Mumps virus matrix, fusion, and nucleocapsid proteins cooperate for efficient production of virus-like particles

Self-assembly and release of peste des petits ruminants virus-like particles in an insect cell-baculovirus system and their immunogenicity in mice and goats

Peste des petits ruminants (PPR) is an acute, febrile, viral disease of small ruminants that has a significant economic impact. For many viral diseases, vaccination with virus-like particles (VLPs) has shown considerable promise as a prophylactic approach; however, the processes of assembly and release of peste des petits ruminants virus (PPRV) VLPs are not well characterized, and their immunogenicity in the host is unknown. In this study, VLPs of PPRV were generated in a baculovirus system through simultaneous expression of PPRV matrix (M) protein and hemaglutin in (H) or fusion (F) protein. The released VLPs showed morphology similar to that of the native virus particles. Subcutaneous injection of these VLPs (PPRV-H, PPRV-F) into mice and goats elicited PPRV-specific IgG production, increased the levels of virus neutralizing antibodies, and promoted lymphocyte proliferation. Without adjuvants, the immune response induced by the PPRV-H VLPs was comparable to that obtained using equivalent amounts of PPRV vaccine. Thus, our results demonstrated that VLPs containing PPRV M protein and H or F protein are potential “differentiating infected from vaccinated animals” (DIVA) vaccine candidates for the surveillance and eradication of PPR.

Paramyxovirus glycoprotein incorporation, assembly and budding: a three way dance for infectious particle production

Paramyxoviruses are a family of negative sense RNA viruses whose members cause serious diseases in humans, such as measles virus, mumps virus and respiratory syncytial virus; and in animals, such as Newcastle disease virus and rinderpest virus. Paramyxovirus particles form by assembly of the viral matrix protein, the ribonucleoprotein complex and the surface glycoproteins at the plasma membrane of infected cells and subsequent viral budding. Two major glycoproteins expressed on the viral envelope, the attachment protein and the fusion protein, promote attachment of the virus to host cells and subsequent virus-cell membrane fusion. Incorporation of the surface glycoproteins into infectious progeny particles requires coordinated interplay between the three viral structural components, driven primarily by the matrix protein. In this review, we discuss recent progress in understanding the contributions of the matrix protein and glycoproteins in driving paramyxovirus assembly and budding while focusing on the viral protein interactions underlying this process and the intracellular trafficking pathways for targeting viral components to assembly sites. Differences in the mechanisms of particle production among the different family members will be highlighted throughout.

Budding of peste des petits ruminants virus-like particles from insect cell membrane based on intracellular co-expression of peste des petits ruminants virus M, H and N proteins by recombinant baculoviruses

Peste des petits ruminants virus (PPRV), an etiological agent of peste des petits ruminants (PPR), is classified into the genus Morbillivirus in the family Paramyxovirida. In this study, two full-length open reading frames (ORF) corresponding to the PPRV matrix (M) and haemagglutinin (H) genes underwent a codon-optimization based on insect cells, respectively. Two codon-optimized ORFs along with one native nucleocapsid (N) ORF were used to construct recombinant baculoviruses co-expressing the PPRV M, H and N proteins in insect cells. Analysis of Western blot, immunofluorescence, confocal microscopy and flow cytometry demonstrated co-expression of the three proteins but at different levels in insect cells, and PPR virus-like particles (VLPs) budded further from cell membrane based on self-assembly of the three proteins by viewing of ultrathin section with a transmission electron microscope (TEM). Subsequently, a small number of VLPs were purified by sucrose density gradient centrifugation for TEM viewing. The PPR VLPs, either purified by sucrose density gradient centrifugation or budding from insect cell membrane on ultrathin section, morphologically resembled authentic PPRVs but were smaller in diameter by the TEM examination.

Structural analysis of respiratory syncytial virus reveals the position of M2-1 between the matrix protein and the ribonucleoprotein complex

