key: cord-0010239-8mon7mz3
authors: Choi, Alex GoEun; Wong, Jerry; Marchant, David; Luo, Honglin
title: The ubiquitin‐proteasome system in positive‐strand RNA virus infection
date: 2012-07-11
journal: Rev Med Virol
DOI: 10.1002/rmv.1725
sha: 604472dc2961647053e3663c586f76ae79f61a26
doc_id: 10239
cord_uid: 8mon7mz3

Positive‐stranded RNA viruses, like many other viruses, have evolved to exploit the host cellular machinery to their own advantage. In eukaryotic cells, the ubiquitin‐proteasome system (UPS) that serves as the major intracellular pathway for protein degradation and modification plays a crucial role in the regulation of many fundamental cellular functions. A growing amount of evidence has suggested that the UPS can be utilized by positive‐sense RNA viruses. The UPS eliminates excess viral proteins that prevent viral replication and modulates the function of viral proteins through post‐translational modification mediated by ubiquitin or ubiquitin‐like proteins. This review will discuss the current understanding of how positive RNA viruses have evolved various mechanisms to usurp the host UPS to modulate the function and stability of viral proteins. In addition to the pro‐viral function, UPS‐mediated viral protein degradation may also constitute a host defense process against some positive‐stranded RNA viral infections. This issue will also be discussed in the current review. Copyright © 2012 John Wiley & Sons, Ltd.

Positive-strand RNA viruses cover more than onethird of all virus genera and infect a wide range of hosts, for example, the plants, animals, and humans. Examples of this class of viruses include poliovirus, coxsackievirus, EMCV, HCV, HAV, SARS-CoV, West Nile virus, and dengue fever virus [1] . Positive-strand RNA viruses, either enveloped or non-enveloped, consist of a single positive-strand of RNA acting as mRNA to direct the synthesis of viral proteins. Viral structural proteins make up the viral capsid, and non-structural proteins, such as viral proteases and RNA-dependent RNA polymerases, function to process the viral polyprotein and catalyze the synthesis of the viral progeny genomes, respectively.

Similar to other viral pathogens, positive-strand RNA viruses utilize and subvert the host cellular machinery to support their life cycle. Among the many host pathways that can be modulated by positive strand RNA viruses, the ubiquitin-proteasome system (UPS), the major intracellular protein degradation pathway, has recently received considerable attention because the interaction between viruses and the UPS has been found to play important roles in many aspects of the viral life cycle [2, 3] . For positive-strand RNA viruses, it has been shown that an appropriate stoichiometric ratio of viral structural to non-structural proteins is essential for effective viral replication [4] [5] [6] . There is evidence that too much of certain viral non-structural proteins are disadvantageous for certain viruses to successfully replicate [6] . To ensure a proper ratio of viral proteins, some positive-strand RNA viruses employ the host regulatory mechanisms, for example, the UPS, to degrade excess viral proteins to prevent interference of these proteins with the viral replication. In addition, post-translational modification of some viral proteins mediated by ubiquitin and ubiquitinlike proteins of the UPS has also been demonstrated as an important means to regulate viral protein function without altering protein stability [2] .

The objective of this review is to present an overview of current knowledge on how positive-strand RNA viruses interact and subvert the host UPS, a central component of the host protein degradation system, to maintain optimal levels of viral proteins and to modify the functions of virus-encoded proteins. The possibility that UPS-mediated viral protein degradation may also constitute a host defense process against some positive-stranded RNA viral infections will be briefly discussed as well. How positive-stranded RNA viruses utilize the UPS to modify the level/function of host proteins to generate a favorable environment for their infection will not be the focus of the current review (please refer to other reviews on this topic [2, 7, 8] ).

