Mutations in the cytoplasmic tail of influenza A virus neuraminidase affect incorporation into virions

N-Linked Glycosylation Plays an Important Role in Budding of Neuraminidase Protein and Virulence of Influenza Viruses

Neuraminidase (NA) has multiple functions in the life cycle of influenza virus, especially in the late stage of virus replication. Both of hemagglutinin (HA) and NA are highly glycosylated proteins. N-linked glycosylation (NLG) of HA has been reported to contribute to immune escape and virulence of influenza viruses. However, the function of NLG of NA remains largely unclear. In this study, we found that NLG is critical for budding ability of NA. Tunicamycin treatment or NLG knockout significantly inhibited the budding of NA. Further studies showed that the NLG knockout caused attenuation of virus in vitro and in vivo Notably, the NLG at 219 position plays an important role in the budding, replication, and virulence of H1N1 influenza virus. To explore the underlying mechanism, the unfolded protein response (UPR) was determined in NLG knockout NA overexpressed cells, which showed that the mutant NA was mainly located in the endoplasmic reticulum (ER), the UPR markers BIP and p-eIF2α were upregulated, and XBP1 was downregulated. All the results indicated that NLG knockout NA was stacked in the ER and triggered UPR, which might shut down the budding process of NA. Overall, the study shed light on the function of NLG of NA in virus replication and budding.IMPORTANCE NA is a highly glycosylated protein. Nevertheless, how the NLG affects the function of NA protein remains largely unclear. In this study, we found that NLG plays important roles in budding and Neuraminidase activity of NA protein. Loss of NLG attenuated viral budding and replication. In particular, the 219 NLG site mutation significantly attenuated the replication and virulence of H1N1 influenza virus in vitro and in vivo, which suggested that NLG of NA protein is a novel virulence marker for influenza viruses.

The cytoplasmic tail domain of influenza B virus hemagglutinin is important for its incorporation into virions but is not essential for virus replication in cell culture in the presence of compensatory mutations

Influenza B virus hemagglutinin (BHA) contains a predicted cytoplasmic tail of 10 amino acids that are highly conserved among influenza B viruses. To understand the role of this cytoplasmic tail in infectious virus production, we used reverse genetics to generate a recombinant influenza B virus lacking the BHA cytoplasmic tail domain. The resulting virus, designated BHATail(-), had a titer approximately 5 log units lower than that of wild-type virus but grew normally when BHA was supplemented in trans by BHA-expressing cells. Although the levels of BHA cell surface expression were indistinguishable between truncated and wild-type BHA, the BHATail(-) virus produced particles containing dramatically less BHA. Moreover, removal of the cytoplasmic tail abrogated the association of BHA with Triton X-100-insoluble lipid rafts. Interestingly, long-term culture of a virus lacking the BHA cytoplasmic tail in Madin-Darby canine kidney (MDCK) cells yielded a mutant with infectivities somewhat similar to that of wild-type virus. Sequencing revealed that the mutant virus retained the original cytoplasmic tail deletion but acquired additional mutations in its BHA, neuraminidase (NA), and M1 proteins. Viral growth kinetic analysis showed that replication of BHA cytoplasmic tailless viruses could be improved by compensatory mutations in the NA and M1 proteins. These findings indicate that the cytoplasmic tail domain of BHA is important for efficient incorporation of BHA into virions and tight lipid raft association. They also demonstrate that the domain is not absolutely required for virus viability in cell culture in the presence of compensatory mutations.

Immunogenicity of a recombinant influenza virus bearing both the CD4+ and CD8+ T cell epitopes of ovalbumin

Recombinant influenza viruses that bear the single immunodominant CD8+ T cell epitope OVA_257−264 or the CD4+ T cell epitope OVA_323−339 of the model antigen ovalbumin (OVA) have been useful tools in immunology. Here, we generated a recombinant influenza virus, WSN-OVA_I/II, that bears both OVA-specific CD8+ and CD4+ epitopes on its hemagglutinin molecule. Live and heat-inactivated WSN-OVA_I/II viruses were efficiently presented by dendritic cells in vitro to OT-I TCR transgenic CD8+ T cells and OT-II TCR transgenic CD4+ T cells. In vivo, WSN-OVA_I/II virus was attenuated in virulence, highly immunogenic, and protected mice from B16-OVA tumor challenge in a prophylactic model of vaccination. Thus, WSN-OVA_I/II virus represents an additional tool, along with OVA TCR transgenic mice, for further studies on T cell responses and may be of value in vaccine design.

The cytoplasmic domain of Marburg virus GP modulates early steps of viral infection

Marburg virus infection is mediated by the only viral surface protein, GP, a trimeric type I transmembrane protein. While its ectodomain mediates receptor binding and fusion of viral and cellular membranes and its transmembrane domain is essential for the recruitment of GP into budding particles by the matrix protein VP40, the role of the short cytoplasmic domain has remained enigmatic. Here we show that a missing cytoplasmic domain did not impair trimerization, intracellular transport, or incorporation of GP into infectious Marburg virus-like particles (iVLPs) but altered the glycosylation pattern as well as the recognition of GP by neutralizing antibodies. These results suggest that subtle conformational changes took place in the ectodomain. To investigate the function of the cytoplasmic domain during viral entry, a novel entry assay was established to monitor the uptake of filamentous VLPs by measuring the occurrence of luciferase-labeled viral nucleocapsids in the cytosol of target cells. This quantitative assay showed that the entry process of VLPs incorporating GP missing its cytoplasmic domain (GPΔCD) was impaired. Supporting these results, iVLPs incorporating a mutant GP missing its cytoplasmic domain were significantly less infectious than iVLPs containing wild-type GP. Taken together, the data indicate that the absence of the short cytoplasmic domain of Marburg virus GP may induce conformational changes in the ectodomain which impact the filoviral entry process.