Respiratory syncytial virus (RSV), a member of the Paramyxoviridae family of nonsegmented, negative-sense, single-stranded RNA genome viruses, is a leading cause of lower respiratory tract infections in infants, young children, and the elderly or immunocompromised. There are many open questions regarding the processes that regulate human RSV (hRSV) assembly and budding. Here, using cryo-electron tomography, we identified virus particles that were spherical, filamentous, and asymmetric in structure, all within the same virus preparation. The three particle morphologies maintained a similar organization of the surface glycoproteins, matrix protein (M), M2-1, and the ribonucleoprotein (RNP). RNP filaments were traced in three dimensions (3D), and their total length was calculated. The measurements revealed the inclusion of multiple full-length genome copies per particle. RNP was associated with the membrane whenever the M layer was present. The amount of M coverage ranged from 24% to 86% in the different morphologies. Using fluorescence light microscopy (fLM), direct stochastic optical reconstruction microscopy (dSTORM), and a proximity ligation assay (PLA), we provide evidence illustrating that M2-1 is located between RNP and M in isolated viral particles. In addition, regular spacing of the M2-1 densities was resolved when hRSV viruses were imaged using Zernike phase contrast (ZPC) cryo-electron tomography. Our studies provide a more complete characterization of the hRSV virion structure and substantiation that M and M2-1 regulate virus organization.

IMPORTANCE

hRSV is a leading cause of lower respiratory tract infections in infants and young children as well as elderly or immunocompromised individuals. We used cryo-electron tomography and Zernike phase contrast cryo-electron tomography to visualize populations of purified hRSV in 3D. We observed the three distinct morphologies, spherical, filamentous, and asymmetric, which maintained comparable organizational profiles. Depending on the virus morphology examined, the amount of M ranged from 24% to 86%. We complemented the cryo-imaging studies with fluorescence microscopy, dSTORM, and a proximity ligation assay to provide additional evidence that M2-1 is incorporated into viral particles and is positioned between M and RNP. The results highlight the impact of M and M2-1 on the regulation of hRSV organization.

Virus budding and the ESCRT pathway

Enveloped viruses escape infected cells by budding through limiting membranes. In the decade since the discovery that HIV recruits cellular ESCRT (endosomal sorting complexes required for transport) machinery to facilitate viral budding, this pathway has emerged as the major escape route for enveloped viruses. In cells, the ESCRT pathway catalyzes analogous membrane fission events required for the abscission stage of cytokinesis and for a series of "reverse topology" vesiculation events. Studies of enveloped virus budding are therefore providing insights into the complex cellular mechanisms of cell division and membrane protein trafficking (and vice versa). Here, we review how viruses mimic cellular recruiting signals to usurp the ESCRT pathway, discuss mechanistic models for ESCRT pathway functions, and highlight important research frontiers.

Rabies virus-like particles expressed in HEK293 cells

Rabies is an infectious viral disease with a mortality rate close to 100%. Currently, there is a need to generate cheaper and more immunogenic vaccines for the effective prevention of rabies, mostly in developing countries. Virus-like particles have been widely used in viral vaccine production due to their high immunogenicity and safety during the production process. Rabies virus glycoprotein is the major antigen to trigger a protective immune response and the only protein capable of generating virus neutralizing antibodies. In this study we describe the development of a recombinant stable cell line for the production of rabies virus-like particles (RV-VLPs) expressing the rabies virus glycoprotein by lentivirus-based transduction of HEK293 cells. Protein expression was analyzed by flow cytometry, fluorescence microscopy, western blot and ELISA. Particles were purified from culture supernatant and their size and morphology were studied. Furthermore, mice were immunized with RV-VLPs, formulated with adjuvant, and these particles were able to produce a specific antibody response, demonstrating that these virus-like particles present a promising rabies vaccine candidate.

Mutations in the FPIV motif of Newcastle disease virus matrix protein attenuate virus replication and reduce virus budding

The FPIV-like late domains identified in the matrix (M) proteins of parainfluenza virus 5 and mumps virus have been demonstrated to be critical for virus budding. In this study, we found that the same FPIV sequence motif is present in the N-terminus of the Newcastle disease virus (NDV) M protein. Mutagenesis experiments demonstrated that mutation of either phenylalanine (F) or proline (P) to alanine led to a more obvious decrease in viral virulence and replication and resulted in poor budding of the mutant viruses. Additionally, evidence for the involvement of cellular multivesicular body (MVB) proteins was obtained, since NDV production was inhibited upon expression of dominant-negative versions of the VPS4A-E228Q protein. Together, these results demonstrate that the FPIV motif, especially the residues F and P, within the NDV M protein, plays a critical role in NDV replication and budding, and this budding process likely involves the cellular MVB pathway.

