The Coxsackievirus B 3CproProtease Cleaves MAVS and
TRIF to Attenuate Host Type I Interferon and Apoptotic
Amitava Mukherjee1, Stefanie A. Morosky2, Elizabeth Delorme-Axford1, Naomi Dybdahl-Sissoko3, M.
Steven Oberste3, Tianyi Wang4, Carolyn B. Coyne2*
1Department of Cell Biology and Physiology, University of Pittsburgh, Pittsburgh, Pennsylvania, United States of America, 2Department of Microbiology and Molecular
Genetics, University of Pittsburgh, Pittsburgh, Pennsylvania, United States of America, 3Picornavirus Laboratory, Centers for Disease Control and Prevention, Atlanta,
Georgia, United States of America, 4Department of Infectious Diseases and Microbiology, University of Pittsburgh, Pittsburgh, Pennsylvania, United States of America
The host innate immune response to viral infections often involves the activation of parallel pattern recognition receptor
(PRR) pathways that converge on the induction of type I interferons (IFNs). Several viruses have evolved sophisticated
mechanisms to attenuate antiviral host signaling by directly interfering with the activation and/or downstream signaling
events associated with PRR signal propagation. Here we show that the 3Cprocysteine protease of coxsackievirus B3 (CVB3)
cleaves the innate immune adaptor molecules mitochondrial antiviral signaling protein (MAVS) and Toll/IL-1 receptor
domain-containing adaptor inducing interferon-beta (TRIF) as a mechanism to escape host immunity. We found that MAVS
and TRIF were cleaved in CVB3-infected cells in culture. CVB3-induced cleavage of MAVS and TRIF required the cysteine
protease activity of 3Cpro, occurred at specific sites and within specialized domains of each molecule, and inhibited both the
type I IFN and apoptotic signaling downstream of these adaptors. 3Cpro-mediated MAVS cleavage occurred within its
proline-rich region, led to its relocalization from the mitochondrial membrane, and ablated its downstream signaling. We
further show that 3Cprocleaves both the N- and C-terminal domains of TRIF and localizes with TRIF to signalosome
complexes within the cytoplasm. Taken together, these data show that CVB3 has evolved a mechanism to suppress host
antiviral signal propagation by directly cleaving two key adaptor molecules associated with innate immune recognition.
Citation: Mukherjee A, Morosky SA, Delorme-Axford E, Dybdahl-Sissoko N, Oberste MS, et al. (2011) The Coxsackievirus B 3CproProtease Cleaves MAVS and TRIF
to Attenuate Host Type I Interferon and Apoptotic Signaling. PLoS Pathog 7(3): e1001311. doi:10.1371/journal.ppat.1001311
Editor: Mark Heise, University of North Carolina at Chapel Hill, United States of America
Received June 23, 2010; Accepted February 2, 2011; Published March 10, 2011
This is an open-access article distributed under the terms of the Creative Commons Public Domain declaration which stipulates that, once placed in the public
domain, this work may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose.
Funding: This work was supported by funding from the NIH [R01AI081759 (CBC)]. The funders had no role in study design, data collection and analysis, decision
to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: firstname.lastname@example.org
The innate immune system is the first line of defense against
pathogen infiltration and is activated by the binding of conserved
microbial ligands to pattern recognition receptors (PRRs). Activa-
tion of these receptors culminates in nuclear factor (NF)-kB and/or
(IFN-a and -b), key components of antimicrobial host defenses.
PRRs, including Toll-like receptors (TLRs) and DExD/H box
RNA helicases, signal through an assortment of downstream
adaptor molecules to propagate innate immune signaling. TLRs
signal through adaptor molecules such as myeloid differentiation
factor 88 (MyD88), Toll/IL-1 receptor domain containing adaptor
protein (TIRAP), Toll/IL-1 receptor domain containing adaptor
inducing interferon-beta (TRIF), and TRIF-related adaptor
molecule (TRAM) to activate cellular defenses . These adaptors
often display specificity with regard to the TLR family members
with whom they interact with and from which they are activated.
The specificity of TLR ectodomain-ligand recognition and
concomitant specificity in the signaling networks that are engaged
by this interaction provides an efficient strategy for microbial
recognition. In contrast, activated DExD/H box RNA helicases,
which include melanoma differentiation associated gene (MDA5)
and retinoic acid induced gene-I (RIG-I), signal to a common
downstream adaptor molecule, mitochondrial antiviral signaling
[(MAVS), also known as VISA/IPS-1/Cardif] to activate NFkB
and IRF3 [2,3,4]. MAVS is localized to the mitochondrial
membrane and to peroxisomes via a C-terminal transmembrane
domain, which is essential for innate immune signaling [5,6]. PRR-
associated adaptor molecules thus serve critical roles in the
activation of cellular defenses associated with microbial recognition.
As host cells have developed highly specialized strategies for
microbial detection and clearance, it is not surprising that many
viruses have evolved strategies to counter these defenses in order to
promote their replication and spread. In some cases, virally-
encoded proteases directly target components of the innate
immune system to abolish antiviral signaling via TLRs and/or
DExD/H box helicases. Targeted proteolysis of adaptor molecules
serves as a powerful means to eliminate antiviral signaling by
suppressing common downstream targets of key innate immune
signaling pathways. For example, MAVS is cleaved by the NS3/
4A serine protease of hepatitis C virus (HCV) , the 3Cpro
cysteine protease of hepatitis A virus (HAV) , the HCV-related
GB virus B NS3/4A protease , and the 2Aproand 3Cpro
proteases of rhinovirus . HCV also utilizes the same NS3/4A
serine protease to cleave TRIF in order to silence TLR3-mediated
PLoS Pathogens | www.plospathogens.org1March 2011 | Volume 7 | Issue 3 | e1001311
signaling . Thus, the targeting of MAVS and/or TRIF by
virally-encoded proteases in order to suppress antiviral signaling is
emerging as a common theme in the evasion of host defenses.
Enteroviruses, which belong to the Picornaviridae family, are small
single-stranded RNA viruses that account for several million
symptomatic infections in the United States each year. Coxsackie-
virus B3 (CVB3), a member of the Enterovirus genus, is associated with
a number of diverse syndromes, including meningitis, febrile illness,
and diabetes  and is an important causative agent of virus-
induced heart disease in adults and children [13,14,15,16]. The
induction of type I IFN signaling is essential for the control of CVB3
infection, as evidenced by enhanced virus-induced lethality in type I
IFN receptor (IFN-a,b R) null mice  and increased susceptibility
to CVB3 infection in IFNb-deficient mice . Both TLR3- and
MDA5-mediated type I IFN signaling have been implicated in the
response to CVB3 infections and mice deficient in either TRIF or
MAVS show an enhanced susceptibility to viral infection [19,20,21].
In this study, we determined the potential mechanisms employed
by CVB3 to antagonize type I IFN signaling. We found that
infection of cells with CVB3 led to the cleavage of the adaptor
molecules MAVS and TRIF. Both MAVS and TRIF were cleaved
by the CVB3-encoded cysteine protease 3Cpro, indicating that a
single protease suppresses innate immune signaling through two
powerful pathways. We found that 3Cprocleaves specific residues
within MAVS and TRIF that render these molecules deficient in
type I IFN signaling and apoptotic signaling. Taken together, these
data suggest that CVB3 has evolved a mechanism to cleave adaptor
components of the innate immune system to escape host immunity.
CVB3 infection does not induce IRF3 nuclear localization
or significant type I IFN responses
The induction of type I IFNs is the earliest cellular immune
response initiated to combat viral infections and is coordinated by
the activation of transcription factors such as interferon regulatory
factor (IRF)-3, IRF7, and NFkB downstream of PRR activation.
We found that CVB3 infection of HEK293 cells led to only a
modest induction of IRF3 activation as assessed by immunoflu-
orescence microscopy for nuclear translocation (Figure 1A),
western blot analysis of nuclear extracts (Figure 1B), and luciferase
activity assays for IFNb (Figure 1C). In contrast, transfection of
cells with poly I:C induced pronounced IRF3 activation
(Figure 1A–C). We also observed little activation of NFkB
signaling in response to CVB3 infection as determined by lucife-
rase activation assay (Figure 1C).
