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Journal of Virology, June 2004, p. 5856-5866, Vol. 78, No. 11
0022-538X/04/$08.00+0 DOI: 10.1128/JVI.78.11.5856-5866.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Proteomics of Herpes Simplex Virus Replication Compartments: Association of Cellular DNA Replication, Repair, Recombination, and Chromatin Remodeling Proteins with ICP8
Travis J. Taylor and David M. Knipe*
Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, Massachusetts 02115
Received 29 October 2003/
Accepted 23 January 2004

ABSTRACT
In
this study, we have used immunoprecipitation and mass spectrometry
to
identify over 50 cellular and viral proteins that are associated
with
the herpes simplex virus 1 (HSV-1) ICP8 single-stranded
DNA-binding
protein. Many of the coprecipitating cellular proteins
are known
members of large cellular complexes involved in (i)
DNA replication or
damage repair, including RPA and MSH6; (ii)
nonhomologous and
homologous recombination, including the catalytic
subunit of the
DNA-dependent protein kinase, Ku86, and Rad50;
and (iii) chromatin
remodeling, including BRG1, BRM, hSNF2H,
BAF155, mSin3a, and histone
deacetylase 2. It appears that DNA
mediates the association of certain
proteins with ICP8, while
more direct protein-protein interactions
mediate the association
with other proteins. A number of these proteins
accumulate in
viral replication compartments in the infected cell
nucleus,
indicating that these proteins may have a role in
viral replication.
WRN, which functions in cellular recombination
pathways via
its helicase and exonuclease activities, is not absolutely
required
for viral replication, as viral yields are only very slightly,
if
at all, decreased in WRN-deficient human primary fibroblasts
compared
to control cells. In Ku70-deficient murine embryonic
fibroblasts,
viral yields are increased by almost 50-fold, suggesting
that
the cellular nonhomologous end-joining pathway inhibits HSV
replication.
We hypothesize that some of the proteins coprecipitating
with
ICP8 are involved in HSV replication and may give new insight
into
viral replication
mechanisms.

INTRODUCTION
Herpes simplex virus 1 (HSV-1) is a large, double-strandedDNA
virus that replicates in the host cell nucleus. HSV encodes
over 80
gene products that contribute to viral replication in
either cultured
cells or animal hosts
(
76). Due to the limited
size
of the HSV-1 genome, the virus cannot code for every function
required
for its propagation; thus, HSV-1 must rely upon factors
supplied
by the host cell for replication. For example, HSV exclusively
uses
the host cell RNA polymerase II for the transcription of viral
genes
(
4,
16). The exact number and
identity of the cellular factors
required for HSV replication is
unknown, but the identification
of such factors is an active area of
research as it may shed
light on mechanisms of viral replication, the
cellular process,
or the factor itself. It is this concept that induced
us to
identify cellular proteins that associate with HSV-1
ICP8.
The HSV-1 single-stranded DNA-binding protein, ICP8, is a
128-kDa multifunctional zinc metalloprotein
(31,
37) encoded by the
UL29 gene
(61). ICP8, in concert
with the other HSV DNA replication proteins, including the
helicase-primase complex (UL5, UL8,
UL52), the origin-binding protein (UL9), and the
polymerase holoenzyme (UL30/UL42), is required
for viral DNA synthesis
(11,
12). While the seven HSV
DNA replication proteins are known, it is currently unclear as to what
host proteins are involved in viral DNA replication. In
addition to its role in DNA synthesis, ICP8 has been shown to affect
viral transcription in at least two ways: (i) by repressing
transcription from input parental viral genomes
(33-35)
and (ii) by stimulating late gene transcription
(32).
ICP8 and a
number of other viral proteins, including the aforementioned viral
replication proteins, the major viral transactivator ICP4, the
immediate-early protein ICP27, and the major capsid protein VP5
accumulate within intranuclear structures referred to as replication
compartments (9,
19,
47,
59,
71,
73). Many of the
processes required for viral replication, including viral DNA synthesis
(18,
71,
74), viral transcription
(47,
57,
71,
74,
75), virion assembly, and
DNA packaging (19,
51,
93,
96), occur within
replication compartments. Because numerous viral processes take place
in replication compartments, it is expected that cellular proteins that
are required for viral replication may accumulate there as well.
Indeed, the host cell RNA polymerase II is redistributed to replication
compartments during HSV infection
(57,
71,
75). Other proteins such
as p53 and the cellular single-stranded DNA-binding protein replication
protein A (RPA) have been observed in replication compartments
(95), but their role in
viral replication remains unknown.