Newcastle disease virus-like particles: preparation, purification, quantification, and incorporation of foreign glycoproteins

Virus-like particles (VLPs) are large particles, the size of viruses, composed of repeating structures that mimic those of infectious virus. Since their structures are similar to that of viruses, they have been used to study the mechanisms of virus assembly. They are also in development for delivery of molecules to cells and in studies of the immunogenicity of particle-associated antigens. However, they have been most widely used for development of vaccines and vaccine candidates. VLPs can form upon the expression of the structural proteins of many different viruses. This chapter describes the generation and purification of VLPs formed with the structural proteins, M, NP, F, and HN proteins, of Newcastle disease virus (NDV). Newcastle disease virus-like particles (ND VLPs) have also been developed as a platform for assembly into VLPs of glycoproteins from other viruses. This chapter describes the methods for this use of ND VLPs.

A stabilized headless measles virus attachment protein stalk efficiently triggers membrane fusion

Paramyxovirus attachment and fusion (F) envelope glycoprotein complexes mediate membrane fusion required for viral entry. The measles virus (MeV) attachment (H) protein stalk domain is thought to directly engage F for fusion promotion. However, past attempts to generate truncated, fusion-triggering-competent H-stem constructs remained fruitless. In this study, we addressed the problem by testing the hypothesis that truncated MeV H stalks may require stabilizing oligomerization tags to maintain intracellular transport competence and F-triggering activity. We engineered H-stems of different lengths with added 4-helix bundle tetramerization domains and demonstrate restored cell surface expression, efficient interaction with F, and fusion promotion activity of these constructs. The stability of the 4-helix bundle tags and the relative orientations of the helical wheels of H-stems and oligomerization tags govern the kinetics of fusion promotion, revealing a balance between H stalk conformational stability and F-triggering activity. Recombinant MeV particles expressing a bioactive H-stem construct in the place of full-length H are viable, albeit severely growth impaired. Overall, we demonstrate that the MeV H stalk represents the effector domain for MeV F triggering. Fusion promotion appears linked to the conformational flexibility of the stalk, which must be tightly regulated in viral particles to ensure efficient virus entry. While the pathways toward assembly of functional fusion complexes may differ among diverse members of the paramyxovirus family, central elements of the triggering machinery emerge as highly conserved.

Assembly and budding of negative-strand RNA viruses

Assembly of negative-strand RNA viruses occurs by budding from host plasma membranes. The budding process involves association of the viral core or nucleocapsid with a region of cellular membrane that will become the virus budding site, which contains the envelope glycoproteins and matrix protein. This region of membrane then buds out and pinches off to become the virus envelope. This review will address the questions of what are the mechanisms that bring the nucleocapsid and envelope glycoproteins together to form the virus budding site, and how does this lead to release of progeny virions? Recent evidence supports the idea that viral envelope glycoproteins and matrix proteins are organized into membrane microdomains that coalesce to form virus budding sites. There has also been substantial progress in understanding the last step in virus release, referred to as the "late budding function," which often involves host proteins of the vacuolar protein sorting apparatus. Key questions are raised as to the mechanism of the initial steps in formation of virus budding sites: How are membrane microdomains brought together and how are nucleocapsids selected for incorporation into these budding sites, particularly in the case of viruses for which genome RNA sequences are important for envelopment of nucleocapsids?

Critical role of the fusion protein cytoplasmic tail sequence in parainfluenza virus assembly

Interactions between viral glycoproteins, matrix protein and nucleocapsid sustain assembly of parainfluenza viruses at the plasma membrane. Although the protein interactions required for virion formation are considered to be highly specific, virions lacking envelope glycoprotein(s) can be produced, thus the molecular interactions driving viral assembly and production are still unclear. Sendai virus (SeV) and human parainfluenza virus type 1 (hPIV1) are highly similar in structure, however, the cytoplasmic tail sequences of the envelope glycoproteins (HN and F) are relatively less conserved. To unveil the specific role of the envelope glycoproteins in viral assembly, we created chimeric SeVs whose HN (rSeVhHN) or HN and F (rSeVh(HN+F)) were replaced with those of hPIV1. rSeVhHN grew as efficiently as wt SeV or hPIV1, suggesting that the sequence difference in HN does not have a significant impact on SeV replication and virion production. In sharp contrast, the growth of rSeVh(HN+F) was significantly impaired compared to rSeVhHN. rSeVh(HN+Fstail) which expresses a chimeric hPIV1 F with the SeV cytoplasmic tail sequence grew similar to wt SeV or rSeVhHN. Further analysis indicated that the F cytoplasmic tail plays a critical role in cell surface expression/accumulation of HN and F, as well as NP and M association at the plasma membrane. Trafficking of nucelocapsids in infected cells was not significantly affected by the origin of F, suggesting that F cytoplasmic tail is not involved in intracellular movement. These results demonstrate the role of the F cytoplasmic tail in accumulation of structural components at the plasma membrane assembly sites.

