A Sendai virus vector leading to the efficient expression of mutant M proteins interfering with virus particle budding

Nipah Virus Matrix Protein Influences Fusogenicity and Is Essential for Particle Infectivity and Stability

Nipah virus (NiV) causes fatal encephalitic infections in humans. To characterize the role of the matrix (M) protein in the viral life cycle, we generated a reverse genetics system based on NiV strain Malaysia. Using an enhanced green fluorescent protein (eGFP)-expressing M protein-deleted NiV, we observed a slightly increased cell-cell fusion, slow replication kinetics, and significantly reduced peak titers compared to the parental virus. While increased amounts of viral proteins were found in the supernatant of cells infected with M-deleted NiV, the infectivity-to-particle ratio was more than 100-fold reduced, and the particles were less thermostable and of more irregular morphology. Taken together, our data demonstrate that the M protein is not absolutely required for the production of cell-free NiV but is necessary for proper assembly and release of stable infectious NiV particles.

IMPORTANCE

Henipaviruses cause a severe disease with high mortality in human patients. Therefore, these viruses can be studied only in biosafety level 4 (BSL-4) laboratories, making it more challenging to characterize their life cycle. Here we investigated the role of the Nipah virus matrix protein in virus-mediated cell-cell fusion and in the formation and release of newly produced particles. We found that even though low levels of infectious viruses are produced in the absence of the matrix protein, it is required for the release of highly infectious and stable particles. Fusogenicity of matrixless viruses was slightly enhanced, further demonstrating the critical role of this protein in different steps of Nipah virus spread.

Patchwork structure-function analysis of the Sendai virus matrix protein

Paramyxoviruses contain a bi-lipidic envelope decorated by two transmembrane glycoproteins and carpeted on the inner surface with a layer of matrix proteins (M), thought to bridge the glycoproteins with the viral nucleocapsids. To characterize M structure-function features, a set of M domains were mutated or deleted. The genes encoding these modified M were incorporated into recombinant Sendai viruses and expressed as supplemental proteins. Using a method of integrated suppression complementation system (ISCS), the functions of these M mutants were analyzed in the context of the infection. Cellular membrane association, localization at the cell periphery, nucleocapsid binding, cellular protein interactions and promotion of viral particle formation were characterized in relation with the mutations. At the end, lack of nucleocapsid binding go together with lack of cell surface localization and both features definitely correlate with loss of M global function estimated by viral particle production.

Mutation of the TYTLE motif in the cytoplasmic tail of the sendai virus fusion protein deeply affects viral assembly and particle production

Enveloped viruses contain glycoproteins protruding from the viral membrane. These proteins play a crucial role in the extra-cellular steps of the virus life cycle, namely attachment to and entry into cells. Their role during the intracellular late phase of virus multiplication has been less appreciated, overlooked by the documented central organizer role of the matrix M protein. Sendai virus, a member of the Paramyxoviridae family, expresses two trans-membrane proteins on its surface, HN and F. In previous work, we have shown that suppression of F in the context of an infection, results in about 70% reduction of virus particle production, a reduction similar to that observed upon suppression of the matrix M protein. Moreover, a TYTLE motif present in F cytoplasmic tail has been proposed essential for virus particle production. In the present work, using original alternate conditional siRNA suppression systems, we generated a double F gene recombinant Sendai virus expressing wt-F and a nonviable mutated TYTLE/5A F protein (F_5A). Suppression of the wild type F gene expression in cells infected with this virus allowed the analysis of F_5A properties in the context of the infection. Coupling confocal imaging analysis to biochemical characterization, we found that F_5A i) was not expressed at the cell surface but restricted to the endoplasmic reticulum, ii) was still capable of interaction with M and iii) had profound effect on M and HN cellular distribution. On the basis of these data, we propose a model for SeV particle formation based on an M/F complex that would serve as nucleation site for virus particle assembly at the cell surface.