Because CVB3 did not elicit a pronounced translocation of
IRF3 into the nucleus during infection of HEK293 cells, we
investigated the role of several PRRs in mediating CVB3
recognition–TLR3, RIG-I, and MDA5. Both MDA5  and
TLR3  have been proposed to act as sensors for CVB3
infection. Although infection of cells with CVB3 is sensitive to
IFNb (Supplemental Figure S1A), we observed less enhancement
of IFNb promoter activity as assessed by luciferase activation in
CVB3-infected HEK293 cells overexpressing MAVS, MDA5,
RIG-I, and TLR3/TRIF than in uninfected controls (Figure 1D).
Instead, we observed the partial ablation of IFNb promoter
activity in response to ectopic expression of MAVS, RIG-I,
MDA5, and TLR3/TRIF in CVB3 infected cells (Figure 1D). We
also found that CVB did not induce potent IFNb production in
HEK293, HeLa, or Caco-2 cells in comparison to VSV controls
MAVS and TRIF are cleaved in CVB3-infected cells
Because CVB3 infection was inefficient at inducing IRF3, we
assessed the pattern of expression of MAVS in CVB3-infected
HEK293 cells. By immunoblot analysis, we found that CVB3
infection induced the cleavage of MAVS (Figure 2A). Similar
results were obtained in HeLa cells (Supplemental Figure S2A).
This effect was specific for CVB3 as infection with VSV did not
alter MAVS migration (Supplemental Figure S2B). In uninfected
cells, full-length MAVS migrated as a single band of ,75 kD.
However, in cells infected with CVB3, there was a decrease in the
expression level of full-length MAVS and the appearance of a
distinct MAVS cleavage fragment migrating at ,40–50 kD
(Figure 2A). Because MAVS cleavage is induced in cells
undergoing apoptosis [23,24] and CVB3 is known to induce
apoptosis in many cell types [25,26], we investigated the role of
apoptosis in CVB3-induced MAVS cleavage. We found that
incubation of CVB3-infected HEK293 cells with the broad
caspase inhibitor z-VAD-FMK and the proteosome inhibitor
MG132 had little effect on CVB3-induced MAVS cleavage
(Figure 2A). (The slight reduction in MAVS cleavage observed
in the presence of MG132 is likely attributable to a reduction in
replication in MG132-exposed cells, consistent with previously
published results [27,28]). The kinetics of MAVS cleavage was also
not consistent with apoptosis: MAVS cleavage was evident by
3 hrs post-infection (p.i.) whereas apoptosis (as measured by
caspase-3 cleavage) did not occur until 5–6 hrs p.i. (Figure 2B).
MAVS is localized to the mitochondrial membrane via a C-
terminal transmembrane domain . We found that CVB3
infection induced a pronounced decrease in MAVS mitochondrial
localization as assessed by immunofluorescence microscopy with a
mitochondrial marker (Figure 2C). We also found that the
expression and mitochondrial localization of ectopically expre-
ssed MAVS was significantly reduced in CVB3-infected cells
(Figure 2D, 2E). The appearance of cleavage fragments was
evident in CVB3-infected cells overexpressing MAVS (Figure 2D).
Moreover, we found that mutation of the caspase cleavage site of
Mammalian cells utilize a variety of defenses to protect
themselves from microbial pathogens. These defenses are
initiated by families of receptors termed pattern recogni-
tion receptors (PRRs) and converge on the induction of
molecules that function to suppress microbial infections.
PRRs respond to essential components of microorganisms
that are broadly expressed within classes of pathogens.
The relative non-specificity of this detection thus allows for
a rapid antimicrobial response to a variety of microorgan-
isms. Coxsackievirus B3 (CVB3), a member of the entero-
virus genus, is associated with a number of diverse
syndromes including meningitis, febrile illness, diabetes,
and is commonly associated with virus-induced heart
disease in adults and children. Despite its significant
impact on human health, there are no therapeutic
interventions to treat CVB3 infections. Here we show that
CVB3 has evolved an effective mechanism to suppress PRR
signal propagation by utilizing a virally-encoded protein,
termed 3Cpro, to directly degrade molecules that function
downstream of PRR signaling. By targeting these mole-
cules, CVB3 can evade host detection and escape antiviral
defenses normally induced by mammalian cells. These
findings will lead to a better understanding of the
mechanisms employed by CVB3 to suppress host antiviral
signaling and could lead to the development of thera-
peutic interventions aimed at modulating CVB3 patho-
3Cpro Cleaves MAVS and TRIF
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MAVS (D429E, ) had no effect on CVB3-induced MAVS
cleavage (Supplemental Figure S2C), indicating a caspase-
independent mechanism of action.
Another common pathway upstream of IRF3 activation is the
engagement of TLR3 by viral dsRNA, which is produced as a
of MAVS in CVB3-infected cells, we sought to determine if CVB3
might also target TRIF, the specific adaptor molecule downstream of
TLR3,to repress IRF3 activation. Similar to our findingswith MAVS,
we found that TRIF expression was significantly reduced in HeLa cells
apoptosis [23,24], we found that z-VAD-FMK and MG132 had little
effect at antagonizing the CVB3-mediated reduction in TRIF
expression (Figure 2F), consistent with our findings with MAVS
(Figure 2A). The kinetics of TRIF cleavage also paralleled that of
MAVS aswe observed amarked reduction in TRIFlevels by3 hrsp.i.
(Figure 2G), a time prior to the induction of caspase-3 cleavage
(Figure 2B). Ectopically expressed CFP-fused TRIF was also signifi-
cantly decreased in cells infected with CVB3 and coincided with the
appearance of several cleavage fragments (Figure 2H).
We next investigated whether cleavage of MAVS and TRIF
occurred incells infected withother enterovirusesincludingechovirus
7 (E7) and enterovirus 71 (EV71). Infection of HeLa cells with both
E7 and EV71 led to the significant reduction of MAVS and TRIF
expression, which corresponded with the appearance of the newly
replicated viral protein VP1 (Supplemental Figure S3A). However, in
contrast to our findings with CVB3 (Figure 2A), we did not observe
the appearance of any significant cleavage fragments in either E7 or
EV71-infected cells. This may indicate that the cleavage fragments
are short-lived in E7 or EV71-infected cells or that cleavage occurs at
different residues within the molecule that alter antibody binding.
These results may indicate that members of the enterovirus family
target MAVS and TRIF to evade host immunity, but further studies
are required to definitively show which members of the enterovirus
family utilize this mechanism.
CVB3 infections are commonly associated with virus-induced
heart disease in adults and children and have been detected in
approximately 20-25% of patients with dilated cardiomyopathy
and myocarditis [13,14,15,16]. To determine whether MAVS and
TRIF are degraded in vivo, mice were infected with CVB3 and the
hearts of infected animals were probed for MAVS and TRIF. In
contrast to uninfected controls, there was an almost complete
absence of both MAVS and TRIF in murine hearts infected with
CVB3 (Supplemental Figure S3B). These data indicate that the
cleavage of MAVS and TRIF may also occur during CVB3
infection in vivo.
Figure 1. CVB3 infection does not induce significant type I IFN responses. HEK293 cells were infected with CVB3 (1 PFU/cell) for 8 hrs or
treated with poly I:C conjugated to transfection reagent [poly I:C/LyoVec (100 ng/mL)] for 12 hrs and (A) fixed and stained for virus (VP1, red) and
IRF3 (green) or (B) western blot analysis for IRF3 performed on nuclear and cytoplasmic fractions. (C) Luciferase assays (expressed in relative luciferase
activity) from HEK293 cells transfected with NFkB and IFNb promoted luciferase constructs and infected with CVB3 (8 hrs) or treated with 100 ng/mL
poly I:C/LyoVec for 12 hrs. Data are shown as mean 6 standard deviation. Asterisks indicate p-values of #0.05. (D) Luciferase assay (expressed in
relative luciferase activity) from HEK293 cells transfected with the indicated constructs and IFNb promoted luciferase constructs for 24 hrs and then
infected with CVB3 (1 PFU/cell) for 14 hrs. (E), IFNb production as measured by ELISA from HEK293, HeLa, or Caco-2 cell culture supernatants infected
with either CVB3 (3PFU/cell) or VSV (5PFU/cell) for the indicated times. Data are shown as the fold IFNb induction compared to no virus (NoV)
controls. Data in (D) and (E) shown as mean 6 standard deviation. Asterisks indicate p-values #0.05.