Arenavirus budding: a common pathway with mechanistic differences

The Arenaviridae is a diverse and growing family of viruses that includes several agents responsible for important human diseases. Despite the importance of this family for public health, particularly in Africa and South America, much of its biology remains poorly understood. However, in recent years significant progress has been made in this regard, particularly relating to the formation and release of new enveloped virions, which is an essential step in the viral lifecycle. While this process is mediated chiefly by the viral matrix protein Z, recent evidence suggests that for some viruses the nucleoprotein (NP) is also required to enhance the budding process. Here we highlight and compare the distinct budding mechanisms of different arenaviruses, concentrating on the role of the matrix protein Z, its known late domain sequences, and the involvement of cellular endosomal sorting complex required for transport (ESCRT) pathway components. Finally we address the recently described roles for the nucleoprotein NP in budding and ribonucleoprotein complex (RNP) incorporation, as well as discussing possible mechanisms related to its involvement.

Virus-like particles as a highly efficient vaccine platform: diversity of targets and production systems and advances in clinical development

Virus-like particles (VLPs) are a class of subunit vaccines that differentiate themselves from soluble recombinant antigens by stronger protective immunogenicity associated with the VLP structure. Like parental viruses, VLPs can be either non-enveloped or enveloped, and they can form following expression of one or several viral structural proteins in a recombinant heterologous system. Depending on the complexity of the VLP, it can be produced in either a prokaryotic or eukaryotic expression system using target-encoding recombinant vectors, or in some cases can be assembled in cell-free conditions. To date, a wide variety of VLP-based candidate vaccines targeting various viral, bacterial, parasitic and fungal pathogens, as well as non-infectious diseases, have been produced in different expression systems. Some VLPs have entered clinical development and a few have been licensed and commercialized. This article reviews VLP-based vaccines produced in different systems, their immunogenicity in animal models and their status in clinical development.

Budding of Enveloped Viruses: Interferon-Induced ISG15-Antivirus Mechanisms Targeting the Release Process

Pathogenic strains of viruses that infect humans are encapsulated in membranes derived from the host cell in which they infect. After replication, these viruses are released by a budding process that requires cell/viral membrane scission. As such, this represents a natural target for innate immunity mechanisms to interdict enveloped virus spread and recent advances in this field will be the subject of this paper.

Gene-specific contributions to mumps virus neurovirulence and neuroattenuation

Mumps virus (MuV) is highly neurotropic and was the leading cause of aseptic meningitis in the Western Hemisphere prior to widespread use of live attenuated MuV vaccines. Due to the absence of markers of virus neuroattenuation and neurovirulence, ensuring mumps vaccine safety has proven problematic, as demonstrated by the occurrence of aseptic meningitis in recipients of certain vaccine strains. Here we examined the genetic basis of MuV neuroattenuation and neurovirulence by generating a series of recombinant viruses consisting of combinations of genes derived from a cDNA clone of the neurovirulent wild-type 88-1961 strain (r88) and from a cDNA clone of the highly attenuated Jeryl Lynn vaccine strain (rJL). Testing of these viruses in rats demonstrated the ability of several individual rJL genes and gene combinations to significantly neuroattenuate r88, with the greatest effect imparted by the rJL nucleoprotein/matrix protein combination. Interestingly, no tested combination of r88 genes, including the nucleoprotein/matrix protein combination, was able to convert rJL into a highly neurovirulent virus, highlighting mechanistic differences between processes involved in neuroattenuation and neurovirulence.

Requirements for Human Respiratory Syncytial Virus Glycoproteins in Assembly and Egress from Infected Cells

Human respiratory syncytial virus (HRSV) is an enveloped RNA virus that assembles and buds from the plasma membrane of infected cells. The ribonucleoprotein complex (RNP) must associate with the viral matrix protein and glycoproteins to form newly infectious particles prior to budding. The viral proteins involved in HRSV assembly and egress are mostly unexplored. We investigated whether the glycoproteins of HRSV were involved in the late stages of viral replication by utilizing recombinant viruses where each individual glycoprotein gene was deleted and replaced with a reporter gene to maintain wild-type levels of gene expression. These engineered viruses allowed us to study the roles of the glycoproteins in assembly and budding in the context of infectious virus. Microscopy data showed that the F glycoprotein was involved in the localization of the glycoproteins with the other viral proteins at the plasma membrane. Biochemical analyses showed that deletion of the F and G proteins affected incorporation of the other viral proteins into budded virions. However, efficient viral release was unaffected by the deletion of any of the glycoproteins individually or in concert. These studies attribute a novel role to the F and G proteins in viral protein localization and assembly.

