M protein (M1) of influenza virus: antigenic analysis and intracellular localization with monoclonal antibodies

Getting on the right track: Interactions between viruses and the cytoskeletal motor proteins

The cytoskeleton is an essential component of the cell and it is involved in multiple physiological functions, including intracellular organization and transport. It is composed of three main families of proteinaceous filaments; microtubules, actin filaments and intermediate filaments and their accessory proteins. Motor proteins, which comprise the dynein, kinesin and myosin superfamilies, are a remarkable group of accessory proteins that mainly mediate the intracellular transport of cargoes along with the cytoskeleton. Like other cellular structures and pathways, viruses can exploit the cytoskeleton to promote different steps of their life cycle through associations with motor proteins. The complexity of the cytoskeleton and the differences among viruses, however, has led to a wide diversity of interactions, which in most cases remain poorly understood. Unveiling the details of these interactions is necessary not only for a better comprehension of specific infections, but may also reveal new potential drug targets to fight dreadful diseases such as rabies disease and acquired immunodeficiency syndrome (AIDS). In this review, we describe a few examples of the mechanisms that some human viruses, that is, rabies virus, adenovirus, herpes simplex virus, human immunodeficiency virus, influenza A virus and papillomavirus, have developed to hijack dyneins, kinesins and myosins.

Molecular association of FtsZ with the intrabacterial nanotransportation system for urease in Helicobacter pylori

Helicobacter pylori possesses intrabacterial nanotransportation system (ibNoTS) for transporting CagA, VacA, and urease within the bacterial cytoplasm, which is controlled by the extrabacterial environment. The route of ibNoTS for CagA is reported to be associated with the MreB filament, whereas the route of ibNoTS for urease is not yet known. In this study, we demonstrated by immunoelectron microscopy that urease along the route of ibNoTS localizes closely with the FtsZ filament in the bacterium. Supporting this, we found by enzyme immunoassay and co-immunoprecipitation analysis that urease interacted with FtsZ. These findings indicate that urease along the route of ibNoTS is closely associated with the FtsZ filament. Since these phenomena were not observed in ibNoTS for CagA, the route of ibNoTS for CagA is different from that of ibNoTS for urease. We propose that the route of ibNoTS for urease is associated with the FtsZ filament in H. pylori.

The C Terminus of Rotavirus VP4 Protein Contains an Actin Binding Domain Which Requires Cooperation with the Coiled-Coil Domain for Actin Remodeling

The interactions between viruses and actin cytoskeleton have been widely studied. We showed that rotaviruses remodel microfilaments in intestinal cells and demonstrated that this was due to the VP4 spike protein. Microfilaments mainly occur in the apical domain of infected polarized enterocytes and favor the polarized apical exit of viral progeny. The present work aims at the identification of molecular determinants of actin-VP4 interactions. We used various deletion mutants of VP4 that were transfected into Cos-7 cells and analyzed interactions by immunofluorescence confocal microscopy. It has been established that the C-terminal part of VP4 is embedded within viral particles when rotavirus assembles. The use of specific monoclonal antibodies demonstrated that VP4 is expressed in different forms in infected cells: classically as spike on the outer layer of virus particles, but also as free soluble protein in the cytosol. The C terminus of free VP4 was identified as interacting with actin microfilaments. The VP4 actin binding domain is unable to promote microfilament remodeling by itself; the coiled-coil domain is also required in this process. This actin-binding domain was shown to dominate a previously identified peroxisomal targeting signal, located in the three last amino acids of VP4. The newly identified actin-binding domain is highly conserved in rotavirus strains from species A, B, and C, suggesting that actin binding and remodeling is a general strategy for rotavirus exit. This provides a novel mechanism of protein-protein interactions, not involving cell signaling pathways, to facilitate rotavirus exit.IMPORTANCE Rotaviruses are causal agents of acute infantile viral diarrhea. In intestinal cells, in vitro as well as in vivo, virus assembly and exit do not imply cell lysis but rely on an active process in which the cytoskeleton plays a major role. We describe here a novel molecular mechanism by which the rotavirus spike protein VP4 drives actin remodeling. This relies on the fact that VP4 occurs in different forms. Besides its structural function within the virion, a large proportion of VP4 is expressed as free protein. Here, we show that free VP4 possesses a functional actin-binding domain. This domain, in coordination with a coiled-coil domain, promotes actin cytoskeleton remodeling, thereby providing the capacity to destabilize the cell membrane and allow efficient rotavirus exit.

A Novel Method of Safely Measuring Influenza Virus Aerosol Using Antigen-Capture Enzyme-Linked Immunosorbent Assay for the Performance Evaluation of Protective Clothing Materials

Currently, threats caused by pathogens are serious public health problems worldwide. Protective clothing is essential when one is treating infected patients or dealing with unknown pathogens. Therefore, it is necessary to evaluate the performance of protective clothing against pathogens. In Japan, some methods for evaluating the performance of protective clothing have been established in the Japanese Industrial Standards (JIS). However, a test method against virus aerosols has not been established. Because there is a risk of infection from a live virus during the test, it is necessary to devise a safe method for the virus-aerosol-based test. Here, we propose a new method of safely measuring virus aerosols for the performance evaluation of protective clothing materials. To ensure safety, an inactivated virus was used. As a model virus, the influenza virus was selected owing to the proper small diameter of the virus particles. To quantitatively measure the particle-amount of the inactivated influenza virus, we developed an antigen-capture enzyme-linked immunosorbent assay (ELISA) targeting the M1 protein. Furthermore, we evaluated two materials using our method. Significant differences in the protection performance against the virus aerosol were observed between different sample materials, thereby confirming the applicability of our new method for performance evaluation.

