Lack of myristoylation of poliovirus capsid polypeptide VP0 prevents the formation of virions or results in the assembly of noninfectious virus particles

Enterovirus D68 capsid formation and stability requires acidic compartments

IMPORTANCE

The respiratory picornavirus enterovirus D68 is a causative agent of acute flaccid myelitis, a childhood paralysis disease identified in the last decade. Poliovirus, another picornavirus associated with paralytic disease, is a fecal-oral virus that survives acidic environments when passing from host to host. Here, we follow up on our previous work showing a requirement for acidic intracellular compartments for maturation cleavage of poliovirus particles. Enterovirus D68 requires acidic vesicles for an earlier step, assembly, and maintenance of viral particles themselves. These data have strong implications for the use of acidification blocking treatments to combat enterovirus diseases.

The Picornaviridae Family: Knowledge Gaps, Animal Models, Countermeasures, and Prototype Pathogens

Picornaviruses are nonenveloped particles with a single-stranded RNA genome of positive polarity. This virus family includes poliovirus, hepatitis A virus, rhinoviruses, and Coxsackieviruses. Picornaviruses are common human pathogens, and infection can result in a spectrum of serious illnesses, including acute flaccid myelitis, severe respiratory complications, and hand-foot-mouth disease. Despite research on poliovirus establishing many fundamental principles of RNA virus biology and the first transgenic animal model of disease for infection by a human virus, picornaviruses are understudied. Existing knowledge gaps include, identification of molecules required for virus entry, understanding cellular and humoral immune responses elicited during virus infection, and establishment of immune-competent animal models of virus pathogenesis. Such knowledge is necessary for development of pan-picornavirus countermeasures. Defining enterovirus A71 and D68, human rhinovirus C, and echoviruses 29 as prototype pathogens of this virus family may provide insight into picornavirus biology needed to establish public health strategies necessary for pandemic preparedness.

Enterovirus D68 capsid formation and stability requires acidic compartments

Enterovirus D68 (EV-D68), a picornavirus traditionally associated with respiratory infections, has recently been linked to a polio-like paralytic condition known as acute flaccid myelitis (AFM). EV-D68 is understudied, and much of the field's understanding of this virus is based on studies of poliovirus. For poliovirus, we previously showed that low pH promotes virus capsid maturation, but here we show that, for EV-D68, inhibition of compartment acidification during a specific window of infection causes a defect in capsid formation and maintenance. These phenotypes are accompanied by radical changes in the infected cell, with viral replication organelles clustering in a tight juxtanuclear grouping. Organelle acidification is critical during a narrow window from 3-4hpi, which we have termed the "transition point," separating translation and peak RNA replication from capsid formation, maturation and egress. Our findings highlight that acidification is crucial only when vesicles convert from RNA factories to virion crucibles.

Incorporating Target-Specific Pharmacophoric Information into Deep Generative Models for Fragment Elaboration

Despite recent interest in deep generative models for scaffold elaboration, their applicability to fragment-to-lead campaigns has so far been limited. This is primarily due to their inability to account for local protein structure or a user's design hypothesis. We propose a novel method for fragment elaboration, STRIFE, that overcomes these issues. STRIFE takes as input fragment hotspot maps (FHMs) extracted from a protein target and processes them to provide meaningful and interpretable structural information to its generative model, which in turn is able to rapidly generate elaborations with complementary pharmacophores to the protein. In a large-scale evaluation, STRIFE outperforms existing, structure-unaware, fragment elaboration methods in proposing highly ligand-efficient elaborations. In addition to automatically extracting pharmacophoric information from a protein target's FHM, STRIFE optionally allows the user to specify their own design hypotheses.

The N-terminal region (VP4) of the foot-and-mouth disease capsid precursor (P1-2A) is not required during its synthesis to allow subsequent processing by the 3C protease

The capsid precursor (P1-2A) of foot-and-mouth disease virus is processed by the 3C protease (3Cpro) to VP0, VP3 and VP1 plus 2A. During capsid assembly, the VP0 is cleaved to VP4 plus VP2. Single amino acid changes in a conserved motif (YCPRP) near the C-terminus of VP1 can block processing of the capsid precursor by the 3Cpro, although the cleavage sites are located hundreds of amino acids distant from this motif, presumably due to misfolding. In contrast, we show here that the absence of the VP4 sequence during the synthesis of the capsid precursor does not affect its subsequent processing. Cleavage of this truncated precursor by 3Cpro at the VP3/VP1 and VP2/VP3 junctions occurred efficiently. Thus, in contrast to the presence of the YCPRP motif in VP1, there are no critical motifs near the N-terminus of the precursor, within VP4, required for correct cleavage by 3Cpro.

Dual-Inhibition of Human N-Myristoyltransferase Subtypes Halts Common Cold Pathogenesis: Atomistic Perspectives from the Case of IMP-1088

The pharmacological inhibition of human N-myristoyltransferase (HsNMT) has emerged as an efficient strategy to completely prevent the replication process of rhinoviruses, a potential treatment for the common cold. This was corroborated by the recent discovery of compound IMP-1088, a novel inhibitor that demonstrated a dual-inhibitory activity against the two HsNMT subtypes 1 and 2 without inducing cytotoxicity. However, the molecular and structural basis for the dual-inhibitory potential of IMP-1088 has not been investigated. As such, we employ molecular modelling techniques to resolve the structural mechanisms that account for the dual-inhibitory prowess of IMP-1088. Sequence and nanosecond-based analyses identified Tyr296, Phe190, Tyr420, Leu453, Gln496, Val181, Leu474, Glu182, and Asn246 as residues common within the binding pockets of both HsNMT1 and HsNMT2 subtypes whose consistent interactions with IMP-1088 underpin the basis for its dual inhibitory potency. Nano-second-based assessment of interaction dynamics revealed that Tyr296 consistently elicited high-affinity π-π stacked interaction with IMP-1088, thus further highlighting its cruciality corroborating previous report. An exploration of resulting structural changes upon IMP-1088 binding further revealed a characteristic impeding of residue fluctuations, structural compactness, and a consequential burial of crucial hydrophobic residues, features required for HsNMT1/2 functionality. Findings present essential structural perspectives that augment previous experimental efforts and could also advance drug development for treating respiratory tract infections, especially those mediated by rhinoviruses.