Minimal features of efficient incorporation of the hemagglutinin-neuraminidase protein into sendai virus particles

Two transmembrane glycoproteins form spikes on the surface of Sendai virus, a member of the Respirovirus genus of the Paramyxovirinae subfamily of the Paramyxoviridae family: the hemagglutinin-neuraminidase (HN) and the fusion (F) proteins. HN, in contrast to F, is dispensable for viral particle production, as normal amounts of particles can be produced with highly reduced levels of HN. This HN reduction can result from mutation of an SYWST motif in its cytoplasmic tail to AFYKD. HNAFYKD accumulates at the infected cell surface but does not get incorporated into particles. In this work, we derived experimental tools to rescue HNAFYKD incorporation. We found that coexpression of a truncated HN harboring the wild-type cytoplasmic tail, the transmembrane domain, and at most 80 amino acids of the ectodomain was sufficient to complement defective HNAFYKD incorporation into particles. This relied on formation of disulfide-bound heterodimers carried out by the two cysteines present in the HN 80-amino-acid (aa) ectodomain. Finally, the replacement of the measles virus H cytoplasmic and transmembrane domains with the corresponding HN domains promoted measles virus H incorporation in Sendai virus particles.

Sendai virus induced cytoplasmic actin remodeling correlates with efficient virus particle production

Cytoplasmic actins have been found interacting with viral proteins and identified in virus particles. We analyzed by confocal microscopy the cytoplasmic β- and γ-actin patterns during the course of Sendai virus infections in polarized cells. We observed a spectacular remodeling of the β-cytoplasmic actin which correlated with productive viral multiplication. Conversely, suppression of M during the course of a productive infection resulted in the decrease of particle production and the absence of β-actin remodeling. As concomitant suppression of β- and γ-actins resulted as well in reduction of virus particle production, we propose that Sendai virus specifically induces actin remodeling in order to promote efficient virion production. Beta- and γ-cytoplasmic actin recruitment could substitute for that of the endosomal sorting complex required for transport (ESCRT) mobilized by other enveloped viruses but apparently not used by Sendai virus.

Sendai virus particle production: basic requirements and role of the SYWST motif present in HN cytoplasmic tail

Sendai virus (SeV) HN protein is dispensable for virus particle production. HN incorporation into virions strictly depends on a cytoplasmic domain SYWST motif. HNAFYKD, with SYWST replaced with the analogous sequence of measles virus (MeV) H (AFYKD), is not incorporated in virus particles produced by LLCMK2 cells, although it is normally expressed at the plasma membrane. Unlike HNSYWST, HNAFYKD is not internalized to late endosomes, raising the possibility that HN internalization is required for uptake into virus particles. Various mosaic MeV-H containing increasing amounts of the SeV-HN all failed to be taken up in SeV virions. However, when co-expressed with HNAFYKD these MeV-H chimera induced HNAFYKD uptake into virions showing that internalization is not a prerequisite for HN uptake into particles. We propose that HN incorporation in virus particles requires first neutralization by HN of a putative inhibitor of infectious particle formation.

Recruitment of Alix/AIP1 to the plasma membrane by Sendai virus C protein facilitates budding of virus-like particles

Sendai virus (SeV) is unique in that one of the viral accessory proteins, C, enhances budding of virus-like particles (VLPs) formed by SeV matrix protein M by physically interacting with Alix/AIP1. C protein itself does not have the ability to form VLPs, while M protein provides viral budding force, like other enveloped viruses. Here we show that SeV C protein recruits Alix/AIP1 to the plasma membrane (PM) to facilitate VLP budding. SeV M-VLP budding is sensitive to overexpression of a dominant-negative (DN) form of VPS4A only in the presence of the C proteins, which is able to recruit Alix/AIP1 to the PM. Our results indicate that SeV M and C proteins play separate roles in the budding process: M protein drives budding and C protein enhances the efficiency of the utilization of cellular MVB sorting machinery for efficient VLP budding.