3Cpro Cleaves MAVS and TRIF
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Figure 2. CVB3 infection induces MAVS and TRIF cleavage. (A) Western blot analysis for MAVS in HEK293 cells infected with CVB3 for 12 hrs in
the absence(NoI) or presence of Z-VAD-FMK(zVAD) or MG132.(B) Time courseofMAVS and caspase-3 cleavage in HEK293 cells infected with CVB3 for the
indicated times. (C) HEK293 cells were infected with CVB3 for 8 hrs and fixed and stained for MAVS (green), mitochondria (red), and VP1 (blue). Asterisks
denote infected cells expressing less MAVS than uninfected controls. (D, E) U2OS cells transfected with Flag-MAVS or EGFP-MAVS were infected with CVB3
(1PFU/cell)for7 hrs(D)or12 hrs(E)andthenlysedandimmunoblottedwithanti-Flagmonoclonalantibody (D)orfixedandstainedformitochondria(red)
andVP1(blue)(E).Inordertobettervisualizecleavagefragmentsin(D),CVB-infectedculturesweretransfectedwith2 mgFlag-MAVS(incomparisonto1 mg
in uninfected controls).(F) Western blot analysis for TRIFin HeLa cells infected with CVB3 for8 hrs in the absence(NoI) or presenceof z-VAD-FMK(zVAD)or
MG132. (G) Time course of TRIF cleavage in HeLa cells infected with CVB3 (1 PFU/cell) for the indicated times. (H) Immunoblot analysis for overexpressed
CFP-TRIF in HEK293 cells infected with CVB3 (1 PFU/cell) for 8 hrs. In (A), (D), and (H), grey arrows denote CVB3-induced cleavage fragments.
3Cpro Cleaves MAVS and TRIF
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CVB3 3Cprocleaves MAVS and TRIF
Enteroviruses encode specific proteases that are required for the
processing of viral proteins and the establishment of replication,
but which also cleave a variety of host cell molecules . Because
we observed the cleavage of MAVS in CVB3-infected cells, we
investigated whether virally-encoded proteases might mediate this
effect. We cotransfected HEK293 cells with N-terminal Flag-
MAVS and various CVB3 viral proteins fused to EGFP. Of these
proteins, we found that expression of the protease 3Cprowas
sufficient to induce a significant reduction in MAVS expression
(Figure 3A). In fact, in order to observe significant levels of full-
length Flag-MAVS (or cleavage fragments) in EGFP-3Cproco-
transfected cells, cells had to be transfected with twice as much
Flag-MAVS as vector control or other CVB3 viral proteins. The
apparent lack of cleavage products in cells overexpressing
proteases is a phenomenon that has also been observed for
HCV-mediated cleavage of TRIF  and likely reflects the high
efficiency of cleavage (which may result from protease overex-
pression) and that cleavage fragments are unstable and/or short-
lived. For our subsequent studies, we transfected cells with
equivalent amounts of MAVS cDNA to compare the level of
full-length MAVS in control versus 3Cpro-transfected cells. The
cleavage of MAVS required the cysteine protease activity of 3Cpro,
as cotransfection of a catalytically inactive N-terminal EGFP-
tagged 3Cpromutant (C147A)  had no effect on MAVS
expression (Figure 3B). In some cases, significant levels of GFP
signal alone can be detected in EGFP-3CproWT transfected
cells which is likely indicative of 3Cprocleaving itself from the N-
terminal EGFP tag.
To confirm that 3Cprowas directly cleaving MAVS, we
incubated recombinant wild-type or C147A mutant 3Cprowith
Flag-MAVS purified by Flag column affinity purification from
overexpressing HEK293 cells. Whereas incubation with wild-type
3Cproinduced the appearance of a MAVS cleavage fragment as
determined by Flag immunoblotting, the C147A mutant did not
induce the appearance of a MAVS cleavage product (Figure 3C).
Figure 3. 3CproCleaves MAVS and TRIF. (A) Immunoblot analysis for Flag-MAVS (top) and GFP (bottom) in HEK293 cells co-transfected with
EGFP-2B, 2C, 3A, or 3Cproand Flag-MAVS. In order to better visualize cleavage fragments, EGFP-3Cproexpressing cells were transfected with 2 mg Flag-
MAVS (in comparison to 1 mg with other constructs). (B) HEK293 cells transfected with Flag-MAVS and control (EGFP-C2), EGFP-3Cpro wild-type or
C147A were lysed and subjected to immunoblotting for MAVS and GFP. (C) 3Cprocleaves MAVS in vitro. Recombinant wild-type or C147A mutant
SUMO-3Cpro(10 mg) was incubated with column purified Flag-MAVS (0.1 mg) for 8 hrs at 37uC, fractionated by SDS-PAGE, and immunoblotted with
anti-Flag monoclonal antibody (top) or commassie stained (bottom). (D) U2OS cells were transfected with EGFP-3Cprowild-type (WT) or the C147A
mutant and Flag-MAVS and immunofluorescence microscopy performed for MAVS (in red). (E) Immunoblot analysis of HEK293 cells transfected
with CFP-TRIF and either vector control, EGFP-2Apro, 3A EGFP-3Cproand lysates immunoblotted for TRIF (top) or GFP (bottom). Grey arrows denote
3Cpro-induced cleavage products. (F) Lysates of HEK293 cells transfected with CFP-TRIF and vector, wild-type (WT), or C147A mutant EGFP-3Cprowere
immunoblotted with anti- TRIF antibody (top) or anti-GFP (bottom) antibodies. Grey arrow denotes 3Cpro-induced cleavage product. (G) 3Cprocleaves
TRIF in vitro. Recombinant wild-type or C147A mutant SUMO-3Cpro(10 mg) was incubated with column purified Flag-TRIF (0.1 mg) for 12 hrs at 37uC,
fractionated by SDS-PAGE, and immunoblotted with anti-Flag monoclonal antibody (top) or commassie stained (bottom).
3Cpro Cleaves MAVS and TRIF
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Moreover, whereas expression of wild-type EGFP-3Cproinduced
the relocalization of MAVS as assessed by immunofluorescence
microscopy, expression of EGFP-3CproC147A had no effect
Because we also observed cleavage of TRIF in CVB3-infected
cells, we determined whether 3Cprowas responsible for its cleavage
as well. We cotransfected HEK293 cells with TRIF and either
EGFP-2Apro, 3A, or -3Cpro. Expression of 3Cpro, but not 2Aproor
3A, led to the cleavage of TRIF, demonstrated by a reduction in
the expression of full-length TRIF and the appearance of several
TRIF cleavage fragments (Figure 3E). 3Cpro-mediated cleavage of
TRIF required the cysteine protease activity of 3Cproas expression
of 3CproC147A did not lead to TRIF cleavage (Figure 3F). We
also confirmed that 3Cprowas directly cleaving TRIF by
incubation of Flag-TRIF purified by Flag column affinity
purification from overexpressing HEK293 cells with recombinant
wild-type or C147A mutant 3Cpro. Similar to our findings with
MAVS (Figure 3C), we found that only recombinant wild-type
3Cproinduced the appearance of TRIF cleavage fragments
(Figure 3G). Note that the pattern of TRIF cleavage by in vitro
proteolysis assay (Figure 3G) differs from our experiments with
overexpressed 3Cproin HEK293 cells (Figure 3E, 3F) due to the
use of C-terminal CFP- versus N-terminal Flag-tagged TRIF
between experiments. Taken together, our data show that 3Cpro
directly cleaves both MAVS and TRIF.
3Cprodisrupts MAVS and TRIF type I IFN and apoptotic
To assess whether expression of 3Cproabrogated MAVS-
dependent signaling, we transfected HEK293 cells with wild-type
or C147A EGFP-3Cproor vector control, with a luciferase reporter
fused to the IFNb promoter region (p-125-Luc), and with either Flag-
MAVS or the caspase activation and recruitment domains (CARDs)
of MDA5 or RIG-I. Expression of the CARDs of MDA5 and RIG-I
alone results inthe constitutive activation of type I IFN signaling even
in the absence of stimulus . We found that whereas there was
pronounced induction of IFNb activity in cells expressing vector
alone or EGFP-3CproC147A, expression of wild-type EGFP-3Cpro
led to a significant reduction in promoter activity (Figure 4A).