The conserved YAGL motif in human metapneumovirus is required for higher-order cellular assemblies of the matrix protein and for virion production

YXXL motifs in cellular and viral proteins have a variety of functions. The matrix (M) protein of the respiratory pathogen human metapneumovirus (hMPV) contains two such conserved motifs--YSKL and YAGL. We mutated these sequences to analyze their contributions to hMPV infectivity. The mutant clones were capable of intracellular replication; however, the YAGL but not YSKL mutants were defective at spreading in infected cultures. We improved the reverse genetics system for hMPV and generated cell lines that stably expressed selectable, replicating full-length genomes for both the wild type and the mutant clones, allowing microscopic and biochemical analyses of these viruses. YAGL mutants produced normal cellular levels of M protein but failed to release virions, while ectopic coexpression of wild-type M generated particles that were restricted to a single cycle of infection. The YAGL motif did not act as a late (L) domain, however, since hMPV budding was independent of the cellular endosomal sorting complex required for transport (ESCRT) machinery and because replacement of the YAGL motif with classical L domains generated defective viruses. Instead, the YAGL mutants had defective M assemblies lacking a normal filamentous appearance and showed poor extractability from the cell compared to the wild-type protein. The mutant proteins were not grossly misfolded, however, as they interacted with cellular membranes and coassembled with wild-type M proteins. Thus, the YAGL motif is an important determinant of hMPV assembly. Furthermore, the selectable hMPV genomes described here should extend the use of reverse genetics systems in the analysis of spreading-defective viruses.

Efficient cellular release of Rift Valley fever virus requires genomic RNA

The Rift Valley fever virus is responsible for periodic, explosive epizootics throughout sub-Saharan Africa. The development of therapeutics targeting this virus is difficult due to a limited understanding of the viral replicative cycle. Utilizing a virus-like particle system, we have established roles for each of the viral structural components in assembly, release, and virus infectivity. The envelope glycoprotein, Gn, was discovered to be necessary and sufficient for packaging of the genome, nucleocapsid protein and the RNA-dependent RNA polymerase into virus particles. Additionally, packaging of the genome was found to be necessary for the efficient release of particles, revealing a novel mechanism for the efficient generation of infectious virus. Our results identify possible conserved targets for development of anti-phlebovirus therapies.

The cellular endosomal sorting complex required for transport pathway is not involved in avian metapneumovirus budding in a virus-like-particle expression system

Avian metapneumovirus (AMPV) is a paramyxovirus that principally causes respiratory disease and egg production drops in turkeys and chickens. Together with its closely related human metapneumovirus (HMPV), they comprise the genus Metapneumovirus in the family Paramyxoviridae. Little is currently known about the mechanisms involved in the budding of metapneumovirus. By using AMPV as a model system, we showed that the matrix (M) protein by itself was insufficient to form virus-like-particles (VLPs). The incorporation of M into VLPs was shown to occur only when both the viral nucleoprotein (N) and the fusion (F) proteins were co-expressed. Furthermore, we provided evidence indicating that two YSKL and YAGL segments encoded within the M protein were not a functional late domain, and the endosomal sorting complex required for transport (ESCRT) machinery was not involved in metapneumovirus budding, consistent with a recent observation that human respiratory syncytial virus, closely related to HMPV, uses an ESCRT-independent budding mechanism. Taken together, these results suggest that metapneumovirus budding is independent of the ESCRT pathway and the minimal budding machinery described here will aid our future understanding of metapneumovirus assembly and egress.