We hypothesized that host
proteins that coprecipitate with viral proteins in replication
compartments might be cellular proteins that play a role in viral
replication. We thought that ICP8 was a good candidate for this
analysis because it is highly expressed in infected cells and it is
believed to interact with multiple cellular or viral complexes to
mediate its various functions during infection. Here, we report the
identification of numerous cellular proteins that coprecipitate with
ICP8, which suggests that they may have a functional role in HSV
replication.

MATERIALS AND METHODS
Cells and viruses.
African green monkey kidney (Vero)
and human epidermoid (HEp-2)
cells obtained from the American Type
Culture Collection (Manassas,
Va.) were grown and maintained in
Dulbecco's modified Eagle's
medium (DMEM; Media Tech Inc.,
Herndon, Va.) supplemented with
5% fetal bovine serum (Gibco,
Carlsbad, Calif.)-5% bovine calf
serum (HyClone, Logan,
Utah), streptomycin (100 µg/ml),
and penicillin (100 U/ml)
(DMEM-10% fetal calf serum [FCS]).
V529 cells
(
17) were grown in
DMEM-10% FCS supplemented with
G418 (400 µg/ml).
Normal and Ku70-deficient murine embryonic
fibroblasts (MEFs)
(
36), kindly provided by
David Sinclair,
Harvard Medical School, Boston, Massachusetts, were
grown and
maintained in DMEM-10% FCS. Normal (number
AG14591) and WRN-deficient
(number AG00780H) primary human fibroblasts
obtained from the
Coriell Institute for Medical Research (Camden, N.J.)
were grown
and maintained in modified Eagle's medium (MEM) with
Earle's
balanced salt solution supplemented with 15%
heat-inactivated
fetal bovine serum, a 2
x
concentration of MEM essential and
nonessential amino acids (Gibco),
and vitamins (Gibco).
The HSV-1 wild-type (wt) KOS strain was
propagated and assayed on Vero cells. The HSV-2-derived dl5
and dl29 mutant strains were propagated on V529 cells as
previously described
(17).
Antibodies.
The anti-HSV ICP8 39S mouse
monoclonal (81) and 3-83
rabbit polyclonal (47)
antibodies were described previously. The anti-RPA p70-9 mouse
monoclonal antibody (23)
was kindly provided by Bruce Stillman, Cold Spring Harbor Laboratory.
The anti-HSV ICP4 4040II rabbit polyclonal antibody was a gift from
Kent Wilcox, Medical College of Wisconsin. Paul Olivo, Washington
University School of Medicine, provided the anti-HSV UL5
R252 rabbit polyclonal antibody
(67). The anti-HSV ICP27
mouse monoclonal antibody H1119 was purchased from the Rumbaugh-Goodwin
Institute for Cancer Research (Plantation, Fla.). The following
antibodies were purchased from Santa Cruz Biotechnology (Santa Cruz,
Calif.): rabbit polyclonal antibodies to BAF155 (H-76), hSNF2H (H-300),
Rad50 (H-300), BRG1 (H-88), histone deacetylase 2 (HDAC2) (H-54), BLM
(H-300), BRCA1 (C-20), Ku86 (H-300), MSH2 (N-20), mSin3a (K-20), and
WRN (H-300) and mouse monoclonal antibodies to BRM (E-1) and the
DNA-dependent protein kinase (DNA-PK) catalytic subunit (DNA-PKcs)
(Ab-2 and G4). As expected, not all of the antibodies or antisera
performed equally well for all assays; therefore, we have included
results of certain assays for only some of the
antibodies.
Viral infections and immunoprecipitations.
HEp-2
cells in 15-cm tissue culture dishes were mock infected or infected
with wt HSV-1 at a multiplicity of infection (MOI) of 20 PFU per cell.
At 6.5 h postinfection, approximately 5 x
107 cells were washed twice with cold phosphate-buffered
saline (PBS) and then harvested by scraping into PBS. The cells were
collected by centrifugation at 4°C and transferred to
microcentrifuge tubes for further preparation. The cells were incubated
on ice for 30 min in 0.5 ml of immunoprecipitation (IP)buffer (120 mM potassium-acetate, 20 mM Tris-acetate [pH
7.9], 5 mM EDTA, 1 mM dithiothreitol, 10% glycerol,
0.1% Nonidet P-40, and 1 Complete protease inhibitor cocktail
tablet [Roche Applied Science, Indianapolis, Ind.] per 25
ml). Ethidium bromide (EtBr) was added to some samples at this step and
was maintained at a concentration of 20 or 50 µg/ml throughout
the entire IP process, including wash steps. The cellular lysates were
sonicated for 5 s and then clarified by centrifugation at
4°C in an Eppendorf microcentrifuge for 10 min. IP was
performed by the addition of 5 µl of the 39S monoclonal
antibody to 0.5 ml of the cellular lysate with gentle agitation at
4°C for 45 min. Protein G-coated paramagnetic beads (Dynal
Biotech, Oslo, Norway) were added, and the lysates were incubated with
gentle agitation for an additional 45 min at 4°C. After 4
washes with IP buffer, the precipitates were boiled in sample buffer
and resolved by sodium dodecyl sulfate-polyacrylamide gel
electrophoresis (SDS-PAGE).