Sendai virus budding in the course of an infection does not require Alix and VPS4A host factors

Closing the Sendai virus C protein open reading frames (rSeV-DeltaC virus) results in the production of virus particles with highly reduced infectivity. Besides, the Sendai virus C proteins interact with Alix/AIP1 and Alix suppression negatively affects Sendai virus like particle (VLP) budding. Similarly, the Sendai virus M protein has been shown to interact with Alix. On this basis, it has been suggested that Sendai virus budding involves recruitment of the multivesicular body formation machinery. We follow, here, the production of SeV particles upon regular virus infection. We find that neither Alix suppression nor dominant negative-VPS4A expression, applied separately or in combination, affects physical or infectious virion production. This contrasts with the observed decrease of SV5 virion production upon dominant negative-VPS4A expression. Finally, we show that suppression of more than 70% of a GFP/C protein in the background of a rSeV-DeltaC virus infection has no effect either on SeV particle production or on virus particle infectivity. Our results contrast with what has been published before. Possible explanations for this discrepancy are discussed.

High level expression of soluble glycoproteins in the allantoic fluid of embryonated chicken eggs using a Sendai virus minigenome system

Background

Embryonated chicken eggs have been used since the mid-20th century to grow a wide range of animal viruses to high titers. However, eggs have found so far only limited use in the production of recombinant proteins. We now describe a system, based on a Sendai virus minigenome, to produce large amounts of heterologous viral glycoproteins in the allantoic cavity of embryonated eggs.

Results

Soluble forms of human respiratory syncytial virus (HRSV) and human metapneumovirus (HMPV) fusion (F) proteins, devoid of their transmembrane and cytoplasmic domains, were produced in allantoic fluids using the Sendai minigenome system. The first step was rescuing in cell cultures Sendai virus minigenomes encoding the proteins of interest, with the help of wild type Sendai virus. The second step was propagating such recombinant defective viruses, together with the helper virus, in the allantoic cavity of chicken embryonated eggs, and passage to optimize protein production. When compared with the production of the same proteins in the culture supernatant of cells infected with vaccinia recombinants, the yield in the allantoic fluid was 5–10 fold higher. Mutant forms of these soluble proteins were easily constructed by site-directed mutagenesis and expressed in eggs using the same approach.

Conclusion

The simplicity and economy of the Sendai minigenome system, together with the high yield achieved in the allantoic fluid of eggs, makes it an attractive method to express soluble glycoproteins aimed for structural studies.

Suppression of the Sendai virus M protein through a novel short interfering RNA approach inhibits viral particle production but does not affect viral RNA synthesis

Short RNA interference is more and more widely recognized as an effective method to specifically suppress viral functions in eukaryotic cells. Here, we used an experimental system that allows suppression of the Sendai virus (SeV) M protein by using a target sequence, derived from the green fluorescent protein gene, that was introduced in the 3' untranslated region of the M protein mRNA. Silencing of the M protein gene was eventually achieved by a small interfering RNA (siRNA) directed against this target sequence. This siRNA was constitutively expressed in a cell line constructed by transduction with an appropriate lentivirus vector. Suppression of the M protein was sufficient to diminish virus production by 50- to 100-fold. This level of suppression had no apparent effect on viral replication and transcription, supporting the lack of M involvement in SeV transcription or replication control.

From assembly to virus particle budding: pertinence of the detergent resistant membranes

Detergent resistant membranes (DRMs) are the site of assembly for a variety of viruses. Here, we make use of Sendai virus mutant proteins that are not packaged into virus particles to determine the involvement of this assembly for the virus particle production. We found that, in the context of an infection, (1) all the Sendai virus proteins associated in part with DRMs, (2) mutant HN and M proteins not packaged into virus particles were similarly part of this association, (3) after M protein suppression resulting in a significant reduction of virus production, the floatation profile of the other viral proteins was not altered and finally (4) cellular cholesterol depletion did not decrease the virus particle production, although it somehow reduced their virus infectivity. These results led us to conclude that the assembly complex found in DRM fractions does not constitute a direct precursor of virus particle budding.