The influenza virus M2 protein cytoplasmic tail interacts with the M1 protein and influences virus assembly at the site of virus budding

The cytoplasmic tail of the influenza A virus M2 proton-selective ion channel has been shown to be important for virus replication. Previous analysis of M2 cytoplasmic tail truncation mutants demonstrated a defect in incorporation of viral RNA (vRNA) into virions, suggesting a role for M2 in the recruitment of M1-vRNA complexes. To further characterize the effect of the M2 cytoplasmic tail mutations on virus assembly and budding, we constructed a series of alanine substitution mutants of M2 with mutations in the cytoplasmic tail, from residues 71 to 97. Mutant proteins M2-Mut1 and M2-Mut2, with mutations of residues 71 to 73 and 74 to 76, respectively, appeared to have the greatest effect on virus-like particle and virus budding, showing a defect in M1 incorporation. Mutant viruses containing M2-Mut1 and M2-Mut2 failed to replicate in multistep growth analyses on wild-type (wt) MDCK cells and were able to form plaques only on MDCK cells stably expressing wt M2 protein. Compared to wt M2 protein, M2-Mut1 and M2-Mut2 were unable to efficiently coimmunoprecipitate with M1. Furthermore, statistical analysis of planar sheets of membrane from cells infected by virus containing M2-Mut1 revealed a reduction in M1-hemagglutinin (HA) and M2-HA clustering as well as a severe loss of clustering between M1 and M2. These results suggest an essential, direct interaction between the cytoplasmic tail of M2 and M1 that promotes the recruitment of the internal viral proteins and vRNA to the plasma membrane for efficient virus assembly to occur.

GP64 of group I nucleopolyhedroviruses cannot readily rescue infectivity of group II f-null nucleopolyhedroviruses

The genus Nucleopolyhedrovirus (NPV) of the family Baculoviridae can be subdivided phylogenetically into two groups. The same division can be made on the basis of their budded virus (BV) envelope fusion protein. Group I NPVs are characterized by the presence of a GP64-like major envelope fusion protein, which is involved in viral attachment and the fusion of virus and cell membrane, and is required for budding of progeny nucleocapsids. Group II NPVs have an envelope fusion protein unrelated to GP64, named F. In contrast to GP64, F proteins are found in all baculoviruses, but they are not functional as envelope fusion proteins in group I NPVs. Autographa californica multiple NPV (AcMNPV) lacking GP64 can be pseudotyped by the F protein of Spodoptera exigua multiple NPV (SeMNPV), suggesting that F proteins are functionally analogous to GP64. GP64 homologues are thought to have been acquired by group I NPVs during evolution, thereby giving these viruses a selective advantage and obviating the need for a functional F protein. To address this supposition experimentally, attempts were made to pseudotype a group II NPV, SeMNPV, with GP64. Transfection of an f-null SeMNPV bacmid into Se301 cells did not result in the production of infectious BVs. This defect was rescued by insertion of SeMNPV f, but not by insertion of AcMNPV gp64. This suggests that the functional analogy between GP64 and F is not readily reciprocal and that F proteins from group II NPVs may provide additional functions in BV formation that are lacking in the GP64 type of fusion protein.

Novel approach to the development of effective H5N1 influenza A virus vaccines: use of M2 cytoplasmic tail mutants

Outbreaks of highly pathogenic H5N1 influenza viruses in avian species began in Asia and have since spread to other continents. Concern regarding the pandemic potential of these viruses in humans is clearly warranted, and there is an urgent need to develop effective vaccines against them. Previously, we and others demonstrated that deletions of the M2 cytoplasmic tail caused a growth defect in A/WSN/33 (H1N1) influenza A virus in vitro (K. Iwatsuki-Horimoto, T. Horimoto, T. Noda, M. Kiso, J. Maeda, S. Watanabe, Y. Muramoto, K. Fujii, and Y. Kawaoka, J. Virol. 80:5233-5240, 2006; M. F. McCown and A. Pekosz, J. Virol. 79:3595-3605, 2005; M. F. McCown and A. Pekosz, J. Virol. 80:8178-8189, 2006). We therefore tested the feasibility of using M2 tail mutants as live attenuated vaccines against H5N1 virus. First we generated a series of highly pathogenic H5N1 (A/Vietnam/1203/04 [VN1203]) M2 cytoplasmic tail deletion mutants and examined their growth properties in vitro and in vivo. We found that one mutant, which contains an 11-amino-acid deletion from the C terminus (M2del11 virus), grew as well as the wild-type virus but replicated in mice less efficiently. We then generated a recombinant VN1203M2del11 virus whose hemagglutinin (HA) gene was modified by replacing sequences at the cleavage site with those of an avirulent type of HA (M2del11-HAavir virus). This M2del11-HAavir virus protected mice against challenge with lethal doses of homologous (VN1203; clade 1) and antigenically distinct heterologous (A/Indonesia/7/2005; clade 2) H5N1 viruses. Our results suggest that M2 cytoplasmic tail mutants have potential as live attenuated vaccines against H5N1 influenza viruses.