In eukaryotic cells, the best-known function of the UPS is to degrade misfolded/damaged proteins or intracellular regulatory proteins that are involved in a variety of cellular activities, including cell-cycle regulation, membrane protein trafficking, transcription, antigen presentation, and signal transduction [9, 10] . Protein degradation via the UPS starts with the covalent attachment of ubiquitin to a target protein (a process referred to as ubiquitylation) ( Figure 1 ). This process consists of multi-step ATP-dependent enzyme reactions. First, the ubiquitin is activated by ubiquitin-activating enzyme E1 to form an E1-ubiquitin thioester intermediate. Figure 1 . Schematic illustration of post-translational modification of viral proteins by the ubiquitin-proteasome system. Post-translational modification mediated by mono-ubiquitylation or sumoylation is necessary for the regulation of viral protein functions. Proteasomal degradation mediated through either ubiquitin-dependent or ubiquitin-independent mechanisms controls the expression levels of viral proteins. The harmony of these modifications is required to maintain the appropriate level and function of viral proteins, which are crucial for successful infection

The activated ubiquitin is then transferred to a ubiquitin-conjugating enzyme E2 that subsequently transfers the ubiquitin to the target protein either directly or with the help of a ubiquitin ligase enzyme E3 by forming an isopeptide bond between the carboxyl terminus of ubiquitin and the e-amino group of a lysine residue on the target protein [11] . The lysine residue of the conjugated ubiquitin is attached by another ubiquitin, consecutively, resulting in the formation of a poly-ubiquitin chain. The ubiquitin-tagged substrate protein is then delivered to the 20S proteasome, and the polypeptide is hydrolyzed into short oligopeptides and the free ubiquitin is recycled through the action of de-ubiquitinating enzymes [12, 13] . De-ubiquitinating enzyme plays important roles in UPS as it proofreads ubiquitinprotein conjugates, processes inactive ubiquitin precursors, and keeps sustainable proteolysis by maintaining sufficient ubiquitin within the cell [14] . The 20S proteasome consists of two 7 subunit-composed outer a-rings and two inner b-rings.

The function of the a-ring is to translocate the target protein to the core of the b-rings. The b-rings possess the catalytic property to conduct the action of the degradation. The 20S proteasome is normally inactive and inaccessible to protein substrates. Proteasome activators (PA), such as PA700 (also known as 19S proteasome), PA28 (also known as REG or 11S proteasome), and PA200, bind and activate the 20S proteasome [15] . Although the function of PA200 remains to be fully characterized, PA700 has been shown to mainly mediate ubiquitin-dependent proteasomal degradation, whereas PA28 facilitates substrate degradation in a ubiquitin and ATP-independent manner [15] (Figure 1 ).

Unlike protein modification by poly-ubiquitylation that mostly influences the stability of substrate protein, attachment of a single ubiquitin, a process called mono-ubiquitylation, regulates the function of target proteins without targeting them for degradation ( Figure 1 ). Mono-ubiquitylation has been shown to alter protein sub-cellular localization, regulate transcriptional and enzymatic activities, and change binding affinities to their partners [16, 17] .

In addition to ubiquitin, several ubiquitin-like (UBL) proteins have been found to also function as protein modifiers to regulate a variety of cellular functions, including transcription, DNA repair, signal transduction, and cell cycle control, by posttranslational modification of target proteins. The best-characterized family of UBL proteins is the small ubiquitin-like modifiers (SUMO), including SUMO-1, SUMO-2, and SUMO-3 [18, 19] . Protein modification by sumoylation is directed by an enzymatic cascade parallel to that involved in ubiquitylation ( Figure 1 ). SUMO is activated in an ATP-dependent manner by an E1-activating enzyme consisting of a heterodimer of the Aos1 and Uba2 proteins. After activation, SUMO is transferred to the SUMO E2-conjugating enzyme Ubc9. Three classes of SUMO E3-ligases have been reported, RanBP2, PIAS and the Polycomb protein Pc2. The sumoylation target is a lysine that occurs in the consensus motif ΨKXE where Ψ is a hydrophobic amino acid and X is any residue. Like mono-ubiquitylation, sumoylation does not target proteins for degradation, but instead, regulates protein function and sub-cellular localization [20, 21] . A growing body of evidence suggests that sumoylation is particularly important in the regulation of transcription [22] . The role of sumoylation in controlling the function of viral proteins by posttranslational modification has also been increasingly recognized [18, 23] .