AIP1/Alix is a binding partner of Sendai virus C protein and facilitates virus budding

The C protein, an accessory protein of Sendai virus (SeV), has anti-interferon capacity and suppresses viral RNA synthesis. In addition, it is thought that the C protein is involved in virus budding because of the low efficiency of release of progeny virions from C-knockout virus-infected cells and because of the requirement of the C protein for efficient release of virus-like particles. Here, we identified AIP1/Alix, a host protein involved in apoptosis and endosomal membrane trafficking, as an interacting partner of the C protein using a yeast two-hybrid system. The amino terminus of AIP1/Alix and the carboxyl terminus of the C protein are important for the interaction in mammalian cells. Mutant C proteins unable to bind AIP1/Alix failed to accelerate the release of virus-like particles from cells. Furthermore, overexpression of AIP1/Alix enhanced SeV budding from infected cells in a C-protein-dependent manner, while the release of nucleocapsid-free empty virions was also enhanced. Finally, AIP1/Alix depletion by small interfering RNA resulted in suppression of SeV budding. The results of this study suggest that AIP1/Alix plays a role in efficient SeV budding and that the SeV C protein facilitates virus budding through interaction with AIP1/Alix.

High throughput screening assay for negative single stranded RNA virus polymerase inhibitors

The Paramyxoviridae form a large family of viruses containing many human and veterinary pathogens for which a need for antiviral treatment is emphasized, particularly following the recent emergence of new viruses. The viral RNA-dependent RNA polymerase constitutes an obvious target for antiviral compounds. An in vitro assay was developed that allows high throughput screening of compounds potentially inhibiting the Sendai virus RNA-dependent RNA polymerase. Screening relies on the detection of the Photinus pyralis luciferase produced in a transcription/translation coupled assay using a mini-replicon virus. It contains an internal control for possible adverse effects of the tested compounds on translation or on luciferase activity. It is estimated that the mini-replicon template produced in one fertilized egg is sufficient to run 5000-10,000 reactions. This assay constitutes a simple, sensitive and easily automated method to perform high throughput screening of Paramyxoviridae RNA-dependent RNA polymerase inhibitors.

Replication and packaging properties of short Paramyxovirus defective RNAs

In the course of a Sendai virus infection, short genome versions (called defective) are frequently produced. These compete with the full length genome for replication (hence called interfering) and decrease the severity of the infection. The portion of the defective genome required for this effect can be limited, much shorter than the size of the defective genome. We postulated then that defective genome size follows from conditions required for their production more than for exerting the protective effect. Therefore, we set up experiments to determine which of the steps involved in their production may require this larger size. We constructed a series of Sendai virus mini-genomes, increasing their size from the smallest possible and we followed their ability to replicate, to amplify through multiples cycles of infection and to be packaged into virus particles, all steps required for their efficient production. We found that the mini-genomes replicated and amplified efficiently regardless of their size. The uptake into virus particles, however, depended on a threshold length. The data suggest no limitation in the ability of the viral RNA polymerase to generate and amplify very short size defective genomes. They however, point to limitations in the ability of short genomes to be packaged into virus particles.

Nontransmissible virus-like particle formation by F-deficient sendai virus is temperature sensitive and reduced by mutations in M and HN proteins