SDS-PAGE and Western blotting.
Proteins
in the IPs were separated on 8 to 10% polyacrylamide gels. The
proteins were visualized by using a colloidal blue staining kit
(Invitrogen, Carlsbad, Calif.) according to manufacturer's
instructions. For immunoblotting, the proteins were transferred to a
polyvinylidene difluoride membrane (Perkin-Elmer Life Sciences, Boston,
Mass.) by electroblotting and the membranes were blocked in
5% milk in Tris-buffered saline with Tween 20 (TBST;
20 mM Tris-Cl [pH 8.0], 150 mM NaCl, 0.1% Tween 20),
probed with the indicated primary antibody in TBST, and visualized by
enhanced chemiluminescence (Perkin-Elmer Life Sciences). The
anti-ICP8 3-83, anti-UL5 R252, anti-ICP4 4040II, and
anti-ICP27 H1119 antibodies were used at a dilution of 1:1,000. All
other antibodies were used at a dilution of 1:200. The goat anti-rabbit
and anti-mouse horseradish peroxidase-conjugated secondary antibodies
(Santa Cruz Biotechnology) were used at a dilution of 1:10,000. The
immunoblots were stripped (4% SDS, 62.5 mM Tris-HCl
[pH 6.8], 100 mM 2-mercaptoethanol) at 55°C for 45
min before reprobing.
Mass spectrometry.
Bands excised
from colloidal blue-stained gels were analyzed at the Taplin Biological
Mass Spectrometry (MS) Facility, Harvard Medical School, by
microcapillary liquid chromatography-tandem MS with a LCQ DECA ion-trap
mass spectrometer (Thermo Finnigan, San Jose,
Calif.).
Indirect immunofluorescence.
For
indirect immunofluorescence, HEp-2 cells plated on 12-mm coverslips in
a 24-well tissue culture plate were mock infected or infected with wt
HSV-1 at an MOI of 20. At 6.5 h postinfection, the cells were
rinsed in cold PBS and then fixed for 10 min in 3.7%
formaldehyde in PBS. After washing with PBS for 5 min, the cells were
subsequently permeabilized with 20°C acetone for 2
min. The cells were rinsed briefly in distilled water and then PBS for
5 min. Primary antibodies were then applied for 30 min in a humid
chamber at 37°C. After a 5-min wash in PBS, the secondary
antibodies were applied for 30 min in a humid chamber at 37°C.
After washing in PBS and rinsing in distilled water, the coverslips
were mounted for observation. The anti-ICP8 39S monoclonal and the 3-83
polyclonal antibodies were used at a dilution of 1:200. All other
primary antibodies were used at a dilution of 1:30. Fluorescein
isothiocyanate or rhodamine-conjugated secondary antibodies (Cappel,
ICN, Irvine, Calif.) were used at a dilution of
1:200.
Measurements of viral growth kinetics.
Cells were
infected at the indicated MOI with wt HSV-1. At 1 h
postinfection, the virus was removed and the cells were washed twice
(135 mM NaCl, 10 mM KCl, 40 mM sodium-citrate, pH 3.0) for 30
s before being overlaid with fresh medium. At 2 or 24 h
postinfection, the cells were harvested by scraping into the medium and
frozen at 80°C. After thawing, the lysate was
sonicated for 15 s on ice. The viral yield was determined by
titration on Vero cells.
Viral recombination assay.
Normal
or WRN-deficient human fibroblasts were infected either singly or
doubly with dl5 or dl29. At 24 h
postinfection, the virus was harvested and titered on Vero and V529
cells. Recombination frequency was determined as the percentage of
viral progeny that contained wt alleles of
UL5 and UL29
and could form plaques on Vero
cells.

RESULTS
Coprecipitation of cellular and viral proteins with ICP8.
To identify potential ICP8-associated
cellular and viral proteins,
we immunoprecipitated ICP8 and associated
molecules from wt
HSV-infected or mock-infected whole-cell HEp-2
lysates prepared
at 6.5 h postinfection using the
conformation-specific 39S monoclonal
antibody, which predominantly
recognizes ICP8 at prereplicative
sites or within HSV replication
compartments (
89). At
this time
of infection, the bulk of ICP8 is in replication compartments
(
18,
72),
and the majority of
ICP8 is associated with replication complexes
or progeny viral DNA
(
48). The IPs were
separated by SDS-PAGE,
and the resolved proteins were visualized by
Coomassie blue
staining (Fig.