A Not-So-Ancient Grease History: Click Chemistry and Protein Lipid Modifications

Protein lipid modification involves the attachment of hydrophobic groups to proteins via ester, thioester, amide, or thioether linkages. In this review, the specific click chemical reactions that have been employed to study protein lipid modification and their use for specific labeling applications are first described. This is followed by an introduction to the different types of protein lipid modifications that occur in biology. Next, the roles of click chemistry in elucidating specific biological features including the identification of lipid-modified proteins, studies of their regulation, and their role in diseases are presented. A description of the use of protein-lipid modifying enzymes for specific labeling applications including protein immobilization, fluorescent labeling, nanostructure assembly, and the construction of protein-drug conjugates is presented next. Concluding remarks and future directions are presented in the final section.

The High Immunity Induced by the Virus-Like Particles of Foot-and-Mouth Disease Virus Serotype O

Foot-and-mouth disease (FMD), caused by FMD virus (FMDV), is a highly contagious and economically devastating viral disease of cloven-hoofed animals worldwide. In this study, the coexpression of small ubiquitin-like modifier (SUMO)–fused capsid proteins of FMDV serotype O by single plasmid in Escherichia coli was achieved with an optimal tandem permutation (VP0–VP3–VP1), showing a protein yield close to 1:1:1. After SUMO removal at a low level of protease activity (5 units), the assembled FMDV virus-like particles (VLPs) could expose multiple epitopes and have a size similar to the naive FMDV. Immunization of pigs with the FMDV VLPs could induce FMDV-specific humoral and cellular immune responses effectively, in a dose-dependent manner. These data suggested that the stable FMDV VLPs with multiple epitope exposure were effective for the induction of an immune response in pigs, which laid a foundation for the further development of the FMDV subunit vaccine.

Human rhinovirus infection and COPD: role in exacerbations and potential for therapeutic targets

INTRODUCTION

Respiratory virus infections (predominantly rhinoviruses) are the commonly identified in COPD exacerbations but debate about their role as a trigger of exacerbations continues. Experimental infection studies have provided significant new evidence establishing a causal relationship between virus infection and COPD exacerbations and contributed to a better understanding of the mechanisms of virus-induced exacerbations. However as yet no anti-viral treatments have undergone clinical trials in COPD patients.

AREAS COVERED

This review discusses the evidence for and against respiratory viruses being the main trigger of COPD exacerbations from both epidemiological studies and experimental infection studies. The host immune response to rhinovirus infection and how abnormalities in host immunity may underlie increased susceptibility to virus infection in COPD are discussed and the role of dual viral-bacterial infection in COPD exacerbations. Finally the current state of anti-viral therapy is discussed and how these may be used in the future treatment of COPD exacerbations.

EXPERT OPINION

Respiratory virus infections are the trigger of a substantial proportion of COPD exacerbations and rhinoviruses are the most common virus type. Clinical trials of anti-viral agents are needed in COPD patients to determine whether they are effective in virus-induced COPD exacerbations.

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.

Foot-and-mouth disease virus: Prospects for using knowledge of virus biology to improve control of this continuing global threat

Understanding of the biology of foot-and-mouth disease virus (FMDV) has grown considerably since the nucleotide sequence of the viral RNA was determined. The ability to manipulate the intact genome and also to express specific parts of the genome individually has enabled detailed analyses of viral components, both RNA and protein. Such studies have identified the requirements for specific functional elements for virus replication and pathogenicity. Furthermore, information about the functions of individual virus proteins has enabled the rational design of cDNA cassettes to express non-infectious empty capsid particles that can induce protective immunity in the natural host animals and thus represent new vaccine candidates. Similarly, attempts to block specific virus activities using antiviral agents have also been performed. However, currently, only the well-established, chemically inactivated FMDV vaccines are commercially available and suitable for use to combat this important disease of livestock animals. These vaccines, despite certain shortcomings, have been used very successfully (e.g. in Europe) to control the disease but it still remains endemic in much of Africa, southern Asia and the Middle East. Hence there remains a significant risk of reintroduction of the disease into highly susceptible animal populations with enormous economic consequences.