Mechanisms for enveloped virus budding: can some viruses do without an ESCRT?

Many enveloped viruses complete their replication cycle by forming vesicles that bud from the plasma membrane. Some viruses encode "late" (L) domain motifs that are able to hijack host proteins involved in the vacuolar protein sorting (VPS) pathway, a cellular budding process that gives rise to multivesicular bodies and that is topologically equivalent to virus budding. Although many enveloped viruses share this mechanism, examples of viruses that require additional viral factors and viruses that appear to be independent of the VPS pathway have been identified. Alternative mechanisms for virus budding could involve other topologically similar process such as cell abscission, which occurs following cytokinesis, or virus budding could proceed spontaneously as a result of lipid microdomain accumulation of viral proteins. Further examination of novel virus-host protein interactions and characterization of other enveloped viruses for which budding requirements are currently unknown will lead to a better understanding of the cellular processes involved in virus assembly and budding.

Biology of influenza a virus

The outbreaks of avian influenza A virus in poultry and humans over the last decade posed a pandemic threat to human. Here, we discuss the basic classification and the structure of influenza A virus. The viral genome contains eight RNA viral segments and the functions of viral proteins encoded by this genome are described. In addition, the RNA transcription and replication of this virus are reviewed.

Influenza virus hemagglutinin and neuraminidase, but not the matrix protein, are required for assembly and budding of plasmid-derived virus-like particles

For influenza virus, we developed an efficient, noncytotoxic, plasmid-based virus-like particle (VLP) system to reflect authentic virus particles. This system was characterized biochemically by analysis of VLP protein composition, morphologically by electron microscopy, and functionally with a VLP infectivity assay. The VLP system was used to address the identity of the minimal set of viral proteins required for budding. Combinations of viral proteins were expressed in cells, and the polypeptide composition of the particles released into the culture media was analyzed. Contrary to previous findings in which matrix (M1) protein was considered to be the driving force of budding because M1 was found to be released copiously into the culture medium when M1 was expressed by using the vaccinia virus T7 RNA polymerase-driven overexpression system, in our noncytotoxic VLP system M1 was not released efficiently into the culture medium. Additionally, hemagglutinin (HA), when treated with exogenous neuraminidase (NA) or coexpressed with viral NA, could be released from cells independently of M1. Incorporation of M1 into VLPs required HA expression, although when M1 was omitted from VLPs, particles with morphologies similar to those of wild-type VLPs or viruses were observed. Furthermore, when HA and NA cytoplasmic tail mutants were included in the VLPs, M1 failed to be efficiently incorporated into VLPs, consistent with a model in which the glycoproteins control virus budding by sorting to lipid raft microdomains and recruiting the internal viral core components. VLP formation also occurred independently of the function of Vps4 in the multivesicular body pathway, as dominant-negative Vps4 proteins failed to inhibit influenza VLP budding.

Role of the transmembrane domain of marburg virus surface protein GP in assembly of the viral envelope

The major protein constituents of the filoviral envelope are the matrix protein VP40 and the surface transmembrane protein GP. While VP40 is recruited to the sites of budding via the late retrograde endosomal transport route, GP is suggested to be transported via the classical secretory pathway involving the endoplasmic reticulum, Golgi apparatus, and trans-Golgi network until it reaches the plasma membrane where most filoviral budding takes place. Since both transport routes target the plasma membrane, it was thought that GP and VP40 join there to form the viral envelope. However, it was recently shown that, upon coexpression of both proteins, GP is partially recruited into peripheral VP40-enriched multivesicular bodies, which contained markers of the late endosome. Accumulation of GP and VP40 in this compartment was presumed to play an important role in the formation of the filoviral envelope. Using a domain-swapping approach, we were able to show that the transmembrane domain of GP was essential and sufficient for (i) partial recruitment of chimeric glycoproteins into VP40-enriched multivesicular bodies and (ii) incorporation into virus-like particles (VLPs) that were released upon expression of VP40. Only those chimeric glycoproteins which were targeted to VP40-enriched endosomal multivesicular bodies were subsequently recruited into VLPs. These data show that the transmembrane domain of GP is critical for the mixing of VP40 and GP in multivesicular bodies and incorporation of GP into the viral envelope. Results further suggest that trapping of GP in the VP40-enriched late endosomal compartment is important for the formation of the viral envelope.

Role of transmembrane domain and cytoplasmic tail amino acid sequences of influenza a virus neuraminidase in raft association and virus budding

