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Journal of Virology, April 2009, p. 3982-3987, Vol. 83, No. 8
0022-538X/09/$08.00+0 doi:10.1128/JVI.02599-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.
Nipah Virus Edits Its P Gene at High Frequency To Express the V and W Proteins
Sachin Kulkarni,1
Valentina Volchkova,2
Christopher F. Basler,1
Peter Palese,1,3
Viktor E. Volchkov,2,4,5* and
Megan L. Shaw1*
Departments of Microbiology,1
Medicine, Mount Sinai School of Medicine, New York, New York 10029,3
INSERM U758, Laboratoire des Filovirus, 21 av. Tony Garnier, Lyon F-69007, France,2
Université de Lyon, Lyon F-69007, France,4
IFR 128 BioSciences Gerland-Lyon Sud, Lyon F-69007, France5
Received 16 December 2008/
Accepted 27 January 2009

ABSTRACT
Nipah virus (NiV) is predicted to encode four proteins from
its P gene (P, V, W, and C) via mRNA editing and an alternate
open reading frame. By use of specific antibodies, the expression
of the V, W, and C proteins in NiV-infected cells has now been
confirmed. Analysis of the P-gene transcripts shows a ratio
of P:V:W mRNA of 1:1:1, but this differs over time, with greater
proportions of V and W transcripts observed as the infection
progresses. Eighty-two percent of transcripts are edited, with
up to 11 G insertions observed. This exceptionally high editing
frequency ensures expression of the V and W proteins.