1A). Protein bands present only in IPs from virus-infected
cells were
excised, digested with trypsin, and analyzed by tandem
MS.
We
identified, in total, over 50 cellular and viral proteins
that
coprecipitated with ICP8 with 39S antibody (Table
1). Table
1 lists the majority
of the coprecipitating proteins categorized
by general function with
the corresponding number of peptides
detected by MS from a
representative experiment. Some proteins,
such as cytoplasmic
constituents or cytoskeletal elements, were
not included in this
analysis, but the entire list of coprecipitating
proteins may be found
at
http://knipelab.med.harvard.edu/.
Certain
proteins identified by one peptide were included in Table
1 because of their
relationship or interaction with other coprecipitating
proteins on the
list.
To confirm the identity of these proteins
and the authenticity
of their association with ICP8, we performed
several lines of
experiments. First, we confirmed the presence of
representative
members of the larger protein complexes in ICP8 IPs by
Western
blotting with specific antibodies (Fig.
1B). Second, we determined
if
DNA plays a role in the coprecipitation (Fig.
1B). Third, we
used
immunofluorescence to determine if the coprecipitating
proteins
colocalize with ICP8 in viral replication compartments
(Fig.
2). Finally, to determine if a cellular protein plays
a role in viral
replication, we have initiated studies to analyze
HSV-1 replication in cells
deficient for the cellular protein
(Tables
2,
3, and
4).
As expected, we identified several coprecipitating viral DNA
replication
proteins, including U
L5, U
L8, and
U
L42 (Table
1),
which are
known to associate with ICP8 in the viral DNA replication
complex
(
6,
7,
9,
39,
59). In agreement with
previous reports (
84,
86,
99),
we also identified
U
L12, ICP27, and ICP4 as ICP8-coprecipitating
proteins.
We also identified numerous cellular proteins (Table 1) that are predominantly
involved in cellular DNA replication and damage repair, including RPA
and MSH6; homologous and nonhomologous recombination, including Rad50,
DNA-PKcs, Mre11, and Ku86; chromatin remodeling, including BRG1, BRM,
hSNF2H, BAF155, mSin3a, and HDAC2; and mRNA splicing, including SAP130,
SAP155, and NMP200. Some of the other coprecipitating proteins have
been identified as possibly playing a role during HSV replication,
including HAUSP, a ubiquitin-specific protease that has been shown to
associate with the HSV-1 immediate-early ICP0 protein
(26); the mRNA splicing
factors SAP130 and SAP155, which interact with SAP145 which in turn
binds HSV-1 ICP27 (8); and
Ku70, which has been demonstrated to bind the downstream activating
sequence present in some HSV late gene promoter sequences
(70).
We isolated
multiple members of individual complexes, arguing that the
coprecipitation of these complexes was specific. For example, we
identified 4 of the 5 known protein partners in the cellular
nonhomologous end-joining (NHEJ) complex
(10), including DNA-PKcs,
Ku86, Ku70, and XRCC4. Based on the coprecipitation of DNA repair and
recombination complexes, we hypothesized that BRCA1, WRN, BLM, and
MSH2, each known to interact with other proteins shown in Table 2
(5,
14,
28,
29,
45,
98,
100), may also
coprecipitate or colocalize with ICP8 in infected cells and were
included in this study as described in more detail
below.
Role of DNA in association of the proteins with ICP8.
Many of
the cellular complexes identified have DNA-binding components, and ICP8
is also bound to viral DNA in infected cells
(48,
53), so we attempted to
determine if the associations were mediated by DNA-protein or
protein-protein interactions by performing the immunoprecipitations in
the presence of EtBr at concentrations known to disrupt DNA-protein but
not protein-protein interactions
(50). The presence of
EtBr did not significantly disrupt the coprecipitation of the host
proteins Ku86, Rad50, RPA, and WRN, all of which are involved in DNA
repair or recombination, or the viral proteins ICP27 and UL5
(Fig. 1B). As observed
previously for ICP8 and ICP27
(99), disrupting DNA
binding increased the coprecipitation of ICP8 with ICP27,
UL5, and WRN (Fig.
1B). This may be due to
the release of ICP8 or one of the proteins from DNA, allowing them to
associate more readily. In contrast, coprecipitation of BAF155, hSNF2H,
mSin3a, and HDAC2, which are all involved in chromatin remodeling, was
reduced by greater than 50%, and ICP4 and BRG1 coprecipitation
was abolished in the presence of EtBr (Fig.