Route of intrabacterial nanotransportation system for CagA in Helicobacter pylori

Helicobacter pylori (H. pylori) possesses an intrabacterial nanotransportation system (ibNoTS) for transporting CagA and urease within the bacterial cytoplasm; this system is controlled by the extrabacterial environment. The transportation routes of the system have not yet been studied in detail. In this study, we demonstrated by immunoelectron microscopy that CagA localizes closely with the MreB filament in the bacterium, and MreB polymerization inhibitor A22 obstructs ibNoTS for CagA. These findings indicate that the route of ibNoTS for CagA is closely associated with the MreB filament. Because these phenomena were not observed in ibNoTS for urease, the route of ibNoTS for CagA is different from that of ibNoTS for urease as previously suggested. We propose that the route of ibNoTS for CagA is associated with the MreB filament in H. pylori.

In vitro and in vivo replication of influenza A H1N1 WSN33 viruses with different M1 proteins

The M1 protein is a major structural protein that has multiple functions in various steps within the life cycle of the influenza A virus (IAV). However, little is currently known about the role of M1 in IAV replication in vivo and the associated pathogenesis. In this study, six isogenic H1N1 WSN33 viruses, constructed to express unique M1 proteins derived from various strains, subtypes or WSN33 itself, were tested to determine in vitro and in vivo functional exchangeability of M1 proteins in the replication and pathogenesis of the WSN33 virus. Despite five chimeric M1 viruses replicating to levels similar to those of the parental WSN33 virus in cell cultures, all M1 chimeras exhibited improved replication and enhanced virulence in mice when compared with the WSN33 virus. Interestingly, M1 proteins derived from swine viruses caused more severe clinical diseases than those from human or quail. These data indicate that the M1 protein is an important determinant of viral replication and pathogenic properties in mice, although the functions of M1 observed in vivo are not adequately reflected in simple infections of cultured cells. Chimeric M1 viruses that are variable in their clinical manifestations described here will aid future understanding of the role of M1 in IAV pathogenesis.

Improvement of H5N1 influenza vaccine viruses: influence of internal gene segments of avian and human origin on production and hemagglutinin content

The H5N1-clade 1 influenza vaccine strain NIBRG-14 produces exceptionally low amounts of antigen, a problem recently encountered also for initial pandemic H1N1-2009 vaccine seeds. Here, we report on a strategy that may contribute to overcome this obstacle. Influenza vaccine viruses usually consist of two segments coding for the antigenic HA and NA proteins of a wild-type strain and the six residual internal gene segments of the vaccine donor strain A/PR/8/34 (PR8). To enhance the antigen yield from H5N1 vaccine virus we generated by reverse genetics a set of PR8-based reassortant viruses expressing the HA and NA segments of the prototypic strain A/Vietnam/1203/2004 and additional replacements of the internal M or PB1 genes of PR8. The reassortants were compared to the parental PR8 and H5N1 viruses in terms of growth in embryonated chicken eggs and the amount of incorporated antigenic HA protein. Compared to NIBRG-14, three out of six viruses displayed an increased replication in embryonated chicken eggs and higher HA content that was also maintained after ether/detergent extraction of virions. Electron microscopic analysis showed that the reassortment hardly affected particle shape and size. Two selected H5N1 reassortant viruses were investigated concerning their pathogenicity in ferrets and found to behave as low pathogenic as the PR8 donor strain. In conclusion, this study shows that replication and antigen content of PR8-derived H5N1 influenza vaccine viruses can be improved by incorporation of heterologous internal gene segments without compromising their attenuated character.

Subversion of the actin cytoskeleton during viral infection

Viral infection converts the normal functions of a cell to optimize viral replication and virion production. One striking observation of this conversion is the reconfiguration and reorganization of cellular actin, affecting every stage of the viral life cycle, from entry through assembly to egress. The extent and degree of cytoskeletal reorganization varies among different viral infections, suggesting the evolution of myriad viral strategies. In this Review, we describe how the interaction of viral proteins with the cell modulates the structure and function of the actin cytoskeleton to initiate, sustain and spread infections. The molecular biology of such interactions continues to engage virologists in their quest to understand viral replication and informs cell biologists about the role of the cytoskeleton in the uninfected cell.

The SUMOylation of matrix protein M1 modulates the assembly and morphogenesis of influenza A virus

SUMOylation is an important posttranslational modification for regulation of cellular functions and viral replication. Here, we report that protein SUMOylation regulates the replication of influenza A virus at the steps of viral maturation and assembly. Knocking down the SUMO-conjugating enzyme Ubc9 resulted in the reduction of virus production. Dissection of the virus life cycle revealed that SUMOylation is involved in the processes of virus maturation and assembly. The viral matrix protein M1 is SUMOylated at K242. A virus carrying the SUMO-defective M1 produced a lower titer of virus, while its viral proteins and viral RNA (vRNA) accumulated in the cells. Furthermore, the mechanistic studies showed that the SUMOylation of M1 is required for the interaction between M1 and viral RNP (vRNP) to form the M1-vRNP complex. The lack of M1 SUMOylation prevented the nuclear export of vRNP and subsequent viral morphogenesis. Taken together, our findings elucidate that the maturation and assembly of influenza A virus is controlled by the SUMO modification of M1 protein. Therefore, we suggest that M1 can serve as a target for developing a new generation of drugs for flu therapy.