Influenza virus neuraminidase (NA), a type II transmembrane glycoprotein, possesses receptor-destroying activity and thereby facilitates virus release from the cell surface. Among the influenza A viruses, both the cytoplasmic tail (CT) and transmembrane domain (TMD) amino acid sequences of NA are highly conserved, yet their function(s) in virus biology remains unknown. To investigate the role of amino acid sequences of the CT and TMD on the virus life cycle, we systematically mutagenized the entire CT and TMD of NA by converting two to five contiguous amino acids to alanine. In addition, we also made two chimeric NA by replacing the CT proximal one-third amino acids of the NA TMD [NA(1T2N)NA] and the entire NA TMD (NATRNA) with that of human transferrin receptor (TR) (a type II transmembrane glycoprotein). We rescued transfectant mutant viruses by reverse genetics and examined their phenotypes. Our results show that all mutated and chimeric NAs could be rescued into transfectant viruses. Different mutants showed pleiotropic effects on virus growth and replication. Some mutants (NA2A5, NA3A7, and NA4A10) had little effect on virus growth while others (NA3A2, NA5A27, and NA5A31) produced about 50- to 100-fold-less infectious virus and still some others (NA5A14, NA4A19, and NA4A23) exhibited an intermediate phenotype. In general, mutations towards the ectodomain-proximal sequences of TMD progressively caused reduction in NA enzyme activity, affected lipid raft association, and attenuated virus growth. Electron microscopic analysis showed that these mutant viruses remained aggregated and bound to infected cell surfaces and could be released from the infected cells by bacterial NA treatment. Moreover, viruses containing mutations in the extreme N terminus of the CT (NA3A2) as well as chimeric NA containing the TMD replaced partially [NA(1T2N)NA] or fully (NATRNA) with TR TMD caused reduction in virus growth and exhibited the morphological phenotype of elongated particles. These results show that although the sequences of NA CT and TMD per se are not absolutely essential for the virus life cycle, specific amino acid sequences play a critical role in providing structural stability, enzyme activity, and lipid raft association of NA. In addition, aberrant morphogenesis including elongated particle formation of some mutant viruses indicates the involvement of NA in virus morphogenesis and budding.

Chimeric influenza A viruses with a functional influenza B virus neuraminidase or hemagglutinin

Reassortment of influenza A and B viruses has never been observed in vivo or in vitro. Using reverse genetics techniques, we generated recombinant influenza A/WSN/33 (WSN) viruses carrying the neuraminidase (NA) of influenza B virus. Chimeric viruses expressing the full-length influenza B/Yamagata/16/88 virus NA grew to titers similar to that of wild-type influenza WSN virus. Recombinant viruses in which the cytoplasmic tail or the cytoplasmic tail and the transmembrane domain of the type B NA were replaced with those of the type A NA were impaired in tissue culture. This finding correlates with reduced NA content in virions. We also generated a recombinant influenza A virus expressing a chimeric hemagglutinin (HA) protein in which the ectodomain is derived from type B/Yamagata/16/88 virus HA, whereas both the cytoplasmic and the transmembrane domains are derived from type A/WSN virus HA. This A/B chimeric HA virus did not grow efficiently in MDCK cells. However, after serial passage we obtained a virus population that grew to titers as high as wild-type influenza A virus in MDCK cells. One amino acid change in position 545 (H545Y) was found to be responsible for the enhanced growth characteristics of the passaged virus. Taken together, we show here that the absence of reassortment between influenza viruses belonging to different A and B types is not due to spike glycoprotein incompatibility at the level of the full-length NA or of the HA ectodomain.

The NB protein of influenza B virus is not necessary for virus replication in vitro

The NB protein of influenza B virus is thought to function as an ion channel and therefore would be expected to have an essential function in viral replication. Because direct evidence for its absolute requirement in the viral life cycle is lacking, we generated NB knockout viruses by reverse genetics and tested their growth properties both in vitro and in vivo. Mutants not expressing NB replicated as efficiently as the wild-type virus in cell culture, whereas in mice they showed restricted growth compared with findings for the wild-type virus. Thus, the NB protein is not essential for influenza B virus replication in cell culture but promotes efficient growth in mice.

A decade after the generation of a negative-sense RNA virus from cloned cDNA - what have we learned?

Since the first generation of a negative-sense RNA virus entirely from cloned cDNA in 1994, similar reverse genetics systems have been established for members of most genera of the Rhabdo- and Paramyxoviridae families, as well as for Ebola virus (Filoviridae). The generation of segmented negative-sense RNA viruses was technically more challenging and has lagged behind the recovery of nonsegmented viruses, primarily because of the difficulty of providing more than one genomic RNA segment. A member of the Bunyaviridae family (whose genome is composed of three RNA segments) was first generated from cloned cDNA in 1996, followed in 1999 by the production of influenza virus, which contains eight RNA segments. Thus, reverse genetics, or the de novo synthesis of negative-sense RNA viruses from cloned cDNA, has become a reliable laboratory method that can be used to study this large group of medically and economically important viruses. It provides a powerful tool for dissecting the virus life cycle, virus assembly, the role of viral proteins in pathogenicity and the interplay of viral proteins with components of the host cell immune response. Finally, reverse genetics has opened the way to develop live attenuated virus vaccines and vaccine vectors.

Influenza A virus can undergo multiple cycles of replication without M2 ion channel activity

Ion channel proteins are common constituents of cells and have even been identified in some viruses. For example, the M2 protein of influenza A virus has proton ion channel activity that is thought to play an important role in viral replication. Because direct support for this function is lacking, we attempted to generate viruses with defective M2 ion channel activity. Unexpectedly, mutants with apparent loss of M2 ion channel activity by an in vitro assay replicated as efficiently as the wild-type virus in cell culture. We also generated a chimeric mutant containing an M2 protein whose transmembrane domain was replaced with that from the hemagglutinin glycoprotein. This virus replicated reasonably well in cell culture but showed no growth in mice. Finally, a mutant lacking both the transmembrane and cytoplasmic domains of M2 protein grew poorly in cell culture and showed no growth in mice. Thus, influenza A virus can undergo multiple cycles of replication without the M2 transmembrane domain responsible for ion channel activity, although this activity promotes efficient viral replication.