1B).
These results
suggested that a number of proteins, including Ku86, Rad50, RPA, WRN,
ICP27, and UL5, associate with ICP8, either directly or
indirectly, via protein-protein interactions. Other proteins, including
BAF155, hSNF2H, mSin3a, HDAC2, BRG1, and ICP4, may require DNA to at
least partially mediate or stabilize the association with
ICP8.
Accumulation of cellular proteins in HSV replication compartments.
If the ICP8-coprecipitating proteins
physically associated with ICP8, it was expected that they would
colocalize with ICP8 within the nucleus of an infected cell. To test
this hypothesis, we used immunofluorescence to examine the distribution
of several representative proteins in HSV-infected HEp-2 cells.
Proteins involved in NHEJ (DNA-PKcs and Ku86), homologous recombination
and DNA repair (BLM, BRCA1, MSH2, Rad50, and WRN), and chromatin
remodeling (BAF155, BRG1, BRM, HDAC2, hSNF2H, and mSin3a) were
redistributed from their normal nuclear distribution (Fig.
2, left panels) to viral
replication compartments after infection (Fig.
2, middle panels) as shown
by their costaining with ICP8 or localization within replication
compartments (Fig. 2,
right panels). Similar staining patterns were observed in
single-stained cells (data not shown). One of the most dramatic
alterations involved the redistribution of WRN from its normal
nucleolar pattern (Fig. 2,
left WRN panel) (90) to
viral replication compartments (Fig.
2, middle WRN panel). The
redistribution of WRN from the nucleolus normally takes place in the
presence of DNA-damaging agents
(78), suggesting that
viral infection may induce intranuclear signaling events that lead to
altered nuclear structure. The accumulation of numerous
ICP8-coprecipitating proteins in viral replication compartments
suggested that they have a functional role in the HSV life cycle or
that these proteins were recruited to damaged viral DNA. It is also
possible that some of the proteins were sequestered within replication
compartments to prevent possible antiviral
functions.
Growth of HSV-1 in WRN-deficient human fibroblasts.
WRN is a mammalian RecQ helicase family
member (41) and is
thought to function in cellular DNA replication, repair, and
recombination pathways via its helicase and exonuclease domains
(68,
80). To determine if WRN
was required for HSV replication, we examined viral growth and
recombination in WRN-deficient primary human fibroblasts. The viral
yield was decreased by three- to fivefold in WRN-deficient cells
compared to control human fibroblast cells (Table 2). This result suggested
that WRN might promote viral replication slightly but that it was not
essential for HSV replication.
We next determined if HSV
recombination was altered in the WRN-deficient cells by infecting
normal or WRN-deficient fibroblasts singly or doubly with HSV-2
dl5 and dl29 strains
(17), which contain
mutations in the UL5 and
UL29 genes, respectively, and measured the
number of wt plaques that formed on Vero cells. We observed a
recombination frequency of
25% (Table 3) in normal fibroblast
cells. In contrast, we observed a lower average recombination frequency
of
18% in WRN-deficient cells (Table 3). This decrease
suggested that HSV recombination was not as efficient or did not occur
as readily in WRN-deficient cells. In agreement with the above results
with HSV-1 infection, we also observed an approximate threefold
decrease in total HSV-2 yields in WRN-deficient cells compared to
control human fibroblast cells.
Growth of HSV-1 in Ku70-deficient MEFs.
Ku70 and Ku86 form the heterodimeric
DNA-binding portion of the DNA-PK complex. Once the Ku70/Ku86
heterodimer is bound to DNA, the other components of the DNA-PK complex
are recruited to the double-strand break to orchestrate NHEJ.
In the absence of the Ku70 subunit in mammalian cells, the NHEJ pathway
is disrupted. To determine the role of Ku70 during viral replication,
we examined HSV growth in Ku70-deficient MEFs.
Viral yields were
increased by 30- to 50-fold in Ku70-deficient MEFs compared with
control MEFs (Table 4).
The increased yield was observed over a range of input virus from an
MOI of 1 to 50 (Table 4).
These results suggested that HSV growth did not depend upon Ku70 but
rather was inhibited by Ku70. This result implied that the NHEJ pathway
inhibited HSV replication in some
manner.

DISCUSSION
We hypothesized that
host proteins involved in HSV replication
may be identified as proteins
that coimmunoprecipitate with
viral proteins in nuclear replication
compartments, and we therefore
used the HSV ICP8 DNA-binding protein as
the prototype viral
protein for IP studies. We first identified protein
bands in
ICP8 IPs by using MS. IP and Western blotting with specific
antibodies
confirmed the association of the host protein with ICP8. We
expected
that proteins associating with ICP8 would colocalize in viral
replication
compartments, so we next demonstrated that representative
ICP8-coprecipitating
proteins accumulated within viral replication
compartments.