Antiserum against the conserved nine amino acid N-terminal peptide of influenza A virus matrix protein 2 is not immunoprotective

The recent emergence and rapid spread of the pandemic H1N1 swine influenza virus reminded us once again of the need for a universal influenza vaccine that can elicit heterosubtypic protection. Here, we show the superior immunogenicity and immunoprotective capacity of the full-length matrix protein 2 ectodomain (M2e) peptide coupled to keyhole limpet haemocyanin (KLH) compared with the N-terminal 9 aa residues of M2e (SP1). Immunization with M2e-KLH protected mice against a lethal challenge with influenza A virus and significantly reduced weight loss and lung virus titres. In addition, passive transfer of serum raised in rabbits against M2e-KLH protected mice against a lethal influenza virus challenge, whereas serum from rabbits immunized with SP1-KLH did not. Nevertheless, immunofluorescence staining revealed that rabbit serum raised against SP1-KLH bound specifically to infected Madin-Darby canine kidney cells. We conclude that the peptide SP1 contains an immunogenic epitope that is not sufficient for immunoprotection.

Evolution of the M gene of the influenza A virus in different host species: large-scale sequence analysis

Background

Influenza A virus infects not only humans, but also other species including avian and swine. If a novel influenza A subtype acquires the ability to spread between humans efficiently, it could cause the next pandemic. Therefore it is necessary to understand the evolutionary processes of influenza A viruses in various hosts in order to gain better knowledge about the emergence of pandemic virus. The virus has segmented RNA genome and 7th segment, M gene, encodes 2 proteins. M1 is a matrix protein and M2 is a membrane protein. The M gene may be involved in determining host tropism. Besides, novel vaccines targeting M1 or M2 protein to confer cross subtype protection have been under development. We conducted the present study to investigate the evolution of the M gene by analyzing its sequence in different species.

Results

Phylogenetic tree revealed host-specific lineages and evolution rates were different among species. Selective pressure on M2 was stronger than that on M1. Selective pressure on M1 for human influenza was stronger than that for avian influenza, as well as M2. Site-by-site analyses identified one site (amino acid position 219) in M1 as positively selected in human. Positions 115 and 121 in M1, at which consensus amino acids were different between human and avian, were under negative selection in both hosts. As to M2, 10 sites were under positive selection in human. Seven sites locate in extracellular domain. That might be due to host's immune pressure. One site (position 27) positively selected in transmembrane domain is known to be associated with drug resistance. And, two sites (positions 57 and 89) locate in cytoplasmic domain. The sites are involved in several functions.

Conclusion

The M gene of influenza A virus has evolved independently, under different selective pressure on M1 and M2 among different hosts. We found potentially important sites that may be related to host tropism and immune responses. These sites may be important for evolutional process in different hosts and host adaptation.

Cyclophilin A interacts with influenza A virus M1 protein and impairs the early stage of the viral replication

Influenza A virus matrix protein (M1) is the most abundant conservative protein that regulates the replication, assembly and budding of the viral particles upon infection. Several host cell factors have been determined to interact with M1 possibly in regulating influenza virus replication. By yeast two-hybrid screening, the isomerase cyclophilin A (CypA) was identified to interact with the M1 protein. CypA specifically interacted with M1 both in vitro and in vivo. The mutagenesis results showed CypA bound to the functional middle (M) domain of M1. The depletion of endogenous CypA by RNA interference resulted in the increase of influenza virus infectivity while overexpression of CypA caused decreasing the infectivity in affected cells. The immunofluorescence assays indicated that overexpressed CypA deduced the infectivity and inhibited the translocation of M1 protein into the nucleus while did not affect nucleoprotein entering the nucleus. Further studies indicated that overexpression of CypA significantly increased M1 self-association. Western blot with purified virions confirmed that CypA was encapsidated within the virus particle. These results together indicated that CypA interacted with the M1 protein and affected the early stage of the viral replication.

Identification of the domains of the influenza A virus M1 matrix protein required for NP binding, oligomerization and incorporation into virions

The matrix (M1) protein of influenza A virus is a multifunctional protein that plays essential structural and functional roles in the virus life cycle. It drives virus budding and is the major protein component of the virion, where it forms an intermediate layer between the viral envelope and integral membrane proteins and the genomic ribonucleoproteins (RNPs). It also helps to control the intracellular trafficking of RNPs. These roles are mediated primarily via protein–protein interactions with viral and possibly cellular proteins. Here, the regions of M1 involved in binding the viral RNPs and in mediating homo-oligomerization are identified. In vitro, by using recombinant proteins, it was found that the middle domain of M1 was responsible for binding NP and that this interaction did not require RNA. Similarly, only M1 polypeptides containing the middle domain were able to bind to RNP–M1 complexes isolated from purified virus. When M1 self-association was examined, all three domains of the protein participated in homo-oligomerization although, again, the middle domain was dominant and self-associated efficiently in the absence of the N- and C-terminal domains. However, when the individual fragments of M1 were tagged with green fluorescent protein and expressed in virus-infected cells, microscopy of filamentous particles showed that only full-length M1 was incorporated into budding virions. It is concluded that the middle domain of M1 is primarily responsible for binding NP and self-association, but that additional interactions are required for efficient incorporation of M1 into virus particles.