We next determined whether 3Cproattenuated TRIF-mediated
signaling. TRIF is involved in the activation of IRF3 and IFNb
induction downstream of dsRNA-TLR3 engagement. While the
expression of TRIF and vector control enhanced IFNb promoter
activity, expression of TRIF in combination with 3Cprosignificantly
impaired IFNb promoter activity (Figure 4B). We found that 3Cpro-
of IRF3 activation as coexpression of wild-type 3Cproand IRF3 had
no effect on IRF3-mediated activation of IFNb promoter activity
(Supplemental Figure S4A). Furthermore, we found that expression
of wild-type, but not C147A 3Cproreduced IFNb activation in
response to infection with VSV (Supplemental Figure S4B).
In addition to their roles in type I IFN signaling, ectopic
expression of MAVS  and TRIF  potently stimulate
intrinsic apoptotic machinery to induce cell death. We found that
expression of MAVS or TRIF induced pronounced apoptosis as
demonstrated by enhanced Annexin V binding [which identifies
the externalization of phosphatidylserine in cells undergoing
apoptosis] (Figure 4C, 4D). In contrast, expression of MAVS or
TRIF in the presence of 3Cpropotently reduced apoptosis
(Figure 4C, 4D). Taken together, these data show that 3Cpro
represses both the apoptotic and type I IFN signaling mediated by
MAVS and TRIF.
3Cprocleaves MAVS within the proline rich region
3Cpro preferentially cleaves glutamine-glycine (Q-G) bonds in
both the viral polyprotein and cellular targets, but may also exhibit
Figure 4. 3Cproabrogates MAVS and TRIF Type I IFN and apoptotic signaling. (A,B) Luciferaseassay(expressed as fold IFNb inductionversus
vector controls) from HEK293 cells transfected with vector, EGFP-3Cprowild-type or C147A mutants, the CARDs of MAVS, MDA5, or RIG-I, and a IFNb
promoted luciferase construct. Data are shown as mean 6 standard deviation. Asterisks indicate p-values #0.05 (C) Representative images of either
untransfected (No Tx) U2OS cells or cells transfected with Flag-MAVS (top row) or TRIF (bottom row) and either vector alone (+Vector) or wild-type (WT)
EGFP-3Cpro. Cells were stained with Alexa Fluor 594-conjugated Annexin V 48 hrs post-transfection. Blue, DAPI-stained nuclei. (D) Quantification of the
extent of apoptosis (shown as the percent of Annexin V positive cells/DAPI) in cells from (C). Asterisks indicate p-values #0.05.
3Cpro Cleaves MAVS and TRIF
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amongst others . In order to identify the residue(s) within
MAVS cleaved by 3Cpro, we constructed a panel of site directed
mutants within MAVS at residues that may serve as 3Cpro
cleavage sites (Q148, Q211, and E480) (Figure 5A). Of these
mutants, only one (Q148A) was resistant to 3Cpro-mediated
cleavage in HEK293 cells and by in vitro protease assay (Figure 5B).
Moreover, whereas wild-type Flag-MAVS was relocalized from
the mitochondrial membrane upon expression of EGFP-3Cpro, the
Q148A Flag-MAVS mutant retained its mitochondrial localization
(Figure 5C). The 3Cprocleavage site within MAVS (Q148) is
located in the proline rich region, which mediates its interaction
with a number of signaling molecules including TRAF2 ,
TRAF3 , TRAF6 , RIP1 , and FADD .
We next determined whether the Q148A mutant of MAVS was
resistant to 3Cpro-mediated abatement of MAVS signaling. We
found that whereas there was a pronounced reduction in IFNb
activity in cells expressing wild-type Flag-MAVS and EGFP-3Cpro,
there was no effect of EGFP-3Cproexpression on IFNb signaling in
cells transfected with Q148A Flag-MAVS (Figure 5D, Supple-
mental Figure S2D). These data show that CVB3 3Cprocleaves
MAVS at Q148 to suppress MAVS signaling.
activity against glutamine-alanine(Q-A) bonds,
3Cpro-induced MAVS cleavage fragments exhibit reduced
MAVS requires an intact CARD and localization to the
mitochondrial membrane (via a C-terminal transmembrane
domain) to remain functionally active . Because we found that
3Cprocleaved MAVS at a specific residue (Q148) within the
proline rich region, we next determined whether either of the
possible 3Cpro-induced cleavage fragments of MAVS would
remain active. To that end, we constructed EGFP-fused constructs
expressing wild-type MAVS, the N-terminal (residues 1-148), or
C-terminal (residues 149-540) fragments of MAVS that would
result from 3Cprocleavage (Figure 5F). We found that the N-
terminal fragment of MAVS (1-148) no longer localized to the
mitochondrial membrane (Figure 5G) and induced NFkB or IFNb
signaling significantly less that full-length MAVS (Figure 5H).
Whereas the C-terminal fragment of MAVS (149-540) retained its
mitochondrial localization (Figure 5G), it also exhibited signifi-
cantly less NFkB and IFNb activation in comparison to full-length
MAVS (Figure 5H). These data indicate that 3Cpro-mediated
cleavage of MAVS likely inactivates MAVS-mediated downstream
signaling by directly cleaving a residue that separates the CARD
and transmembrane regions.
3Cprolocalizes to the TRIF signalosome and interacts with
the C-terminus of TRIF
Overexpressed TRIF forms multimers and localizes to punctate
cytoplasmic structures referred to as the TRIF ‘signalosome’ .
Downstream components of TRIF signaling localize to signalo-
somes as a mechanism to stimulate TRIF signaling [35,36]. We
found that EGFP-3Cproand EGFP-3CproC147A were recruited
to TRIF signalosomes when co-expressed with TRIF (Figure 6A).
This recruitment was specific for 3Cproas we did not observe the
recruitment of either EGFP-2Apro(not shown) or EGFP-3A
(Supplemental Figure S5) to TRIF signalosomes. Although TLR3
(and presumably TRIF) can localize to endosomal membranes
, we did not observe any colocalization of overexpressed TRIF
with markers of both early and late endosomes (Supplemental
Because we observed the relocalization of 3Cproto the signalo-
some complex, we next determined whether 3Cproand TRIF
interact within this specialized complex. HEK293 cells were
transfected with TRIF and either vector (EGFP alone), EGFP-
3Cprowild-type, or EGFP-3CproC147A and co-immunoprecipi-
tation studies were performed. We found that whereas EGFP-
3CproC147A and TRIF co-immunoprecipitated, wild-type EGFP-
3Cproand TRIF did not (Figure 6B). These findings indicate that
3Cproforms an interaction with TRIF that is likely abolished upon
TRIF contains a proline-rich N-terminal region, a Toll/
Interleukin-1 receptor (TIR) domain, and a C-terminal region.
To determine which TRIF domain is responsible for interacting
with 3Cproand recruiting it to the signalosome, we constructed
N-terminal (NT, 1–359aa), C-terminal (CT, 360–712aa), and TIR
(390–460aa) domain expression constructs of TRIF containing a
HA-tag at the N-terminus and a Flag-tag at the C-terminus
(Figure 6C). We then coexpressed these constructs with wild-type
and C147A versions of 3Cproand performed fluorescence micro-
scopy and immunoprecipitation analysis. We found that 3Cpro
C147A specifically interacted with the C-terminal domain of
TRIF, but not the N-terminus (Figure 6D). However, the TIR
domain did not mediate the interaction between TRIF and 3Cpro
as we observed no co-immunoprecipitation between HA-TIR-Flag
and 3Cpro(not shown).