We also initiated studies to analyze the functional role
of
these host proteins during HSV replication by examining viral
growth
and recombination in mutant cell lines. We hope to extend
these studies
by depleting specific host factors by RNA interference
techniques.
In this study, we report the identification of
numerous cellular proteins that coprecipitated with HSV ICP8. The
majority of the proteins were components of cellular complexes that
coordinate DNA recombination and repair or chromatin remodeling (for
reviews, see references 2,
21,
27,
41,
46,
63, and
83). We examine the
potential roles of the coprecipitating proteins in HSV replication in
greater detail
below.
Interactions with cellular DNA repair and recombination complexes.
Double-strand DNA breaks (DSB) and
other DNA damage are experimentally induced by irradiation, UV
exposure, and chemical treatment; however, this damage may also arise
naturally as a consequence of DNA replication. Many of the repair and
recombination proteins reported to redistribute to DSB after
irradiation or chemical treatment colocalize with cellular DNA
replication sites during S phase. For example, the Rad50/Mre11/NBS
complex localizes to cellular replication structures during S phase as
shown by costaining with RPA or BrdU
(13,
60,
62). It is believed that
the presence of stalled replication forks or stretches of
single-stranded DNA are responsible for recruiting these factors. In
the absence of such repair functions, genomic integrity degrades. As
evidence for the critical role of these proteins in genomic integrity,
many of the proteins involved in DNA repair and recombination are
mutated or deleted in human genetic disorders that are associated with
an increased risk of cancer
(58,
87).
Two major
pathways in mammalian cells repair DSB: (i) NHEJ and (ii) homologous
recombination repair (HRR). The NHEJ and HRR pathways are believed to
compete within the cell as they both act on DSB
(3,
38,
44). HSV replication and
recombination are coupled in infected cells
(24,
25,
94), but the significance
of this process remains unclear. It is known that the HSV genome
undergoes an inversion of the L and S segments
(20,
40) and that
recombination frequencies are high in HSV-infected cells
(24,
25,
79,
82), indicating that
recombination may have a role in HSV replication. Members of both NHEJ
and HRR complexes coprecipitated with ICP8, suggesting that these
factors may also have a role in repairing DSB that occur during HSV DNA
synthesis or have an as-yet-undefined function required for viral DNA
replication. It has been recently proposed that HSV DNA replication
occurs initially on a linear molecule
(43); thus, recombination
may play a role in HSV DNA replication as observed for bacteriophage T4
DNA replication
(49).
We found that
HSV replication and recombination levels were decreased modestly in
WRN-deficient cells. WRN-deficient cells have a decreased ability to
resolve homologous recombination products
(77), which may account
for the lower levels of homologous recombination that we observed in
our assays. The presence of other RecQ helicases in the cell, such as
BLM, raises the possibility that other cellular proteins may
functionally substitute for WRN, providing a possible explanation for
why we did not see a significant decrease in viral replication in
WRN-deficient cells. We are currently investigating whether HSV
replication is inhibited in other cell lines that have reduced
homologous recombination activity.
In contrast to the
WRN-deficient cells, we found that HSV replication is significantly
increased in Ku70-deficient cells, which suggested that NHEJ may
inhibit HSV replication. In support of this, it has been shown that HSV
growth is increased in cells lacking another NHEJ component, DNA-PKcs
(69). Although it has
been suggested that HSV-1 infection causes a degradation of DNA-PKcs
(54,
69), the extent of this
effect is cell type and/or virus strain specific
(69). We were surprised
to see DNA-PKcs as one of the major proteins coprecipitating with ICP8,
but further studies have shown that DNA-PKcs decreases by less than
50% in HEp-2 and other cells infected with HSV-1 KOS
(T. J Taylor, G. Melroe, and D. M. Knipe,
unpublished data). Therefore, DNA-PKcs could exert an effect during
infection under our conditions. Indeed, we found that DNA-PKcs
accumulated in viral replication compartments in HEp-2 cells infected
with HSV-1 KOS. The accumulation of proteins involved in NHEJ within
replication compartments may not reflect a functional role in HSV
replication but rather the nonspecific binding of these proteins to
DSBs within replication compartments.
The increased growth of HSV
in cell lines deficient for NHEJ suggests that this pathway inhibits
HSV replication. The disruption of the NHEJ pathway may increase HSV
replication in at least two ways. First, it has been suggested that HSV
may utilize a recombination-dependent replication mechanism similar to
that used by bacteriophage T4 to replicate linear viral DNA later
during infection (66). If
so, then the Ku70/Ku86 DNA-binding heterodimer may block access of
viral or cellular recombination proteins to DNA by competitively
binding DSB. In the absence of Ku70, the competition for DSB would
decrease, thus allowing greater access of other recombination factors.