The membrane-proximal stem region of vesicular stomatitis virus G protein confers efficient virus assembly

In this report, we show that the glycoprotein of vesicular stomatitis virus (VSV G) contains within its extracellular membrane-proximal stem (GS) a domain that is required for efficient VSV budding. To determine a minimal sequence in GS that provides for high-level virus assembly, we have generated a series of recombinant DeltaG-VSVs which express chimeric glycoproteins having truncated stem sequences. The recombinant viruses having chimeras with 12 or more membrane-proximal residues of the G stem, and including the G protein transmembrane-cytoplasmic tail domains, produced near-wild-type levels of particles. In contrast, viruses encoding chimeras with shorter or no G-stem sequences produced approximately 10- to 20-fold less. This budding domain when present in chimeric glycoproteins also promoted their incorporation into the VSV envelope. We suggest that the G-stem budding domain promotes virus release by inducing membrane curvature at sites where virus budding occurs or by recruiting condensed nucleocapsids to sites on the plasma membrane which are competent for efficient virus budding.

Influenza type B neuraminidase can replace the function of type A neuraminidase

Influenza A and B viruses do not form reassortants with each other, presumably due to selection at either the RNA or protein level. Although differences in the promoter sequences of type A and B viruses have been studied, selection at the protein level has not been addressed. In this paper we describe experiments to determine whether differences in structure and/or function of the neuraminidase (NA) protein preclude formation of A/B NA reassortants. Influenza type A (N9) NA or B/Lee/40 NA expressed from plasmids can support multicycle growth of a NA-deficient type A virus (NWS-Mvi), indicating that their function in tissue culture is similar. To determine whether the type A or B NA supplied in trans can be incorporated into the virion of NWS-Mvi, the virus grown in NA-expressing cells was purified by sucrose gradient centrifugation. In each case there was a peak of NA activity coincident with the virus peak, indicating that some NA protein is packaged into the virion. The experiments suggest that, in spite of large sequence differences, the functions of the head, stalk, signal-anchor, and cytoplasmic domains of type A and B NAs are similar in tissue culture. Thus, lack of formation of A/B NA reassortant viruses is not due to restriction at the protein level.

Isolation and characterization of a transfectant influenza B virus altered in RNA segment 6

This report describes the successful generation of an influenza B transfectant virus altered in RNA segment 6, which encodes the neuraminidase (NA) protein. The procedure for selection of the transfectant virus relies on the use of strain-specific anti-NA monoclonal antibodies to inhibit growth of the helper virus within the system. A transfectant virus has been engineered which has a coding change in the NA protein. This change resulted in attenuated growth in vitro that could be rescued by addition of exogenous bacterial NA. The mutant virus-associated NA activity was unstable as a result of the engineered changes. The ability to genetically manipulate influenza B virus segment 6 will allow us to assess the function of both NA and the small protein NB, also coded from this RNA, within the context of the virus infectious cycle.

Phosphorylation of the M2 protein of influenza A virus is not essential for virus viability

M2 is a minor component of the influenza A virus envelope. The cytoplasmic tail of the M2 protein is posttranslationally modified in the infected cell by palmitylation and phosphorylation. The primary site for phosphorylation of the M2 cytoplasmic tail is serine 64, which is highly conserved yet not required for the activity of the M2 ion channel. Using an exogenous incorporation assay, we have shown that incorporation of M2 into virus particles is type-specific and does not require phosphorylation of the cytoplasmic tail. In addition, phosphorylation of the cytoplasmic tail is not required for the directional transport of M2 in polarized MDCK cells. Using a reverse genetics and reassortment procedure, we generated a virus (Ra) specifically mutated in segment 7 such that the M2 cytoplasmic tail could no longer be phosphorylated. The virus was found to grow as well as wild-type virus in tissue culture and in eggs, was stable on passage in these systems, and possessed no second-site mutations in the engineered RNA segment. In vivo Ra replicated in Balb/c mice at least as well as the parent strain A/WSN/33. These studies indicate that phosphorylation of the M2 cytoplasmic tail is not required for in vitro or in vivo replication of influenza A virus.

Coronavirus particle assembly: primary structure requirements of the membrane protein

Coronavirus-like particles morphologically similar to normal virions are assembled when genes encoding the viral membrane proteins M and E are coexpressed in eukaryotic cells. Using this envelope assembly assay, we have studied the primary sequence requirements for particle formation of the mouse hepatitis virus (MHV) M protein, the major protein of the coronavirion membrane. Our results show that each of the different domains of the protein is important. Mutations (deletions, insertions, point mutations) in the luminal domain, the transmembrane domains, the amphiphilic domain, or the carboxy-terminal domain had effects on the assembly of M into enveloped particles. Strikingly, the extreme carboxy-terminal residue is crucial. Deletion of this single residue abolished particle assembly almost completely; most substitutions were strongly inhibitory. Site-directed mutations in the carboxy terminus of M were also incorporated into the MHV genome by targeted recombination. The results supported a critical role for this domain of M in viral assembly, although the M carboxy terminus was more tolerant of alteration in the complete virion than in virus-like particles, likely because of the stabilization of virions by additional intermolecular interactions. Interestingly, glycosylation of M appeared not essential for assembly. Mutations in the luminal domain that abolished the normal O glycosylation of the protein or created an N-glycosylated form had no effect. Mutant M proteins unable to form virus-like particles were found to inhibit the budding of assembly-competent M in a concentration-dependent manner. However, assembly-competent M was able to rescue assembly-incompetent M when the latter was present in low amounts. These observations support the existence of interactions between M molecules that are thought to be the driving force in coronavirus envelope assembly.