Another well-studied UBL molecule is interferonstimulated gene 15 (ISG15). Expression of ISG15 is highly induced upon viral infection and interferon stimulation [24] . Protein modification by ISG15, termed ISGylation, occurs in a mode similar to ubiquitylation and sumoylation that requires the sequential action of E1 (UBE1L), E2 (UbcH8), and E3 enzymes [24] . The process of ISG15 conjugation can be reversed by de-ISGylating enzymes [24] . Function of ISGylation has not been fully understood, but it is thought to alter the biological activities of the target proteins or compete for ubiquitylation and play significant roles in many cellular functions [ , there were reports that the host ISG15 conjugations system can also be exploited by HCV to facilitate viral production [29-31]. The exact mechanism for the pro-viral function of ISGylation remains unclear; however, it is speculated that ISGylation of HCV proteins or host proteins critical for HCV life-cycle promotes HCV production by altering the function or preventing target proteins from ubiquitinmediated degradation [31].

RNA-dependent RNA polymerase (RdRp) catalyzes the synthesis of new viral RNA genomes from the original viral RNA [32] . Appropriate balance of viral RdRp concentration in infected cells was found to be a key factor affecting the membrane rearrangements, RNA replication, and RNA recombination efficiencies [33] [34] [35] . Transgenic overexpression of the 3D RdRp of the Theiler's murine encephalomyelitis virus has been shown to have antiviral effects against the infection of this virus itself and other viruses [6, 36] . Although the detailed mechanisms remain to be elucidated, changes of the stoichiometry have been proposed to be one mechanism responsible for such antiviral properties of overexpressed 3D [37] . These studies imply that maintaining an appropriate level of RdRp is important for viral growth [6] . Indeed, the expression levels of RdRp of Sindbis virus, TYMV, HCV, and HAV have been reported to be regulated via UPS-regulated protein proteolysis [5, 19, [38] [39] [40] . It was shown that the RNA polymerase (NSP4) of Sindbis virus is degraded by the proteasome through the ubiquitin-dependent N-end rule pathway [38] . The N-end rule is a highly conserved degradation mechanism relating the stability of a protein with the nature of its N-terminal residue [41]. TYMV RdRp was also shown to be degraded by the proteasome at a late stage of viral infection [5] . This UPS-mediated TYMV RdRp degradation requires prerequisite phosphorylation at two residues (threonine-64 and serine-80) localized within the putative N-terminal PEST sequence (P, proline; E, glutamic acid; S, serine; T, threonine), a known protein degradation motif [19, 42] .

The RdRps of HAV and HCV are another two examples of UPS-mediated degradation [40] . The HAV 3D polymerase and its precursors were observed to be present at low levels in infected cells [43] and was demonstrated to be poly-ubiquitinated for rapid proteasomal degradation [40] . Nonstructural 5B (NS5B) protein, the RdRp of HCV, was reported to bind to a cellular ubiquitin-like protein [39] . Overexpression of this protein, but not mutant lacking the NS5B-binding domain, increases the poly-ubiquitylation of NS5B and significantly decreases its stability [39] .

What is the significance of RdRp degradation during viral infection? Low amounts of RdRp may be required for regulating the switch between the negative-strand and the positive-strand viral RNA synthesis during viral replication [5] . It is also possible that maintaining a low concentration of RdRp is a strategy for virus to preserve its genome integrity [5] . Furthermore, it is conceivable that keeping a low quantity of viral proteins may be beneficial for virus to escape host immune surveillance.

As discussed earlier, in addition to UPS-mediated rapid turnover of viral proteins, viral protein modification by ubiquitin and/or UBLs also plays a key role in the regulation of viral protein function. It has been recently shown that coxsackieviral 3D RdRp can be post-translationally modified by mono-ubiquitylation [44] . Although the specific E3 ligase responsible for 3D ubiquitylation and the exact ubiquitylation site remain undetermined, this modification appears to be required for its function in regulating transcription of viral genome [44] . Besides ubiquitylation, it has also been found that the 3D polymerase of coxsackievirus can be sumoylated with SUMO-1, SUMO-2, and SUMO-3 (unpublished data). DNA alignment reveals one highly conserved sequence Valine-Lysine-Aspartic acid-Glutamic acid (VKDE), which could be found in the 3D of several different species of enteroviral 3D polymerases, that matches the ΨKXE consensus motif for sumoylation, suggesting that sumoylation may also be required for the regulation of 3D transcriptional activity.