Cellular N-myristoyltransferases play a crucial picornavirus genus-specific role in viral assembly, virion maturation, and infectivity

In nearly all picornaviruses the precursor of the smallest capsid protein VP4 undergoes co-translational N-terminal myristoylation by host cell N-myristoyltransferases (NMTs). Curtailing this modification by mutation of the myristoylation signal in poliovirus has been shown to result in severe assembly defects and very little, if any, progeny virus production. Avoiding possible pleiotropic effects of such mutations, we here used pharmacological abrogation of myristoylation with the NMT inhibitor DDD85646, a pyrazole sulfonamide originally developed against trypanosomal NMT. Infection of HeLa cells with coxsackievirus B3 in the presence of this drug decreased VP0 acylation at least 100-fold, resulting in a defect both early and late in virus morphogenesis, which diminishes the yield of viral progeny by about 90%. Virus particles still produced consisted mainly of provirions containing RNA and uncleaved VP0 and, to a substantially lesser extent, of mature virions with cleaved VP0. This indicates an important role of myristoylation in the viral maturation cleavage. By electron microscopy, these RNA-filled particles were indistinguishable from virus produced under control conditions. Nevertheless, their specific infectivity decreased by about five hundred fold. Since host cell-attachment was not markedly impaired, their defect must lie in the inability to transfer their genomic RNA into the cytosol, likely at the level of endosomal pore formation. Strikingly, neither parechoviruses nor kobuviruses are affected by DDD85646, which appears to correlate with their native capsid containing only unprocessed VP0. Individual knockout of the genes encoding the two human NMT isozymes in haploid HAP1 cells further demonstrated the pivotal role for HsNMT1, with little contribution by HsNMT2, in the virus replication cycle. Our results also indicate that inhibition of NMT can possibly be exploited for controlling the infection by a wide spectrum of picornaviruses.

Picornaviruses are important human and animal pathogens. Protective vaccines are only available against very few representatives. Furthermore, antiviral drugs have not made it to the market because of serious side effects and viral mutational escape. We here show that pharmacological inhibition of cellular myristoyltransferases severely decreased myristoylation of enteroviral structural proteins as exemplified by coxsackievirus B3, a prominent pathogen causing virus-induced acute and chronic heart disease. The drug DDD85646 substantially diminished virus yield and almost abolished the infectivity of the residual progeny virus. It is highly effective against several other picornaviruses, except those two included in our study that naturally do not process VP0. Our work provides new insight into the role of myristoylation in the life cycle of picornaviruses and identifies the responsible cellular enzyme as a promising candidate for antiviral therapy.

Fragment-derived inhibitors of human N-myristoyltransferase block capsid assembly and replication of the common cold virus

Rhinoviruses (RVs) are the pathogens most often responsible for the common cold, and are a frequent cause of exacerbations in asthma, chronic obstructive pulmonary disease and cystic fibrosis. Here we report the discovery of IMP-1088, a picomolar dual inhibitor of the human N-myristoyltransferases NMT1 and NMT2, and use it to demonstrate that pharmacological inhibition of host-cell N-myristoylation rapidly and completely prevents rhinoviral replication without inducing cytotoxicity. The identification of cooperative binding between weak-binding fragments led to rapid inhibitor optimization through fragment reconstruction, structure-guided fragment linking and conformational control over linker geometry. We show that inhibition of the co-translational myristoylation of a specific virus-encoded protein (VP0) by IMP-1088 potently blocks a key step in viral capsid assembly, to deliver a low nanomolar antiviral activity against multiple RV strains, poliovirus and foot and-mouth disease virus, and protection of cells against virus-induced killing, highlighting the potential of host myristoylation as a drug target in picornaviral infections.

The Cellular Chaperone Heat Shock Protein 90 Is Required for Foot-and-Mouth Disease Virus Capsid Precursor Processing and Assembly of Capsid Pentamers

Productive picornavirus infection requires the hijacking of host cell pathways to aid with the different stages of virus entry, synthesis of the viral polyprotein, and viral genome replication. Many picornaviruses, including foot-and-mouth disease virus (FMDV), assemble capsids via the multimerization of several copies of a single capsid precursor protein into a pentameric subunit which further encapsidates the RNA. Pentamer formation is preceded by co- and posttranslational modification of the capsid precursor (P1-2A) by viral and cellular enzymes and the subsequent rearrangement of P1-2A into a structure amenable to pentamer formation. We have developed a cell-free system to study FMDV pentamer assembly using recombinantly expressed FMDV capsid precursor and 3C protease. Using this assay, we have shown that two structurally different inhibitors of the cellular chaperone heat shock protein 90 (hsp90) impeded FMDV capsid precursor processing and subsequent pentamer formation. Treatment of FMDV permissive cells with the hsp90 inhibitor prior to infection reduced the endpoint titer by more than 10-fold while not affecting the activity of a subgenomic replicon, indicating that translation and replication of viral RNA were unaffected by the drug.IMPORTANCE FMDV of the Picornaviridae family is a pathogen of huge economic importance to the livestock industry due to its effect on the restriction of livestock movement and necessary control measures required following an outbreak. The study of FMDV capsid assembly, and picornavirus capsid assembly more generally, has tended to be focused upon the formation of capsids from pentameric intermediates or the immediate cotranslational modification of the capsid precursor protein. Here, we describe a system to analyze the early stages of FMDV pentameric capsid intermediate assembly and demonstrate a novel requirement for the cellular chaperone hsp90 in the formation of these pentameric intermediates. We show the added complexity involved for this process to occur, which could be the basis for a novel antiviral control mechanism for FMDV.

Inhibition of enterovirus VP4 myristoylation is a potential antiviral strategy for hand, foot and mouth disease

The Hand, Foot and Mouth Disease (HFMD) can result from infections by a plethora of human enteroviruses of the species Enterovirus A and B. These infections are highly contagious, resulting in regular outbreaks especially in the Asia-Pacific Region in the recent decade. Although this disease is generally a childhood affliction which manifests as a mild, febrile illness accompanied by the vesicles on the hands, feet and mouth, permanent morbidity or even fatality can result from severe forms of the disease in a subset of the infected patients. The N-terminal myristoylation signal (MGXXXS) of viral capsid protein VP4, one of the four viral structural proteins, is an extremely well conserved feature of enteroviruses, a potential antiviral target that may yield broad-spectrum inhibitors of HFMD. In this study, we have confirmed through the use of small interfering RNAs, human N-myristoyltransferase 1 plays an integral role in human Enterovirus 71 replication. Subsequent studies by inhibition of myristoylation using different myristic acid analogues elicited differential effects on the virus replication in human rhabdomyosarcoma cells. In particular, 2-hydroxymyristic acid specifically inhibited the cleavage between VP4 and VP2, part of the virion maturation process required to ensure infectivity of progeny virions while 4-oxatetradecanoic acid reduced the synthesis of viral RNA. These findings suggest that the requirement of a myristate moiety in viral structural protein precursor cleavage can serve as a viable antiviral target for further research.