The cysteine residues of the M2 protein are not required for influenza A virus replication

The M2 protein of influenza A virus functions as an ion channel. It contains three cysteine residues: cysteines 17 and 19, which form disulfide bonds in the ectodomain, and cysteine 50 which is acylated. To understand the role of these cysteine residues in virus replication, we used reverse genetics to create influenza viruses in which the individual cysteines were mutated and a virus in which all three cysteines were changed to serine. The M2 cysteine mutants that lacked either of the cysteine residues in the ectodomain and the mutant that lacked all three residues had appreciably lower amounts of M2 oligomers than did the wild-type virus when examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. None of the mutants, however, were defective in replication, either in vitro or in ferrets and mice. These findings demonstrate that noncovalent interactions are sufficient for the M2 protein to form functional oligomers for virus replication and that its cysteine residues are dispensable for influenza virus replication in vitro and in vivo.

An influenza A live attenuated reassortant virus possessing three temperature-sensitive mutations in the PB2 polymerase gene rapidly loses temperature sensitivity following replication in hamsters

The purpose of the present study was to produce an influenza A H2N2 donor virus from which an attenuating PB2 gene bearing three discrete temperature sensitive (ts) mutations could be readily transferred to currently epidemic influenza A H1N1 and H3N2 viruses via genetic reassortment. An influenza A transfectant virus was first produced that contained site-directed ts mutations at amino acids 112, 265, and 556 in the PB2 gene of influenza A/AA/60 virus origin in a background of the other seven RNA segments from the influenza A/LA/87 (H3N2) virus. The A/LA/87 PB2 ts transfectant virus (clone 22B1) was mated with the A/AA/60 (H2N2) wild type virus, and six H2N2 ts reassortants were obtained. One reassortant virus, clone 25A1, possessed the triple ts PB2 gene in the context of all seven other genes of homologous A/AA/60 origin. Isolation of this reassortant permitted an examination of the contribution of the ts mutations present in a triple ts PB2 transfectant virus to its attenuation and phenotypic stability independent from an effect of the A/AA/60-A/LA/87 gene constellation on attenuation. It was found that the A/AA/60 triple ts reassortant virus was less ts, less attenuated, and less phenotypically stable than the A/LA/87 triple ts transfectant virus from which it was derived. The A/AA/60 reassortant possessing the PB2 gene containing three introduced ts mutations underwent rapid and significant loss of its temperature sensitivity following replication in the lungs of immunocompetent hamsters. This indicated that the A/AA/60-A/LA/87 gene constellation contributed significantly to the overall level of temperature-sensitivity, attenuation, and stability of the A/LA/87 triple ts transfectant virus. It is likely that the instability of the ts phenotype exhibited by the A/AA/60 triple ts reassortant virus would not be acceptable for a vaccine to be used in humans. The implications of these findings for the usefulness of ts mutations as the sole attenuating mutation in influenza virus vaccines is discussed.

Influenza virus hemagglutinin and neuraminidase glycoproteins stimulate the membrane association of the matrix protein

We have analyzed the mechanism by which the matrix (M1) protein associates with cellular membranes during influenza A virus assembly. Interaction of the M1 protein with the viral hemagglutinin (HA) or neuraminidase (NA) glycoprotein was extensively analyzed by using wild-type and transfectant influenza viruses as well as recombinant vaccinia viruses expressing the M1 protein, HA, or NA. Membrane binding of the M1 protein was significantly stimulated at the late stage of virus infection. Using recombinant vaccinia viruses, we found that a relatively small fraction (20 to 40%) of the cytoplasmic M1 protein associated with cellular membranes in the absence of other viral proteins, while coexpression of the HA and the NA stimulated membrane binding of the M1 protein. The stimulatory effect of the NA (>90%) was significant and higher than that of the HA (>60%). Introduction of mutations into the cytoplasmic tail of the NA interfered with its stimulatory effect. Meanwhile, the HA may complement the defective NA and facilitate virus assembly in cells infected with the NA/TAIL(-) transfectant. In conclusion, the highly conserved cytoplasmic tails of the HA and NA play an important role in virus assembly.

An internalization signal in the simian immunodeficiency virus transmembrane protein cytoplasmic domain modulates expression of envelope glycoproteins on the cell surface