Parainfluenza virus 5 m protein interaction with host protein 14-3-3 negatively affects virus particle formation

Paramyxovirus matrix (M) proteins organize virus assembly, linking viral glycoproteins and viral ribonucleoproteins together at virus assembly sites on cellular membranes. Using a yeast two-hybrid screening approach, we identified 14-3-3 as a binding partner for the M protein of parainfluenza virus 5 (PIV5). Binding in both transfected and PIV5-infected cells was confirmed by coimmunoprecipitation and was mapped to a C-terminal region within the M protein, namely, 366-KTKSLP-371. This sequence resembles known 14-3-3 binding sites, in which the key residue for binding is a phosphorylated serine residue. Mutation of S369 within the PIV5 M protein disrupted 14-3-3 binding and improved the budding of both virus-like particles (VLPs) and recombinant viruses, suggesting that 14-3-3 binding impairs virus budding. 14-3-3 protein overexpression reduced the budding of VLPs. Using (33)P labeling, phosphorylated M protein was detected in PIV5-infected cells, and this phosphorylation was nearly absent in cells infected with a recombinant virus harboring an S369A mutation within the M protein. Assembly of the M protein into clusters and filaments at infected cell surfaces was enhanced in cells infected with a recombinant virus defective in 14-3-3 binding. These findings support a model in which a portion of M protein within PIV5-infected cells is phosphorylated at residue S369, binds the 14-3-3 protein, and is held away from sites of virus budding.

The C-terminal end of parainfluenza virus 5 NP protein is important for virus-like particle production and M-NP protein interaction

Enveloped virus particles are formed by budding from infected-cell membranes. For paramyxoviruses, viral matrix (M) proteins are key drivers of virus assembly and budding. However, other paramyxovirus proteins, including glycoproteins, nucleocapsid (NP or N) proteins, and C proteins, are also important for particle formation in some cases. To investigate the role of NP protein in parainfluenza virus 5 (PIV5) particle formation, NP protein truncation and substitution mutants were analyzed. Alterations near the C-terminal end of NP protein completely disrupted its virus-like particle (VLP) production function and significantly impaired M-NP protein interaction. Recombinant viruses with altered NP proteins were generated, and these viruses acquired second-site mutations. Recombinant viruses propagated in Vero cells acquired mutations that mainly affected components of the viral polymerase, while recombinant viruses propagated in MDBK cells acquired mutations that mainly affected the viral M protein. Two of the Vero-propagated viruses acquired the same mutation, V/P(S157F), found previously to be responsible for elevated viral gene expression induced by a well-characterized variant of PIV5, P/V-CPI(-). Vero-propagated viruses caused elevated viral protein synthesis and spread rapidly through infected monolayers by direct cell-cell fusion, bypassing the need to bud infectious virions. Both Vero- and MDBK-propagated viruses exhibited infectivity defects and altered polypeptide composition, consistent with poor incorporation of viral ribonucleoprotein complexes (RNPs) into budding virions. Second-site mutations affecting M protein restored interaction with altered NP proteins in some cases and improved VLP production. These results suggest that multiple avenues are available to paramyxoviruses for overcoming defects in M-NP protein interaction.

Significance of the YLDL motif in the M protein and Alix/AIP1 for Sendai virus budding in the context of virus infection

Sendai virus (SeV) M protein has a YLDL motif, which is essential for budding of virus-like particles (VLPs) by expression of the M protein. We investigated the importance of the YLDL motif for SeV budding. Virus budding of an M-deficient SeV was not rescued by transient expression of motif mutants, M-A2 (ALDA) and M-A4 (AAAA), and viruses possessing those mutations hardly propagated in cultured cells. However, a budding-competent revertant virus, SeV M-A2R, was obtained from SeV M-A2, and nucleotide sequencing showed an ALDV sequence at the motif instead of the ALDA sequence derived from M-A2. The M-A2R protein rescued budding of an M-deficient SeV, formed VLPs when expressed with viral C protein, and restored the capacity to bind with Alix/AIP1. The results indicate that the YLDL motif is essential for efficient budding in the context of virus infection and suggest involvement of Alix/AIP1 in SeV budding.

Newcastle disease virus-like particles as a platform for the development of vaccines for human and agricultural pathogens

Vaccination is the single most effective way to control viral diseases. However, many currently used vaccines have safety concerns, efficacy issues or production problems. For other viral pathogens, classic approaches to vaccine development have, thus far, been unsuccessful. Virus-like particles (VLPs) are increasingly being considered as vaccine candidates because they offer significant advantages over many currently used vaccines or developing vaccine technologies. VLPs formed with structural proteins of Newcastle disease virus, an avian paramyxovirus, are a potential vaccine candidate for Newcastle disease in poultry. More importantly, these VLPs are a novel, uniquely versatile VLP platform for the rapid construction of effective vaccine candidates for many human pathogens, including genetically complex viruses and viruses for which no vaccines currently exist.