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.

The VP4 peptide of hepatitis A virus ruptures membranes through formation of discrete pores

Membrane-active peptides, components of capsid structural proteins, assist viruses in overcoming the host membrane barrier in the initial stages of infection. Several such peptides have been identified, and their roles in membrane fusion or disruption have been characterized through biophysical studies. In several members of the Picornaviridae family, the role of the VP4 structural peptide in cellular-membrane penetration is well established. However, there is not much information on the membrane-penetrating capsid components of hepatitis A virus (HAV), an unusual member of this family. The VP4 peptide of HAV differs from its analogues in other picornaviruses in being significantly shorter in length and in lacking a signal for myristoylation, thought to be a critical requisite for VP4-mediated membrane penetration. Here we report, for the first time, that the atypical VP4 in HAV contains significant membrane-penetrating activity. Using a combination of biophysical assays and molecular dynamics simulation studies, we show that VP4 integrates into membrane vesicles through its N-terminal region to finally form discrete pores of 5- to 9-nm diameter, which induces leakage in the vesicles without altering their overall size or shape. We further demonstrate that the membrane activity of VP4 is specific toward vesicles mimicking the lipid content of late endosomes at acidic pH. Taken together, our data indicate that VP4 might be essential for the penetration of host endosomal membranes and release of the viral genome during HAV entry.

IMPORTANCE

Hepatitis A virus causes acute hepatitis in humans through the fecal-oral route and is particularly prevalent in underdeveloped regions with poor hygienic conditions. Although a vaccine for HAV exists, its high cost makes it unsuitable for universal application in developing countries. Studies on host-virus interaction for HAV have been hampered due to a lack of starting material, since the virus is extremely slow growing in culture. Among the unknown aspects of the HAV life cycle is its manner of host membrane penetration, which is one of the most important initial steps in viral infection. Here, we present data to suggest that a small peptide, VP4, a component of the HAV structural polyprotein, might be essential in helping the viral genome cross cell membranes during entry. It is hoped that this work might help in elucidating the manner of initial host cell interaction by HAV.

An interaction between glutathione and the capsid is required for the morphogenesis of C-cluster enteroviruses

Glutathione (GSH) is the most abundant cellular thiol playing an essential role in preserving a reduced cellular environment. Cellular GSH levels can be efficiently reduced by the GSH biosynthesis inhibitor, L-buthionine sulfoximine (BSO). The aim of our study was to determine the role of GSH in the growth of two C-cluster enteroviruses, poliovirus type 1 (PV1) and coxsackievirus A20 (CAV20). Our results show that the growth of both PV1 and CAV20 is strongly inhibited by BSO and can be partially reversed by the addition of GSH. BSO has no effect on viral protein synthesis or RNA replication but it strikingly reduces the accumulation of 14S pentamers in infected cells. GSH-pull down assays show that GSH directly interacts with capsid precursors and mature virus made in the absence of BSO whereas capsid precursors produced under GSH-depletion do not bind to GSH. In particular, the loss of binding of GSH may debilitate the stability of 14S pentamers, resulting in their failure to assemble into mature virus. Immunofluorescence cell imaging demonstrated that GSH-depletion did not affect the localization of viral capsid proteins to the replication complex. PV1 BSO resistant (BSOr) mutants evolved readily during passaging of the virus in the presence of BSO. Structural analyses revealed that the BSOr mutations, mapping to VP1 and VP3 capsid proteins, are primarily located at protomer/protomer interfaces. BSOr mutations might, in place of GSH, aid the stability of 14S particles that is required for virion maturation. Our observation that BSOr mutants are more heat resistant and need less GSH than wt virus to be protected from heat inactivation suggests that they possess a more stable capsid. We propose that the role of GSH during enterovirus morphogenesis is to stabilize capsid structures by direct interaction with capsid proteins both during and after the formation of mature virus particles.

Enteroviruses are plus stranded RNA viruses in the Picornaviridae family that cause as many as 3 billion infections per year. Enterovirus morphogenesis, which involves the encapsidation of newly made viral RNAs, has been studied for many years but the process is still poorly understood. Elucidation of this process is important for the development of drug treatments for a variety of human diseases. We describe the role of glutathione, an important cellular reducing agent, in enterovirus morphogenesis by studying the inhibition of GSH biosynthesis with BSO on viral proliferation. We discovered that GSH directly interacts with viral capsid precursors and the mature virus. In the presence of BSO the accumulation of a small capsid precusor (pentamer) is reduced and consequently no mature viruses are produced in virus-infected cells. Drug resistant viruses are easily isolated with mutations located in two of the capsid proteins, VP1 and VP3. We propose a model to explain the role of GSH in enterovirus morphogenesis, which is to stabilize the capsid precursors and the mature virus during and after the encapsidation of the progeny viral RNA by direct interaction with capsid proteins.