A Tyr to Cys mutation at amino acid position 723 in the cytoplasmic domain of the simian immunodeficiency virus (SIV) transmembrane (TM) molecule has been shown to increase expression of envelope glycoproteins on the surface of infected cells. Here we show that Tyr-723 contributes to a sorting signal that directs the rapid endocytosis of viral glycoproteins from the plasma membrane via coated pits. On cells infected by SIVs with a Tyr at position 723, envelope glycoproteins were transiently expressed on the cell surface and then rapidly endocytosed. Similar findings were noted for envelope molecules expressed in the absence of other viral proteins. Immunoelectron microscopy demonstrated that these molecules were localized in patches on the cell surface and were frequently associated with coated pits. In contrast, envelope glycoproteins containing a Y723C mutation were diffusely distributed over the entire plasma membrane. To determine if an internalization signal was present in the SIV TM, chimeric molecules were constructed that contained the CD4 external and membrane spanning domains and a SIV TM cytoplasmic tail with a Tyr or other amino acids at SIV position 723. In Hela cells stably expressing these molecules, chimeras with a Tyr-723 were rapidly endocytosed, while chimeras containing other amino acids at position 723, including a Phe, were internalized at rates only slightly faster than a CD4 molecule that lacked a cytoplasmic domain. In addition, the biological effects of the internalization signal were evaluated in infectious viruses. A mutation that disrupted the signal and as a result, increased the level of viral envelope glycoprotein on infected cells, was associated with accelerated infection kinetics and increased cell fusion during viral replication. These results demonstrate that a Tyr-dependent motif in the SIV TM cytoplasmic domain can function as an internalization signal that can modulate expression of the viral envelope molecules on the cell surface and affect the biological properties of infectious viruses. The conservation of an analogous Tyr in all human and simian immunodeficiency viruses suggests that this signal may be present in other primate lentiviruses and could be important in the pathogenesis of these viruses in vivo.

The cytoplasmic tail of influenza A virus neuraminidase (NA) affects NA incorporation into virions, virion morphology, and virulence in mice but is not essential for virus replication

In this study, we investigated the role of the conserved neuraminidase (NA) cytoplasmic tail residues in influenza virus replication. Mutants of influenza A virus (A/WSN/33 [H1N1]) with deletions of the NA cytoplasmic tail region were generated by reverse genetics. The resulting viruses, designated NOTAIL, contain only the initiating methionine of the conserved six amino-terminal residues. The mutant viruses grew much less readily and produced smaller plaques than did the wild-type virus. Despite similar levels of NA cell surface expression by the NOTAIL mutants and wild-type virus, incorporation of mutant NA molecules into virions was decreased by 86%. This reduction resulted in less NA activity per virion, leading to the formation of large aggregates of progeny mutant virions on the surface of infected cells. A NOTAIL virus containing an additional mutation (Ser-12 to Pro) in the transmembrane domain incorporated three times more NA molecules into virions than did the NOTAIL parent but approximately half of the amount incorporated by the wild-type virus. However, aggregation of the progeny virions still occurred at the cell surface. All NOTAIL viruses were attenuated in mice. We conclude that the cytoplasmic tail of NA is not absolutely essential for virus replication but exerts important effects on the incorporation of NA into virions and thus on the aggregation and virulence of progeny virus. In addition, the relative abundance of long filamentous particles formed by the NOTAIL mutants, compared with the largely spherical wild-type particles, indicates a role for the NA cytoplasmic tail in virion morphogenesis.

Sequential addition of temperature-sensitive missense mutations into the PB2 gene of influenza A transfectant viruses can effect an increase in temperature sensitivity and attenuation and permits the rational design of a genetically engineered live influenza A virus vaccine

We have previously described a strategy for the recovery of a synthetic influenza A virus wild-type (wt) PB2 gene (derived from influenza A/Ann Arbor/6/60 [AA] virus) into an infectious virus. It was possible to introduce an attenuating temperature-sensitive (ts) mutation at amino acid residue 265 of the AA wt PB2 gene and to rescue this mutant gene into infectious virus. Application of this new technology to influenza A virus vaccine development requires that multiple attenuating mutations be introduced to achieve a satisfactorily attenuated virus that retains the attenuation (att) phenotype following replication in vivo. In this report, we demonstrate that putative ts mutations at amino acids 112, 556, and 658 each indeed specify the ts and att phenotypes. Each of these mutations was introduced into a cDNA copy of the AA mutant mt265 PB2 gene to produce three double-mutant PB2 genes, each of which was rescued into an infectious virus. In general, the double-mutant PB2 transfectant viruses were more ts and attenuated in the lower respiratory tracts of hamsters than the single-mutant transfectant viruses, and the ts phenotype of two of three double-mutant PB2 transfectant viruses was stable even after prolonged replication in the upper respiratory tracts of immunocompromised mice. Two triple-mutant PB2 transfectant viruses with three predicted amino acid substitutions resulting from five nucleotide substitutions in the cDNA were then generated. The triple-mutant PB2 transfectant viruses were more ts and more attenuated than the double-mutant PB2 transfectant viruses. These results indicate that sequential introduction of additional ts mutations into the PB2 gene can yield mutants that exhibit a stepwise increase in temperature sensitivity and attenuation compared with the preceding mutant(s) in the series. Furthermore, the level of temperature sensitivity of the transfectant viruses correlated significantly with the level of attenuation of these viruses in hamsters. Although the triple-mutant PB2 transfectant viruses were attenuated in hamsters, intranasal administration of these viruses elicited a vigorous serum hemagglutination-inhibiting antibody response, and this was associated with resistance of the lower respiratory tract to subsequent wt virus challenge. These observations suggest the feasibility of using PB2 reverse genetics to generate a live influenza A virus vaccine donor strain that contains three attenuating mutations in one gene. It is predicted that reassortant viruses derived from such a donor virus would have the properties of attenuation, genetic stability, immunogenicity, and protective efficacy against challenge with wt virus.