Previous studies have shown that expression of the C-terminus
of TRIF is required for the formation of the TRIF signalosome
. We found that expression of HA-CT-Flag was sufficient to
induce the relocalization of 3CproC147A to signalosomes
(Figure 6E). In contrast, 3CproC147A did not localize with either
HA-NT-Flag or HA-TIR-Flag (Figure 6E). The formation of
tubule-like structures induced by the expression of the TRIF TIR
is consistent with previous work by others . Taken together,
these data indicate that 3Cprointeracts with the C-terminus of
TRIF that is sufficient for its recruitment into the TRIF
3Cprocleaves the N- and C-terminal regions of TRIF
We did not observe any interaction between wild-type 3Cpro
and either full-length or C-terminal TRIF (Figure 6D and 6E)
suggesting that the interaction between TRIF and 3Cprois
diminished following cleavage. Interestingly, we observed the
appearance of cleavage fragments of both HA-NT-Flag and HA-
CT-Flag when coexpressed with wild-type 3Cpro(Figure 6D). To
further define the extent of 3Cpro-mediated proteolysis of the
N- and C-terminal regions of TRIF, we coexpressed dually HA-
and Flag-tagged constructs of TRIF (described in Figure 6C) and
wild-type or C147A EGFP-3Cproand subjected lysates to dual-
color (700 nm and 800 nm) immunoblot analysis using a LI-COR
Odyssey infrared imaging system and antibodies specific for HA
and Flag. This technique could therefore allow for the detection of
a variety of TRIF cleavage fragments simultaneously. We found
that expression of wild-type 3Cpro(but not the C147A mutant)
induced the cleavage of both the N- and C-termini of TRIF
(Figure 7A). In contrast, we observed no cleavage of the TIR
domain (Figure 7A). Additionally, our data indicate that the
C-terminus of TRIF is cleaved more abundantly than the
N-terminus as we observed a marked decrease in the expression
of full-length HA-CT-Flag and the appearance of several HA- or
Flag-tag-positive cleavage products induced by 3Cprooverexpres-
sion (Figure 7A).
3Cprosuppresses NFkB and Apoptotic signaling and via
the C-terminus of TRIF
The N- and C-terminal regions of TRIF differ in their capacities
to induce type I IFN and NFkB signaling—whereas overexpres-
3Cpro Cleaves MAVS and TRIF
PLoS Pathogens | www.plospathogens.org7March 2011 | Volume 7 | Issue 3 | e1001311
Figure 5. Q148 is the site of 3Cpro-mediated cleavage of MAVS. (A) Schematic of MAVS showing the locations of possible 3Cprocleavage sites.
(B) Left, Immunoblot analysis for overexpressed Flag-MAVS wild-type and the Q148A mutant in HEK293 cells cotransfected with either vector or
EGFP-3Cpro. Right, recombinant wild-type SUMO-3Cpro(10 mg) was incubated with wild-type or Q148A column purified Flag-MAVS (0.1 mg) for 12 hrs
at 37uC, fractionated by SDS-PAGE, and immunoblotted with anti-Flag monoclonal antibody (top) or commassie stained (bottom).
(C) Immunofluorescence microscopy for Flag-MAVS wild-type (WT) of the Q148A mutant in U2OS cells co-transfected with EGFP-3Cpro.
(D) Luciferase assay (expressed in relative luciferase activity) from HEK293 cells transfected with wild-type or Q148A Flag-MAVS and vector control or
EGFP-3Cproand IFNb promoted luciferase constructs. Data are shown as mean 6 standard deviation. Asterisks indicate p-values of #0.05.
(E), Schematic of EGFP-fused MAVS constructs of 3Cpro-induced MAVS cleavage fragments. (F), Western blot analysis of HEK293 cells transfected with
MAVS constructs depicted in (E). Lysates were immunoblotted with anti-monoclonal GFP antibody (top) or GAPDH as a loading controls (bottom).
(G), Confocal microscopy of U2OS cells transfected with EGFP-fused MAVS 3Cprocleavage fragments shown in (E). Cells were fixed and stained with
anti-mitochondria monoclonal antibody (MITO) (red). Blue, DAPI stained nuclei. (H), Luciferase assay (expressed in relative luciferase activity) from
HEK293 cells transfected with EGFP-fused MAVS 3Cprocleavage fragments [from (E)] and NFkB or IFNb promoted luciferase constructs. Data are
shown as mean 6 standard deviation. Asterisks indicate p-values #0.05.
3Cpro Cleaves MAVS and TRIF
PLoS Pathogens | www.plospathogens.org8 March 2011 | Volume 7 | Issue 3 | e1001311
sion of the N-terminal region of TRIF activates both IFNb and
NFkB signaling, the C-terminal domain fails to activate IFNb but
potently induces NFkB activation [38,39]. Moreover, the
C-terminus of TRIF is sufficient to induce apoptosis . The
RIP homotypic interaction motif (RHIM) at the C-terminus of
TRIF is essential for both NFkB and apoptotic signaling [33,40].
Because we observed pronounced 3Cpro-mediated cleavage of the
C-terminus of TRIF (Figure 7A), we investigated whether NFkB
and apoptotic signaling mediated by the C-terminus of TRIF was
abolished. We found that expression of wild-type 3Cpropotently
abrogated NFkB and apoptotic signaling induced by expression of
the C-terminus of TRIF (Figure 7B, 7C). These findings are
consistent with those indicating that 3Cproalso inhibits full-length
TRIF-mediated apoptotic signaling (Figure 4C, 4D).
3Cprocleaves specific sites in the N- and C-terminal
regions of TRIF
In order to identify the residue(s) within TRIF cleaved by 3Cpro,
we constructed a panel of site directed mutants within the TRIF
N- and C-terminal domains at residues that may serve as 3Cpro
cleavage sites (Figure 7D). [We omitted any potential sites within
the TRIF TIR domain as we did not observe 3Cpro-induced
cleavage of this domain (Figure 7A)]. We found that a specific
residue (Q190) within the N-terminal region of TRIF was targeted
by 3Cproas mutagenesis of this site abolished 3Cpro-induced
cleavage (Figure 7E). Because several sites in the C-terminal
domain of TRIF can serve as possible 3Cprocleavage sites, and
because these sites lie within close proximity to one another, we
mutated these sites simultaneously. We found that simultaneous
mutagenesis of four potential 3Cprocleavage sites (Q653, Q659,
Q671, and Q702) was sufficient to prevent 3Cprocleavage
(Figure 7F). These findings are consistent with our observation
that the C-terminus of TRIF likely undergoes 3Cprocleavage at
several sites (Figure 7A).
3Cpro-induced TRIF cleavage fragments are nonfunctional
in NFkB and apoptotic signaling
Because the N- and C-terminal domains of TRIF function in
unique capacities to induce IRF3, NFkB, and apoptotic signaling,
we next explored whether possible 3Cprocleavage fragments of
TRIF could remain functional in these pathways. We constructed
EGFP-fused full-length TRIF and various possible cleavage
fragments of TRIF (encoding residues 190-653, 190-671, or 190-
702). We found that all three possible 3CproTRIF cleavage
fragments maintained their capacity to activate type I IFN
signaling (as assessed by luciferase assays for IFN- stimulated
response element (ISRE),an
(Figure 7G, 7H). In contrast, two of these fragments, 190–653
Figure 6. 3Cprolocalizes to TRIF signalosomes and interacts with the C-terminal domain of TRIF. (A) Immunofluorescence microscopy for
overexpressed TRIF (in red) in U2OS cells coexpressing vector control (EGFP-C2), or wild-type or C147A mutant EGFP-3Cpro. (B) HEK293 cells
transfected with TRIF and vector, or wild-type or C147A mutant EGFP-3Cprowere lysed and subjected to immunoprecipitation with an anti-GFP
monoclonal antibody. Immunoprecipitates were subjected to immunoblot analysis for TRIF and GFP. (C), Top, schematic of N-terminal HA-tagged
and C-terminal Flag-tagged TRIF constructs. (D), HEK293 cells transfected with the indicated TRIF construct and either vector, wild-type or C147A
mutant EGFP-3Cprowere lysed and subjected to immunoprecipitation with an anti-GFP monoclonal antibody. Immunoprecipitates were subjected to
immunoblot analysis for HA and GFP. Arrows denote full-length (black) or cleaved (grey) TRIF. (E) Immunofluorescence microscopy for HA (in red),
Flag (in purple), and EGFP and DAPI-stained nuclei in U2OS cells transfected with EGFP-3CproC147A and the indicated TRIF construct.