The loss of DNA-PKcs activity would also disrupt the NHEJ pathway
because DNA-PKcs regulates the function of proteins involved in DNA
repair by phosphorylation
(3,
44); however, the effect
on HSV replication might not be as pronounced in DNA-PKcs-deficient
cells compared to Ku70-deficient cells in that Ku70 acts upstream of
DNA-PKcs in the NHEJ pathway and could still bind DSB to some degree in
DNA-PKcs-deficient cells. If HSV uses a recombination-dependent
replication mechanism for replication, then it might be expected that
viral replication occurs more readily in the absence of the competing
NHEJ pathway. Second, it has recently been suggested that HSV genome
circularization may actually repress the onset of HSV infection
(43). If the NHEJ pathway
is responsible for genome circularization, then the absence of this
cellular pathway may promote the initiation of the viral life cycle.
Further studies are needed to define the mechanisms of the inhibitory
effect of NHEJ during HSV replication.
At least two proteins
involved in DNA repair and recombination, NBS and BLM, accumulate in or
near intranuclear ND10 sites
(64,
91). As it is believed
that HSV DNA targets to ND10 sites and replication compartments form
adjacent to the ND10 sites
(42,
88), it is possible that
ICP8 or other viral proteins associate with one or more of these
proteins for proper intranuclear localization early during infection.
It will be of interest to determine if a recently identified region of
ICP8 required for proper intranuclear localization
(85) mediates the
interaction with one or more of the ICP8-coprecipitating
proteins.
Interactions with cellular chromatin remodeling complexes.
Two main groups of chromatin remodeling
complexes exist in mammalian cells: (i) those that require ATP
hydrolysis to alter histone-DNA contacts, such as the mating type
switch/sucrose nonfermenting (SWI/SNF) and imitation switch (ISWI)
complexes, and (ii) those that covalently modify histone proteins by a
variety of posttranslational modifications (phosphorylation,
acetylation, methylation), such as histone acetyltransferases and
histone deacetylase complexes. We identified members of both types of
complexes as ICP8-coprecipitating proteins.
The SWI/SNF and ISWI
complexes have related core subunits but differ in the makeup of
accessory proteins that are believed to regulate function or
specificity (83). The
SWI/SNF and ISWI complexes have similar but nonoverlapping functions
that differ by substrate specificity and mechanism of action
(1,
52,
83), again possibly
regulated by various binding partners. Several ATPase subunits
associated with SWI/SNF or ISWI complexes, such as BRM, BRG1, hSNF2H,
and hSNF2L, coprecipitated with ICP8, suggesting a role for these
proteins in viral replication. The role of chromatin remodeling
proteins in HSV productive infection is not immediately obvious because
it has been reported that HSV genomic DNA is kept in a relatively
nucleosome-free form during lytic replication
(55,
56,
72). It is possible that
SWI/SNF complexes may be involved in maintaining this nucleosome-free
state. In contrast to lytic infection, it is thought that latent HSV
genomes are nucleosome associated
(22), suggesting that
SWI/SNF complexes may be involved in generating nucleosome-free genomes
during HSV reactivation.
There is evidence that interactions with
transcription factors may target SWI/SNF complexes to specific
promoters to regulate transcription
(1,
15,
52,
65,
83,
97). It is possible that
the virus uses a similar form of targeted regulation by recruiting
SWI/SNF complexes to weak viral promoters early in infection or during
reactivation to enhance
transcription.
Other ICP8-coprecipitating proteins.
Many other proteins
that do not fall into the above categories also coprecipitated with
ICP8. The most numerous of those were proteins involved in mRNA
splicing or transcription factors. HSV has been shown to alter host
cell mRNA maturation through the immediate-early protein ICP27. As we
also identified ICP27 as a coprecipitating protein, it is possible that
the mRNA splicing proteins were interacting directly with ICP27 and not
ICP8. The transcription factors may have been associated with viral DNA
or with RNA polymerase II, which has been shown to associate with ICP8
(99). We postulate that
these interactions may play a role in virus early and/or, more likely,
late gene transcription.
Role of DNA in mediating ICP8 coprecipitation.
The majority of the
ICP8-coprecipitating proteins tested associated with ICP8 in a
DNA-independent manner, suggesting that they physically associate with
ICP8 directly or indirectly through other binding partners. The
coprecipitation of proteins involved in chromatin remodeling required
DNA for optimal association. In agreement with a recent study
(84), we coprecipitated
ICP4 with ICP8; however, this did not appear to be an authentic direct
protein-protein interaction as it was dependent upon the presence of
DNA.
