Capsid protein VP4 of poliovirus is N-myristoylated

Myristoylated GRASP55 dimerizes in the presence of model membranes

The Golgi Reassembly and Stacking Proteins (GRASPs) are engaged in various functions within the cell, both in unconventional secretion mechanisms and structuring and organizing the Golgi apparatus. Understanding their specific role in each situation still requires more structural and functional data at the molecular level. GRASP55 is one of the GRASP members in mammals, anchored to the membrane via the myristoylation of a Gly residue at its N-terminus. Therefore, co-translational modifications, such as myristoylation, are fundamental when considering a strategy to obtain detailed information on the interactions between GRASP55 and membranes. Despite its functional relevance, the N-terminal myristoylation has been underappreciated in the studies reported to date, compromising the previously proposed models for GRASP-membrane interactions. Here, we investigated the synergy between the presence of the membrane and the formation of oligomeric structures of myristoylated GRASP55, using a series of biophysical techniques to perform the structural characterization of the lipidated GRASP55 and its interaction with biological lipid model membranes. Our data fulfill an unexplored gap: the adequate evaluation of the presence of lipidations and lipid membranes on the structure-function dyad of GRASPs.Communicated by Ramaswamy H. Sarma.

Differences in Oligomerization of the SARS-CoV-2 Envelope Protein, Poliovirus VP4, and HIV Vpu

Viroporins constitute a class of viral membrane proteins with diverse roles in the viral life cycle. They can self-assemble and form pores within the bilayer that transport substrates, such as ions and genetic material, that are critical to the viral infection cycle. However, there is little known about the oligomeric state of most viroporins. Here, we use native mass spectrometry in detergent micelles to uncover the patterns of oligomerization of the full-length SARS-CoV-2 envelope (E) protein, poliovirus VP4, and HIV Vpu. Our data suggest that the E protein is a specific dimer, VP4 is exclusively monomeric, and Vpu assembles into a polydisperse mixture of oligomers under these conditions. Overall, these results revealed the diversity in the oligomerization of viroporins, which has implications for the mechanisms of their biological functions as well as their potential as therapeutic targets.

Differences in Oligomerization of the SARS-CoV-2 Envelope Protein, Poliovirus VP4, and HIV Vpu

Viroporins constitute a class of viral membrane proteins with diverse roles in the viral life cycle. They can self-assemble and form pores within the bilayer that transport substrates, such as ions and genetic material, that are critical to the viral infection cycle. However, there is little known about the oligomeric state of most viroporins. Here, we use native mass spectrometry (MS) in detergent micelles to uncover the patterns of oligomerization of the full-length SARS-CoV-2 envelope (E) protein, poliovirus VP4, and HIV Vpu. Our data suggest that the E protein is a specific dimer, VP4 is exclusively monomeric, and Vpu assembles into a polydisperse mixture of oligomers under these conditions. Overall, these results revealed the diversity in the oligomerization of viroporins, which has implications for mechanisms of their biological functions as well as their potential as therapeutic targets.

Efficacy of an isoxazole-3-carboxamide analog of pleconaril in mouse models of Enterovirus-D68 and Coxsackie B5

Enteroviruses (EV) cause a number of life-threatening infectious diseases. EV-D68 is known to cause respiratory illness in children that can lead to acute flaccid myelitis. Coxsackievirus B5 (CVB5) is commonly associated with hand-foot-mouth disease. There is no antiviral treatment available for either. We have developed an isoxazole-3-carboxamide analog of pleconaril (11526092) which displayed potent inhibition of EV-D68 (IC50 58 nM) as well as other enteroviruses including the pleconaril-resistant Coxsackievirus B3-Woodruff (IC50 6-20 nM) and CVB5 (EC50 1 nM). Cryo-electron microscopy structures of EV-D68 in complex with 11526092 and pleconaril demonstrate destabilization of the EV-D68 MO strain VP1 loop, and a strain-dependent effect. A mouse respiratory model of EV-D68 infection, showed 3-log decreased viremia, favorable cytokine response, as well as statistically significant 1-log reduction in lung titer reduction at day 5 after treatment with 11526092. An acute flaccid myelitis neurological infection model did not show efficacy. 11526092 was tested in a mouse model of CVB5 infection and showed a 4-log TCID50 reduction in the pancreas. In summary, 11526092 represents a potent in vitro inhibitor of EV with in vivo efficacy in EV-D68 and CVB5 animal models suggesting it is worthy of further evaluation as a potential broad-spectrum antiviral therapeutic against EV.

The Art of Viral Membrane Fusion and Penetration

As obligate pathogens, viruses have developed diverse mechanisms to deliver their genome across host cell membranes to sites of virus replication. While enveloped viruses utilize viral fusion proteins to accomplish fusion of their envelope with the cellular membrane, non-enveloped viruses rely on machinery that causes local membrane ruptures and creates an opening through which the capsid or viral genome is released. Both membrane fusion and membrane penetration take place at the plasma membrane or in intracellular compartments, often involving the engagement of the cellular machinery and antagonism of host restriction factors. Enveloped and non-enveloped viruses have evolved intricate mechanisms to enable virus uncoating and modulation of membrane fusion in a spatiotemporally controlled manner. This chapter summarizes and discusses the current state of understanding of the mechanisms of viral membrane fusion and penetration. The focus is on the role of lipids, viral scaffold uncoating, viral membrane fusion inhibitors, and host restriction factors as physicochemical modulators. In addition, recent advances in visualizing and detecting viral membrane fusion and penetration using cryo-electron microscopy methods are presented.

Plant-made poliovirus vaccines - Safe alternatives for global vaccination

Human polioviruses are highly infectious viruses that are spread mainly through the fecal-oral route. Infection of the central nervous system frequently results in irreversible paralysis, a disease called poliomyelitis. Children under five years are mainly affected if they have not acquired immunity through natural infection or via vaccination. Current polio vaccines comprise the injectable inactivated polio vaccine (IPV, also called the Salk vaccine) and the live-attenuated oral polio vaccine (OPV, also called the Sabin vaccine). The main limitations of the IPV are the reduced protection at the intestinal mucosa, the site of virus replication, and the high costs for manufacturing due to use of live viruses. While the OPV is more effective and stimulates mucosal immunity, it is manufactured using live-attenuated strains that can revert into pathogenic viruses resulting in major safety concerns and vaccine-derived outbreaks. During the last fifteen years, plant-based poliovirus vaccines have been explored by several groups as a safe and low-cost alternative, and promising results in protection against challenges with viruses and induction of neutralizing antibodies have been obtained. However, low yields and a high frequency in dose administration highlight the need for improvements in polioviral antigen production. In this review, we provide insights into recent efforts to develop plant-made poliovirus candidates, with an emphasis on strategies to optimize the production of viral antigens.

Enzymatic Production of Chondroitin Oligosaccharides and Its Sulfate Derivatives

Chondroitin sulfate (CS) has a wide range of physiological functions and clinical applications. However, the biosynthesis of chondroitin oligosaccharides (o-CHs) and sulfate derivatives with specific length is always challenging. Herein, we report enzymatic strategies for producing homogeneous o-CHs and its sulfate derivatives from microbial sourced chondroitin. Chondroitin disaccharides, tetrasaccharides, hexasaccharides, octasaccharides, and decasaccharides with defined structure were produced by controllably depolymerizing microbial sourced chondroitin with an engineered chondroitinase ABC I. The highest conversion rates of the above corresponding o-CHs were 65.5%, 32.1%, 12.7%, 7.2%, and 16.3%, respectively. A new efficient enzymatic sulfation system that directly initiates from adenosine 5′-triphosphate (ATP) and sulfate was developed and improved the sulfation of chondroitin from 8.3% to 85.8% by optimizing the temperature, sulfate and ATP concentration. o-CHs decasaccharide, octasaccharide, hexasaccharide, tetrasaccharide and disaccharide were modified and the corresponding sulfate derivatives with one sulfate group were prepared. The enzymatic approaches constructed here for preparing o-CHs and its sulfate derivatives pave the way for the study of structure-activity relationship and applications.