Infectious salmon anemia virus (ISAV) genomic segment 3 encodes the viral nucleoprotein (NP), an RNA-binding protein with two monopartite nuclear localization signals (NLS)

Infectious salmon anemia virus (ISAV) is the type species of the genus Isavirus belonging to the Orthomyxoviridae, and causes serious disease in Atlantic salmon (Salmo salar). This study presents the expression and functional analysis of the ISAV genome segment 3, and provides further evidence that it encodes the viral nucleoprotein (NP). The encoded protein was expressed in a baculovirus system, and Western blot analysis showed that it corresponds to the 66-71 kDa structural protein previously found in purified ISAV preparations. RNA-binding activity was established by the interaction of viral and recombinant NP with single-stranded RNA transcribed in vitro. Immunofluorescence studies of infected cells showed the ISAV NP to be an early protein. It locates to the nucleus of infected cells before it is transported to the cytoplasm prior to virus assembly. A similar localization pattern was observed in cells transfected with the NP gene, confirming that the encoded protein has an intrinsic ability to be imported into the nucleus. Two monopartite nuclear localization signals (NLS) at amino acids (230)RPKR(233) and (473)KPKK(476) were identified by computer analysis, and validated by site-directed mutagenesis. In contrast to other orthomyxovirus-NPs, that have several NLSs that function independent of each other, both NLSs had to be present for the ISAV NP protein to be transported into the nucleus, indicating that these motifs cooperate to target the protein to the nucleus.

Assembly and budding of influenza virus

Influenza viruses are causative agents of an acute febrile respiratory disease called influenza (commonly known as "flu") and belong to the Orthomyxoviridae family. These viruses possess segmented, negative stranded RNA genomes (vRNA) and are enveloped, usually spherical and bud from the plasma membrane (more specifically, the apical plasma membrane of polarized epithelial cells). Complete virus particles, therefore, are not found inside infected cells. Virus particles consist of three major subviral components, namely the viral envelope, matrix protein (M1), and core (viral ribonucleocapsid [vRNP]). The viral envelope surrounding the vRNP consists of a lipid bilayer containing spikes composed of viral glycoproteins (HA, NA, and M2) on the outer side and M1 on the inner side. Viral lipids, derived from the host plasma membrane, are selectively enriched in cholesterol and glycosphingolipids. M1 forms the bridge between the viral envelope and the core. The viral core consists of helical vRNP containing vRNA (minus strand) and NP along with minor amounts of NEP and polymerase complex (PA, PB1, and PB2). For viral morphogenesis to occur, all three viral components, namely the viral envelope (containing lipids and transmembrane proteins), M1, and the vRNP must be brought to the assembly site, i.e. the apical plasma membrane in polarized epithelial cells. Finally, buds must be formed at the assembly site and virus particles released with the closure of buds. Transmembrane viral proteins are transported to the assembly site on the plasma membrane via the exocytic pathway. Both HA and NA possess apical sorting signals and use lipid rafts for cell surface transport and apical sorting. These lipid rafts are enriched in cholesterol, glycosphingolipids and are relatively resistant to neutral detergent extraction at low temperature. M1 is synthesized on free cytosolic polyribosomes. vRNPs are made inside the host nucleus and are exported into the cytoplasm through the nuclear pore with the help of M1 and NEP. How M1 and vRNPs are directed to the assembly site on the plasma membrane remains unclear. The likely possibilities are that they use a piggy-back mechanism on viral glycoproteins or cytoskeletal elements. Alternatively, they may possess apical determinants or diffuse to the assembly site, or a combination of these pathways. Interactions of M1 with M1, M1 with vRNP, and M1 with HA and NA facilitate concentration of viral components and exclusion of host proteins from the budding site. M1 interacts with the cytoplasmic tail (CT) and transmembrane domain (TMD) of glycoproteins, and thereby functions as a bridge between the viral envelope and vRNP. Lipid rafts function as microdomains for concentrating viral glycoproteins and may serve as a platform for virus budding. Virus bud formation requires membrane bending at the budding site. A combination of factors including concentration of and interaction among viral components, increased viscosity and asymmetry of the lipid bilayer of the lipid raft as well as pulling and pushing forces of viral and host components are likely to cause outward curvature of the plasma membrane at the assembly site leading to bud formation. Eventually, virus release requires completion of the bud due to fusion of the apposing membranes, leading to the closure of the bud, separation of the virus particle from the host plasma membrane and release of the virus particle into the extracellular environment. Among the viral components, M1 contains an L domain motif and plays a critical role in budding. Bud completion requires not only viral components but also host components. However, how host components facilitate bud completion remains unclear. In addition to bud completion, influenza virus requires NA to release virus particles from sialic acid residues on the cell surface and spread from cell to cell. Elucidation of both viral and host factors involved in viral morphogenesis and budding may lead to the development of drugs interfering with the steps of viral morphogenesis and in disease progression.