Autoproteases are virus-encoded proteases that not only proteolytically process viral polyprotein to yield individual structural and non-structural proteins but also mediate the cleavage of many host proteins essential for transcription/translation and maintenance of cellular structures [45, 46] . The levels of mature 3C protease of EMCV and HAV, both belonging to the picornavirus family, have been found to decrease rapidly after they reach maximum proteolytic activity, about halfway through the infectious cycle [47] [48] [49] [50] . It was later demonstrated that low concentrations of 3C in infected cells are largely attributed to increased protein turnover through ubiquitin-mediated proteasome degradation [48,51-53]. In the presence of proteasome inhibitors, poly-ubiquitinated 3C protein accumulated and the rate of 3C degradation was significantly reduced [48, 52] . Evidence also suggests that EMCV and HAV exploit the same ubiquitin enzymes for 3C ubiquitylation as 3C proteases of these two viruses were shown to compete with each other for ubiquitin conjugation [48] . Further investigation identified the amino acid sequences 34 LLVRGRTLVV 43 and 32 LGVKDDWLLV 41 that serve as the protein destruction signals for recognition and ubiquitylation of EMCV and HAV 3C proteases, respectively, by the host ubiquitinconjugating system [40,51-53]. Mutations within these sequences lead to increased stability of 3C proteases. The E3 ubiquitin ligase, E3a, has been shown to recognize these destruction signals of EMCV and HAV 3C proteases and catalyze the conjugation of ubiquitin to them [40,51,53]. The 3C protein of another virus in the picornavirus family, enterovirus 71 (EV71) has also been reported to be regulated by ubiquitin-mediated degradation [54] . It was found that sumoylation of EV71 3C at lysine 52 promotes its ubiquitylation and subsequent degradation [54] .

Virus-encoded proteases are required for successful virus replication via cleaving viral polyprotein precursors and can trigger host cell apoptosis by activating pro-apoptotic mediators and suppressing host protein translation and transcription [55, 56] . Apoptosis at late stages of viral replication can promote viral progeny release. But premature cell death will perturb viral replication before the virus has completed its life cycle. It is therefore speculated that keeping a small amount of 3C protease is necessary, at least at the early stage of viral infection, to prevent premature cell death and allow efficient viral replication. However, the potential significance of UPS targeting viral protease for degradation in host antiviral defense cannot be completely ruled out.

In the study of the regulation of HCV proteases, it was found that both enzymatically inactive nonstructural 2/3 (NS2/3) protein and cleaved NS2 protein of HCV are rapidly degraded during the course of viral infection, underlining the importance of tight regulation of these proteins during the viral life cycle [57-59]. The degradation of NS2 was shown to be regulated in a phosphorylation-dependent manner mediated by casein kinase 2 on serine 164 residue [57] . Notably, ubiquitylation appears not to be required for NS2 degradation as NS2 lysine-toarginine mutagenesis does not affect its stability and expression levels [57, 59] . A non-proteasomal degradation pathway, for example, lysosomemediated proteolysis, has also been suggested to be involved in the rapid turnover of NS2 [59] .

Not all proteases in the family of positivestranded RNA viruses are regulated by the UPS. The 3C proteases of rhinovirus and poliovirus, similar to those of EMCV and HAV, are present in low concentrations in infected cell [35,60,61]. However, it was found that poliovirus 3C is not conjugated with ubiquitin and remains stable during the viral life cycle [48] . Further studies are warranted to elucidate the mechanisms involved in maintaining low levels of polioviral 3C protease. Possible mechanisms may include blockage of ribosomal read-through and differential processing of polyproteins, leading to reduced production of 3C protease. A recent report has suggested that pseudoknot structures of the SARS-CoV RNA can stop the translation from upstream to downstream encoded viral proteins [62] . Some viral proteases have been shown to possess de-ubiquitinating enzyme activity. It was reported that the papain-like cysteine proteases of coronavirus [63] [64] [65] [66] [67] [68] , hepatitis E virus [69] , and foot-and-mouth disease virus [70] have structural similarity to the cellular de-ubiquitinating enzymes and are able to efficiently hydrolyze ubiquitinated substrates. Although the viral and cellular targets remain largely unknown, the de-ubiquitinating activity of these proteases appears to play a significant role in viral life cycle and in blockage of host innate immunity.