TEXT
A characteristic feature of paramyxoviruses is the presence
of an editing site in their P genes. This A
nG
n stretch of residues
marks the position at which nontemplated G residues are added
into a proportion of the P-gene mRNA transcripts in a process
known as mRNA editing (
10,
13). The addition of one or two extra
G residues causes a frameshift such that the resulting proteins
contain the same amino-terminal domain as that expressed from
an unedited transcript but have a unique C-terminal domain that
is expressed from either the +1 or +2 frame (
13). Members of
the
Morbillivirus, Respirovirus, and
Avulavirus genera express
their P proteins from an unedited transcript, while the V protein
is expressed from transcripts with one additional G residue
and the W/D proteins are expressed from transcripts with two
additional G residues (
2,
13,
19,
28). Rubulaviruses have a
different coding strategy, since their P proteins are expressed
from the +2 transcript while the V protein is expressed from
the unedited transcript and the W/I protein from the +1 transcript
(
5,
16,
18,
26,
27). Nipah virus (NiV) and Hendra virus, the
two members of the
Henipavirus genus, appear to conform to the
same pattern as the morbilliviruses. Genome analysis and plasmid-based
expression studies have shown that the cysteine-rich C-terminal
domain, characteristic of all V proteins, is accessed via addition
of one extra G nucleotide following the editing site (
8,
9,
17,
20-
23,
25). The addition of two G residues results in the
expression of the W protein (
17,
22,
23,
25). The W-encoding
transcripts of other paramyxoviruses contain a stop codon shortly
following the editing site, essentially producing a truncated
protein representing the common N-terminal domain. In contrast,
the henipavirus W proteins possess a substantial 43-residue
unique C-terminal domain, and for NiV W, this domain has been
shown to contain a nuclear localization signal (
22). In this
respect the henipavirus W protein seems analogous to the D protein
of parainfluenza virus 3 (a respirovirus), whose 131-amino-acid
C-terminal domain is also expressed from the +2 frame and which
has also been reported to localize to the nucleus (
19,
31).
An additional P-gene product, the C protein, is expressed from
an alternate open reading frame in paramyxoviruses of the
Respirovirus, Morbillivirus, and
Henipavirus genera (
13).
The majority of these alternative P-gene products have been shown to function as inhibitors of the host innate immune response (7, 12). The NiV V and W proteins prevent interferon signaling by interacting with STAT1 and preventing its activation (20, 21, 23). NiV V, like other paramyxovirus V proteins, binds to mda-5 and thereby inhibits the downstream signaling events leading to beta interferon (IFN) synthesis (4). NiV W can also prevent IFN production through an unidentified mechanism that requires its nuclear localization (22). The NiV C protein has also been shown to inhibit the activation of an antiviral state, but the mechanism is unknown (17). All of this work, as well as experiments showing that the C, V, and W proteins can inhibit replication of a minigenome (25), has been performed using plasmid-based expression studies. Information regarding the expression of these proteins in NiV-infected cells is lacking, and given the roles of V and W as IFN antagonists, the extent of mRNA editing may play an important role in NiV pathogenesis.
To address these issues, 293T cells were infected with NiV (isolate UMMC1 [3]; GenBank accession no. AY029767) under biosafety level 4 conditions. The infection was performed in duplicate at a multiplicity of 0.4, and the NiV- or mock-infected cells were collected at 2.5, 6, 9.5, 24, and 30 h postinfection. The cells were lysed and processed for RNA extraction according to the protocol for the RNAeasy and Qiashredder kits (Qiagen). Proteins were precipitated from the eluate with 20% trichloroacetic acid and dissolved in 2x sodium dodecyl sulfate-polyacrylamide gel electrophoresis sample buffer. These proteins were subjected to Western blot analysis using antibodies raised against the common N-terminal domain of the P, V, and W proteins (Fig. 1A), the unique C-terminal domain of the V protein (Fig. 1B), the unique C-terminal domain of the W protein (Fig. 1C), and the C protein (Fig. 1D). Equal loading of the samples was monitored with an antibody against actin (A4700; Sigma). As controls for the mobility of P, V, W, and C and for antibody specificity, whole-cell extracts from 293T cells transfected with expression constructs for hemagglutinin-tagged P, V, W, and FLAG-tagged C were included in each blot. All NiV antibodies detected specific proteins in the 24- and 30-h samples for both infections, indicating that all four of the predicted protein products of the NiV P gene are expressed in an infected cell. In Fig. 1A, bands consistent with the sizes of P and V/W can be observed. V and W differ by only six amino acids in length and so cannot be distinguished from one another on the basis of molecular weight. The expression of these proteins can be better observed using antibodies specific for V or W (Fig. 1B and C), and it is clear that in infected cells, a higher-molecular-weight species of both V and W is seen that is not present in transfected cells. This is most likely due to a modification that occurs only in the context of the virus infection, and we can speculate that differential phosphorylation or acetylation may be the cause since these modifications have been described for the NiV P protein in the virion (24). In contrast, the NiV C protein migrates as a single species corresponding to its predicted size of 18 kDa (Fig. 1D). The slower migration of the plasmid-expressed C protein is most likely due to the presence of a tag on the C terminus.
To assess the extent of editing that occurs in NiV-infected
cells and to calculate the proportion of P:V:W transcripts,
total RNA extracted from the infected cells at each time point
was reverse transcribed with Superscript II reverse transcriptase
(Invitrogen) using an oligo(dT) primer. A 341-bp region spanning
the editing site in the NiV P gene was amplified by PCR using
Taq DNA polymerase (Invitrogen) and the following primers: NiV-P
1032 (5'-CAGCAAGGGAAAGATGCTCAGCCT) and NiV-P
1373 (5'-TTAACCGCAGTGGAAGCATTCAGTTG).
The product was cloned into pGEMT (Promega), and where available,
more than 40 individual clones for each sample were sequenced.
No visible PCR product was observed for any of the mock-infected
samples, and no NiV-positive clones were obtained. For the 2.5-h