The formation of nontransmissible virus-like particles (NTVLP) by cells infected with F-deficient Sendai virus (SeV/deltaF) was found to be temperature sensitive. Analysis by hemagglutination assays and Western blotting demonstrated that the formation of NTVLP at 38 degrees C was about 1/100 of that at 32 degrees C, whereas this temperature-sensitive difference was only moderate in the case of F-possessing wild-type SeV. In order to reduce the NTVLP formation with the aim of improving SeV for use as a vector for gene therapy, amino acid substitutions found in temperature-sensitive mutant SeVs were introduced into the M (G69E, T116A, and A183S) and HN (A262T, G264R, and K461G) proteins of SeV/deltaF to generate SeV/M(ts)HN(ts)deltaF. The use of these mutations allows vector production at low temperature (32 degrees C) and therapeutic use at body temperature (37 degrees C) with diminished NTVLP formation. As expected, the formation of NTVLP by SeV/M(ts)HN(ts)deltaF at 37 degrees C was decreased to about 1/10 of that by SeV/deltaF, whereas the suppression of NTVLP formation did not cause either enhanced cytotoxicity or reduced gene expression of the vector. The vectors showed differences with respect to the subcellular distribution of M protein in the infected cells. Clear and accumulated immunocytochemical signals of M protein on the cell surface were not observed in cells infected by SeV/deltaF at an incompatible temperature, 38 degrees C, or in those infected by SeV/M(ts)HN(ts)deltaF at 37 or 38 degrees C. The absence of F protein in SeV/deltaF and the additional mutations in M and HN in SeV/M(ts)HN(ts)deltaF probably weaken the ability to transport M protein to the plasma membrane, leading to the diminished formation of NTVLP.

Assembly of Sendai virus: M protein interacts with F and HN proteins and with the cytoplasmic tail and transmembrane domain of F protein

Sendai virus matrix protein (M protein) is critically important for virus assembly and budding and is presumed to interact with viral glycoproteins on the outer side and viral nucleocapsid on the inner side. However, since M protein alone binds to lipid membranes, it has been difficult to demonstrate the specific interaction of M protein with HN or F protein, the Sendai viral glycoproteins. Using Triton X-100 (TX-100) detergent treatment of membrane fractions and flotation in sucrose gradients, we report that the membrane-bound M protein expressed alone or coexpressed with heterologous glycoprotein (influenza virus HA) was totally TX-100 soluble but the membrane-bound M protein coexpressed with HN or F protein either individually or together was predominantly detergent-resistant and floated to the top of the density gradient. Furthermore, both the cytoplasmic tail and the transmembrane domain of F protein facilitated binding of M protein to detergent-resistant membranes. Analysis of the membrane association of M protein in the early and late phases of the Sendai virus infectious cycle revealed that the interaction of M protein with mature glycoproteins that associated with the detergent-resistant lipid rafts was responsible for the detergent resistance of the membrane-bound M protein. Immunofluorescence analysis by confocal microscopy also demonstrated that in Sendai virus-infected cells, a fraction of M protein colocalized with F and HN proteins and that some M protein also became associated with the F and HN proteins while they were in transit to the plasma membrane via the exocytic pathway. These studies indicate that F and HN interact with M protein in the absence of any other viral proteins and that F associates with M protein via its cytoplasmic tail and transmembrane domain.

Structure-function analysis of the Sendai virus F and HN cytoplasmic domain: different role for the two proteins in the production of virus particle

The role of the cytoplasmic domain (cytd) of the Sendai virus HN and F glycoproteins in the process of virus assembly and budding are evaluated. Recombinant Sendai virus (rSeV) mutants are generated carrying modifications in the cytd of each of the glycoprotein separately. The modifications include increasing truncations and/or amino acid sequence substitutions. Following steady-state (35)[S]methionine/cysteine labeling of the infected cells, the virus particle production is estimated. The radioactive virions in the cell supernatants are measured relative to the extent of the infection, assessed by the intracellular N protein signal. For both the F and HN cytd truncation mutants, the largest cytd deletions lead to a 20- to 50-fold reduction in virion production. This reduction cannot be explained by a reduction of the cell surface expression of the glycoproteins. For the F protein mutants, the virions produced in reduced amount always exhibit a normal F protein composition. It is then concluded that a threshold level of F is required for SeV assembly and budding. The rate or the efficiency with which this threshold is reached up appears to depend on the nature of the F cytd. A minimal cytd length is required as well as a specific sequence. The analysis of HN protein mutants brings to light an apparent paradox. The larger cytd truncations result in significant reduction of virion production. On the other hand, a normal virion production can take place with an underrepresentation of or, even, an undetectable HN in the particles. The HN uptake in virion is confirmed to depend on the previously proposed cytd SYWST signal (T. Takimoto, T. Bousse, E. C. Coronel, R. A Scroggs, and A. Portner. 1998. J. Virol. 72, 9747-9754.).