Although the structural proteins need to be expressed at higher levels than non-structural proteins [71] , the levels and functions of some viral structural proteins can also be regulated by the UPS. The core protein of HCV is a structural protein that not only packages the viral genomic RNA but also modulates multiple cellular functions, including apoptosis, cell proliferation, cell transformation, and signal transduction, and contributes to HCV pathogenesis [72, 73] . The stability of HCV core protein has been reported to be regulated by the proteasome in both ubiquitin-dependent and ubiquitinindependent manner [74] [75] [76] [77] [78] [79] [80] . It was demonstrated that the E3 ligase, E6-associated protein (E6AP), binds to the core protein of HCV and promotes its ubiquitylation and subsequent degradation by the proteasome [78] . Ubiquitin-independent proteasomal degradation mediated by PA28g has also been shown to facilitate the degradation of HCV core protein [76, 79] . PA28g interacts directly with the core protein of HCV in the nucleus and regulates its degradation [76] . In animal experiments, this interaction has been demonstrated to play an important role in HCV pathogenesis as PA28g knockout disrupts the progression of steatosis, hepatocarcinoma, and insulin resistance induced by HCV core protein [74, 75] . Moreover, recent evidence revealed a role for PA28g-mediated and E6AP-mediated degradation of HCV core protein in the regulation of HCV propagation [77] .

Recently discovered F protein, a frame-shift product of the core protein of HCV, has also been reported to be directly degraded by the 20S proteasome through a mechanism independent of ubiquitin [81, 82] . The function of HCV F protein remains unclear. Available data suggest a role for F protein in HCV pathogenesis by regulating the expression and activity of several pro-inflammatory cytokines, transcriptional factors, and oncogenes [5, 83, 84] . HCV F protein was found to be very labile with a short half-life (~10 min) and degraded by the proteasome [81, 85] . Further experimentation revealed that the degradation of F protein is ubiquitinindependent as a lysine-less F protein mutant remains unstable and its protein levels do not appears to be affected in a cell line with a temperature-sensitive E1 [82] . It was demonstrated that HCV F protein binds to the a3 subunit of the 20S proteasome and is degraded directly by the 20S proteasome in vitro [82] . The functional significance of F protein degradation in viral replication (pro-viral strategy versus host defense mechanism) warrants future investigation.

West Nile virus capsid protein is another example of a viral structural protein being the target of the UPS [86, 87] . The Makorin ring finger protein 1, a member of the Makorin family of proteins, was identified as the E3 ligase facilitating the ubiquitylation and consequent degradation of this capsid protein by the proteasome [86] . In addition to its role in nucleocapsid assembly, the West Nile capsid protein is also involved in viral pathogenesis by inducing apoptosis [88, 89] . Overexpression of Makorin ring finger protein 1 has been shown to result in reduced apoptosis triggered by West Nile virus infection, whereas depletion of this protein promotes viral cytotoxicity [86] . It remains unclear whether such modification and degradation of the capsid protein is a host defense mechanism or a viral strategy to prevent premature cell death.

The envelope (E) protein of SARS-CoV has been reported to be ubiquitinated in vitro and in cells, likely through its interaction with the N-terminal ubiquitin-like domain-1 of non-structural protein (NSP3) [90] . The coat protein of TMV has also been shown to be modified by mono-ubiquitylation [91] . However, the functional consequence of these posttranslational modifications in the regulation of the viral life cycle and viral pathogenesis is not known and requires further investigation.