RP-UPLC method for Oncolytic Coxsackievirus Viral Protein Separation and Empty to Full Capsid Quantification

Oncolytic virus immunotherapy is emerging as a novel therapeutic approach for cancer treatment. Immunotherapy clinical drug candidate V937 is currently in phase I/II clinical trials and consists of a proprietary formulation of Coxsackievirus A21 (CVA21), which specifically infects and lyses cells with overexpressed ICAM-1 receptors in a range of tumors. Mature Coxsackievirus virions, consisting of four structural virion proteins, (VPs) VP1, VP2, VP3, and VP4, and the RNA genome, are the only viral particles capable of being infectious. In addition to mature virions, empty procapsids with VPs, VP0, VP1, and VP3, and other virus particles are produced in V937 production cell culture. Viral protein VP0 is cleaved into VP2 and VP4 after RNA genome encapsidation to form mature virions. Clearance of viral particles containing VP0, and quantification of viral protein distribution are important in V937 downstream processing. Existing analytical methods for the characterization of viral proteins and particles may lack sensitivity or are low throughput. We developed a sensitive and robust reverse-phase ultra-performance chromatography method to separate, identify, and quantify all five CVA21 VPs. Quantification of virus capsid concentration and empty/full capsid ratio was achieved with good linearity, accuracy, and precision. ClinicalTrials.gov ID: NCT04521621 and NCT04152863.

Mapping the myristoylome through a complete understanding of protein myristoylation biochemistry

Protein myristoylation is a C14 fatty acid modification found in all living organisms. Myristoylation tags either the N-terminal alpha groups of cysteine or glycine residues through amide bonds or lysine and cysteine side chains directly or indirectly via glycerol thioester and ester linkages. Before transfer to proteins, myristate must be activated into myristoyl coenzyme A in eukaryotes or, in bacteria, to derivatives like phosphatidylethanolamine. Myristate originates through de novo biosynthesis (e.g., plants), from external uptake (e.g., human tissues), or from mixed origins (e.g., unicellular organisms). Myristate usually serves as a molecular anchor, allowing tagged proteins to be targeted to membranes and travel across endomembrane networks in eukaryotes. In this review, we describe and discuss the metabolic origins of protein-bound myristate. We review strategies for in vivo protein labeling that take advantage of click-chemistry with reactive analogs, and we discuss new approaches to the proteome-wide discovery of myristate-containing proteins. The machineries of myristoylation are described, along with how protein targets can be generated directly from translating precursors or from processed proteins. Few myristoylation catalysts are currently described, with only N-myristoyltransferase described to date in eukaryotes. Finally, we describe how viruses and bacteria hijack and exploit myristoylation for their pathogenicity.

Picornaviruses: A View from 3A

Picornaviruses are comprised of a positive-sense RNA genome surrounded by a protein shell (or capsid). They are ubiquitous in vertebrates and cause a wide range of important human and animal diseases. The genome encodes a single large polyprotein that is processed to structural (capsid) and non-structural proteins. The non-structural proteins have key functions within the viral replication complex. Some, such as 3D^pol (the RNA dependent RNA polymerase) have conserved functions and participate directly in replicating the viral genome, whereas others, such as 3A, have accessory roles. The 3A proteins are highly divergent across the Picornaviridae and have specific roles both within and outside of the replication complex, which differ between the different genera. These roles include subverting host proteins to generate replication organelles and inhibition of cellular functions (such as protein secretion) to influence virus replication efficiency and the host response to infection. In addition, 3A proteins are associated with the determination of host range. However, recent observations have challenged some of the roles assigned to 3A and suggest that other viral proteins may carry them out. In this review, we revisit the roles of 3A in the picornavirus life cycle. The 3AB precursor and mature 3A have distinct functions during viral replication and, therefore, we have also included discussion of some of the roles assigned to 3AB.

Myristoylation of EV71 VP4 is Essential for Infectivity and Interaction with Membrane Structure

The Enterovirus 71 (EV71) VP4 is co-translationally linked to myristic acid at its amino-terminal glycine residue. However, the role of this myristoylation in the EV71 life cycle remains largely unknown. To investigate this issue, we developed a myristoylation-deficient virus and reporter (luciferase) pseudovirus with a Gly-to-Ala mutation (G2A) on EV71 VP4. When transfecting the EV71-G2A genome encoding plasmid in cells, the loss of myristoylation on VP4 did not affect the expression of viral proteins and the virus morphology, however, it did significantly influence viral infectivity. Further, in myristoylation-deficient reporter pseudovirus-infected cells, the luciferase activity and viral genome RNA decreased significantly as compared to that of wild type virus; however, cytopathic effect and viral capsid proteins were not detected in myristoylation-deficient virus-infected cells. Also, although myristoylation-deficient viral RNA and proteins were detected in the second blind passage of infection, they were much fewer in number compared to that of the wild type virus. The replication of genomic RNA and negative-strand viral RNA were both blocked in myristoylation-deficient viruses, suggesting that myristoylation affects viral genome RNA release from capsid to cytoplasm. Besides, loss of myristoylation on VP4 altered the distribution of VP4-green fluorescent protein protein, which disappeared from the membrane structure fraction. Finally, a liposome leakage assay showed that EV71 myristoylation mediates the permeability of the model membrane. Hence, the amino-terminal myristoylation of VP4 is pivotal to EV71 infection and capsid-membrane structure interaction. This study provides novel molecular mechanisms regarding EV71 infection and potential molecular targets for antiviral drug design.

The life cycle of non-polio enteroviruses and how to target it

The genus Enterovirus (EV) of the family Picornaviridae includes poliovirus, coxsackieviruses, echoviruses, numbered enteroviruses and rhinoviruses. These diverse viruses cause a variety of diseases, including non-specific febrile illness, hand-foot-and-mouth disease, neonatal sepsis-like disease, encephalitis, paralysis and respiratory diseases. In recent years, several non-polio enteroviruses (NPEVs) have emerged as serious public health concerns. These include EV-A71, which has caused epidemics of hand-foot-and-mouth disease in Southeast Asia, and EV-D68, which recently caused a large outbreak of severe lower respiratory tract disease in North America. Infections with these viruses are associated with severe neurological complications. For decades, most research has focused on poliovirus, but in recent years, our knowledge of NPEVs has increased considerably. In this Review, we summarize recent insights from enterovirus research with a special emphasis on NPEVs. We discuss virion structures, host-receptor interactions, viral uncoating and the recent discovery of a universal enterovirus host factor that is involved in viral genome release. Moreover, we briefly explain the mechanisms of viral genome replication, virion assembly and virion release, and describe potential targets for antiviral therapy. We reflect on how these recent discoveries may help the development of antiviral therapies and vaccines.

Evolutionary and Structural Overview of Human Picornavirus Capsid Antibody Evasion

Picornaviruses constitute one of the most relevant viral groups according to their impact on human and animal health. Etiologic agents of a broad spectrum of illnesses with a clinical presentation that ranges from asymptomatic to fatal disease, they have been the cause of uncountable epidemics throughout history. Picornaviruses are small naked RNA-positive single-stranded viruses that include some of the most important pillars in the development of virology, comprising poliovirus, rhinovirus, and hepatitis A virus. Picornavirus infectious particles use the fecal-oral or respiratory routes as primary modes of transmission. In this regard, successful viral spread relies on the capability of viral capsids to (i) shelter the viral genome, (ii) display molecular determinants for cell receptor recognition, (iii) facilitate efficient genome delivery, and (iv) escape from the immune system. Importantly, picornaviruses display a substantial amount of genetic variability driven by both mutation and recombination. Therefore, the outcome of their replication results in the emergence of a genetically diverse cloud of individuals presenting phenotypic variance. The host humoral response against the capsid protein represents the most active immune pressure and primary weapon to control the infection. Since the preservation of the capsid function is deeply rooted in the virus evolutionary dynamics, here we review the current structural evidence focused on capsid antibody evasion mechanisms from that perspective.

