baculovirus-expressed mu1 and sigma3 onto purified cores that lack mu1, sigma3, and sigma1. The resulting particles (recoated cores, or r-cores) closely resembled native virions in protein composition (except for lacking cell attachment protein sigma1), buoyant density, and particle morphology by scanning cryoelectron microscopy. Transmission cryoelectron microscopy and image reconstruction of r-cores confirmed that they closely resembled virions in the structure of the outer capsid and revealed that assembly of mu1 and sigma3 onto cores had induced rearrangement of the pentameric lambda2 turrets into a conformation approximating that in virions. r-cores, like virions, underwent proteolytic conversion to particles resembling native ISVPs (infectious subvirion particles) in protein composition, particle morphology, and capacity to permeabilize membranes in vitro. r-cores were 250- to 500-fold more infectious than cores in murine L cells and, like virions but not ISVPs or cores, were inhibited from productively infecting these cells by the presence of either NH4Cl or E-64. The latter results suggest that r-cores and virions used similar routes of entry into L cells, including processing by lysosomal cysteine proteinases, even though the former particles lacked the sigma1 protein. To examine the utility of r-cores for genetic dissections of mu1 functions in reovirus entry, we generated r-cores containing a mutant form of mu1 that had been engineered to resist cleavage at the delta:phi junction during conversion to ISVP-like particles by chymotrypsin in vitro. Despite their deficit in delta:phi cleavage, these ISVP-like particles were fully competent to permeabilize membranes in vitro and to infect L cells in the presence of NH4Cl, providing new evidence that this cleavage is dispensable for productive infection.

Reovirus virion-like particles obtained by recoating infectious subvirion particles with baculovirus-expressed sigma3 protein: an approach for analyzing sigma3 functions during virus entry

Structure-function studies with mammalian reoviruses have been limited by the lack of a reverse-genetic system for engineering mutations into the viral genome. To circumvent this limitation in a partial way for the major outer-capsid protein sigma3, we obtained in vitro assembly of large numbers of virion-like particles by binding baculovirus-expressed sigma3 protein to infectious subvirion particles (ISVPs) that lack sigma3. A level of sigma3 binding approaching 100% of that in native virions was routinely achieved. The sigma3 coat in these recoated ISVPs (rcISVPs) appeared very similar to that in virions by electron microscopy and three-dimensional image reconstruction. rcISVPs retained full infectivity in murine L cells, allowing their use to study sigma3 functions in virus entry. Upon infection, rcISVPs behaved identically to virions in showing an extended lag phase prior to exponential growth and in being inhibited from entering cells by either the weak base NH4Cl or the cysteine proteinase inhibitor E-64. rcISVPs also mimicked virions in being incapable of in vitro activation to mediate lysis of erythrocytes and transcription of the viral mRNAs. Last, rcISVPs behaved like virions in showing minor loss of infectivity at 52 degrees C. Since rcISVPs contain virion-like levels of sigma3 but contain outer-capsid protein mu1/mu1C mostly cleaved at the delta-phi junction as in ISVPs, the fact that rcISVPs behaved like virions (and not ISVPs) in all of the assays that we performed suggests that sigma3, and not the delta-phi cleavage of mu1/mu1C, determines the observed differences in behavior between virions and ISVPs. To demonstrate the applicability of rcISVPs for genetic studies of protein functions in reovirus entry (an approach that we call recoating genetics), we used chimeric sigma3 proteins to localize the primary determinants of a strain-dependent difference in sigma3 cleavage rate to a carboxy-terminal region of the ISVP-bound protein.

Characterization of the thermosensitive ts453 reovirus mutant: increased dsRNA binding of sigma 3 protein correlates with interferon resistance

The mutation harbored by the reovirus ts453 thermosensitive mutant has been assigned to the S4 gene encoding the major outer capsid protein sigma 3. Previous gene sequencing has identified a nonconservative amino acid substitution located near the zinc finger of sigma 3 protein in the mutant. Coexpression in COS cells of the sigma 3 protein presenting this amino acid substitution (N16K), together with the other major capsid protein mu 1, has also revealed an altered interaction between the two proteins; this altered interaction prevents the sigma 3-dependent cleavage of mu 1 to mu 1C. This could explain the lack of outer capsid assembly observed during ts453 virus infection at nonpermissive temperature. In the present study, we pursued the characterization of this mutant sigma 3 protein. Although the N16K mutation is located close to the zinc finger region, it did not affect the ability of the protein to bind zinc. In contrast, this mutation, as well as mutations within the zinc finger motif itself, can increase the binding of the protein to double-stranded RNA (dsRNA). It also appears that the N16K mutant protein is more efficiently transported to the nucleus than the wild-type protein, an observation consistent with the postulated role of dsRNA binding in sigma 3 nuclear presence. The lack of association with mu 1, and/or the increased dsRNA-binding activity of sigma 3, could be responsible for a partial resistance of the ts453 virus to interferon treatment and this could have important consequences in the context of protein synthesis regulation during natural reovirus infection.

Enhancement of aquareovirus infectivity by treatment with proteases: mechanism of action

The effects of protease digestion on the polypeptide composition and on the infectivity of striped bass virus, an aquareovirus, were examined. Both trypsin and chymotrypsin enhanced the infectivity of the virus. Enhancement of infectivity was correlated with the digestion of the outer capsid protein, VP7. These studies support the assertion that VP7 is the outermost capsid protein and suggest that VP4 and VP5 are exposed on the outer surface of infectious particles. The possible role of VP7 in the variation in virulence observed among aquareovirus isolates is discussed.

Stoichiometry of reovirus structural proteins in virus, ISVP, and core particles

All eight reovirus structural proteins were resolved in a new tris, glycine, and urea (TGU) electrophoretic gel system. The specific identities of proteins were determined immunologically, biochemically, and genetically. Structural proteins of reovirus type 1 Lang had different mobilities in the TGU gel than did type 3 Dearing proteins. Intertypic reassortant viruses that contained various combinations of parental genes were used to identify each of the viral protein bands. Type 1 Lang virions were metabolically-labelled with either 3H-amino acids or 35S-methionine/cysteine and gradient purified. Aliquots of purified virions were treated to generate infectious subviral particles (ISVPs) and core particles. Radiolabelled virus, ISVP, and core proteins were resolved in the TGU gel and protein band intensities were used to determine copy numbers of each structural protein. These studies confirmed the copy numbers and locations of most reovirus proteins. However, important new findings include the discovery that virions contain approximately 120 copies of major core protein sigma 2 and 20 copies of the polymerase cofactor protein mu 2, and ISVP particles contain about 24 copies of mu 1 C that has not been processed to the delta peptide. These data are used to generate a new model of the arrangement of structural proteins with the reovirus particle.