Escherichia coliExpressing Recombinant Antigen and Listeriolysin O Stimulate Class I-Restricted CD8+T Cells following Uptake by Human APC

Vaccination against cancer or intracellular pathogens requires stimulation of class I-restricted CD8(+) T cells. It is therefore important to develop Ag delivery vectors that will promote cross-presentation by APCs and stimulate appropriate inflammatory responses. Toward this goal, we tested the potential of Escherichia coli as an Ag delivery vector in in vitro human culture. Bacteria expressing enhanced green fluorescent protein were internalized efficiently by dendritic cells, as shown by flow cytometry and fluorescence microscopy. Phenotypic changes in DC were observed, including up-regulation of costimulatory molecules and IL-12p40 production. We tested whether bacteria expressing recombinant Ags could stimulate human T cells using the influenza matrix protein as a model Ag. Specific responses against an immunodominant epitope were seen using IFN-gamma ELISPOT assays when the matrix protein was coexpressed with listeriolysin O, but not when expressed alone. THP-1 macrophages were also capable of stimulating T cells after uptake of bacteria, but showed slower kinetics and lower overall levels of T cell stimulation than dendritic cells. Increased phagocytosis of bacteria induced by differentiation of THP-1 increased their ability to stimulate T cells, as did opsonization. Presentation was blocked by proteasome inhibitors, but not by lysosomal protease inhibitors leupeptin and E64. These results demonstrate that recombinant E. coli can be engineered to direct Ags to the cytosol of human phagocytic APCs, and suggest possible vaccine strategies for generating CD8(+) T cell responses against pathogens or tumors.

Localization of influenza virus proteins to nuclear dot 10 structures in influenza virus-infected cells

We studied influenza virus M1 protein by generating HeLa and MDCK cell lines that express M1 genetically fused to green fluorescent protein (GFP). GFP-M1 was incorporated into virions produced by influenza virus infected MDCK cells expressing the fusion protein indicating that the fusion protein is at least partially functional. Following infection of either HeLa or MDCK cells with influenza A virus (but not influenza B virus), GFP-M1 redistributes from its cytosolic/nuclear location and accumulates in nuclear dots. Immunofluorescence revealed that the nuclear dots represent nuclear dot 10 (ND10) structures. The colocalization of authentic M1, as well as NS1 and NS2 protein, with ND10 was confirmed by immunofluorescence following in situ isolation of ND10. These findings demonstrate a previously unappreciated involvement of influenza virus with ND10, a structure involved in cellular responses to immune cytokines as well as the replication of a rapidly increasing list of viruses.

Nuclear export of influenza viral ribonucleoprotein is temperature-dependently inhibited by dissociation of viral matrix protein

The influenza virus copies its genomic RNA in the nuclei of host cells, but the viral particles are formed at the plasma membrane. Thus, the export of new genome from the nucleus into the cytoplasm is essential for viral production. Several viral proteins, such as nucleoprotein (NP) and RNA polymerases, synthesized in the cytoplasm, are imported into the nucleus, and form viral ribonucleoprotein (vRNP) with new genomic RNA. vRNP is then exported into the cytoplasm from the nucleus to produce new viral particles. M1, a viral matrix protein, is suggested to participate in the nuclear export of vRNP. It was found unexpectedly that the production of influenza virus was suppressed in MDCK cells at 41 degrees C, although viral proteins were synthesized and the cytopathic effect was observed in host cells. Indirect immunofluorescent staining with anti-NP or M1 monoclonal antibody showed that NP and M1 remained in the nuclei of infected cells at 41 degrees C, suggesting that a suppression of viral production was caused by inhibition of the nuclear export of these proteins. The cellular machinery for nuclear export depending on CRM1, which mediates the nuclear export of influenza viral RNP, functioned normally at 41 degrees C. Glycerol-density gradient centrifugation demonstrated that vRNP also formed normally at 41 degrees C. However, an examination of the interaction between vRNP and M1 by immunoprecipitation indicated that M1 did not associate with vRNP at 41 degrees C, suggesting that the association is essential for the nuclear export of vRNP. Furthermore, when infected cells incubated at 41 degrees C were cultured at 37 degrees C, the interaction between vRNP and M1 was no longer detected even at 37 degrees C. The results suggest that M1 synthesized at 41 degrees C is unable to interact with vRNP and the dissociation of M1 from vRNP is one of the reasons that the transfer of vRNP into the cytoplasm from the nucleus is prevented at 41 degrees C.

Role of matrix and fusion proteins in budding of Sendai virus

Paramyxoviruses are assembled at the surface of infected cells, where virions are formed by the process of budding. We investigated the roles of three Sendai virus (SV) membrane proteins in the production of virus-like particles. Expression of matrix (M) proteins from cDNA induced the budding and release of virus-like particles that contained M, as was previously observed with human parainfluenza virus type 1 (hPIV1). Expression of SV fusion (F) glycoprotein from cDNA caused the release of virus-like particles bearing surface F, although their release was less efficient than that of particles bearing M protein. Cells that expressed only hemagglutinin-neuraminidase (HN) released no HN-containing vesicles. Coexpression of M and F proteins enhanced the release of F protein by a factor greater than 4. The virus-like particles containing F and M were found in different density gradient fractions of the media of cells that coexpressed M and F, a finding that suggests that the two proteins formed separate vesicles and did not interact directly. Vesicles released by M or F proteins also contained cellular actin; therefore, actin may be involved in the budding process induced by viral M or F proteins. Deletion of C-terminal residues of M protein, which has a sequence similar to that of an actin-binding domain, significantly reduced release of the particles into medium. Site-directed mutagenesis of the cytoplasmic tail of F revealed two regions that affect the efficiency of budding: one domain comprising five consecutive amino acids conserved in SV and hPIV1 and one domain that is similar to the actin-binding domain required for budding induced by M protein. Our results indicate that both M and F proteins are able to drive the budding of SV and propose the possible role of actin in the budding process.