Lipopeptides: a novel antigen repertoire presented by major histocompatibility complex class I molecules

Post-translationally modified peptides, such as those containing either phosphorylated or O-glycosylated serine/threonine residues, may be presented to cytotoxic T lymphocytes (CTLs) by MHC class I molecules. Most of these modified peptides are captured in the MHC class I groove in a similar manner to that for unmodified peptides. N-Myristoylated 5-mer lipopeptides have recently been identified as a novel chemical class of MHC class I-presented antigens. The rhesus classical MHC class I allele, Mamu-B*098, was found to be capable of binding N-myristoylated lipopeptides and presenting them to CTLs. A high-resolution X-ray crystallographic analysis of the Mamu-B*098:lipopeptide complex revealed that the myristic group as well as conserved C-terminal serine residue of the lipopeptide ligand functioned as anchors, whereas the short stretch of three amino acid residues located in the middle of the lipopeptides was only exposed externally with the potential to interact directly with specific T-cell receptors. Therefore, the modes of lipopeptide-ligand interactions with MHC class I and with T-cell receptors are novel and fundamentally distinct from that for MHC class I-presented peptides. Another lipopeptide-presenting MHC class I allele has now been identified, leading us to the prediction that MHC class I molecules may be separated on a functional basis into two groups: one presenting long peptides and the other presenting short lipopeptides. Since the N-myristoylation of viral proteins is often linked to pathogenesis, CTLs capable of sensing N-myristoylation may serve to control pathogenic viruses, raising the possibility for the development of a new type of lipopeptide vaccine.

A Novel Open and Infectious Form of Echovirus 1

One of the hallmarks of enterovirus genome delivery is the formation of an uncoating intermediate particle. Based on previous studies of mostly heated picornavirus particles, intermediate particles were shown to have externalized the innermost capsid protein (VP4) and exposed the N terminus of VP1 and to have reduced infectivity. Here, in addition to the native and intact particle type, we have identified another type of infectious echovirus 1 (E1) particle population during infection. Our results show that E1 is slightly altered during entry, which leads to the broadening of the major virion peak in the sucrose gradient. In contrast, CsCl gradient separation revealed that in addition to the light intact and empty particles, a dense particle peak appeared during infection in cells. When the broad peak from the sucrose gradient was subjected to a CsCl gradient, it revealed light and dense particles, further suggesting that the shoulder represents the dense particle. The dense particle was permeable to SYBR green II, it still contained most of its VP4, and it was able to bind to its receptor α2β1 integrin and showed high infectivity. A thermal assay further showed that the α2β1 integrin binding domain (I-domain) stabilized the virus particle. Finally, heating E1 particles to superphysiological temperatures produced more fragile particles with aberrant ultrastructural appearances, suggesting that they are distinct from the dense E1 particles. These results describe a more open and highly infectious E1 particle that is naturally produced during infection and may represent a novel form of an uncoating intermediate.

IMPORTANCE

In this paper, we have characterized a possible uncoating intermediate particle of E1 that is produced in cells during infection. Before releasing their genome into the host cytosol, enteroviruses go through structural changes in their capsid, forming an uncoating intermediate particle. It was shown previously that structural changes can be induced by receptor interactions and, in addition, by heating the native virion to superphysiological temperatures. Here, we demonstrate that an altered, still infectious E1 particle is found during infection. This particle has a more open structure, and it cannot be formed by heating. It still contains the VP4 protein and is able to bind to its receptor and cause infection. Moreover, we show that in contrast to some other enteroviruses, the receptor-virion interaction has a stabilizing effect on E1. This paper highlights the differences between enterovirus species and further increases our understanding of various uncoating forms of enteroviruses.

Comparative Genetic Analyses of Human Rhinovirus C (HRV-C) Complete Genome from Malaysia

Human rhinovirus-C (HRV-C) has been implicated in more severe illnesses than HRV-A and HRV-B, however, the limited number of HRV-C complete genomes (complete 5′ and 3′ non-coding region and open reading frame sequences) has hindered the in-depth genetic study of this virus. This study aimed to sequence seven complete HRV-C genomes from Malaysia and compare their genetic characteristics with the 18 published HRV-Cs. Seven Malaysian HRV-C complete genomes were obtained with newly redesigned primers. The seven genomes were classified as HRV-C6, C12, C22, C23, C26, C42, and pat16 based on the VP4/VP2 and VP1 pairwise distance threshold classification. Five of the seven Malaysian isolates, namely, 3430-MY-10/C22, 8713-MY-10/C23, 8097-MY-11/C26, 1570-MY-10/C42, and 7383-MY-10/pat16 are the first newly sequenced complete HRV-C genomes. All seven Malaysian isolates genomes displayed nucleotide similarity of 63–81% among themselves and 63–96% with other HRV-Cs. Malaysian HRV-Cs had similar putative immunogenic sites, putative receptor utilization and potential antiviral sites as other HRV-Cs. The genomic features of Malaysian isolates were similar to those of other HRV-Cs. Negative selections were frequently detected in HRV-Cs complete coding sequences indicating that these sequences were under functional constraint. The present study showed that HRV-Cs from Malaysia have diverse genetic sequences but share conserved genomic features with other HRV-Cs. This genetic information could provide further aid in the understanding of HRV-C infection.

Picornavirus morphogenesis

The Picornaviridae represent a large family of small plus-strand RNA viruses that cause a bewildering array of important human and animal diseases. Morphogenesis is the least-understood step in the life cycle of these viruses, and this process is difficult to study because encapsidation is tightly coupled to genome translation and RNA replication. Although the basic steps of assembly have been known for some time, very few details are available about the mechanism and factors that regulate this process. Most of the information available has been derived from studies of enteroviruses, in particular poliovirus, where recent evidence has shown that, surprisingly, the specificity of encapsidation is governed by a viral protein-protein interaction that does not involve an RNA packaging signal. In this review, we make an attempt to summarize what is currently known about the following topics: (i) encapsidation intermediates, (ii) the specificity of encapsidation (iii), viral and cellular factors that are required for encapsidation, (iv) inhibitors of encapsidation, and (v) a model of enterovirus encapsidation. Finally, we compare some features of picornavirus morphogenesis with those of other plus-strand RNA viruses.

[Polio, the long walk to the endgame]

Although the WHO original target date for the global eradication of poliomyelitis was the year 2000 -thanks to vaccination and institutional, public and private, resources for that purpose-, in 2013 the disease remained endemic in three countries, Afghanistan, Pakistan and Nigeria, and some cases were described in five others. The circulation of wild type 1 poliovirus in Israel, Gaza and the West Bank and the cases in Syria were a wakeup call, as at that time there were polioviruses derived from the oral vaccine that are still circulating among the human population and can cause the development of the disease. Travelling "from" and "to" endemic areas are factors to consider in poliovirus exportation and in its spread when it reaches areas with poor immunogenicity. Wars, terrorism, intolerance, lack of culture and proliferation of anti-vaccine groups and the rise of the anti-vaccination movement are important factors in the maintenance and expansion of the virus and in the "non-vaccination" against it. Based on the international situation to date, the Emergency Committee of WHO met in May 2014 to address the problem. It is still necessary to enhance the knowledge of the disease and its agent. In the first case to perform a differential diagnosis of flaccid paralysis and to continue vaccination programs, and in the second case to keep studying and looking for the poliovirus in environmental samples, which is a model for the study of many other viruses.

Comparison of lipid-containing bacterial and archaeal viruses

Lipid-containing bacteriophages were discovered late and considered to be rare. After further phage isolations and the establishment of the domain Archaea, several new prokaryotic viruses with lipids were observed. Consequently, the presence of lipids in prokaryotic viruses is reasonably common. The wealth of information about how prokaryotic viruses use their lipids comes from a few well-studied model viruses (PM2, PRD1, and ϕ6). These bacteriophages derive their lipid membranes selectively from the host during the virion assembly process which, in the case of PM2 and PRD1, culminates in the formation of protein capsid with an inner membrane, and for ϕ6 an outer envelope. Several inner membrane-containing viruses have been described for archaea, and their lipid acquisition models are reminiscent to those of PM2 and PRD1. Unselective acquisition of lipids has been observed for bacterial mycoplasmaviruses and archaeal pleolipoviruses, which resemble each other by size, morphology, and life style. In addition to these shared morphotypes of bacterial and archaeal viruses, archaea are infected by viruses with unique morphotypes, such as lemon-shaped, helical, and globular ones. It appears that structurally related viruses may or may not have a lipid component in the virion, suggesting that the significance of viral lipids might be to provide viruses extended means to interact with the host cell.