3Cpro Cleaves MAVS and TRIF
PLoS Pathogens | www.plospathogens.org9 March 2011 | Volume 7 | Issue 3 | e1001311
Figure 7. 3Cprocleaves the N- and C-terminal domains of TRIF at specific sites. (A) Dual-color immunoblot analysis using a LI-COR Odyssey
infrared imaging system and antibodies specific for HA (800 nm, green) and Flag (700 nm, red) in HEK293 cells transfected with vector, EGFP-3Cprowild-
type or C147A and the indicated TRIF plasmids (for schematic, see Figure 6C). Black arrows denote full-length TRIF and grey arrows denote cleavage
fragments. An overlay of both channels is shown below (with yellow indicating overlapping signals). (B) Luciferase assay (expressed in relative luciferase
activity) from HEK293 cells transfected with vector, wild-type or C147A EGFP-3Cpro, the indicated domains of TRIF, and a NFkB promoted luciferase
construct. (C), Quantification of the extent of apoptosis (shown as the percent of Annexin V positive cells/DAPI) in HEK293 cells transfected with the
indicated domains of TRIF and vector alone or wild-type of C147A mutant EGFP-3Cpro. (D) Schematic of TRIF showing the locations of possible 3Cpro
cleavage sites. (E,F) Immunoblot analysis for wild-type or 3Cpro-resistant mutants of the N-terminal (HA-NT-Flag) (E) or C-terminal (HA-CT-Flag) (F) domains
of TRIF from lysates of HEK293 cells transfected with the indicate constructs and EGFP-3Cpro. Immunblots were performed with anti-TRIF [NT-TRIF, (E)] or
anti-HA [CT-TRIF, (F)]. (G). HEK293 cells were transfected with EGFP-fused full-length TRIF (Full) or possible possible 3Cpro-induced cleavage fragments of
TRIF. Cells were either co-transfected NFkB and IFNb promoted luciferase constructs and luciferase assays performed or the extent of apoptosis was
measured by AnnexinV binding. Data are presented as fold-induction versus vector controls. (H), Lysates from cells described in (G) were harvested and
immunoblotted with anti-GFP monoclonal antibody. Data in (B), (C), and (G) are shown as mean 6 standard deviation. Asterisks indicate p-values #0.05.
3Cpro Cleaves MAVS and TRIF
PLoS Pathogens | www.plospathogens.org10 March 2011 | Volume 7 | Issue 3 | e1001311
and 190-671, lost their ability to activate NFkB or induce
apoptotic signaling (Figure 7G, 7H). These data indicate that
3Cpro-mediated cleavage of TRIF may primarily function to
suppress TRIF-mediated NFkB and apoptotic signal propagation.
The host innate immune response to viral infections often
involves the activation of parallel PRR pathways that converge on
the induction of type I IFNs and NFkB activation. Several viruses
have evolved sophisticated mechanisms to evade the host innate
immune response by directly interfering with the activation and/
or downstream signaling events associated with PRR signal
propagation. Here we show that the 3Cprocysteine protease of
CVB3 targets MAVS and TRIF, two key adaptor molecules in the
innate immune response as a mechanism to suppress type I IFN
and apoptotic signaling. By targeting these adaptors, CVB3 has
evolved a strategy to suppress antiviral signal propagation through
two powerful pathways—TLR3 and RIG-I/MDA5. 3Cprocleaves
MAVS at a specific site within its proline-rich region (at Q148) and
suppresses MAVS-mediated induction of type I IFNs and
apoptosis. Moreover, 3Cprotargets both the N- and C-terminal
domains of TRIF to abrogate its type I IFN, NFkB, and apoptotic
signaling capacities. Interestingly, we found that 3Cprolocalized to
TRIF signalosomes and interacted with the C-terminal domain of
TRIF. Taken together, these data highlight the strategies used by
CVB3 to evade the host innate immune response.
Many viruses target molecules upstream of IFN induction as a
means to escape host immunity. Similar to our findings with
CVB3 3Cpro, the 3Cproof HAV directly cleaves MAVS to escape
host immunity , but it is not known if HAV 3Cproalso cleaves
TRIF. However, although HAV 3Cprois responsible for mediating
MAVS cleavage, the protease must be localized to the mitochon-
drial membrane via a transmembrane domain within the 3A viral
protein in order to facilitate this event . In contrast, CVB3 3A
localizes to the ER membrane where it disrupts ER-Golgi
vesicular trafficking [41,42] and is thus not targeted to the
mitochondrial membrane. Our studies indicate that in contrast to
HAV, CVB3 3Cproalone is sufficient to induce MAVS cleavage
despite it not being localized to the mitochondrial membrane.
Although MAVS and TRIF are potent inducers of type I IFN
signaling downstream of PRR activation, they have also been
shown to induce apoptotic signaling–another powerful pathway
used by host cells to suppress viral replication and progeny release.
Enteroviruses are lytic viruses, and as such, possess no known
mechanism for progeny release other than the destruction of the
host cell membrane. Lytic viruses often develop efficient strategies
to tightly regulate host cell death pathways in order to avoid killing
the host cell prematurely (and terminating viral replication). CVB3
possesses anti-apoptotic strategies, which are mediated by the 2B
and 2BC viral proteins [43,44]. In addition, it has been shown that
3Cprotargets the inhibitor of kBa as a means to stimulate
apoptosis and suppress viral replication . Our results show that
3Cpromay also serve in an anti-apoptotic capacity to suppress
MAVS- and TRIF-mediated apoptotic signaling as a means to
tightly regulate host cell apoptotic pathways. The pro-apoptotic
signaling mediated by MAVS requires its localization to the
mitochondrial membrane and the presence of intact CARDs, but
not the presence of an intact proline-rich region . Although
3Cprocleaves MAVS within the proline-rich region (Q148,
Figure 5B, 5C), this cleavage both induces the relocalization of
MAVS from the mitochondrial membrane (Figure 5C) and
inhibits MAVS signals (Figure 5D). Furthermore, 3Cprocleavage
fragments of MAVS are non-functional (Figure 5H). Thus, the loss
of MAVS-induced apoptosis in CVB3 3Cpro-expressing cells is
likely the result of both the relocalization of MAVS from the
mitochondrial membrane and the inhibition of signaling via the
CARD regions. Moreover, CVB3 3Cprotargets the C-terminal
region of TRIF, which has been shown to induce apoptosis via
direct binding to receptor interacting protein 1 (RIP1) via its RIP
homotypic interaction motif (RHIM) . Specifically, we found
that 3Cprotargeted several sites within the C-terminal domain of
TRIF that could effectively remove the RHIM domain, a domain
of TRIF known to be critically involved in NFkB and apoptotic
signaling (Figure 7D, F). In support of this, we found that 3Cpro
cleavage fragments were deficient in NFkB activation and
apoptosis (Figure 7G). Taken together, these data indicate that
3Cprosuppresses MAVS and TRIF-induced apoptotic signals both
by their direct cleavage and by their relocalization from either the
mitochondria or signalosome, respectively.
The N- and C-terminal domains of TRIF serve disparate
functions in the initiation of innate immune signaling. Whereas the
N-terminus of TRIF activates type I IFN induction via the
phosphorylation of IRF3, the C-terminal domain activates NFkB
[38,39]. Interestingly, we found that 3Cprocleaves both of these
domains—likely as a mechanism to suppress global TRIF-
generated signaling capacities. Upon ligand stimulation of TLR3
(or upon overexpression), activated TRIF forms signalosomes
enriched in TRIF-associated signaling components including RIP1
and NFkB -activating kinase-associated protein 1 (NAP1) [35,36].
We found that 3Cprolocalizes to the TRIF signalosome and that
expression of the C-terminal domain of TRIF is sufficient to
induce this localization (Figure 6A, 6E). Moreover, we found that
3Cprointeracts with the C-terminal domain of TRIF (Figure 6D).
However, it remains unclear whether this interaction is direct or
mediated via an adaptor molecule that also localizes to the
signalosome. Additionally, we found that 3Cprocleavage of the
TRIF C-terminal domain leads to the disruption of TRIF
signalosome formation (Supplemental Figure S7), which is
required for the initiation of TRIF-mediated IRF3 and NFkB
activation . It is thus conceivable that 3Cproattenuates TRIF-
dependent signaling via direct cleavage, the degradation of the
signalosome complex, and inhibition of the interactions between
TRIF and downstream molecules that are required to propagate
Although we found that 3Cprocleavage fragments of TRIF were
deficient in NFkB and apoptotic signaling, they retained their
capacity to induce type I IFN signaling (Figure 7G). These data
may indicate that the cleavage fragments of TRIF generated by
3Cprocleavage are short-lived and do not accumulate within the
cell. In support of this, we failed to identify TRIF cleavage
products induced by CVB3 infection endogenously (Figure 2F,
2G). Alternatively, it remains possible that 3Cpro-mediated
disruption of TRIF signaling is not involved in the suppression
of type I IFN signaling, but may instead target type II IFN
signaling. Previous studies in TLR3 and TRIF deficient mouse
models indicate that TLR3- and TRIF-mediated IFNc production
plays an important role in CVB3 infections in vivo . Thus,
TLR3 signaling via TRIF to induce type II IFNs may function as a
parallel pathway to MDA5 and/or RIG-I-mediated induction of
type I IFNs. In this scenario, 3Cprowould suppress the down-
stream propagation of both type I and II IFN signaling in order to
evade host immunity.