A persistent variant of influenza C virus fails to interact with actin filaments during viral assembly

C/AA-pi virus, a variant of influenza C/Ann Arbor/1/50 virus, establishes persistent infections in MDCK cells, characterized by low levels of progeny production. During viral assembly, nucleoprotein (NP) was found homogeneously distributed over cytoplasmic and nuclear compartments and matrix (M) protein was likewise localized in a barely structured fashion. In contrast, infections with nonpersistent influenza A, B and C viruses produced cytoplasmic granular structures, which typically consisted of colocalized NP and M proteins. Studies on the in vitro interaction between NP and M proteins revealed identical binding capacities comparing influenza C wild-type virus with the persistent variant. Cytochalasin D treatment of infected cells demonstrated that NP protein of the wild-type virus, but not of the persistent variant, was distinctly associated with cellular actin filaments. Moreover, the assembly characteristics of wild-type virus were modulated in the presence of recombinant persistent-type NP protein towards a behaviour similar to persistent infection. Cell type specificity was particularly illustrated in C/AA-pi virus-infected Vero cells, which did not support viral persistence, but produced granular wild-type-like complexes. Thus, interaction between NP, M and actin proteins (i) is a basic part of the viral assembly process, (ii) is dominantly modulated by NP protein and (iii) is specifically altered in the case of persistent infection.

Covalent chromatography of influenza virus membrane M1 protein on activated thiopropyl Sepharose-6B

The M1 protein of influenza virus is a highly hydrophobic polypeptide that is resistant to enzyme cleavage during incubation in water solutions. We show here that the M1 protein that is immobilized on an insoluble activated support (thiopropyl Sepharose-6B) by means of a thiol-disulfide exchange reaction acquires sensitivity to trypsin. After tryptic digestion noncysteine-containing peptides of M1 were removed by washing the support, while cysteine-containing ones were detached from the support by reduction. As a result, 24 unique tryptic peptides of M1 protein were clearly separated by reversed-phase high-performance liquid chromatography. The described method opens a new way to the investigation of functional properties of distinct domains of viral thiol proteins.

Association of influenza virus NP and M1 proteins with cellular cytoskeletal elements in influenza virus-infected cells

We have investigated the association of the influenza virus matrix (M1) and nucleoprotein (NP) with the host cell cytoskeletal elements in influenza virus-infected MDCK and MDBK cells. At 6.5 h postinfection, the newly synthesized M1 was Triton X-100 (TX-100) extractable but became resistant to TX-100 extraction during the chase with a t1/2 of 20 min. NP, on the other hand, acquired TX-100 resistance immediately after synthesis. Significant fractions of both M1 and NP remained resistant to differential detergent (Triton X-114, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate [CHAPS], octylglucoside) extraction, suggesting that M1 and NP were interacting with the cytoskeletal elements. However, the high-molecular-weight form of the viral transmembrane protein hemagglutinin (HA), which had undergone complex glycosylation, also became resistant to TX-100 extraction but was sensitive to octylglucoside detergent extraction, indicating that HA, unlike M1 or NP, was interacting with TX-100-insoluble lipids and not with cytoskeletal elements. Morphological analysis with cytoskeletal disrupting agents demonstrated that M1 and NP were associated with microfilaments in virus-infected cells. However, M1, expressed alone in MDCK or HeLa cells from cloned cDNA or coexpressed with NP, did not become resistant to TX-100 extraction even after a long chase. NP, on the other hand, became TX-100 insoluble as in the virus-infected cells. M1 also did not acquire TX-100 insolubility in ts 56 (a temperature-sensitive mutant with a defect in NP protein)-infected cells at the nonpermissive temperature. Furthermore, early in the infectious cycle in WSN-infected cells, M1 acquired TX-100 resistance very slowly after a long chase and did not acquire TX-100 resistance at all when chased in the presence of cycloheximide. On the other hand, late in the infectious cycle, M1 acquired TX-100 resistance when chased in either the presence or absence of cycloheximide. Taken together, these results demonstrate that M1 and NP interact with host microfilaments in virus-infected cells and that M1 requires other viral proteins or subviral components (possibly viral ribonucleoprotein) for interaction with host cytoskeletal components. The implication of these results for viral morphogenesis is discussed.

Viral manipulations of the actin cytoskeleton

Viruses succeed as intracellular parasites because of their ability to invade cells and appropriate the cellular machinery required during their life cycle. The actin cytoskeleton of the host cell does not escape viral infection unscathed, but is often co-opted by the virus at many different stages of its life cycle to facilitate the infection process.