Nonenveloped capsid: mechanisms of viral entry

ABSTRACT 

Viruses are a diverse class of nanoparticles. However, they have evolved a few common mechanisms that enable successful infection of their host cells. The first stage of this process involves entry into the cell. For enveloped viruses this process has been well characterized. For nonenveloped viruses, the focus of this review, the entry mechanisms are less well understood. For these viruses, a typical pathway involves receptor attachment followed by internalization into cellular vesicles and subsequent viral escape to the cytosol and transport to the site of genome replication. Significantly, these viruses have evolved numerous mechanisms to fulfill this seemingly simple infection scheme. We focus on the latest observations for several families of nonenveloped viruses and highlight specific members for eukaryotic families: Adenoviridae, Papillomaviridae, Parvoviridae, Picornaviridae, Polyomaviridae and Reoviridae; and prokaryotic families: Microviridae, Myoviridae, Podoviridae and Siphoviridae.

Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry applied to virus identification

Virus detection and/or identification traditionally rely on methods based on cell culture, electron microscopy and antigen or nucleic acid detection. These techniques are good, but often expensive and/or time-consuming; furthermore, they not always lead to virus identification at the species and/or type level. In this study, Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) was tested as an innovative tool to identify human polioviruses and to identify specific viral protein biomarkers in infected cells. The results revealed MALDI-TOF MS to be an effective and inexpensive tool for the identification of the three poliovirus serotypes. The method was firstly applied to Sabin reference strains, and then to isolates from different clinical samples, highlighting its value as a time-saving, sensitive and specific technique when compared to the gold standard neutralization assay and casting new light on its possible application to virus detection and/or identification.

How viruses hijack cell regulation

Viruses, as obligate intracellular parasites, are the pathogens that have the most intimate relationship with their host, and as such, their genomes have been shaped directly by interactions with the host proteome. Every step of the viral life cycle, from entry to budding, is orchestrated through interactions with cellular proteins. Accordingly, viruses will hijack and manipulate these proteins utilising any achievable mechanism. Yet, the extensive interactions of viral proteomes has yielded a conundrum: how do viruses commandeer so many diverse pathways and processes, given the obvious spatial constraints imposed by their compact genomes? One important approach is slowly being revealed, the extensive mimicry of host protein short linear motifs (SLiMs).

Foot-and-mouth disease virus assembly: processing of recombinant capsid precursor by exogenous protease induces self-assembly of pentamers in vitro in a myristoylation-dependent manner

The assembly of foot-and-mouth disease virus (FMDV) particles is poorly understood. In addition, there are important differences in the antigenic and receptor binding properties of virus assembly and dissociation intermediates, and these also remain unexplained. We have established an experimental model in which the antigenicity, receptor binding characteristics, and in vitro assembly of capsid precursor can be studied entirely from purified components. Recombinant capsid precursor protein (P1 region) was expressed in Escherichia coli as myristoylated or unmyristoylated protein. The protein sedimented in sucrose gradients at 5S and reacted with monoclonal antibodies which recognize conformational or linear antigen determinants on the virion surface. In addition, it bound the integrin alpha(v)beta(6), a cellular receptor for FMDV, indicating that unprocessed recombinant capsid precursor is both structurally and antigenically similar to native virus capsid. These characteristics were not dependent on the presence of 2A at the C terminus but were altered by N-terminal myristoylation and in mutant precursors which lacked VP4. Proteolytic processing of myristoylated precursor by recombinant FMDV 3C(pro) in vitro induced a shift in sedimentation from 5S to 12S, indicating assembly into pentameric capsid subunits. Nonmyristoylated precursor still assembled into higher-order structures after processing with 3C(pro), but these particles sedimented in sucrose gradients at approximately 17S. In contrast, mutant precursors lacking VP4 were antigenically distinct, were unable to form pentamers, and had reduced capacity for binding integrin receptor. These studies demonstrate the utility of recombinant capsid precursor protein for investigating the initial stages of assembly of FMDV and other picornaviruses.

Virion associated proteins of equine rhinitis B virus 1 (ERBV1): the non-structural protein 3C(pro) co-purifies with virions

Equine rhinitis B virus (ERBV), genus Erbovirus, is most closely related to the Cardiovirus genus in the family Picornaviridae. The structural proteins (VP1-4) of erboviruses are not well described, but are predicted by sequence to be 35, 29, 26 and 7 kDa. Methods for the purification of cardioviruses (polyethylene glycol, trypsin treatment) were used to characterise the structural proteins of ERBV1. Only one of the virus proteins detected was an expected molecular mass, and this 26 kDa protein was identified as VP3 by N-terminal amino acid sequencing. N-terminal sequencing of the 56 and a 29 kDa protein identified sequences consistent with VP2 and VP1 respectively, despite these being 27 kDa larger and 6 kDa smaller than predicted. Virus purified without trypsin showed proteins more consistent with masses predicted for VP1, VP2 and VP3 at 35, 29 and 26 kDa respectively. These proteins were further identified with antibodies affinity purified to recombinant VP1, VP2, VP3 produced in E. coli. Interestingly, antibodies affinity purified to the non-structural protein 3C(pro), produced in insect cells, strongly detected a 27 kDa protein in western blots of virus purified with and without trypsin treatment, suggesting the non-structural 27 kDa 3C(pro) co-purifies with ERBV1 virions.

Crystal structure of CD155 and electron microscopic studies of its complexes with polioviruses

When poliovirus (PV) recognizes its receptor, CD155, the virus changes from a 160S to a 135S particle before releasing its genome into the cytoplasm. CD155 is a transmembrane protein with 3 Ig-like extracellular domains, D1-D3, where D1 is recognized by the virus. The crystal structure of D1D2 has been determined to 3.5-A resolution and fitted into approximately 8.5-A resolution cryoelectron microscopy reconstructions of the virus-receptor complexes for the 3 PV serotypes. These structures show that, compared with human rhinoviruses, the virus-receptor interactions for PVs have a greater dependence on hydrophobic interactions, as might be required for a virus that can inhabit environments of different pH. The pocket factor was shown to remain in the virus during the first recognition stage. The present structures, when combined with earlier mutational investigations, show that in the subsequent entry stage the receptor moves further into the canyon when at a physiological temperature, thereby expelling the pocket factor and separating the viral subunits to form 135S particles. These results provide a detailed analysis of how a nonenveloped virus can enter its host cell.

HIV-1 production is specifically associated with human NMT1 long form in human NMT isozymes

The N-myristoylation of the N-terminal of human immunodeficiency virus type-1 (HIV-1) Pr55(gag) by human N-myristoyltransferase (hNMT) is a prerequisite modification for HIV-1 production. hNMT consists of multiple isozymes encoded by hNMT1 and hNMT2. The hNMT1 isozyme consists of long, medium, and short forms. Here, we investigated which isozyme is crucial for HIV-1 production. Human embryonic kidney (HEK) 293 cells transfected with infectious HIV-1 vectors were used as models of HIV-1-infected cells in this study. The significant reduction in HIV-1 production and the failure of the specific localization of Pr55(gag) in a detergent-resistant membrane fraction were dependent on the knockdown of the different forms of the hNMT1 isozyme but not of the hNMT2 isozyme. Additionally, the coexpression of an inactive mutant hNMT1 isozyme, namely the hNMT1 long form (hNMT1(L)), but not that of other hNMT mutants resulted in a significant reduction in HIV-1 production. These results strongly suggest that HIV-1 production is specifically associated with hNMT1, particularly hNMT1(L), but not with hNMT2 in vivo, contributing to the understanding of a step in HIV-1 replication.