[Envelope virus assembly and budding]

For many enveloped viruses, viral matrix and retroviral Gag proteins are able to bud from the cell surface by themselves in the form of lipid-enveloped, virus-like particles (VLPs), suggesting that these proteins play important roles in viral assembly and budding. The major three-types of L-domain motifs, PPxY, P(T/S)AT, and YP(x)(n)L have been identified within these proteins. Many viruses have been shown to commonly utilize cellular ESCRT pathway via direct interaction between the L-domains and the components of the pathway for efficient viral budding. However, for many enveloped viruses, L-domain motifs have not yet been identified, and the involvement of the ESCRT pathway in virus budding is still unknown. Among such viruses, we have been focusing on Sendai virus (SeV) and shown that (i) SeV M functionally and physically interact with a component of the ESCRT complex, Alix/AIP1, although budding of M-VLPs does not seem to be dependent on the pathway; (ii) one of the accessory proteins of SeV, C, also interact with Alix/AIP1, and recruit it to the plasma membrane for efficient budding of M-VLPs; (iii) the C protein regulate balance of viral genome and antigenome RNA synthesis for optimized production of infectious virus particles. These results demonstrate a unique mechanism for budding of SeV as well as a novel mechanism of regulated synthesis of viral genome RNAs for efficient production of infectious particles.

Tsg101 is recruited by a late domain of the nucleocapsid protein to support budding of Marburg virus-like particles

The nucleoprotein NP of Marburg virus (MARV) is the major component of the viral nucleocapsid, which also consists of the viral proteins VP35, L, and VP30, as well as the viral genome. During virus assembly at the plasma membrane, the nucleocapsids are enwrapped by the major matrix protein VP40 and the viral envelope, which contains the transmembrane glycoprotein GP. Upon recombinant expression, VP40 alone is able to induce the formation and release of virus-like particles (VLPs) that closely resemble the filamentous morphology of MARV particles. Release of these VP40-induced VLPs is partially dependent on the cellular ESCRT machinery, which interacts with a late-domain motif in VP40. Coexpression with NP significantly enhances the budding of VP40-induced VLPs by an unknown mechanism. In the present study we analyzed the impact of late domains present in NP on the release of VLPs. We observed that the ESCRT I protein Tsg101 was recruited by NP into NP-induced inclusions in the perinuclear region. In the presence of VP40, NP was then recruited to VP40-positive membrane clusters and, in turn, recruited Tsg101 via a C-terminal PSAP late-domain motif in NP. This PSAP motif also mediated a dramatically enhanced incorporation of Tsg101 into VLPs, and its deletion significantly diminished the positive effect of NP on the release of VLPs. Taken together, these data indicate that NP enhances budding of VLPs by recruiting Tsg101 to the VP40-positive budding site through a PSAP late-domain motif.

Paramyxovirus assembly and budding: building particles that transmit infections

The paramyxoviruses define a diverse group of enveloped RNA viruses that includes a number of important human and animal pathogens. Examples include human respiratory syncytial virus and the human parainfluenza viruses, which cause respiratory illnesses in young children and the elderly; measles and mumps viruses, which have caused recent resurgences of disease in developed countries; the zoonotic Hendra and Nipah viruses, which have caused several outbreaks of fatal disease in Australia and Asia; and Newcastle disease virus, which infects chickens and other avian species. Like other enveloped viruses, paramyxoviruses form particles that assemble and bud from cellular membranes, allowing the transmission of infections to new cells and hosts. Here, we review recent advances that have improved our understanding of events involved in paramyxovirus particle formation. Contributions of viral matrix proteins, glycoproteins, nucleocapsid proteins, and accessory proteins to particle formation are discussed, as well as the importance of host factor recruitment for efficient virus budding. Trafficking of viral structural components within infected cells is described, together with mechanisms that allow for the selection of specific sites on cellular membranes for the coalescence of viral proteins in preparation of bud formation and virion release.