Role of the myristoylation site in expressing exogenous functional proteins in coxsackieviral vector

We generated a cardiotropic replication-competent chimeric coxsackievirus B3 (CVB3) to express alcohol dehydrogenase (ADH). Although exogenously expressed ADH was found by Western blot analysis, its enzyme function was repressed. To define the factor that inhibits the enzymatic function of ADH, we introduced a site-directed mutation at the second amino acid (MGAQEF···) of the CVB3 VP0 capsid protein, effectively changing glycine to alanine. This glycine is known to be a myristoylation site during viral capsid protein maturation in infected cells. In contrast to the unmodified virus, ADH expression and enzymatic function were readily detectable in the mutated rCVB3-ADH (G2A) virus. While expression of ADH required mutation of the CVB3 VP0 myristoylation site for proper function, another chimeric virus that expresses green fluorescent protein (rCVB3-GFP (G or A)) worked independently of the myristoylation site. Indeed, infected HeLa cells displayed GFP under a fluorescent microscope. These results indicate that the myristoylation site in the VP0 capsid protein inhibited the expression of enzymatically active ADH but not GFP. VP0 myristoylation is dispensable for chimeric CVB3 virus replication.

N-terminal myristoylation-dependent masking of neutralizing epitopes in the preS1 attachment site of hepatitis B virus

BACKGROUNDS & AIMS

The N-terminally myristoylated preS1 domain of the large hepatitis B surface protein (LHBs) mediates specific attachment of hepatitis B virus (HBV) to hepatocytes. Its B-cell epitopes leading to neutralization of infectivity are not yet characterized.

METHODS

We inserted C- and N-terminal preS1 peptides into the most immunogenic region of HBV core particles, therewith immunized Balb/c mice and determined binding properties and neutralization potential of resulting antibodies in vitro.

RESULTS

The particles with preS1 inserts were highly immunogenic and the corresponding anti-preS antibodies strongly bound to HBV particles from chronic carriers infected with different HBV genotypes A-F. However, antibodies binding to the C-terminal part of preS1 did not neutralize HBV infectivity for susceptible hepatocyte cultures. In contrast, antibodies elicited by the complete N-terminal attachment site of preS1(2-48) strongly neutralized with an IC50<3μg/ml of total immunoglobulin. Interestingly, antibodies against the very N-terminal part of preS1(1-21) could not neutralize infectivity although this sequence contains the most conserved and essential part of the attachment site. These antibodies reacted well with non-myristoylated preS1 peptides but only weakly with myristoylated preS1 peptides, natural HBsAg or HBV.

CONCLUSIONS

N-terminal myristic acid obviously favors a topology of LHBs that makes the most essential part of the preS1 attachment site inaccessible for neutralizing antibodies, whereas antibodies to neighbouring sequences neutralized very well. Thus, addition of the preS1(2-48) peptide in a highly immunogenic form to the current hepatitis B vaccine may improve protective immunity and reduce selection of escape mutations.

Uncoating of human rhinoviruses

Human rhinoviruses (HRVs) are a major cause of the common cold. The more than one hundred serotypes, divided into species HRV-A and HRV-B, either bind intercellular adhesion molecule 1 (major group viruses) or members of the low-density lipoprotein receptor (minor group viruses) for cell entry. Some major group HRVs can also access the host cell via heparan sulphate proteoglycans. The cell attachment protein(s) of the recently discovered phylogenetic clade HRV-C is unknown. The respective receptors direct virus uptake via clathrin-dependent or independent endocytosis or via macropinocytosis. Triggered by ICAM-1 and/or the low pH environment in endosomes the virions undergo conformational alterations giving rise to hydrophobic subviral particles. These are handed over from the receptors to the endosomal membrane. According to the current view, the RNA genome is released through an opening at one of the fivefold axes of the icosahedral capsid and crosses the membrane through a pore presumably formed by viral proteins. Alternatively, the membrane may be ruptured allowing subviral particles and RNA to enter the cytosol. Whether a channel is formed or the membrane is disrupted most probably depends on the respective HRV receptor.

Frog virus 3 ORF 53R, a putative myristoylated membrane protein, is essential for virus replication in vitro

Although previous work identified 12 complementation groups with possible roles in virus assembly, currently only one frog virus 3 protein, the major capsid protein (MCP), has been linked with virion formation. To identify other proteins required for assembly, we used an antisense morpholino oligonucleotide to target 53R, a putative myristoylated membrane protein, and showed that treatment resulted in marked reductions in 53R levels and a 60% drop in virus titers. Immunofluorescence assays confirmed knock down and showed that 53R was found primarily within viral assembly sites, whereas transmission electron microscopy detected fewer mature virions and, in some cells, dense granular bodies that may represent unencapsidated DNA-protein complexes. Treatment with a myristoylation inhibitor (2-hydroxymyristic acid) resulted in an 80% reduction in viral titers. Collectively, these data indicate that 53R is an essential viral protein that is required for replication in vitro and suggest it plays a critical role in virion formation.