Potential role of N-myristoyltransferase in cancer

Colorectal cancer is the second leading cause of malignant death, and better preventive strategies are needed. The treatment of colonic cancer remains difficult because of the lack of effective chemotherapeutic agents; therefore it is important to continue to search for cellular functions that can be disrupted by chemotherapeutic drugs resulting in the inhibition of the development and progression of cancer. The current knowledge of the modification of proteins by myristoylation involving myristoyl-CoA: protein N-myristoyltransferase (NMT) is in its infancy. This process is involved in the pathogenesis of cancer. We have reported for the first time that NMT activity and protein expression were higher in human colorectal cancer, gallbladder carcinoma and brain tumors. In addition, an increase in NMT activity appeared at an early stage in colonic carcinogenesis. It is conceivable therefore that NMT can be used as a potential marker for the early detection of cancer. These observations lead to the possibility of developing NMT specific inhibitors, which may be therapeutically useful. We proposed that HSC70 and/or enolase could be used as an anticancer therapeutic target. This review summarized the status of NMT in cancer which has been carried in our laboratory.

Analysis of translational initiation in coxsackievirus B3 suggests an alternative explanation for the high frequency of R+4 in the eukaryotic consensus motif

Translational initiation of most eukaryotic mRNAs occurs when a preinitiation complex binds to the 5' cap, scans the mRNA, and selects a particular AUG codon as the initiation site. Selection of the correct initiation codon relies, in part, on its flanking residues; in mammalian cells, the core of the "Kozak" consensus is R-3CCAUGG+4 (R=purine; the A residue is designated position +1). The R-3 is considered the most important flanking residue, followed by G+4. Picornaviral mRNAs differ from most cellular mRNAs in several ways; they are uncapped, and they contain an internal ribosome entry site that allows the ribosome to bind near the initiation codon. The initiation codon of coxsackievirus B3 (CVB3) is flanked by both R-3 and G+4 (AAAATGG). Here, we report the construction of full-length CVB3 genomes that vary at these two positions, and we evaluate the effects of these variant sequences in vitro, in tissue culture cells, and in vivo. A virus with an A-->C transversion at position -3 replicates as well as wild-type CVB3, both in tissue culture and in vivo. This virus is highly pathogenic, and its sequence is stable throughout the course of an in vivo infection. Furthermore, the in vitro translation products from this RNA are very similar to the wild type. Thus, R-3-thought to be the most functionally important component of the Kozak consensus-appears to be dispensable in CVB3. In contrast, a G-to-C transversion at G+4 is lethal; RNAs carrying this mutation fail to generate infectious virus either in tissue culture or in vivo. However, in vitro analysis indicates that G+4 has only a marginal effect on translational initiation, especially if R-3 is present; instead, the G+4 is required mainly because the second triplet of the polyprotein open reading frame must encode glycine, without which infectious virus production cannot proceed. In summary, our data indicate that CVB3 remains viable, even in vivo, in the absence of R-3, and we propose that the most important factor contributing to the high frequency of G+4-not only in CVB but also in other eukaryotic mRNAs, and thus in the consensus motif itself-may be the constraint upon the second amino acid rather than the requirements for translational initiation.

Biochemical and genetic studies of the VPg uridylylation reaction catalyzed by the RNA polymerase of poliovirus

The first step in poliovirus (PV) RNA synthesis is the covalent linkage of UMP to the terminal protein VPg. This reaction can be studied in vitro with two different assays. The simpler assay is based on a poly(A) template and requires synthetic VPg, purified RNA polymerase 3D(pol), UTP, and a divalent cation. The other assay uses specific viral sequences [cre(2C)] as a template for VPg uridylylation and requires the addition of proteinase 3CD(pro). Using one or both of these assays, we analyzed the VPg specificities and metal requirements of the uridylylation reactions. We determined the effects of single and double amino acid substitutions in VPg on the abilities of the peptides to serve as substrates for 3D(pol). Mutations in VPg, which interfered with uridylylation in vitro, were found to abolish viral growth. A chimeric PV containing the VPg of human rhinovirus 14 (HRV14) was viable, but substitutions of HRV2 and HRV89 VPgs for PV VPg were lethal. Of the three rhinoviral VPgs tested, only the HRV14 peptide was found to function as a substrate for PV1(M) 3D(pol) in vitro. We also examined the metal specificity of the VPg uridylylation reaction on a poly(A) template. Our results show a strong preference of the RNA polymerase for Mn(2+) as a cofactor compared to Mg(2+) or other divalent cations.

Influence of hydrophobic interactions on the conformational adaptability of the beta-Ala residue

The chemical synthesis and X-ray crystal structure analysis of a model peptide incorporating a conformationally flexible beta-Ala residue: Boc-beta-Ala-Pda, 1 (C23H46N2O3: molecular weight = 398.62) have been described. The peptide crystallized in the crystal system triclinic with space group P21: a = 5.116(3) A, b = 5.6770(10) A, c = 21.744(5) A; alpha = 87.45 degrees, beta = 86.87 degrees, gamma = 90.0 degrees; Z = 1. An attractive feature of the crystal molecular structure of 1 is the induction of a reasonably extended backbone conformation of the beta-Ala moiety, i.e. the torsion angles phi approximately -115 degrees, mu approximately 173 degrees and psi approximately 122 degrees, correspond to skew-, trans and skew+ conformation, respectively, by an unbranched hydrophobic alkyl chain, Pda, which prefers an all-anti orientation (theta1 approximately -153 degrees, theta2 approximately ellipsis theta14 approximately +/-178 degrees ). The observation is remarkable because, systematic conformational investigations of short linear beta-Ala peptides of the type Boc-beta-Ala-Xaa-OCH3 (Xaa = Aib or Acc6) have shown that the chemical and stereochemical characters of the neighboring moieties may be critical in dictating the overall folded and/or unfolded conformational features of the beta-Ala residue. The overall conformation of 1 is typical of a 'bar'. It appears convincing that, in addition to a number of hydrophobic contacts between the parallel arranged molecules, an array of conventional N-HellipsisO=C intermolecular H-bonding interactions stabilize the crystal molecular structure. Moreover, the resulting 14-membered pseudo-ring motif, generated by the amide-amide interactions between the adjacent molecules, is completely devoid of nonconventional C-HellipsisO interaction. The potentials of the conformational adaptation of the beta-Ala residue, to influence and stabilize different structural characteristics have been highlighted.

Using recombinant coxsackievirus B3 to evaluate the induction and protective efficacy of CD8+ T cells during picornavirus infection

Coxsackievirus B3 (CVB3) is a common human pathogen that has been associated with serious diseases including myocarditis and pancreatitis. To better understand the effect of cytotoxic T-lymphocyte (CTL) responses in controlling CVB3 infection, we have inserted well-characterized CTL epitopes into the CVB3 genome. Constructs were made by placing the epitope of interest upstream of the open reading frame encoding the CVB3 polyprotein, separated by a poly-glycine linker and an artificial 3Cpro/3CDpro cleavage site. This strategy results in the foreign protein being translated at the amino- terminus of the viral polyprotein, from which it is cleaved prior to viral assembly. In this study, we cloned major histocompatibility complex class I-restricted CTL epitopes from lymphocytic choriomeningitis virus (LCMV) into recombinant CVB3 (rCVB3). In vitro, rCVB3 growth kinetics showed a 1- to 2-h lag period before exponential growth was initiated, and peak titers were approximately 1 log unit lower than for wild-type virus. rCVB3 replicated to high titers in vivo and caused severe pancreatitis but minimal myocarditis. Despite the high virus titers, rCVB3 infection of naive mice failed to induce a strong CD8+ T-cell response to the encoded epitope; this has implications for the proposed role of "cross-priming" during virus infection and for the utility of recombinant picornaviruses as vaccine vectors. In contrast, rCVB3 infection of LCMV-immune mice resulted in direct ex vivo cytotoxic activity against target cells coated with the epitope peptide, demonstrating that the rCVB3-encoded LCMV-specific epitope was expressed and presented in vivo. The preexisting CD8+ memory T cells could limit rCVB replication; compared to naive mice, infection of LCMV-immune mice with rCVB3 resulted in approximately 50-fold-lower virus titers in the heart and approximately 6-fold-lower virus titers in the pancreas. Although the inserted CTL epitope was retained by rCVB3 through several passages in tissue culture, it was lost in an organ-specific manner in vivo; a substantial proportion of viruses from the pancreas retained the insert, compared to only 0 to 1.8% of myocardial viruses. Together, these results show that expression of heterologous viral proteins by recombinant CVB3 provides a useful model for determining the mechanisms underlying the immune response to this viral pathogen.