Viruses often utilize elegant strategies to attenuate innate
immune signaling in order to promote their propagation. Here we
show that the 3Cprocysteine protease of CVB3 (and likely other
enteroviruses) attenuates innate immune signaling mediated by
two potent antiviral adapter molecules, MAVS and TRIF. By
3Cpro Cleaves MAVS and TRIF
PLoS Pathogens | www.plospathogens.org11March 2011 | Volume 7 | Issue 3 | e1001311
utilizing a variety of methods to abate MAVS and TRIF signaling,
including both cleavage and retargeting from sites of signal
propagation, 3Cprocan efficiently suppress both type I IFN and
apoptotic signals aimed at clearing CVB3 infections. A better
understanding of the mechanisms employed by enteroviruses to
suppress host antiviral signaling could lead to the development of
therapeutic interventions aimed at modulating viral pathogenesis.
Cells and viruses
Human embryonic kidney (HEK) 293, HeLa, and U2OS cells
were cultured in DMEM-H supplemented with 10% FBS and
penicillin/streptomycin. Human intestinal Caco-2 cells were
cultured in MEM supplemented with 10% FBS and penicillin/
streptomycin. Cells were screened for mycoplasma using a PCR-
based mycoplasma test (Takara Bio USA) to prevent abnormalities
in cellular morphology, transfection, and growth.
All experiments were performed with CVB3-RD, expanded as
described . Vesicular stomatitis virus (VSV) was kindly
provided by Sara Cherry (University of Pennsylvania, Philadel-
phia, PA). Experiments measuring productive virus infection were
performed with 0.1-1 plaque forming units (PFU)/cell for the
indicated times. HeLa cells were infected with echovirus 7 and
enterovirus 71 at a MOI=0.1 for the indicated times.
Mouse infections were performed as described previously 
and lysates kindly provided to us by Jeffrey M. Bergelson,
Children’s Hospital of Philadelphia.
Plasmid transfections were performed using FuGENE 6
according to the manufacturer’s protocol (Roche Applied Science).
Following transfection, cells were plated as described above and
used 48–72 hrs later.
Cells cultured in collagen-coated chamber slides (LabTek,
Nunc) were washed and fixed with either 4% paraformaldehyde or
with ice-cold methanol. Cells were then permeabilized with 0.1%
Triton X-100 in phosphate buffered saline (PBS) and incubated
with the indicated primary antibodies for 1 hr at room
temperature (RT). Following washing, cells were incubated with
secondary antibodies for 30 min at room temperature, washed,
and mounted with Vectashield (Vector Laboratories) containing
49,6-diamidino-2-phenylindole (DAPI). For detection of apoptosis,
cells were washed in cold PBS and incubated with Alexa-Fluor-
488 conjugated-annexin V and propidium iodide for 15 min at
room temperature. Cells were then washed, fixed in 4% parafor-
maldehyde, and images captured as described below. Images were
captured using an Olympus IX81 inverted microscope equipped
with a motorized Z-axis drive. Images were generated by multiple-
section stacking (0.2 mm stacks) and deconvolved using a
calculated point-spread function (Slidebook 5.0). Confocal micros-
copy was performed with a FV1000 confocal laser scanning
Rabbit polyclonal and mouse monoclonal antibodies directed
against GFP (FL, B-2), GAPDH, HA (Y-11, F-7) and IRF3 (FL-
425) were purchased from Santa Cruz Biotechnology. Mouse
monoclonal anti-Flag (M2) was purchased from Sigma. Rabbit
polyclonal antibodies to TRIF and MAVS (human and rodent
specific) were purchased from Cell Signaling Technologies or
Bethyl Laboratories, respectively. Mouse anti-enterovirus VP1
(Ncl-Entero) was obtained from Novocastra Laboratories (New-
castle upon Tyne, United Kingdom). Mitochondria antibody
[MTC02] was purchased from Abcam. Mouse anti-enterovirus 71
antibody was purchased from Millipore. Alexa Fluor-conjugated
secondary antibodies were purchased from Invitrogen.
Flag-MDA5, Flag-MAVS, and Flag-TRIF plasmids were kindly
provided by Tianyi Wang (University of Pittsburgh). pUNO2-
hTRIF was purchased from Invivogen. EGFP-2Apro2B, 2C, 3A
and -3Cprowere constructed by amplification from CVB33 cDNA
(kindly provided by Jeffrey Bergelson, Children’s Hospital of
Philadelphia) and cloned into the NT-GFP TOPO fusion vector
(Invitrogen) following PCR amplification. EGFP-fusion constructs
expressing cleavage fragments of MAVS and TRIF were
generated by PCR amplification from Flag-MAVS or pUNO2-
TRIF and cloned into the NT-GFP TOPO fusion vector
(Invitrogen). CFP-TRIF was purchased from Addgene (plasmid
13644). Dual HA- and Flag-tagged TRIF constructs were
generated by amplification of TRIF cDNA with primers encoding
a N-terminal HA or C-terminal Flag tags and cloned into the
XhoI and EcoRI sites of pcDNA3.1(+). Mutagenesis of 3Cpro,
MAVS and TRIF constructs were performed using Quickchange
mutagenesis kit following the manufacturer’s protocol (Stratagene).
Primer sequences are available upon request.
Immunoblots and immunoprecipitations
Cell lysates were prepared with RIPA buffer (50 mM Tris-HCl
[pH 7.4]; 1% NP-40; 0.25% sodium deoxycholate; 150 mM
NaCl; 1 mM EDTA; 1 mM phenylmethanesulfonyl fluoride;
1 mg/ml aprotinin, leupeptin, and pepstatin; 1 mM sodium
orthovanadate), and insoluble material was cleared by centrifuga-
tion at 7006g for 5 min at 4uC. Lysates (30-50 mg) were loaded
onto 4–20% Tris-HCl gels (Bio-Rad, Hercules, CA) and transfer-
red to polyvinylidene difluoride membranes. Membranes were
blocked in 5% nonfat dry milk or 3% bovine serum albumin,
probed with the indicated antibodies, and developed with
horseradish peroxidase-conjugated secondary antibodies (Santa
Cruz Biotechology), and SuperSignal West Pico or West Dura
chemiluminescent substrates (Pierce Biotechnology).
Immunoblots in isolated mouse hearts and dually HA- and Flag-
tagged TRIF constructs were conducted using an Odyssey
Infrared Imaging System (LI-COR Biosciences). Tissue homoge-
nized in lysis buffer (100 mg) or whole-cell lysates from transfected
HEK293 cells (30 mg) were loaded onto 4–20% Tris-HCl gels,
separated electrophoretically, and transferred to nitrocellulose
membranes. Membranes were blocked in Odyssey Blocking buffer
and then incubated with the appropriate antibodies overnight
at 4uC in Odyssey Blocking buffer. Following washing, membranes
were incubated with anti-rabbit or anti-mouse antibodies conju-
gated to IRDye 680 or 800CW and visualized with the Odyssey
Infrared Imaging System according to the manufacturer’s
For immunoprecipitations, HEK293 cells transiently transfected
with the indicated plasmids were lysed with EBC buffer (50 mM
Tris [pH 8.0], 120 mM NaCl, 0.5% Nonidet P-40, 1 mm
phenylmethylsulfonyl fluoride, 0.5 mg/ml leupeptin, and 0.5 mg/
ml pepstatin). Insoluble material was cleared by centrifugation.