A single amino acid change in the cytoplasmic domain of the simian immunodeficiency virus transmembrane molecule increases envelope glycoprotein expression on infected cells

We have described a virus termed CP-MAC, derived from the BK28 molecular clone of simian immunodeficiency virus, that was remarkable for its ability to infect Sup-T1 cells with rapid kinetics, cell fusion, and CD4 down-modulation (C. C. LaBranche, M. M. Sauter, B. S. Haggarty, P. J. Vance, J. Romano, T. K. Hart, P. J. Bugelski, and J. A. Hoxie, J. Virol. 68:5509-5522, 1994 [Erratum 68:7665-7667]). Compared with BK28, CP-MAC exhibited a number of changes in its envelope glycoproteins, including a highly stable association between the external (SU) and transmembrane (TM) molecules, a more rapid electrophoretic mobility of TM, and, of particular interest, a marked increase in the level of envelope protein expression on the surface of infected cells. These changes were shown to be associated with 11 coding mutations in the env gene (5 in SU and 6 in TM). In this report, we demonstrate that a single amino acid mutation of a Tyr to a Cys at position 723 (Y723C) in the TM cytoplasmic domain of CP-MAC is the principal determinant for the increased expression of envelope glycoproteins on the cell surface. When introduced into the env gene of BK28, the Y723C mutation produced up to a 25-fold increase in the levels of SU and TM on chronically infected cells, as determined by fluorescence-activated cell sorter analysis with monoclonal and polyclonal antibodies. A similar effect was observed when a Tyr-to-Cys change was introduced at the analogous position (amino acid 721) in the SIVmac239 molecular clone, which, unlike BK28 does not contain a premature stop codon in its TM cytoplasmic tail. Substituting other amino acids, including Ala, Ile, and Ser, at this position produced increases in surface envelope glycoproteins that were similar to that observed for the Cys substitution, while a Tyr-to-Phe mutation produced a smaller increase. These results could not be accounted for by differences in the kinetics or efficiency of envelope glycoprotein processing or by shedding of SU from infected cells. However, immunoelectron microscopy demonstrated that the Y723C mutation in BK28 produced a striking redistribution of cell surface envelope molecules from localized patches to a diffuse pattern that covered the entire plasma membrane. This finding suggests that mutation of a Tyr residue in the simian immunodeficiency virus TM cytoplasmic domain may disrupt a structural element that can modulate envelope glycoprotein expression on the surface of infected cells.

The cytoplasmic tail of the neuraminidase protein of influenza A virus does not play an important role in the packaging of this protein into viral envelopes

We have rescued a transfectant influenza virus, NA/TAIL(-), whose neuraminidase (NA) protein lacks the predicted cytoplasmic tail. The virus was attenuated (one log10 reduction) both in tissue culture and in mouse lungs. Attenuation correlated with a 50% reduction of the level of NA in infected cells and levels of incorporation of the tail-less NA protein into viral particles paralleled that in infected cells. This result indicates that the signal for packaging of the NA protein into the viral envelope is not located in its cytoplasmic domain.

Expression of a foreign protein by influenza A virus

In this report we describe the rescue of a transfectant influenza A virus which stably expresses a heterologous protein, bacterial chloramphenicol acetyltransferase (CAT). The foreign sequences encoding CAT are expressed as part of an essential influenza virus segment, that coding for the neuraminidase (NA) protein. The novel way by which this was achieved involved inserting in frame the 16-amino-acid self-cleaving 2A protease of foot-and-mouth disease virus between the CAT and the NA coding sequences. The resultant gene produces a polyprotein which is proteolytically cleaved to release both CAT and NA. The intramolecular cleavage occurs at the C terminus of the 2A sequence between a glycine-proline dipeptide motif such that the released NA protein has an additional N-terminal proline residue. The transfectant virus is stable upon passage in tissue culture. CAT activity is expressed at high levels in cell culture supernatants and in the allantoic fluid of infected eggs. Since the chimeric segment must maintain the heterologous reading frame to retain viability, the virus stability is dependent upon concomitant synthesis of the heterologous protein. This design may be particularly appropriate for utilization of influenza virus as a mammalian expression vector.

Reverse genetics provides direct evidence for a correlation of hemagglutinin cleavability and virulence of an avian influenza A virus

To obtain direct evidence for a relationship between hemagglutinin (HA) cleavability and the virulence of avian influenza A viruses, we generated a series of HA cleavage mutants from a virulent virus, A/turkey/Ontario/7732/66 (H5N9), by reverse genetics. A transfectant virus containing the wild-type HA with R-R-R-K-K-R at the cleavage site, which was readily cleaved by endogenous proteases in chicken embryo fibroblasts (CEF), was highly virulent in intramuscularly or intranasally/orally inoculated chickens. By contrast, a mutant containing the HA with an avirulent-like sequence (R-E-T-R) at the cleavage site, which was not cleaved by the proteases in CEF, was avirulent in chickens, indicating that a genetic alteration confined to the HA cleavage site can affect cleavability and virulence. Mutant viruses with HA cleavage site sequences of T-R-R-K-K-R or T-T-R-K-K-R were as virulent as viruses with the wild-type HA, whereas a mutant with a two-amino-acid deletion but retention of four consecutive basic residues (R-K-K-R) was as avirulent as a virus with the avirulent-type HA. Interestingly, although a mutant containing an HA with R-R-R-K-T-R, which has reduced cleavability in CEF, was as virulent as viruses with high HA cleavability when given intramuscularly, it was less virulent when given intranasally/orally. We conclude that the degree of HA cleavability in CEF predicts the virulence of avian influenza viruses.