Poliovirus/Hepatitis C virus (internal ribosomal entry site-core) chimeric viruses: improved growth properties through modification of a proteolytic cleavage site and requirement for core RNA sequences but not for core-related polypeptides

H.-H. Lu and E. Wimmer (Proc. Natl. Acad. Sci. USA 93:1412-1417, 1996) have demonstrated that the internal ribosomal entry site (IRES) of poliovirus (PV) can be functionally replaced by the related genetic element from hepatitis C virus (HCV). One important finding of this study was that open reading frame sequences 3' of the initiating AUG, corresponding to the open reading frame of the HCV core polypeptide, are required to create a viable chimeric virus. This made necessary the inclusion of a PV 3C protease (3Cpro) cleavage site for proper polyprotein processing to create the authentic N terminus of the PV capsid precursor. Chimeric PV/HCV (P/H) viruses, however, grew poorly relative to PV. The goal of this study was to determine the molecular basis of impaired replication and enhance the growth properties of this chimeric virus. Genetic modifications leading to a different proteinase (PV 2Apro) cleavage site between the HCV core sequence and the PV polyprotein (P/H701-2A) proved far superior with respect to viral protein expression, core-PV fusion polyprotein processing, plaque phenotype, and viral titer than the original prototype PV/HCV chimera containing the PV 3Cpro-specific cleavage site (P/H701). We have used this new virus model to answer two questions concerning the role of the HCV core protein in P/H chimeric viral proliferation. First, a derivative of P/H701-2A with frameshifts in the core-encoding sequence was used to demonstrate that production of the core protein was not necessary for the translation and replication of the P/H chimera. Second, a viral construct with a C-terminal truncation of 23 amino acids of the core gene was used to show that a signal sequence for signal peptidase processing, when present in the viral construct, is detrimental to P/H virus growth. The novel P/H chimera described here are suitable models for analyzing the function(s) of the HCV elements by genetic analyses in vivo and for antiviral drug discovery.

Expression of foreign proteins by poliovirus polyprotein fusion: analysis of genetic stability reveals rapid deletions and formation of cardioviruslike open reading frames

Using a strategy developed by R. Andino, D. Silvera, S. D. Suggett, P. I. Achacoso, C. J. Miller, D. Baltimore, and M. B. Feinberg (Science 265:1448-1451, 1994), we constructed recombinant polioviruses by fusing the open reading frame (ORF) of the green fluorescent protein gene (gfp) of Aequorea victoria or the gag gene (encoding p17-p24) of human immunodeficiency virus type 1 (HIV-1) to the N terminus of the poliovirus polyprotein. All poliovirus expression vectors constructed by us and those obtained from Andino et al. were found to be severely impaired in viral replication and genetically unstable. Upon replication, inserted sequences were rapidly deleted as early as the first growth cycle in HeLa cells. However, the vector viruses did not readily revert to the wild-type sequence but rather retained some of the insert plus the artificial 3Cpro/3CDpro cleavage site, engineered between the heterologous sequence and the poliovirus polyprotein, to give rise to genotypes reminiscent of cardioviruses. These virus variants that carry a small leader polypeptide were now relatively stable, and they grew better than their progenitor strains. Reverse transcription followed by PCR and sequence analysis of the genomic RNAs reproducibly revealed a few preferred genotypes among the isolated deletion variants. The remaining truncated inserts were retained through subsequent passages. In the immediate vicinity of the deletion borders, we observed short direct sequence repeats that we propose are involved in aligning RNA strands for illegitimate (nonhomologous) RNA recombination during minus-strand synthesis. On the basis of our results, which are at variance with published data, the utility of poliovirus vectors to express proteins > 10 kDa in size through fusion with the polyprotein needs to be reevaluated.

Myristoylation

N-myristoylation is an acylation process absolutely specific to the N-terminal amino acid glycine in proteins. This maturation process concerns about a hundred proteins in lower and higher eukaryotes involved in oncogenesis, in secondary cellular signalling, in infectivity of retroviruses and, marginally, of other virus types. Thy cytosolic enzyme responsible for this activity, N-myristoyltransferase (NMT), studied since 1987, has been purified from different sources. However, the studies of the specificities of the various NMTs have not progressed in detail except for those relating to the yeast cytosolic enzyme. Still to be explained are differences in species specificity and between various putative isoenzymes, also whether the data obtained from the yeast enzyme can be transposed to other NMTs. The present review discusses data on the various addressing processes subsequent to myristoylation, a patchwork of pathways that suggests myristoylation is only the first step of the mechanisms by which a protein associates with the membrane. Concerning the enzyme itself, there are evidences that NMT is also present in the endoplasmic reticulum and that its substrate specificity is different from that of the cytosolic enzyme(s). These differences have major implications for their differential inhibition and for their respective roles in several pathologies. For instance, the NMTs from mammalians are clearly different from those found in several microorganisms, which raises the question whether the NMT may be a new targets for fungicides. Finally, since myristoylation has a central role in virus maturation and oncogenesis, specific NMT inhibitors might lead to potent antivirus and anticancer agents.

The poliovirus 135S particle is infectious

The molecular mechanism of cell entry by unenveloped viruses is poorly understood. The picornaviruses poliovirus, human rhinovirus, and coxsackievirus convert to an altered form (the 135S or A particle) upon interaction with receptors on susceptible cells at 37 degrees C. The 135S particle is thought to be a necessary intermediate because it accumulates inside susceptible cells soon after infection and drugs which inhibit conversion of the virus to this form also prevent infection. However, since a variable fraction of the altered 135S particles is reported to elute unproductively from the surface of susceptible cells, their precise role remains unclear. We have found that poliovirus 135S particles can infect Chinese hamster ovary (CHO) and murine L cells, neither of which are susceptible to infection by native poliovirus. The infectivity of the particles in tissue culture appears to be between 10(3) to 10(5) times less than that of poliovirus on HeLa cells. The 135S particle infectivity was not sensitive to RNase but was greatly reduced by proteolytic treatment. Proteolysis specifically removed the newly exposed N terminus of VP1, a feature which has previously been shown to mediate interactions of the particle with lipid membranes. These results demonstrate that although the infectivity of the 135S particle appears to be receptor independent, it nonetheless requires some property associated with the protein coat. In particular, the N terminus of VP1 plays an important role in the infection process. Our findings are consistent with the hypothesis that the 135S particle is an intermediate in the normal cell entry pathway of poliovirus infection.

Release of virus-like particles from cells infected with poliovirus replicons which express human immunodeficiency virus type 1 Gag

The effectiveness of attenuated poliovirus vaccines when given orally to induce both systemic and mucosal immune responses against poliovirus has resulted in an effort to develop poliovirus-based vectors to express foreign proteins. We have previously described the construction of poliovirus genomes (referred to as replicons) in which the complete human immunodeficiency virus type 1 (HIV-1) gag gene was substituted for the capsid gene (P1) (D.C. Porter, D.C. Ansardi, and C.D. Morrow, J. Virol. 69:1548-1555, 1995). Infection of cells with encapsidated replicons resulted in the expression of a 55-kDa protein. To further characterize the biological features of the HIV-1 Gag proteins expressed in cells infected with encapsidated replicons, we utilized biochemical analysis and electron microscopy. Expression of the 55-kDa protein in cells infected with encapsidated replicons resulted in myristylation of the Pr55gag protein. The Gag precursor protein was released from infected cells; analysis on sucrose density gradients revealed that the precursor sedimented at a density consistent with that of an HIV-1 virus-like particle. Analysis of replicon-infected cells by electron microscopy demonstrated the presence of condensed structures at the plasma membrane and the release of virus-like particles. These studies demonstrate that poliovirus-based vectors can be used to express foreign proteins which require posttranslational modifications, such as myristylation, and assemble into higher-order structures, providing a foundation for the future use of poliovirus replicons as vaccine vectors.