Pathogenicity of Sendai viruses adapted into polarized MDCK cells

Apically and basally released Sendai viruses (SeV) were obtained after infection of polarized Madin-Darby canine kidney (MDCK) cells grown on permeable membrane culture inserts. After 20 passages of adaptation in MDCK cells, we compared their in vivo and in vitro pathogenicity with the parental Mol-strain of SeV. These viruses had comparable in vitro pathogenicity, but the in vivo pathogenicities were varied. The apically released MDCK-adapted virus showed comparable pathogenicity with the parental virus, in contrast with the basally released MDCK-adapted virus, which showed in vivo attenuation.

Characterization of Sendai virus M protein mutants that can partially interfere with virus particle production

Substitution of Val(113) in Sendai virus (SeV) M protein generates non-functional polypeptides, characterized by their exclusion from virus particles and by their ability to interfere with virus particle production. These phenotypic traits correlate with a single-band PAGE migration profile, in contrast to wild-type M (M(wt )), which separates into two species, one of which is a phosphorylated form. The single-band migration is likely to result from a conformational change, as evidenced by the lack of maturation of a native epitope and by a particular tryptic digestion profile, and not from the phosphorylation of all M molecules, an assumption consistent with the PAGE migration feature. One of the M mutants (HA-M(30 ), an M protein carrying Thr(112)Met and Val(113) Glu substitutions tagged with an influenza virus haemagglutinin epitope) was characterized further in the context of SeV infection, i.e. under conditions of co-expression with M(wt). HA-M (30) is shown (i) to bind mainly to membrane fractions, (ii) not to co-precipitate M(wt), as HA-M(wt) does, (iii) to interfere with the binding of nucleocapsids to membranes and (iv) to accumulate in perinuclear regions, in contrast to HA-M(wt ), which is also found at the cell periphery. Such mutants constitute potential tools for the identification of critical steps in paramyxovirus assembly and budding.

Virus promoters determine interference by defective RNAs: selective amplification of mini-RNA vectors and rescue from cDNA by a 3' copy-back ambisense rabies virus

Typical defective interfering (DI) RNAs are more successful in the competition for viral polymerase than the parental (helper) virus, which is mostly due to an altered DI promoter composition. Rabies virus (RV) internal deletion RNAs which possess the authentic RV terminal promoters, and which therefore are transcriptionally active and can be used as vectors for foreign gene expression, are poorly propagated in RV-infected cells and do not interfere with RV replication. To allow DI-like amplification and high-level gene expression from such mini-RNA vectors, we have used an engineered 3' copy-back (ambisense) helper RV in which the strong replication promoter of the antigenome was replaced with the 50-fold-weaker genome promoter. In cells coinfected with ambisense helper virus and mini-RNAs encoding chloramphenicol acetyltransferase (CAT) and luciferase, mini-RNAs were amplified to high levels. This was correlated with interference with helper virus replication, finally resulting in a clear predominance of mini-RNAs over helper virus. However, efficient successive passaging of mini-RNAs and high-level reporter gene activity could be achieved without adding exogenous helper virus, revealing a rather moderate degree of interference not precluding substantial HV propagation. Compared to infections with recombinant RV vectors expressing CAT, the availability of abundant mini-RNA templates led to increased levels of CAT mRNA such that CAT activities were augmented up to 250-fold, while virus gene transcription was kept to a minimum. We have also exploited the finding that internal deletion model RNAs behave like DI RNAs and are selectively amplified in the presence of ambisense helper virus to demonstrate for the first time RV-supported rescue of cDNA after transfection of mini-RNA cDNAs in ambisense RV-infected cells expressing T7 RNA polymerase.