Measles virus M protein-driven particle production does not involve the endosomal sorting complex required for transport (ESCRT) system

Assembly and budding of enveloped RNA viruses rely on viral matrix (M) proteins and host proteins involved in sorting and vesiculation of cellular cargoes, such as the endosomal sorting complex required for transport (ESCRT). The measles virus (MV) M protein promotes virus-like particle (VLP) production, and we now show that it shares association with detergent-resistant or tetraspanin-enriched membrane microdomains with ebolavirus VP40 protein, yet accumulates less efficiently at the plasma membrane. Unlike VP40, which recruits ESCRT components via its N-terminal late (L) domain and exploits them for particle production, the M protein does this independently of this pathway, as (i) ablation of motifs bearing similarity to canonical L domains did not affect VLP production, (ii) it did not redistribute Tsg101, AIP-1 or Vps4A to the plasma membrane, and (iii) neither VLP nor infectious virus production was sensitive to inhibition by dominant-negative Vps4A. Importantly, transfer of the VP40 L domain into the MV M protein did not cause recruitment of ESCRT proteins or confer sensitivity of VLP release to Vps4A, indicating that MV particle production occurs independently of and cannot be routed into an ESCRT-dependent pathway.

Efficient budding of the tacaribe virus matrix protein z requires the nucleoprotein

The Z protein has been shown for several arenaviruses to serve as the viral matrix protein. As such, Z provides the principal force for the budding of virus particles and is capable of forming virus-like particles (VLPs) when expressed alone. For most arenaviruses, this activity has been shown to be linked to the presence of proline-rich late-domain motifs in the C terminus; however, for the New World arenavirus Tacaribe virus (TCRV), no such motif exists within Z. It was recently demonstrated that while TCRV Z is still capable of functioning as a matrix protein to induce the formation of VLPs, neither its ASAP motif, which replaces a canonical PT/SAP motif in related viruses, nor its YxxL motif is involved in budding, leading to the suggestion that TCRV uses a novel budding mechanism. Here we show that in comparison to its closest relative, Junin virus (JUNV), TCRV Z buds only weakly when expressed in isolation. While this budding activity is independent of the ASAP or YxxL motif, it is significantly enhanced by coexpression with the nucleoprotein (NP), an effect not seen with JUNV Z. Interestingly, both the ASAP and YxxL motifs of Z appear to be critical for the recruitment of NP into VLPs, as well as for the enhancement of TCRV Z-mediated budding. While it is known that TCRV budding remains dependent on the endosomal sorting complex required for transport, our findings provide further evidence that TCRV uses a budding mechanism distinct from that of other known arenaviruses and suggest an essential role for NP in this process.

Viral protein determinants of Lassa virus entry and release from polarized epithelial cells

The epithelium plays a key role in the spread of Lassa virus. Transmission from rodents to humans occurs mainly via inhalation or ingestion of droplets, dust, or food contaminated with rodent urine. Here, we investigated Lassa virus infection in cultured epithelial cells and subsequent release of progeny viruses. We show that Lassa virus enters polarized Madin-Darby canine kidney (MDCK) cells mainly via the basolateral route, consistent with the basolateral localization of the cellular Lassa virus receptor alpha-dystroglycan. In contrast, progeny virus was efficiently released from the apical cell surface. Further, we determined the roles of the glycoprotein, matrix protein, and nucleoprotein in directed release of nascent virus. To do this, a virus-like-particle assay was developed in polarized MDCK cells based on the finding that, when expressed individually, both the glycoprotein GP and matrix protein Z form virus-like particles. We show that GP determines the apical release of Lassa virus from epithelial cells, presumably by recruiting the matrix protein Z to the site of virus assembly, which is in turn essential for nucleocapsid incorporation into virions.

PIV5 M protein interaction with host protein angiomotin-like 1

Paramyxovirus matrix (M) proteins organize virus assembly, functioning as adapters that link together viral ribonucleoprotein complexes and viral glycoproteins at infected cell plasma membranes. M proteins may also function to recruit and manipulate host factors to assist virus budding, similar to retroviral Gag proteins. By yeast two-hybrid screening, angiomotin-like 1 (AmotL1) was identified as a host factor that interacts with the M protein of parainfluenza virus 5 (PIV5). AmotL1-M protein interaction was observed in yeast, in transfected mammalian cells, and in virus-infected cells. Binding was mapped to a 83-amino acid region derived from the C-terminal portion of AmotL1. Overexpression of M-binding AmotL1-derived polypeptides potently inhibited production of PIV5 VLPs and impaired virus budding. Expression of these polypeptides moderately inhibited production of mumps VLPs, but had no effect on production of Nipah VLPs. siRNA-mediated depletion of AmotL1 protein reduced PIV5 budding, suggesting that this interaction is beneficial to paramyxovirus infection.