Interactions among the major and minor coat proteins of polyomavirus

Murine polyomavirus contains two related minor coat proteins, VP2 and VP3, in addition to the major coat protein, VP1. The sequence of VP3 is identical to that of the carboxy-terminal two-thirds of VP2. VP2 may serve a role in uncoating of the virus, and both minor coat proteins may be important for viral assembly. In this study, we show that VP3 and a series of deletion mutants of VP3 can be expressed in Escherichia coli as fusion proteins to glutathione S-transferase and partially solubilized with a mild detergent. Using an in vitro binding assay, we demonstrate that a 42-amino-acid fragment near the carboxy terminus of VP3 (residues 140 to 181) is sufficient for binding to purified VP1 pentamers. This binding interaction is rapid, saturable, and specific for the common carboxy terminus of VP2 and VP3. The VP1-VP3 complex can be coimmunoprecipitated with an antibody specific to VP1, and a purified VP3 fragment can selectively extract VP1 from a crude cell lysate. The stoichiometry of the binding reaction suggests that each VP1 pentamer in the virus binds either one VP2 or one VP3, with the VP1-VP2/3 complex stabilized by hydrophobic interactions. These results, taken together with studies from other laboratories on the expression of polyomavirus capsid proteins in mouse and insect cells (S. E. Delos, L. Montross, R. B. Moreland, and R. L. Garcea, Virology, 194:393-398, 1993; J. Forstova, N. Krauzewicz, S. Wallace, A. J. Street, S. M. Dilworth, S. Beard, and B. E. Griffin, J. Virol. 67:1405-1413, 1993), support the idea that a VP1-VP2/3 complex forms in the cytoplasm and, after translocation into the nucleus, acts as the unit for viral assembly.

Polioviruses containing picornavirus type 1 and/or type 2 internal ribosomal entry site elements: genetic hybrids and the expression of a foreign gene

A picornavirus hybrid genome was constructed in which the internal ribosomal entry site (IRES) of encephalomyocarditis virus was inserted between the 5' non-translated region and the open reading frame of poliovirus (PV), type 1 (Mahoney). Upon transfection into HeLa cells, the hybrid RNA replicated and yielded a derivative of PV (W1-PNENPO). The PV IRES could be removed from pPNENPO, which resulted in a hybrid picornavirus (W1-P108ENPO) in which the translation of the PV open reading frame normally promoted by the type 1 IRES of PV was promoted by the type 2 IRES of encephalomyocarditis virus. This result indicates that these elements are not likely to contain cis-acting elements necessary for PV replication or encapsidation. A foreign gene (bacterial chloramphenicol acetyltransferase, CAT) was inserted into pPNENPO cDNA between the PV and encephalomyocarditis virus IRES elements. The dicistronic RNA replicated in HeLa cells and yielded a derivative of PV (W1-DICAT) with a genome 17% longer than that of wild-type PV. CAT assays and immunoblot analyses showed that the viral RNA efficiently expressed the foreign gene in cell culture. The CAT activity diminished somewhat with each passage of the dicistronic virus, an observation which suggested that the inserted gene had a deleterious effect on viral replication. However, even after five virus passages, a significant quantity of the foreign gene was still expressed. Insertion of the open reading frame of luciferase (67 kDa) resulted in an RNA species that replicated and expressed luciferase for up to 20 hr after transfection. However, this elongated RNA was not encapsidated.

Synthesis of myristoyl CoA analogues and myristoyl peptides as inhibitors of myristoyl CoA:protein N-myristoyltransferase

To develop inhibitors of myristoyl CoA:protein N-myristoyltransferase (NMT), a series of myristoyl coenzyme A analogues and myristoyl peptides were synthesized, including S-(2-oxopentadecyl)-CoA (1), S-(2-hydroxypentadecyl)-CoA (2), S-(2-oxopentadecyl)-pantetheine (3), Myr-N-Gly-(L)-Phe (4), Myr-N-Gly-(L)-Tyr (5), and Myr-N-Gly-(L)-Asn-Ala- Ala-Ser-Ala-Arg-(NH2) (6). Biological evaluation of these compounds in an in vitro NMT enzyme assay revealed that the nonhydrolyzable acyl CoA analogue 1 was the most potent inhibitor [inhibitor dissociation constant (Ki) = 24 nM]. A preliminary structure-activity relationship study showed that the adenosine moiety and the 2-keto group in this nonhydrolyzable analogue were necessary for inhibitory activity. A possible mechanism for the inhibition of NMT by 1 was proposed, in which 1 might block the reaction at the stage of an acyl-CoA-NMT-peptide complex. Product analogues such as the myristoylated peptides 4-6 were poor inhibitors of NMT.

Reovirus M2 gene is associated with chromium release from mouse L cells

In this study, we investigated the interaction of reovirus particles with cell membranes by using a 51Cr release assay. We confirmed prior observations (J. Borsa, B. D. Morash, M. D. Sargent, T. P. Copps, P. A. Lievaart, and J. G. Szekely, J. Gen. Virol. 45:161-170, 1979) that intermediate subviral particles (ISVPs) of reovirus type 3 strain Abney (T3A) induced the release of 51Cr from preloaded L cells and showed that the intact virion and core forms did not. Reovirus type 1 strain Lang (T1L) ISVPs were found to be less efficient at 51Cr release than T3A ISVPs. Reassortants between these strains indicated that the 51Cr release phenotype segregates with the M2 gene segment. Biochemical studies indicated that the ISVPs' acquisition of the capacity to induce 51Cr release followed the cleavage of the viral M2 gene product mu 1/mu 1C to fragments delta and phi during virion conversion to ISVP but did not directly correlate with this cleavage. These studies suggest that the reovirus M2 gene product (in its cleaved form) plays a role in interacting with cell membranes.

Molecular characterization of mouse-virulent poliovirus type 1 Mahoney mutants: involvement of residues of polypeptides VP1 and VP2 located on the inner surface of the capsid protein shell

Most poliovirus (PV) strains, including PV PV-1/Mahoney, are unable to cause paralysis in mice. Determinants for restriction of PV-1/Mahoney in mice have been identified by manipulating PV-1 cDNA and located on the viral capsid protein VP1. These determinants consist of a highly exposed amino acid sequence on the capsid surface corresponding to the B-C loop (M. Murray, J. Bradley, X. Yang, E. Wimmer, E. Moss, and V. Racaniello, Science 241:213-215, 1988; A. Martin, C. Wychowski, T. Couderc, R. Crainic, J. Hogle, and M. Girard, EMBO J. 7:2839-2847, 1988) and of residues belonging to the N-terminal sequence located on the inner surface of the protein shell (E. Moss and V. Racaniello, EMBO J. 10:1067-1074, 1991). Using an in vivo approach, we isolated two mouse-neurovirulent PV-1 mutants in the mouse central nervous system after a single passage of PV-1/Mahoney inoculated by the intracerebral route. Both mutants were subjected to two additional passages in mice, plaque purified, and subsequently characterized. The two cloned mutants, Mah-NK13 and Mah-NL32, retained phenotypic characteristics of the parental PV-1/Mahoney, including epitope map, heat lability, and temperature sensitivity. Mah-NK13 exhibited slightly smaller plaques than did the parental virus. The nucleotide sequences of the mutant genomes were determined, and mutations were identified. Mutations were independently introduced into the parental PV-1/Mahoney genome by single-site mutagenesis. Mutated PV-1/Mahoney viruses were then tested for their neurovirulence in mice. A single amino acid substitution in the capsid proteins VP1 (Thr-22-->Ile) and VP2 (Ser-31-->Thr) identified in the Mah-NK13 and Mah-NL32 genomes, respectively, conferred the mouse-virulent phenotype to the mouse-avirulent PV-1/Mahoney. Ile-22 in VP1 was responsible for the small-plaque phenotype of Mah-NK13. Both mutations arose during the first passage in the mouse central nervous system. We thus identified a new mouse adaptation determinant on capsid protein VP1, and we showed that at least one other capsid protein, VP2, could also express a mouse adaptation determinant. Both determinants are located in the inside of the three-dimensional structure of the viral capsid. They may be involved in the early steps of mouse nerve cell infection subsequent to receptor attachment.