Antiviral activity of influenza virus M1 zinc finger peptides

Matrix protein (M1) of influenza virus inhibits its own polymerase; this suggested that a peptide segment of M1 with inhibitory properties could serve as an antiviral agent. A peptide synthesized to the Zn2+ finger region of the M1 sequence of influenza virus strain A/PR/8/34 centered around amino acids residues 148 to 166 was shown earlier to be 1,000-fold more effective as a polymerase inhibitor than M1. This peptide, designated peptide 6, represents a Zn2+ finger which includes a 7-residue "loop" and a 4-residue "tail" in addition to the 4 residues on either side of the loop involved in coordination of Zn2+. We have now demonstrated antiviral activity for this peptide in microassays measuring inhibition of the viral cytopathic effect. When the peptide was introduced into tissue culture 5 min after viral challenge with A/PR/8/34, antiviral activity was seen at levels as low as 0.1 nM; on a molar basis, the peptide was shown to be 1,000- to 2,500-fold more effective than ribavirin or amantadine. Antiviral activity was seen with addition of the peptide up to 1 h after viral infection; however, little or no activity was seen at later times, suggesting that viral replication is inhibited at an early stage, possibly at the level of transcription. Reduction in the finger loop or tail length reduced antiviral activity; reduction in the number of residues involved in coordination of Zn2+ abolished antiviral activity. In addition to A/PR/8/34, peptide 6 was shown to have antiviral activity against other type A influenza viruses, including those representing H1N1, H2N2, and H3N2 subtypes. Antiviral activity against type B influenza viruses was also seen. A low level of activity against vesicular stomatitis virus was observed. Zn2+ finger peptides or analogs of Zn2+ finger peptides may provide a new class of antiviral agents effective against influenza virus and possibly other viruses.

Mechanism for inhibition of influenza virus RNA polymerase activity by matrix protein

Influenza virus M1 protein has been shown to inhibit the transcription catalyzed by viral ribonucleoprotein complexes isolated from virions. Here, this inhibition mechanism was studied with the recombinant M1 protein purified from Escherichia coli expressing it from cDNA. RNA mobility shift assays indicated that both soluble and aggregate forms of the recombinant M1, which were separated by the glycerol density gradient, were bound to RNA. Once an M1-RNA complex was formed, free M1 was bound to the M1-RNA complex cooperatively rather than to free RNA. In addition, the recombinant M1 was capable of binding to preformed RNA-nucleocapsid protein complexes. The mechanism for inhibition of the viral RNA polymerase activity was analyzed by the in vitro RNA synthesis systems that depend on an exogenously added RNA template. These systems were more sensitive for evaluating the inhibition by M1 than the RNA synthesis system depending on an endogenous RNA template. The RNA synthesis inhibition was examined at four steps: cleavage of capped RNA; incorporation of the first nucleotide, GMP; limited elongation; and synthesis of full-size product. M1 inhibited RNA synthesis mainly at the early steps. The experiments with M1 mutant proteins containing amino acid deletions suggested that the M1 region between amino acid residues 91 and 111 was essential for anti-RNA synthesis activity, RNA binding, and oligomerization of M1 on RNA.

Hyperphosphorylation of mutant influenza virus matrix protein, M1, causes its retention in the nucleus

The matrix (M1) protein of influenza virus is a major structural component, involved in regulation of viral ribonucleoprotein transport into and out of the nucleus. Early in infection, M1 is distributed in the nucleus, whereas later, it is localized predominantly in the cytoplasm. Using immunofluorescence microscopy and the influenza virus mutant ts51, we found that at the nonpermissive temperature M1 was retained in the nucleus, even at late times after infection. In contrast, the viral nucleoprotein (NP), after a temporary retention in the nucleus, was distributed in the cytoplasm. Therefore, mutant M1 supported the release of the viral ribonucleoproteins from the nucleus, but not the formation of infectious virions. The point mutation in the ts51 M1 gene was predicted to encode an additional phosphorylation site. We observed a substantial increase in the incorporation of 32Pi into M1 at the nonpermissive temperature. The critical role of this phosphorylation site was demonstrated by using H89, a protein kinase inhibitor; it inhibited the expression of the mutant phenotype, as judged by M1 distribution in the cell. Immunofluorescence analysis of ts51-infected cells after treatment with H89 showed a wild-type phenotype. In summary, the data indicated that the ts51 M1 protein was hyperphosphorylated at the nonpermissive temperature and that this phosphorylation was responsible for its aberrant nuclear retention.

Growth control of influenza A virus by M1 protein: analysis of transfectant viruses carrying the chimeric M gene

Analysis of fast-growing reassortants (AWM viruses) of influenza A virus produced by mixed infection with a fast-growing WSN strain and a slowly growing Aichi strain indicated that the M gene plays a role in the regulation of virus growth rate at an early step of infection (J. Yasuda, T. Toyoda, M. Nakayama, and A. Ishihama, Arch. Virol. 133:283-294, 1993). To determine which of the two M gene products, M1 or M2, is responsible for the growth rate control, one recombinant WSN virus (CWA) clone possessing a chimeric M gene (WSN M1-Aichi M2) was generated by using an improved reverse genetics and transfection system. The recombinant CWA virus retained the phenotype of both large plaque formation and early onset of virus growth. This indicates that the WSN M1 protein is responsible for rapid virus growth.

Nuclear entry and nucleolar localization of the Newcastle disease virus (NDV) matrix protein occur early in infection and do not require other NDV proteins

A large proportion of the Newcastle disease virus (NDV) matrix (M) protein is found in the nuclei of infected chicken embryo cells. Kinetic analysis indicated that much of the M protein enters the nucleus early in infection, concentrating in discrete regions of the nucleus and remaining there throughout infection. The M protein was found in localized regions of the nuclei of a variety of cell lines infected with NDV. Immunostaining for both M protein and nucleolar antigens indicated that most of these regions represent nucleoli. Moreover, this nucleolar localization of the M protein was observed in chicken embryo cells infected with 11 different strains of NDV. Only the M protein of strain HP displayed a modified pattern, concentrating in the nucleolus early in infection but in the cytoplasm late in infection. M protein transiently expressed in COS-1 cells also localized to the nucleus and nucleolus, indicating that the M protein does not require other NDV proteins for this localization.