Besides ubiquitylation, UBL-mediated modification has also been reported for viral structural proteins. The nucleocapsid (N) protein, a structural protein of SARS-CoV, was demonstrated to undergo post-translational modification by sumoylation [92, 93] . Using a yeast two-hybrid system, Ubc9, the E2-conjugating enzyme for sumoylation, was identified as the cellular protein interacting with the SARS-CoV N protein [92] . It was further demonstrated that N protein is covalently modified mainly at lysine 62 residue by SUMO [94] . Further investigation using wild-type N protein and a sumoylation mutant revealed that sumoylation of this protein increases its homo-oligomerization that may play a role in controlling viral replication cycle [94] . Swine fever virus core protein was also found to interact with the intracellular sumoylation pathway [95] . It binds to Ubc9 and SUMO-1 and disruption of these interactions results in attenuated viral virulence, suggesting a regulatory role of SUMO modification in viral infectivity [95] . Dengue virus is a member of the virus family Flaviviridae. Its envelope protein that is responsible for the virus attachment and entry to host cells has been shown to interact directly with SUMO E2 enzyme Ubc9. Further investigation demonstrates that overexpression of Ubc9 reduces the production of infectious virus, suggesting a role for SUMO modification in attenuating viral infectivity [96] .

Movement protein (MP) is a non-structural protein encoded by plant viruses to facilitate cell to cell movement. TMV MP was previously observed to be only transiently expressed during virus infection [97] . Further investigation demonstrates that TMV MP is poly-ubiquitinated and subsequently degraded by the proteasome and inhibition Li et al.

Li et al. [94] of proteasome function leads to accumulation of MP preferentially on the perinuclear ER [98] . The exact role of UPS-mediated degradation of MP in TMV infection remains to be elucidated. It is speculated that such modification of MP plays a critical role in regulating virus spread by attenuating its damage on ER structure [98] . Ubiquitin-conjugating enzyme Cdc34p has been identified as one of the host proteins binding to the p33 replication protein of Tombusvirus [99] . It was shown that p33 can be ubiquitinated both in vitro and in vivo, and overexpression of Cdc34p increases, whereas down-regulation of Cdc34p reduces Tombusvirus replication [99] . Further investigation identified the lysine residues required for p33 mono-ubiquitylation and demonstrated the functional significance of p33 ubiquitylation in Tombusvirus replication [100] .

The interaction between the host UPS and positivestranded RNA viruses is summarized in Table 1 .

Current evidence, reviewed herein, strongly supports a notion that the maintenance of appropriate levels of certain viral proteins within infected cells is crucial for successful viral reproduction. Autoproteases at high concentrations have been suggested to trigger apoptosis of the host cells by cleaving cellular proteins [55,56], whereas high amounts of RdRps have been shown to perturb the process of appropriate viral packaging and even become antiviral when expressed at high levels [6, 36] . Growing studies have suggested that positive-stranded RNA viruses manipulate the host UPS for the degradation of excess viral proteins that disturb efficient viral growth and for the modulation of viral protein function through ubiquitin-mediated or UBL-mediated modification ( Table 1 and Figure 1 ). These findings will not only be able to explain the differential expression of various viral proteins but also provide a drug target through a thorough understanding of the mechanism regulating the levels and activities of viral proteins. 

The ubiquitin-proteasome pathway in viral infections

Protein degradation systems in viral myocarditis leading to dilated cardiomyopathy

Regulation of poliovirus 3C protease by the 2C polypeptide

The ubiquitin-proteasome system regulates the accumulation of turnip yellow mosaic virus RNA-dependent RNA polymerase during viral infection

Antiviral effects of a transgenic RNAdependent RNA polymerase

Viral avoidance and exploitation of the ubiquitin system

Viral hijacking of the host ubiquitin system to evade interferon responses

Natural substrates of the ubiquitin proteolytic pathway

Ubiquitin and ubiquitin-like proteins as multifunctional signals

The ubiquitin-proteasome proteolytic pathway: destruction for the sake of construction

Ubiquitin in chains

Ubiquitin enters the new millennium

Mechanism and function of deubiquitinating enzymes

Regulation of monoubiquitinated PCNA by DUB autocleavage

The novel functions of ubiquitination in signaling

Viruses and sumoylation: recent highlights

Evidence for phosphorylation and ubiquitinylation of the turnip yellow mosaic virus RNA-dependent RNA polymerase domain expressed in a baculovirus-insect cell system