Complementation of a poliovirus defective genome by a recombinant vaccinia virus which provides poliovirus P1 capsid precursor in trans

Defective interfering (DI) RNA genomes of poliovirus which contain in-frame deletions in the P1 capsid protein-encoding region have been described. DI genomes are capable of replication and can be encapsidated by capsid proteins provided in trans from wild-type poliovirus. In this report, we demonstrate that a previously described poliovirus DI genome (K. Hagino-Yamagishi and A. Nomoto, J. Virol. 63:5386-5392, 1989) can be complemented by a recombinant vaccinia virus, VVP1 (D. C. Ansardi, D. C. Porter, and C. D. Morrow, J. Virol. 65:2088-2092, 1991), which expresses the poliovirus capsid precursor polyprotein, P1. Stocks of defective polioviruses were generated by transfecting in vitro-transcribed defective genome RNA derived from plasmid pSM1(T7)1 into HeLa cells infected with VVP1 and were maintained by serial passage in the presence of VVP1. Encapsidation of the defective poliovirus genome was demonstrated by characterizing poliovirus-specific protein expression in cells infected with preparations of defective poliovirus and by Northern (RNA) blot analysis of poliovirus-specific RNA incorporated into defective poliovirus particles. Cells infected with preparations of defective poliovirus expressed poliovirus protein 3CD but did not express capsid proteins derived from a full-length P1 precursor. Poliovirus-specific RNA encapsidated in viral particles generated in cells coinfected with VVP1 and defective poliovirus migrated slightly faster on formaldehyde-agarose gels than wild-type poliovirus RNA, demonstrating maintenance of the genomic deletion. By metabolic radiolabeling with [35S]methionine-cysteine, the defective poliovirus particles were shown to contain appropriate mature-virion proteins. This is the first report of the generation of a pure population of defective polioviruses free of contaminating wild-type poliovirus. We demonstrate the use of this recombinant vaccinia virus-defective poliovirus genome complementation system for studying the effects of a defined mutation in the P1 capsid precursor on virus assembly. Following removal of residual VVP1 from defective poliovirus preparations, processing and assembly of poliovirus capsid proteins derived from a nonmyristylated P1 precursor expressed by a recombinant vaccinia virus, VVP1 myr- (D. C. Ansardi, D. C. Porter, and C. D. Morrow, J. Virol. 66:4556-4563, 1992), in cells coinfected with defective poliovirus were analyzed. Capsid proteins generated from nonmyristylated P1 did not assemble detectable levels of mature virions but did assemble, at low levels, into empty capsids.(ABSTRACT TRUNCATED AT 400 WORDS)

The nonmyristylated Pr160gag-pol polyprotein of human immunodeficiency virus type 1 interacts with Pr55gag and is incorporated into viruslike particles

The expression of the pol gene of human immunodeficiency virus type 1 occurs via a ribosomal frameshift between the gag and pol genes. The resulting protein, a Gag-Pol polyprotein, is produced at a level 5 to 10% of that of the Gag protein. The Gag-Pol polyprotein is incorporated into virions and provides viral protease, reverse transcriptase, and integrase, which are essential for infectivity. It is generally believed that the Gag-Pol polyprotein is incorporated into virions via interaction with the Gag protein, although the details of the mechanism are unknown. To further study this problem, we have constructed a human immunodeficiency virus type 1 proviral genome which overexpresses the Gag-Pol polyprotein (Pr160gag-pol). Transfection of this proviral genome (pGPpr-) into COS-1 cells resulted in the expression of full-length Pr160gag-pol polyprotein. Although the majority of the Pr160gag-pol was confined to the cells, low levels of reverse transcriptase activity were detectable in the cell supernatants. The cotransfection of pGPpr- with a second plasmid which expresses only the Pr55gag precursor (pGAG) resulted in a significantly higher level of Pr160gag-pol in the medium of transfected cells. Sedimentation analysis using sucrose density gradients demonstrated that most Pr160gag-pol was found in fractions corresponding to the density of virion particles, indicating that the Pr160gag-pol polyprotein was released in association with a Pr55gag viruslike particle. To further characterize the requirements for the release, a mutation was constructed to express an unmyristylated Pr160gag-pol polyprotein. Coexpression with Pr55gag demonstrated that the unmyristylated Pr160gag-pol was also incorporated into virion particles. Subcellular fractionation experiments revealed that the distributions of the Pr160gag-polmyr- and Pr160gag-pol in the membrane and cytosol were similar under low- or high-ionic-strength conditions. Taken together, these results suggest that myristylation of the Pr160gag-pol polyprotein is not required for the interaction with the Pr55gag necessary for packaging into a viruslike particle.

Myristylation of poliovirus capsid precursor P1 is required for assembly of subviral particles

The poliovirus capsid precursor polyprotein, P1, is cotranslationally modified by the addition of myristic acid. We have examined the importance of myristylation of the P1 capsid precursor during the poliovirus assembly process by using a recently described recombinant vaccinia virus expression system which allows the independent production of the poliovirus P1 protein and the poliovirus 3CD proteinase (D. C. Ansardi, D. C. Porter, and C. D. Morrow, J. Virol. 65:2088-2092, 1991). We constructed a site-directed mutation in the poliovirus cDNA encoding an alanine at the second amino acid position of P1 in place of the glycine residue required for the myristic acid addition and isolated a recombinant vaccinia virus (VVP1myr-) that expressed a nonmyristylated form of the P1 capsid precursor. The 3CD proteinase expressed by a coinfecting vaccinia virus, VVP3, proteolytically processed the nonmyristylated precursor P1 expressed by VVP1myr-. However, the processed capsid proteins, VP0, VP3, and VP1, did not assemble into 14S or 75S subviral particles, in contrast to the VP0, VP3, and VP1 proteins derived from the myristylated P1 precursor. When cells were coinfected with VVP1myr- and poliovirus type 1, the nonmyristylated P1 precursor expressed by VVP1myr- was processed by 3CD expressed by poliovirus, and the nonmyristylated VP0-VP3-VP1 (VP0-3-1) protomers were incorporated into capsid particles and virions which sedimented through a 30% sucrose cushion. Thus, the nonmyristylated P1 precursor and VP0-3-1 protomers were not excluded from sites of virion assembly, and the assembly defects observed for the nonmyristylated protomers were overcome in the presence of myristylated capsid protomers expressed by poliovirus. We conclude that myristylation of the poliovirus P1 capsid precursor plays an important role during poliovirus assembly by facilitating the appropriate interactions required between 5S protomer subunits to form stable 14S pentamers. The results of these studies demonstrate that the independent expression of the poliovirus P1 and 3CD proteins by using recombinant vaccinia viruses provides a unique experimental tool for analyzing the dynamics of the poliovirus assembly process.

Association of heat shock protein 70 with enterovirus capsid precursor P1 in infected human cells

Members of the human heat shock (HSP) family of related proteins are involved in the intracellular folding, transport, and assembly of proteins and protein complexes. We have observed that human heat shock protein 70 (HSP70) is associated with the capsid precursor P1 of poliovirus and coxsackievirus B1 in infected HeLa cells. Antiserum generated against HSP70 coimmunoprecipitated the poliovirus protein P1, an intermediate in capsid assembly. Similarly, alpha-virion serum coimmunoprecipitated HSP70 from virus-infected cell extracts, but not from mock-infected cell extracts. The HSP70-P1 complex was stable in high-salt medium but was sensitive to incubation with 2 mM ATP, which is a characteristic of other known functional complexes between HSP70 and cellular proteins. The P1 in the complex was predominantly newly synthesized, and the half-life of complexed P1 was nearly twice as long as that of total P1. The HSP70-P1 complex was found to sediment at 3S to 6S, suggesting that it may be part of, or a precursor to, the "5S promoter particles" thought to be an assembly intermediate of picornaviruses. The finding that HSP70 was associated with the capsid precursors of at least two enteroviruses may suggest a functional role of these complexes in the viral life cycles.