Subtype H7 influenza viruses: comparative antigenic and molecular analysis of the HA-, M-, and NS-genes

Antigenic analysis of the haemagglutinin and matrix protein with corresponding sets of monoclonal antibodies as well as sequence analysis of HA-, M-, and NS-genes were carried out to establish antigenic and genetic relationships between four fowl plague virus (FPV) strains of H7 subtype. The data obtained revealed close genetic relatedness between the oldest known influenza A virus, A/chicken/Brescia/1902 (H7N7), and two FPV strains, A/FPV/Dobson (H7N7) and A/FPV/Weybridge (H7N7). These three strains apparently differ in all genes investigated from the A/FPV/Rostock isolate.

Isolation of matrix protein M1 from influenza viruses by acid-dependent extraction with nonionic detergent

Influenza viruses were disrupted layer by layer with the nonionic detergent NP-40 at fixed pH. Treatment of the virions with NP-40 at neutral or mildly alkaline pH (6.8-8.0) yielded viral core structures containing M1 protein. The matrix M1 protein was selectively extracted from cores at acidic pH 3.0-4.5 with citrate, acetate, and phosphate buffers or with morpholinoethanesulfonic acid. The resulting M1 protein sedimented in a glycerol gradient with a coefficient of 2.8 S and most likely existed as a monomeric form of the 27,000-Da polypeptide. An antigenic map of the monomeric protein M1 tested with a panel of monoclonal anti-M1 antibodies was found to be similar to those of the assembled M1 protein in whole virions. The isolated M1 protein retained biological properties and inhibited the RNA polymerase activity of viral RNP. This transcription-inhibition function of M1 monomers was specifically restricted by one of the monoclonal antibodies studied.

Rapid detection of type A influenza viruses with monoclonal antibodies to the M protein (M1) by enzyme-linked immunosorbent assay and time-resolved fluoroimmunoassay

Monoclonal antibodies (MAbs) to the M protein (M1) were used in the development of direct detection systems for type A influenza viruses in clinical specimens. Optimal detection by an enzyme-linked immunosorbent assay was achieved when MAbs were used as capture antibodies and rabbit polyclonal antibodies were used as sandwich antibodies. Detection by the enzyme-linked immunosorbent assay required amplification of the virus. direct detection in clinical specimens (nasopharyngeal aspirates) was accomplished when MAbs recognizing two distinct antigenic sites of M1 were used in a time-resolved fluoroimmunoassay. Type A influenza viruses could be detected equally well in specimens obtained during epidemics of both H3N2 and H1N1 influenza viruses.

The vaccinia virus HindIII F fragment: nucleotide sequence of the left 6.2 kb

The nucleotide sequence of the left 6.2 kb of the 13.2-kb HindIII F fragment of vaccinia virus was determined. Translation of the sequence revealed nine closely spaced, tandemly oriented open reading frames (ORFs), all reading leftward. The transcriptional organization of this region was determined by Northern blot and S1 nuclease mapping. The analysis suggested that ORFs 1, 2, 4, 5, 6, 7, and 8 are transcribed early in infection, whereas ORFs 3 and 9 are probably late genes. Two of these ORFs have been reported previously. ORF F4L encodes the small subunit of ribonucleotide reductase and ORF F2L is homologous to a retroviral protease-like gene.

The phosphorylation of the integral membrane (M1) protein of influenza virus

The phosphorylation of the internal and integral membrane (M1) protein of influenza virus was studied. Four points can be made based on the data: (1) The M1 contains at least two moles of phosphate per mole of M1. (2) Phosphorylation of M1 is conserved between influenza A, B and C viruses. Other characteristics of the M1 are also conserved, such as solubility in organic solvent, heterogeneity and ability to partition into lipid vesicles. (3) M1 is phosphorylated in cells infected with a vaccinia recombinant (vP273) containing only the gene of M1, either as a result of a vaccinia virus associated kinase or a cellular one. (4) The phosphate is located within or in close proximity to the major stretch of neutral and hydrophobic amino acids found in M1, as determined by analyzing cyanogen bromide fragments.

Role of matrix protein in cytopathogenesis of vesicular stomatitis virus

The matrix (M) protein of vesicular stomatitis virus (VSV) plays an important structural role in viral assembly, and it also has a regulatory role in viral transcription. We demonstrate here that the M protein has an additional function. It causes visible cytopathic effects (CPE), as evidenced by the typical rounding of polygonal cells after VSV infection. We have analyzed a temperature-sensitive mutant of the M protein of VSV (tsG33) which is defective in viral assembly and which fails to cause morphological changes of the cells after infection at the nonpermissive temperature (40 degrees C). Interestingly, this defect in viral assembly as well as the CPE were reversible. Microinjection of antisense oligonucleotides which specifically inhibit M protein translation also inhibited the occurrence of CPE. Most importantly, when cells were transfected with a cDNA encoding the temperature-sensitive M protein of tsG33, no CPE was observed at the nonpermissive temperature. However, when these cells were shifted to the permissive temperature (32 degrees C), they rounded up and detached from the dish. These results demonstrate that M protein in the absence of the other viral proteins causes rounding of the cells, probably through a disorganization of the cytoskeleton. The absence of CPE at the nonpermissive temperature is correlated with an abnormal dotted staining pattern of M in these cells, suggesting that the mutant M protein may self-aggregate or associate with membranes rather than interact with cytoskeletal elements.