As we precipitated ICP8 using the conformation-specific 39S
monoclonal antibody that preferentially recognized ICP8 within
replication compartments
(89), we believe that
viral DNA, not contaminating cellular DNA, mediated these interactions
because ICP8 is bound to viral DNA in replication compartments
(48). It is possible that
ICP8 recruited these factors to replication compartments or viral DNA
and was not required for their retention once assembled onto viral
DNA.
Functional role of cellular proteins in HSV replication compartments.
The presence of representative members
of the coprecipitating proteins in replication compartments argued for
a potential role of these proteins in HSV replication. We observed that
the distribution and number of foci within replication compartments
differed from protein to protein. For example, there were relatively
few MSH2, BRM, or hSNF2H foci within the boundaries of replication
compartments compared to BRG1, mSin3a, or BAF155 (Fig.
2, middle panels). This
finding suggested that these cellular complexes targeted to distinct
subcompartments or structures within replication compartments. This
sort of subcompartmentalization has been described for viral proteins
in that ICP4, ICP8, and VP5 have distinct distribution patterns within
replication compartment boundaries
(19). The different
distributions of cellular proteins may reflect functional domains
within replication compartments in which similar or related cellular
machinery are recruited to aid HSV replication or sequestered to
prevent possible antiviral activities.
It remains to be
established that the ICP8-coprecipitating proteins have a role in viral
replication. We demonstrated that the WRN and Ku70 proteins were not
absolutely required for HSV growth. Further investigation is needed to
examine the necessity of the other coprecipitating proteins during HSV
replication. To do so, cells deficient in one or more proteins listed
in Table 1 would be
invaluable. This may prove to be difficult, as the proteins may be
absolutely required for viability, making isolation of such cell lines
difficult, or as is the case for WRN, redundant protein functions may
complement the deficiency, thus masking any phenotype. However, with
the increasing number of knockout mice, small-interfering RNA
technology, and repositories that supply cells from humans with certain
genetic disorders, it will rapidly become more feasible to investigate
the role of the ICP8-coprecipitating proteins in HSV
replication.
We believe that the large number of coprecipitating
proteins reflects the multiple roles of ICP8 and replication
compartments in viral DNA replication, viral gene expression, and
capsid assembly. Not all of the coprecipitating proteins can interact
with ICP8 directly, so we believe that some of the interactions are
dependent upon intermediate binding partners. It remains to be
determined which ICP8-coprecipitating proteins physically interact with
ICP8 directly. Many of the proteins identified associate with numerous
other proteins including other ICP8-coprecipitating proteins. It is
possible that the association of ICP8 with one or two proteins may lead
to the recruitment of numerous other proteins or complexes with a
variety of functions. For example, the BRCA1-associated genome
surveillance complex (BASC) is a 2-MDa "super complex"
that is believed to contain over 40 proteins that are involved in
genome integrity surveillance
(92). The many components
of BASC can detect and repair DNA damage as well as transduce signals
to cell cycle control proteins to halt cell growth
(30,
92). The association of
ICP8 with BRCA1 or another member of this complex could recruit the
numerous other BASC-associated proteins to viral replication
compartments.
We hypothesize that HSV recruits some of these
cellular proteins to replication compartments to participate directly
in HSV replication. Alternatively, they may be targeted to damaged
viral DNA that arises during replication. This demonstrates the
complexity of determining the function of these proteins in the context
of HSV infection. As many of the ICP8-coprecipitating proteins function
in the major recombination repair pathways, it will be of interest to
determine if these proteins have a necessary role in HSV replication or
are recruited to sites of DNA damage or stalled replication forks and
are not required for HSV replication. Further studies are needed to
determine the specific role of the proteins identified in viral
infection.

ACKNOWLEDGMENTS
We thank Kent Wilcox,
Bruce Stillman, Haim Cohen, and David
Sinclair for generously providing
reagents used in this study.
We also thank Ross Tomaino and Steven Gygi
for their aid in
the MS analysis.
This study was supported by NIH
grant AI
63106.

FOOTNOTES
* Corresponding
author. Mailing address: Department of Microbiology and Molecular
Genetics, Harvard Medical School, 200 Longwood Ave., Boston, MA 02115.
Phone: (617) 432-1934. Fax: (617) 432-0223. E-mail:
david_knipe{at}hms.harvard.edu.


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Journal of Virology, June 2004, p. 5856-5866, Vol. 78, No. 11
0022-538X/04/$08.00+0 DOI: 10.1128/JVI.78.11.5856-5866.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
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