Deubiquitinating and interferon antagonism activities of coronavirus papain-like proteases

The papain-like protease from the severe acute respiratory syndrome coronavirus is a deubiquitinating enzyme

A noncovalent class of papain-like protease/deubiquitinase inhibitors blocks SARS virus replication

Severe acute respiratory syndrome coronavirus papain-like protease: structure of a viral deubiquitinating enzyme

Deubiquitination activity associated with hepatitis E virus putative papain-like cysteine protease

The leader proteinase of foot-andmouth disease virus negatively regulates the type I interferon pathway by acting as a viral deubiquitinase

Requirements for RNA replication of a poliovirus replicon by coxsackievirus B3 RNA polymerase

Sequence variability of HCV Core region: important predictors of HCV induced pathogenesis and viral production

The roles of hepatitis C virus proteins in modulation of cellular functions: a novel action mechanism of the HCV core protein on gene regulation by nuclear hormone receptors

Involvement of the PA28gamma-dependent pathway in insulin resistance induced by hepatitis C virus core protein

Critical role of PA28gamma in hepatitis C virus-associated steatogenesis and hepatocarcinogenesis

Proteasome activator PA28gamma-dependent nuclear retention and degradation of hepatitis C virus core protein

Involvement of PA28gamma in the propagation of hepatitis C virus

ubiquitin ligase mediates ubiquitylation and degradation of hepatitis C virus core protein

Proteasomal turnover of hepatitis C virus core protein is regulated by two distinct mechanisms: a ubiquitin-dependent mechanism and a ubiquitin-independent but PA28gamma-dependent mechanism

Ubiquitin-mediated degradation of hepatitis C virus core protein is regulated by processing at its carboxyl terminus

Hepatitis C virus f protein is a short-lived protein associated with the endoplasmic reticulum

Ubiquitin-independent degradation of hepatitis C virus F protein

Expression of the alternative reading frame protein of Hepatitis C virus induces cytokines involved in hepatic injuries

Hepatitis C virus F protein up-regulates c-myc and downregulates p53 in human hepatoma HepG2 cells

Characterization of the expression of the hepatitis C virus F protein

MKRN1 induces degradation of West Nile virus capsid protein by functioning as an E3 ligase

Jab1 mediates cytoplasmic localization and degradation of West Nile virus capsid protein

Induction of inflammation by West Nile virus capsid through the caspase-9 apoptotic pathway

Nile virus capsid protein induces p53-mediated apoptosis via the sequestration of HDM2 to the nucleolus

The envelope protein of severe acute respiratory syndrome coronavirus interacts with the non-structural protein 3 and is ubiquitinated

Tobacco mosaic virus particles contain ubiquitinated coat protein subunits

SARS-CoV nucleocapsid protein binds to hUbc9, a ubiquitin conjugating enzyme of the sumoylation system

Sumoylation of the nucleocapsid protein of severe acute respiratory syndrome coronavirus

Sumoylation of the nucleocapsid protein of severe acute respiratory syndrome coronavirus by interaction with Ubc9. Advances in Experimental

Effects of the interactions of classical swine fever virus Core protein with proteins of the SUMOylation pathway on virulence in swine

The type 2 dengue virus envelope protein interacts with small ubiquitin-like modifier-1 (SUMO-1) conjugating enzyme 9 (Ubc9)

Synthesis of TMV-specific RNAs and proteins at the early stage of infection in tobacco protoplasts: transient expression of the 30K protein and its mRNA

Degradation of tobacco mosaic virus movement protein by the 26S proteasome

Cdc34p ubiquitin-conjugating enzyme is a component of the tombusvirus replicase complex and ubiquitinates p33 replication protein

Ubiquitination of tombusvirus p33 replication protein plays a role in virus replication and binding to the host Vps23p ESCRT protein

The authors have no competing interest.ACKNOWLEDGEMENT This work was supported by the Canadian Institutes of Health Research (to HL).