Lysates were incubated with the indicated antibodies in EBC
buffer for 1 hr at 4uC followed by the addition of Sepharose G
beads for an additional 1 hr at 4uC. After centrifugation, the beads
were washed in NETN buffer (150 mm NaCl, 1 mm EDTA,
50 mm Tris-HCl (pH 7.8), 1% Nonidet P-40, 1 mm phenyl-
methylsulfonyl fluoride, 0.5 mg/ml leupeptin, and 0.5 mg/ml
3Cpro Cleaves MAVS and TRIF
PLoS Pathogens | www.plospathogens.org12March 2011 | Volume 7 | Issue 3 | e1001311
pepstatin), then heated at 95uC for 10 min in Laemmli sample
buffer. Following a brief centrifugation, the supernatant was
immunblotted with the indicated antibodies as described above.
Expression and purification of recombinant proteins
Bacterial expression vectors encoding wild-type or C147A 3Cpro
were constructed in pET-SUMO (which encodes a linear fusion
consisting of an N-terminal 6xHis tag for affinity purification
followed by SUMO) following PCR amplification according to the
manufacturers protocol (Invitrogen). pET-SUMO-3Cproconstructs
were expressed in bacteria and purified by metal chelation resin
the manufacturer’s instructions (Invitrogen) and then dialyzed.
For purification of MAVS and TRIF, 10 cm dishes of
HEK2393 cells were transfected with Flag-MAVS or Flag-TRIF
and lysed 48 hrs post-transfection. Lysates were purified over anti-
Flag affinity gel columns, washed several times, and protein eluted
by competition with five washes of 3x Flag peptide using the Flag
M purification kit for mammalian expression systems (Sigma-
Aldrich). Eluted protein was quantified by BCA protein assay and
verified for purity by SDS-PAGE and immunoblot analysis.
Reporter gene assay
Activation of the NFkB and IFNb promoter was measured by
reporter assay. Cells were transfected in 24-well plates with p-125
luc (IFNb) or NFkB reporter plasmid together with the indicated
plasmids. Luciferase activity was measured by the Dual-Luciferase
assay kit (Promega). All experiments were performed in triplicate
and conducted a minimum of three times.
To measure IFNb production, the indicated cells were infected
with either CVB or VSV and samples of culture supernatant
removed at the indicated times. IFNb levels in culture supernatant
were determined by IFNb ELISA according to the manufacturer’s
instructions (PBL Biomedical Laboratories).
Isolation of nuclear extracts
Nuclear extracts were prepared from HEK293 cells after infection
with CVB3 for 12 hr. Cells were washed in ice-cold PBS and isolated
by incubation in 10 mM EDTA for 10 min. Cells were pelleted at
A (10 mM HEPES [pH 7.9], 1.5 mM MgCl2, 10 mM KCl,
0.5 mM DTT, 0.5 mM PMSF, and 0.1% NP-40). The pellets were
then resuspended in buffer B (20 mM HEPES [pH 7.9], 25%
glycerol, 0.42 M NaCl, 1.5 mM MgCl2, 0.2 mM EDTA, 0.5 mM
DTT, 0.5 mM PMSF, 5-mg/ml leupeptin, 5-mg/ml pepstatin, 5-mg/
centrifuged at 10,0006g. Nuclear extract supernatants were diluted
with buffer C (20 mM HEPES [pH 7.9], 20% glycerol, 0.2 mM
EDTA, 50 mM KCl, 0.5 mM DTT, 0.5 mM PMSF).
Data are presented as mean 6 standard deviation. One-way
analysis of variance (ANOVA) and Bonferroni’s correction for
multiple comparisons were used to determine statistical signifi-
(numbers were taken from GenBank at Pubmed): mitochondrial
antiviral signaling protein (MAVS) 57506; Toll/IL-1 receptor
domain-containing adaptor inducing interferon-beta (TRIF)
148022, toll-like receptor 3 (TLR3) 7098; Retinoic acid-inducible
gene-I (RIG-I) 23586; melanoma-differentiation-associated gene 5
(MDA5) 64135; interferon regulatory factor 3 (IRF3) 3661.
Western blot analysis for VP1 in HeLa cells pretreated with
medium alone (Con) or medium containing 100 U or 1000 U of
purified IFNb for 24 hrs and then infected with CVB (1PFU/cell)
for 10 hrs. (B) As a control, similar studies were performed with
VSV. Western blot analysis for VSV-G in HeLa cells pretreated
with medium alone (Con) or medium containing 100 U or 1000 U
of purified IFNb for 24 hrs and then infected with VSV (5PFU/
cell) for 10 hrs.
Found at: doi:10.1371/journal.ppat.1001311.s001 (0.13 MB TIF)
CVB infection is sensitive to type I interferons. (A)
cleavage. (A) Western blot analysis for MAVS in lysates from
HeLa cells infected with CVB for 10 hrs in the absence (NoI) or
presence of Z-VAD-FMK (zVAD) or MG132. (B) Western blot
analysis for MAVS in lysates from HEK293 cells infected with
CVB or VSV for 12 hrs. (C),HEK293 cells with transfected with
D429E Flag-MAVS (1 mg in no virus (NoV) controls or 2 mg in
CVB-infeceted culures) and then infected with CVB (1PFU/cell
for 12 hrs) 48 hrs following transfection. Lysates were harvested
and immunoblotted for Flag, VP1, or GAPDH (as a loading
control). (D), Lysates from Figure 5D were immunoblotted for Flag
Found at: doi:10.1371/journal.ppat.1001311.s002 (0.36 MB TIF)
CVB, but not VSV, infection induces MAVS
and are absent from the hearts of CVB-infected mice. (A)
Immunoblot analysis for MAVS and TRIF in HeLa cells infected
with echovirus 7 (E7) or enterovirus 71 (EV71) for the indicated
times (0.1 PFU/cell). (B) Hearts of three mice infected by
intraperitoneal injection with CVB for 7 days were removed,
homogenized, and lysed. Lysates were subjected to immunblot
analysis for MAVS and TRIF using an Odyssey Infrared Imaging
System (immunoblots are shown as grey scale images).
Found at: doi:10.1371/journal.ppat.1001311.s003 (0.17 MB TIF)
MAVS and TRIF are cleaved by other enteroviruses
induced IFNb activation. (A), HEK293 cells were transfected with
an IFNb-luciferase construct and IRF3 either with or without
3Cpro. Lysates were harvested 48 hrs post-transfection and
luciferase activity measured. (B), HEK293 cells were transfected
with an IFNb-luciferase construct and either vector control, or
wild-type of C147A 3Cpro. 48 hrs post-transfections, cells were
infected with VSV for 12 hrs, lysates collected and and luciferase
Found at: doi:10.1371/journal.ppat.1001311.s004 (0.21 MB TIF)
3Cpro acts upstream of IRF3 and attenuates VSV-
Immunofluoescence microscopy of U2OS cells transfected with
EGFP-3A and TRIF (red).
Found at: doi:10.1371/journal.ppat.1001311.s005 (0.25 MB TIF)
CVB 3A does not localize to the TRIF signalosome.
transfected with TRIF were stained for early endosome antigen-1
(EEA1), the lysosomal marker LAMP2, or Alexa Fluor 488-
conjugated transferrin (TRANS).
Found at: doi:10.1371/journal.ppat.1001311.s006 (1.72 MB TIF)
TRIF does not localize to endosomes. U2OS cells
cence microscopy of EGFP-3Cpro wild-type and HA-CT-Flag
(HA, red) in transfected U2OS cells.
Found at: doi:10.1371/journal.ppat.1001311.s007 (0.38 MB TIF)
3Cpro inhibits signalsome formation. Immunofluoes-
3Cpro Cleaves MAVS and TRIF
PLoS Pathogens | www.plospathogens.org13March 2011 | Volume 7 | Issue 3 | e1001311
Acknowledgments Download full-text
We are grateful to Jeffrey Bergelson and Nicole K. LeLay for providing us
with lysates of CVB3-infected hearts. We thank Eckard Wimmer and
Chunling Wang for helpful discussions and Jeff Bergelson and Fred Homa
for careful review of the manuscript.
Conceived and designed the experiments: AM CBC. Performed the
experiments: AM SAM EDA NDS CBC. Analyzed the data: AM CBC.
Contributed reagents/materials/analysis tools: NDS MSO TW. Wrote the
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PLoS Pathogens | www.plospathogens.org14 March 2011 | Volume 7 | Issue 3 | e1001311