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Journal of Virology, February 2006, p. 1588-1591, Vol. 80, No. 3
0022-538X/06/$08.00+0 doi:10.1128/JVI.80.3.1588-1591.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Direct Stimulation of Translation by the Multifunctional Herpesvirus ICP27 Protein
Osmany Larralde,1,
Richard W. P. Smith,1,
Gavin S. Wilkie,2,
Poonam Malik,1,
Nicola K. Gray,2,3* and
J. Barklie Clements1
Division of Virology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G11 5JR, Scotland, United Kingdom,1
MRC Human Genetics Unit, Western General Hospital, Edinburgh EH4 2XU, Scotland, United Kingdom,2
School of Molecular and Clinical Medicine, 3rd Floor Outpatients Department, Western General Hospital, University of Edinburgh, Edinburgh, Scotland EH4 2XU, United Kingdom3
Received 10 July 2005/
Accepted 11 November 2005

ABSTRACT
Herpes simplex virus type 1 (HSV-1) ICP27 protein is an essential
regulator of viral gene expression with roles at various levels
of RNA metabolism in the nucleus. Using the tethered function
assay, we showed a cytoplasmic activity for ICP27 in directly
enhancing mRNA translation in vivo in the absence of other viral
factors. The region of ICP27 required for translational stimulation
maps to the C terminus. Furthermore, in infected cells, ICP27
is associated with polyribosomes, indicating a function in translation
during the lytic cycle.

TEXT
The multifunctional ICP27 phosphoprotein (
40) is essential for
herpes simplex virus type 1 (HSV-1) replication, has counterparts
in all the herpesvirus families (
3,
15), and is necessary for
efficient expression of early (
37) and late (
17,
20,
26,
27,
31) viral genes. While ICP27 influences viral transcription
(
12,
17,
24), much evidence indicates that it acts posttranscriptionally.
An RNA binding protein (
23,
36), ICP27 inhibits pre-mRNA splicing
(
2,
11,
14,
34,
35), stimulates pre-mRNA 3' processing (
18,
19), affects mRNA stability (
1), and shuttles between the nucleus
and the cytoplasm (
25), promoting viral RNA nuclear export (
4,
13,
32,
35). An RGG box is required for RNA binding (
23,
32),
and C-terminal regions, including a zinc finger-like motif,
have roles in transactivation and repression of reporter and
viral genes (
1,
27-
28). Interestingly, two recent reports have
provided indirect evidence for a role in the cytoplasm. First,
ICP27 was shown to stimulate the polysomal association of HSV-1
VP16 RNA and the levels of VP16 protein (
6); however, it remains
to be determined whether this is due to a direct effect of ICP27
on translation. Second, association of ICP27 with translation
factors eIF3, eIF4G, and cytoplasmic poly(A)-binding protein
1 (PABP) was reported (
7), suggesting a potential role in viral
and/or host mRNA translation or mRNA stability. Here we present
data demonstrating that ICP27 directly stimulates mRNA translation.
ICP27 expressed from wild-type (wt) virus cosediments with polyribosomes.
To determine whether ICP27 protein is associated with the translation machinery in HSV-1-infected cells, sucrose gradient analysis was used to separate polyribosomes from monoribosomes and uncomplexed ribosomal subunits. BHK-21 cells were either mock infected or infected with wt virus or the ICP27 mutant M15, carrying missense mutations (P465L and G466E) within the C-terminal zinc finger region, which is deficient in expression of certain viral late genes (28). After 5 h, cycloheximide was added to arrest protein synthesis and to fix polyribosomes (30). Cell extracts were prepared and fractionated on sucrose gradients, and fractions were analyzed by Western blotting as described previously (33). The UV absorbance profile from mock-infected cells is shown in Fig. 1A; infected-cell extracts showed slightly reduced levels of polyribosomes (data not shown), presumably due to shutoff of host protein synthesis as a consequence of infection. The polyribosome distribution of ICP27 was compared with that of PABP, a general translation factor for poly(A)+ mRNA (8, 16). With all extracts, PABP was present across the gradient from polyribosomal to mRNP fractions (Fig. 1B) (33) and moves to the lighter fractions following EDTA treatment (33), which dissociates monosomes and polyribosomes into ribosomal subunits (data not shown). In extracts of wt-infected cells, ICP27 was also present in the heavier 80S ribosome and polyribosomal fractions (Fig. 1B). Furthermore, ICP27 that was associated with the polysomal and heavier fractions was released following EDTA treatment and moved to the lighter fractions, similar to PABP (data not shown), indicating an association with the cellular translational machinery. In contrast, the M15 mutant protein was absent from the polyribosome fractions (Fig. 1B). As M15 protein has been reported to have an exclusively nuclear distribution (22), we cannot rule out a degree of nuclear contamination during extract preparation contributing to the M15 protein cosedimenting with mRNP fractions. The association of ICP27 with polyribosomes is indicative of a function in mRNA stability or translation.
ICP27 enhances translation in vivo.
The widely used tethered function assay of
Xenopus oocytes (
5,
10,
33,
41) was employed to test the effect of ICP27 on translation
in vivo. This allows putative translational regulators to be
studied independently of any action on transcription, polyadenylation,
or mRNA export and without knowledge of any natural RNA binding
target. The assay has two components: a luciferase reporter
mRNA containing binding sites for the bacteriophage MS2 coat
protein within its 3' untranslated region and a fusion of MS2
coat protein with ICP27. Interaction of the MS2 coat protein
with its target mRNA binding sites brings ICP27 to the mRNA.
Luciferase activity is normalized against an internal standard
by coinjection of a ß-galactosidase reporter mRNA
lacking MS2 sites. Reporter mRNAs were introduced directly into
the cytoplasm of stage VI oocytes containing the following proteins
expressed from previously injected RNAs (
10): MS2 coat protein
alone, MS2-tethered splicing factor U1A (MS2-U1A), PABP (MS2-PABP),
or ICP27 (MS2-ICP27). Importantly, MS2-ICP27 stimulated reporter
gene expression more than 10-fold compared with MS2 coat protein
(Fig.
2A). By contrast, MS2-U1A, an unrelated RNA binding protein,
showed no stimulation, while MS2-PABP, a known activator of
translation in this assay (
10,
41), also enhanced expression.
Furthermore, the stimulatory effect of ICP27 required MS2 binding,
as no effect was observed in the absence of MS2 binding sites
in the reporter mRNA (Fig.
2B). Quantitative reverse transcription-PCR
showed that the levels of reporter mRNA remained unchanged at
12 h postinjection in oocytes expressing either MS2 coat protein
or MS2-ICP27 (Fig.
2C), and thus stimulation of luciferase activity
is not a consequence of altered mRNA stability. These data show
that ICP27 can directly stimulate the translation of mRNAs to
which it is bound and establish that ICP27 is sufficient to
stimulate translation in the absence of any other viral factors.
The ICP27 C terminus is required for stimulation of translation.
To define the ICP27 regions that contribute to this novel function,
we examined the ability of MS2 fused with various ICP27 mutants
to stimulate translation in the tethered function assay. A diagram
of ICP27 and the mutants used is shown in Fig.
3A. Two well-studied
ICP27 N-terminal deletions,
d1-2 (
29) and
d4-5 (
21), stimulated
translation to an extent similar to that for the wt protein
(Fig.
3B), suggesting that these regions do not contribute to
the translational activity. In contrast, deletion of the C-terminal
105 amino acids (