Myristoylation of the arterivirus E protein: the fatty acid modification is not essential for membrane association but contributes significantly to virus infectivity

The envelope of equine arteritis virus (EAV) contains two glycoprotein complexes (GP2b/GP3/GP4 and GP5/M) and the small, non-glycosylated E protein. As E is essential for the production of infectious progeny but dispensable for assembly and release of virus-like particles, it probably mediates virus entry into cells, putatively in concert with the GP2b/GP3/GP4 complex. The E protein contains a central hydrophobic domain and a conserved potential site for N-terminal myristoylation, a hydrophobic modification usually pivotal for membrane targeting of the modified protein. Here, it was shown by radiolabelling that E is myristoylated at glycine-2, both in transfected cells as a fusion protein with yellow fluorescent protein (YFP) and in virus particles. Biochemical fractionation revealed that E–YFP with an inactivated acylation site was still completely membrane-bound, indicating that the putative transmembrane domain of E mediates membrane targeting. Confocal microscopy showed that both myristoylated and non-myristoylated E–YFP were localized to the endoplasmic reticulum and Golgi complex, the membranes from which EAV buds. The presence of a myristoylation inhibitor during replication of EAV, whilst completely blocking E acylation, reduced virus titres by 1.5 log10. Similarly, a mutant EAV with non-myristoylatable E grew to a titre five- to sevenfold lower than that of the wild-type virus and exhibited a reduced plaque size. Western blotting of cell-culture supernatants showed that N and M, the major structural proteins of EAV, are released in similar amounts by cells transfected with wild-type and mutant genomes. Thus, E myristoylation is not required for budding of particles and probably has a function during virus entry.

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.

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.

JC virus minor capsid proteins Vp2 and Vp3 are essential for virus propagation

Virus-encoded capsid proteins play a major role in the life cycles of all viruses. The JC virus capsid is composed of 72 pentamers of the major capsid protein Vp1, with one of the minor coat proteins Vp2 or Vp3 in the center of each pentamer. Vp3 is identical to two-thirds of Vp2, and these proteins share a DNA binding domain, a nuclear localization signal, and a Vp1-interacting domain. We demonstrate here that both the minor proteins and the myristylation site on Vp2 are essential for the viral life cycle, including the proper packaging of its genome.

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.

Genome delivery and ion channel properties are altered in VP4 mutants of poliovirus

During entry into host cells, poliovirus undergoes a receptor-mediated conformational transition to form 135S particles with irreversible exposure of VP4 capsid sequences and VP1 N termini. To understand the role of VP4 during virus entry, the fate of VP4 during infection by site-specific mutants at threonine-28 of VP4 (4028T) was compared with that of the parental Mahoney type 1 virus. Three virus mutants were studied: the entry-defective, nonviable mutant 4028T.G and the viable mutants 4028T.S and 4028T.V, in which residue threonine-28 was changed to glycine, serine, and valine, respectively. We show that mutant and wild-type (WT) VP4 proteins are localized to cellular membranes after the 135S conformational transition. Both WT and viable 4028T mutant particles interact with lipid bilayers to form ion channels, whereas the entry-defective 4028T.G particles do not. In addition, the electrical properties of the channels induced by the mutant viruses are different from each other and from those of WT Mahoney and Sabin type 3 viruses. Finally, uncoating and/or cytoplasmic delivery of the viral genome is altered in the 4028T mutants: the 4028T.G lethal mutant does not release its genome into the cytoplasm, and genome delivery is slower during infection by mutant 4028T.V 135S particles than by mutant 4028T.S or WT 135S particles. The distinctive electrical characteristics of the different 4028T mutant channels indicate that VP4 sequences might form part of the channel structure. The different entry phenotypes of these VP4 mutants suggest that the ion channels may be related to VP4's role during genome uncoating and/or delivery.

Analysis of mouse polyomavirus mutants with lesions in the minor capsid proteins

Polyomavirus mutants E, Q and H, expressing non-myristylated VP2, were generated by replacing the N-terminal glycine residue with glutamic acid, glutamine or histidine, respectively. Viruses mutated in either VP2 or VP3 translation initiation codons were also prepared. All mutated genomes, when transfected into murine host cells, gave rise to viral particles. Infectivity of VP2- and VP3- viruses, as measured by the number of cells expressing viral antigens, was dramatically diminished, indicative of defects in the early stages of infection. In contrast, the absence of a myristyl moiety on VP2 did not substantially affect the early steps of virus infection. No differences in numbers of cells expressing early or late viral antigens were observed between wild-type (wt) and E or Q myr- viruses during the course of a life cycle. Furthermore, no delay in virus DNA replication was detected. However, when cells were left for longer in culture, the number of infected cells, measured by typical virus bursts, was much lower when mutant rather than wt genomes were used. In situ, cell fractionation studies revealed differences in the interaction of viral particles with host cell structures. The infectivity of mutants was affected not only by loss of the myristyl group on VP2, but also, and to a greater extent, by alterations of the N-terminal amino acid composition.

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.

Uncoating kinetics of hepatitis A virus virions and provirions

When the growth kinetics of immature hepatitis A virus provirions and mature virions were monitored, distinct eclipse phases were noted for both types of particles. Strikingly, uncoating of virions occurred around 4 h postinfection, while uncoating of provirions occurred predominantly between 8 and 10 h postinfection. It is proposed that the heterogeneous mixture of infectious hepatitis A virus particles (virions and provirions) typically present in inocula is responsible for the normally asynchronous nature of hepatitis A virus uncoating kinetics.

The amino-terminal nine amino acid sequence of poliovirus capsid VP4 protein is sufficient to confer N-myristoylation and targeting to detergent-insoluble membranes

The confinement of membrane proteins by lipid-lipid interactions into specialized detergent-insoluble membrane (DIM) microdomains has been proposed as a general mechanism to recruit selectively lipid-modified proteins and specific transmembrane proteins. Poliovirus capsid VP4 protein and its precursors are myristoylated at the NH(2)-terminal Gly residue. To determine whether poliovirus uses DIMs during its replicative cycle, we isolated DIMs from poliovirus-infected HeLa cells and identified the presence of capsid proteins and their precursors, proteinases 2A and 3C, and other viral proteins involved in poliovirus RNA replication such as protein 2C and the polymerase 3D. The morphology of these DIMs was similar to that of the previously described rosette-like vesicles associated with replication complexes isolated from poliovirus-infected cells. To examine the possible role of the myristoyl moiety in the targeting of poliovirus structural proteins to DIMs, we generated a chimeric protein consisting of the nine amino-terminal amino acids from VP4 fused to the amino terminus of the green fluorescent protein (GFP). The selected VP4 sequence was sufficient to confer N-myristoylation and targeting to DIMs to the GFP chimera. Mutations within this sequence known to affect both myristoylation and poliovirus assembly abrogated the targeting of the GFP chimera. These results indicate that the myristoylated amino-terminal nonapeptide from poliovirus VP4 protein constitutes a signal for incorporation into DIMs.