The activity of Sendai virus genomic and antigenomic promoters requires a second element past the leader template regions: a motif (GNNNNN)3 is essential for replication

The paramyxovirus genome, a nonsegmented, negative-polarity, single-stranded RNA of approximately 15 kb, contains six transcription units flanked at the 3' and 5' ends by a short (approximately 50- to 60-nucleotide) extracistronic sequence, dubbed the positive and negative leader regions. These leader template regions, present at the 3' end of the genome and the antigenome, have been shown to contain essential signals governing RNA replication activity. Whether they are sufficient to promote replication is still open to question. By using a series of Sendai virus defective interfering RNAs carrying a nested set of deletions in the promoter regions, it is shown here that for both the genomic and antigenomic promoters, a 3'-end RNA sequence of 96 nucleotides is required to allow replication. Sequence comparison of active and inactive promoters led to the identification of a set of three nucleotide hexamers (nucleotides 79 to 84, 85 to 90, and 91 to 96) containing a repeated motif RXXYXX [shown as 5'-3' positive-strand]. Sequential mutation of each hexamer into its complementary sequence confirmed their essential role. The three hexamers are required, and their relative positioning is important, since displacing them by 6 nucleotides destroyed promoter function. RNAs carrying degenerate nucleotides in the three hexamers were used as replication templates. They led to the selection of actively replicating RNA species exclusively carrying the basic motif (GNNNNN)3 from nucleotides 79 to 96. These results clearly show that, apart from the region from nucleotides 1 to 31, previously identified as governing Sendai virus replication activity, a second element, spanning at the most nucleotides 79 to 96, appears essential. Thus, the paramyxovirus replication promoters are not confined to the leader template regions, as seems to be the case for the rhabdoviruses.

Inhibition of Sendai virus genome replication due to promoter-increased selectivity: a possible role for the accessory C proteins

The role of the negative-stranded virus accessory C proteins is difficult to assess because they appear sometimes as nonessential and thereby of no function. On the other hand, when a function is found, as in the case of Sendai virus, it represents an enigma, in that the C proteins inhibit replication under conditions where the infection follows an exponential course. Furthermore, this inhibitory function is exerted differentially: in contrast to the replication of internal deletion defective interfering (DI) RNAs, that of copy-back DI RNAs appears to escape inhibition, under certain experimental conditions (in vivo assay). In a reexamination of the C effect by the reverse genetics approach, it was found that copy-back RNA replication is inhibited by C in vivo as well, under conditions where the ratio of C to copy-back template is increased. This effect can be reversed by an increase in P but not L protein. The "rule of six" was differentially observed in the presence or absence of C. Finally, a difference in the ability of the replicating complex to tolerate promoter modifications in RNA synthesis initiation was shown to occur in the presence or the absence of C as well. We propose that C acts by increasing the selectivity of the replicating complex for the promoter cis-acting elements governing its activity. The inhibitory effect of C becomes the price to pay for this increased selectivity.

Phosphorylation of the Sendai virus M protein is not essential for virus replication either in vitro or in vivo

A large proportion of intracellular Sendai virus (SeV) M proteins is phosphorylated, but in mature virions the M protein is not phosphorylated or dephosphorylated. Phosphorylated M protein in cells is bound to the cytoskeletal components more firmly than unphosphorylated M protein. Thus it has been hypothesized that M protein phosphorylation plays an important role in the virus life cycle, especially in the step of maturation. Here, a transient expression-mutation experiment of the M gene demonstrated that a change of the Ser residue at the 70th position from the N-terminus to Ala (S70A) totally abolished M protein phosphorylation, strongly suggesting that this residue is phosphorylated. The mutated M gene was then placed in the corresponding region in the cDNA plasmid which generates a full-length antigenome SeV RNA, and a mutant SeV M-S70A was successfully recovered from the cDNA. This mutant virus was indeed defective in M protein phosphorylation but did not differ at all from the wild-type SeV recovered from the parental cDNA either in the replication kinetics and plaque morphology in cultured cells or in in vivo replication and pathogenicity for mice. We thus concluded that no phosphorylation of the M protein was required for SeV replication either in vitro or in vivo.