407) led to a complete loss of translational
stimulation (Fig.
3C); this deletion mutant lacks the putative
zinc finger that is conserved in ICP27 counterparts of various
herpesviruses (
1,
38). Interestingly, the ICP27 missense mutations
in M15 (P465L and G466E) and M16 (C488L) (
28), which map to
this region, also fail to stimulate translation (Fig.
3B), although
they are efficiently expressed in oocytes (Fig.
3D). Importantly,
ICP27 expressed from the M15 mutant is absent from polyribosomes
(Fig.
1B). Thus, the activity of ICP27 as a translational activator
correlates with its ability to associate with polyribosomes
in HSV-1-infected cells, and both assays implicate the C-terminal
region as essential for the ability of ICP27 to stimulate mRNA
translation.
The ICP27 zinc binding region is involved in protein self-interaction
(
39,
42) and in interactions with cellular partners such as
splicing factor SRp20 (
34) and the nuclear export factor TAP
(
3), but whether any of these properties relate to translational
stimulation remains to be determined. It is likely, however,
that this region mediates an interaction between ICP27 and a
component of the translation machinery. Currently we are elucidating
the viral RNA targets, protein cofactors, and mechanism of ICP27-mediated
translational stimulation and evaluating the contribution of
this novel cytoplasmic effect to the viral life cycle.

ACKNOWLEDGMENTS
This work was supported by MRC grant G9826324 to J.B.C. and
MRC Ph.D. studentship award G78/7826 to O.L. N.K.G. is funded
by an MRC Career Development Award and an MRC Senior Fellowship;
G.S.W. is funded by a Beit Memorial Fellowship.
We thank William Richardson, Ross Anderson, Barbara Gorgoni, and Brian Collier for technical assistance.

FOOTNOTES
* Corresponding author. Mailing address: MRC Human Genetics Unit, Western General Hospital, Edinburgh EH4 2XU, Scotland, United Kingdom. Phone: 44-131-332-2471. Fax: 44-131-467-8456. E-mail:
Nicola.Gray{at}hgu.mrc.ac.uk.

This work is dedicated to the memory of J. Barklie Clements, who was co-corresponding author and died during revision of the manuscript. We dedicate this article to Barklie for his long-standing contribution to the herpesvirus field and for being a wonderful person and a great scientist. He will be sadly missed by all of us in his group, his friends, and his colleagues for his enthusiasm, encouragement, and support. 
These authors contributed equally to this work. 

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Journal of Virology, February 2006, p. 1588-1591, Vol. 80, No. 3
0022-538X/06/$08.00+0 doi:10.1128/JVI.80.3.1588-1591.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
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