The leader polypeptide of Theiler's murine encephalomyelitis virus is required for the assembly of virions in mouse L cells

Deletion of the entire leader polypeptide of the GDVII strain of Theiler's murine encephalomyelitis virus (TMEV) results in the production of an attenuated virus that grows in baby hamster kidney (BHK) cells but cannot grow at all in mouse L-929 cells. This study examined the reasons for the failure of dl-L, the GDVII variant that lacks the leader polypeptide, to grow in mouse cells. At low multiplicities of infection, it was difficult to detect any viral proteins in mouse cells. However, levels of positive- and negative-strand RNA molecules were only moderately reduced in these infections. Viral RNA showed no major defect in translatability, as the mutant viral RNA was nearly as efficient as that of the wild-type (WT) virus in directing protein synthesis in vitro in assays using extracts prepared from mouse L cells. Viral protein synthesis was detected in dl-L-infected mouse cells as multiplicities of infection were increased and approached the levels observed in WT infections. Despite this, there was a total lack of virus production in high-multiplicity infections, and this was found to correlate with the failure of viral proteins and early virion precursors to assemble into virions in mouse cells. Thus, the inability of dl-L to grow in mouse cells reflects complex effects on various stages of the virus infection but is primarily a defect in virus assembly.

Intrinsic signals for the assembly of hepatitis A virus particles. Role of structural proteins VP4 and 2A

Capsid assembly is the final event of virus replication, and its understanding is pivotal for the design of empty capsid-based recombinant vaccines and drug delivery systems. Although the capsid structure of several members of the picornavirus family has been elucidated, little is known about the structural elements governing the assembly process that is tightly associated with proteolytic processing of the viral polyprotein. Among the picornaviruses, hepatitis A virus (HAV) is unique in that it contains VP1-2A as a structural component and the small structural protein VP4, which argues for an assembly pathway different from that proposed for other picornaviruses. Using a recombinant system we show here that proteolytic processing of the HAV capsid proteins' precursor P1-2A is independent of the terminal domains 2A and VP4 of the substrate. However, both terminal domains play distinct roles in the assembly of viral particles. 2A as part of P1-2A is a primary signal for the assembly of pentameric structures which only further aggregate to empty viral capsids when VP4 is present as the N terminus of the precursor. Particle formation in the hepatovirus genus is thus regulated by two intrinsic signals that are distinct from those described for other picornaviruses.

Identification and analysis of three myristylated vaccinia virus late proteins

Previous studies have shown that at least three vaccinia virus (VV) late proteins (with apparent molecular asses of 37, 35, and 25 kDa) label with myristic acid. Time course labeling of VV-infected cells with [3H]myristic acid reveals at least three additional putative myristylproteins, with apparent molecular masses of 92, 17, and 14 kDa. The 25-kDa protein has previously been identified as that encoded by the L1R open reading frame, leaving the identities of the remaining proteins to be determined. Sequence analysis led to the preliminary identification of the 37-, 35-, and 17-kDa proteins as G9R, A16L, and E7R, respectively. Using synthetic oligonucleotides and PCR techniques, each of these open reading frames was amplified by using VV DNA as a template and then cloned individually into expression vectors behind T7 promoters. These plasmid constructs were then transcribed in vitro, and the resulting mRNAs were translated in wheat germ extracts and radiolabeled with either [35S]methionine or [3H]myristic acid. Each wild-type polypeptide was labeled with [35S]methionine or [3H]myristic acid in the translation reactions, while mutants containing an alanine in place of glycine at the N terminus were labeled only with [35S]methionine, not with myristic acid. This result provided strong evidence that the open reading frames had been correctly identified and that each protein is myristylated on a glycine residue adjacent to the initiating methionine. Subcellular fractionations of VV-infected cells suggested that A16L and E7R are soluble, in contrast to L1R, which is a membrane-associated protein.

In vitro antiviral activities of myristic acid analogs against human immunodeficiency and hepatitis B viruses

A group of myristic acid analogs, designed as alternative substrates for N-myristoyltransferase (NMT), were evaluated against human immunodeficiency virus (HIV), hepatitis B virus (HBV) and duck hepatitis B virus (DHBV) in vitro. Antiviral potency was increased when S or O was substituted for -CH2- in myristic acid and selectivity was affected by the presence and position of the heteroatoms and phenyl groups. A correlation was established among anti-HIV activity, Log P and Log D7.4 and between anti-HIV activity and carbonyl-heteroatom interatomic distances in the myristoyl analogs. 12-Thioethyldodecanoic acid 6 was moderately active (EC50 = 9.37 microM) against HIV-infected T4-lymphocytes (CEM-SS cell line), and it exhibited in vitro activity (EC50 = 17.8 microM) against HBV-producing 2.2.15 cell cultures derived from a human hepatoblastoma cell line (Hep G2). 12-Methoxydodecanoic acid 1 exhibited in vitro activity (EC50 = 20-30 microM) against hepatitis B in the HBV DNA-transfected 2.2.15 cell line. At a concentration of 10 microg/ml, none of the fatty acids significantly inhibited the replication of DHBV in infected hepatocytes.