infection time point, only 1 positive clone was obtained from
the first infection and 11 positive clones were obtained from
the second infection. The sequences of all positive clones were
analyzed, and the number of G insertions that follow the editing
site was determined. Based on this information, the protein
product encoded by each clone was determined. Figure
2 shows
the relative percentages of P, V, and W transcripts detected
in NiV-infected cells at each time point for the duplicate infections.
Although the pattern is not identical for the two infections,
the trend is similar, with a greater proportion of P transcripts
present at early times postinfection. As the infection progresses,
there is a marked increase in the percentages of V and W transcripts,
which at times equal or outnumber the P transcripts. The small
increase in the number of P transcripts seen at the last time
point is probably reflective of a second cycle of infection.
No difference was observed in the levels of V and W transcripts,
indicating equal expression levels. To our knowledge, this is
the first kinetic analysis of paramyxovirus mRNA editing, and
the data suggest that there may be temporal regulation of P,
V, and W expression at the level of transcription and editing.
Further exploration of this possibility using nonpathogenic
paramyxoviruses, which allow for more-rigorous experimental
conditions, will be necessary. Of course, the relative stability
of the mRNA transcripts and of the protein may also affect the
final protein levels in the infected cells.
A striking observation was that the G-residue insertions were
not restricted to +1 or +2 but instead ranged from +1 to +11
(Fig.
3). In other words P transcripts include those with 0,
3, 6, and 9 insertions, V transcripts include those with 1,
4, 7, and 10 insertions, and W transcripts include those with
2, 5, 8, and 11 insertions. Transcripts with these extra G residues
are predicted to encode proteins with one, two, or three additional
glycine amino acids at the end of the N-terminal domain. It
remains to be seen whether this has any functional consequences
for the proteins. Combining the data from the different time
points and the duplicate infections (383 clones), 18% of NiV
P-gene transcripts are unedited (0 insertions) and 82% are edited
(+1 to +11). The most dominant insertions (>10%) are +1,
+2, +4, and +5, and this can be seen particularly at the 9.5-
and 24-h time points, when 94% and 91% of the transcripts, respectively,
are edited (Fig.
3B). Overall, the ratio of P:V:W transcripts
is about 1:1:1 (33%:36%:31%), and of the P transcripts, 44%
are edited (+3, +6, and +9 insertions). This editing frequency
is higher than that described for any other paramyxovirus to
date, and the abundance of NiV V and W transcripts relative
to P transcripts is much higher (Table
1). By comparison, a
recent report on four strains of measles virus shows a combined
editing frequency of 42%, with 60% of transcripts encoding P,
35% of transcripts encoding V, and the remaining 5% encoding
the hypothetical W protein (
1). Similar numbers have been reported
for Newcastle disease virus (
15). The only other paramyxoviruses
known to hyperedit their P genes are human and bovine parainfluenza
virus 3 (PIV3) viruses, which predominantly insert 1 to 6 G
residues at equal frequency, although up to 12 insertions have
been documented (
6,
19). For bovine PIV3, the editing frequency
has been calculated as 65%, with 52% P transcripts, 25% V transcripts,
and 23% D transcripts (
19). It is striking that NiV and PIV3,
which both encode a unique protein (W/D) from the +2 open reading
frame in their P genes, both also display a significantly higher
editing frequency than other paramyxoviruses whose P proteins
are encoded by faithful P-gene transcripts. We speculate that
this is to ensure adequate expression of the W/D proteins, and
for NiV W at least, we know that it plays an important role
in allowing the virus to evade the innate immune response (
22,
23). Of note, transcriptional RNA editing is also required for
expression of the Ebola virus full-length glycoprotein, and
here too, a random editing pattern of the GP gene is observed
(
29,
30). Therefore, different negative-strand RNA viruses use
similar strategies to express important replication or pathogenicity
factors.
View this table:
[in this window]
[in a new window]
|
TABLE 1. Paramyxovirus P-gene mRNA editing in members of the Henipavirus, Morbillivirus, Avulavirus, and Respirovirus genera
|
A comparison of the editing sites of NiV and PIV3 does not reveal
any common feature that may account for the increased editing
rate observed in these two paramyxoviruses. The henipavirus
editing sites are actually very similar to that of measles virus,
differing only by two nucleotides at positions –7 and
–8 relative to the A
nG
n run. The nucleotides at the –1
and –2 positions, which have been identified as the reason
for the differing editing frequencies of Sendai virus and PIV3
(
11), are identical in PIV3, henipaviruses, and morbilliviruses.
Additional features, such as the processivity of the viral polymerase
and its ability to pause at the editing site, may also play
a role in determining the editing rate and perhaps in the observed
temporal expression ratio of P:V/W mRNA in NiV-infected cells.
ADDENDUM
Following submission of this paper, similar results were published by Lo et al. (14).

ACKNOWLEDGMENTS
All experiments involving live Nipah virus were carried out
in the INSERM BSL-4 Laboratory Jean Merieux in Lyon, France.
We thank the biosafety team members for their support and assistance
in conducting experiments.
This work was supported by National Institute of Health grant 1 UC19 AI062623 (Center for Investigating Viral Immunity and Antagonism) to P.P. and by INSERM (France).

FOOTNOTES
* Corresponding author. Mailing address for Megan L. Shaw: Department of Microbiology, Mount Sinai School of Medicine, New York, NY 10029. Phone: (212) 241-8931. Fax: (212) 534-1684. E-mail:
megan.shaw{at}mssm.edu. Mailing address for Viktor E. Volchkov: Filovirus Laboratory, INSERM U758, UCBL-1, 21 av. Tony Garnier, 69365 Lyon, Cedex 07, France. Phone: 33 437282450. Fax: 33 437282459. E-mail:
viktor.volchkov{at}inserm.fr 
Published ahead of print on 11 February 2009. 

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Journal of Virology, April 2009, p. 3982-3987, Vol. 83, No. 8
0022-538X/09/$08.00+0 doi:10.1128/JVI.02599-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.
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