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.

Myristoyl-CoA:protein N-myristoyltransferase activity in cancer cells. Purification and characterization of a cytosolic isoform from the murine leukemia cell line L1210

Myristoylation is a co-translational maturation process of proteins. It is extremely specific for the cosubstrate (myristoyl-CoA) and for the substrate protein that should bear a glycine at the N-terminus of the protein to be myristoylated. This acylation is catalyzed by the myristoyl-CoA:protein N-myristoyltransferase. Most of the molecular biochemistry and biology concerning this enzyme has been done on Saccharomyces cerevisiae. Because of the major importance of this pathway in several types of pathology, it is essential to study intensively the enzyme(s) isolated from mammalian tissue(s) to confirm that the enormous amount of work done on the yeast enzyme can be transposed to mammalian tissues. In earlier studies, we demonstrated the existence of a microsomal N-myristoyltransferase from the murine leukemia cell line L1210 [Boutin, J. A., Clarenc, J.-P., Ferry, G., Ernould, A. P., Remond, G., Vincent, M. & Atassi, G. (1991) Eur. J. Biochem. 201, 257-263], a feature which is not shared by yeast, and examined the N-myristoyltransferase activities associated with L1210 cytosol. In the present work, we purified to homogeneity one of the isoforms (A) of the transferase from L1210 cytosol. The purified enzyme showed on SDS/PAGE an apparent molecular mass of 67.5 kDa, distinct from the 53-kDa yeast cytosolic enzyme. The purified enzyme from L1210 cytosol could be labeled with [14C]myristoyl-CoA. Rabbit antibodies were raised against the A isoform and used to immunoprecipitate the enzyme and immunoinhibit the activity from the same source. A survey of the specificity of the partially and completely purified isoforms was performed using peptides derived from the NH2-terminus of 42 proteins which are potential substrates for myristoylation, including oncogene products and virus structural proteins. We synthesized a series of compounds capable of inhibiting the cytosol activities of the enzyme. For example, a myristoyltetrahydroquinolein derivative showed an IC50 of about 0.1 microM. Based on both biophysical and biochemical evidence, the N-myristoyltransferases extracted from mammalian cell cytosols seem to be different from the extensively studied yeast enzyme.

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)

Viral acylproteins: greasing the wheels of assembly

Viruses take advantage of the host's protein modification and targeting pathways to modify their own proteins and to ensure that they assume active configurations and locate appropriately for assembly. In many viruses, one recurrent theme in such processes is exploitation of cellular protein acylation pathways for the addition of myristic and palmitic acid to capsid or envelope proteins.

Efficient analysis of nonviable poliovirus capsid mutants

Nonviable poliovirus capsid mutants were studied by an efficient infection-transfection system. Phenotypically, nonviable poliovirus capsid mutants appear to segregate into three classes: those that form only protomers, those that can form pentamers, and one that can form completed virions.

Myristate-protein interactions in poliovirus: interactions of VP4 threonine 28 contribute to the structural conformation of assembly intermediates and the stability of assembled virions

The VP4 capsid protein of poliovirus is N-terminally modified with myristic acid. Within the poliovirus structure, a hydrogen bond is observed between the myristate carbonyl and the hydroxyl side chain of threonine 28 of VP4. This interaction is between two fivefold symmetry-related copies of VP4 and is one of several myristoyl-mediated interactions that appears to structurally link the promoters within the pentamer subunit of the virus particle. Site-specific substitutions of the threonine residue were constructed to investigate the biological relevance of these myristate-protein interactions. Replacement of the threonine with glycine or lysine is lethal, generating nonviable viruses. Substitution with serine or valine led to viable viruses, but these mutants displayed anomalies during virus assembly. In addition, both assembled serine- and valine-substituted virion particles showed reduced infectivity and were more sensitive to thermal inactivation and antibody neutralization. Thus the threonine residue provides interactions necessary for efficient assembly of the virus and for virion stability.

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.

Reovirus polypeptide sigma 3 and N-terminal myristoylation of polypeptide mu 1 are required for site-specific cleavage to mu 1C in transfected cells

N-myristoylated viral polypeptide mu 1 was produced in COS cells transfected with a transient expression vector containing a DNA copy of the reovirus M2 gene. The mu 1 product was specifically cleaved to polypeptide mu 1C in cells that were cotransfected with the reovirus S4 gene and that expressed polypeptide sigma 3. Studies with site-specific mutants of the M2 gene demonstrated that conversion of mu 1 to mu 1C was dependent on myristoylation and the presence of the proteolytic cleavage sequence asparagine 42-proline 43 in mu 1, as well as on the presence of polypeptide sigma 3. The mu 1C product and polypeptide sigma 3 formed complexes that were immunoprecipitated by sigma 3-directed antibody, and a myristoylation-negative M2 double mutant, G2A-N42T, yielded mu 1 that did not undergo cleavage to mu 1C or bind sigma 3. However, the N42T single mutant did form immunoprecipitable complexes with sigma 3, indicating that binding can occur in the absence of cleavage. Polypeptide sigma 3 alternatively can bind double-stranded RNA and in COS cells stimulates translation of reporter chloramphenicol acetyltransferase mRNA translation, presumably by blocking double-stranded RNA-mediated activation of the eukaryotic initiation factor 2 alpha subunit kinase which inhibits the initiation of protein synthesis. Consistent with these observations and with the formation of mu 1C-sigma 3 complexes, coexpression of M2 with S4 DNA prevented the translational stimulatory effect of polypeptide sigma 3.