Nonproteolytic K29-Linked Ubiquitination of the PB2 Replication Protein of Influenza A Viruses by Proviral Cullin 4-Based E3 Ligases

Ubiquitination of NS1 Confers Differential Adaptation of Zika Virus in Mammalian Hosts and Mosquito Vectors

Arboviruses, transmitted by medical arthropods, pose a serious health threat worldwide. During viral infection, Post Translational Modifications (PTMs) are present on both host and viral proteins, regulating multiple processes of the viral lifecycle. In this study, a mammalian E3 ubiquitin ligase WWP2 (WW domain containing E3 ubiquitin ligase 2) is identified, which interacts with the NS1 protein of Zika virus (ZIKV) and mediates K63 and K48 ubiquitination of Lys 265 and Lys 284, respectively. WWP2-mediated NS1 ubiquitination leads to NS1 degradation via the ubiquitin-proteasome pathway, thereby inhibiting ZIKV infection in mammalian hosts. Simultaneously, it is found Su(dx), a protein highly homologous to host WWP2 in mosquitoes, is capable of ubiquitinating NS1 in mosquito cells. Unexpectedly, ubiquitination of NS1 in mosquitoes does not lead to NS1 degradation; instead, it promotes viral infection in mosquitoes. Correspondingly, the NS1 K265R mutant virus is less infectious to mosquitoes than the wild-type (WT) virus. The above results suggest that the ubiquitination of the NS1 protein confers different adaptations of ZIKV to hosts and vectors, and more importantly, this explains why NS1 K265-type strains have become predominantly endemic in nature. This study highlights the potential application in antiviral drug and vaccine development by targeting viral proteins' PTMs.

AIMP2 restricts EV71 replication by recruiting SMURF2 to promote the degradation of 3D polymerase

Hand, foot and mouth disease (HFMD), mainly caused by enterovirus 71 (EV71), has frequently occurred in the Asia-Pacific region, posing a significant threat to the health of infants and young children. Therefore, research on the infection mechanism and pathogenicity of enteroviruses is increasingly becoming important. The 3D polymerase, as the most critical RNA-dependent RNA polymerase (RdRp) for EV71 replication, is widely targeted to inhibit EV71 infection. In this study, we identified a novel host protein, AIMP2, capable of binding to 3D polymerase and inhibiting EV71 infection. Subsequent investigations revealed that AIMP2 recruits the E3 ligase SMURF2, which mediates the polyubiquitination and degradation of 3D polymerase. Furthermore, the antiviral effect of AIMP2 extended to the CVA16 and CVB1 serotypes. Our research has uncovered the dynamic regulatory function of AIMP2 during EV71 infection, revealing a novel antiviral mechanism and providing new insights for the development of antienteroviral therapeutic strategies.

SARS-CoV-2 and UPS with potentials for therapeutic interventions

The Ubiquitin proteasome system (UPS), an essential eukaryotic/host/cellular post-translational modification (PTM), plays a critical role in the regulation of diverse cellular functions including regulation of protein stability, immune signaling, antiviral activity, as well as virus replication. Although UPS regulation of viral proteins may be utilized by the host as a defense mechanism to invade viruses, viruses may have adapted to take advantage of the host UPS. This system can be manipulated by viruses such as the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) to stimulate various steps of the viral replication cycle and facilitate pathogenesis, thereby causing the respiratory disease COVID-19. Many SARS-CoV-2 encoded proteins including open reading frame 3a (ORF3a), ORF6, ORF7a, ORF9b, and ORF10 interact with the host's UPS machinery, influencing host immune signaling and apoptosis. Moreover, SARS-CoV-2 encoded papain-like protease (PLpro) interferes with the host UPS to facilitate viral replication and to evade the host's immune system. These alterations in SARS-CoV-2 infected cells have been revealed by various proteomic studies, suggesting potential targets for clinical treatment. To provide insight into the underlying causes of COVID-19 and suggest possible directions for therapeutic interventions, this paper reviews the intricate relationship between SARS-CoV-2 and UPS. Promising treatment strategies are also investigated in this paper including targeting PLpro with zinc-ejector drugs, as well as targeting viral non-structural protein (nsp12) via heat treatment associated ubiquitin-mediated proteasomal degradation to reduce viral pathogenesis.

Ubiquitination in viral entry and replication: Mechanisms and implications

The ubiquitination process is a reversible posttranslational modification involved in many essential cellular functions, such as innate immunity, cell signaling, trafficking, protein stability, and protein degradation. Viruses can use the ubiquitin system to efficiently enter host cells, replicate and evade host immunity, ultimately enhancing viral pathogenesis. Emerging evidence indicates that enveloped viruses can carry free (unanchored) ubiquitin or covalently ubiquitinated viral structural proteins that can increase the efficiency of viral entry into host cells. Furthermore, viruses continuously evolve and adapt to take advantage of the host ubiquitin machinery, highlighting its importance during virus infection. This review discusses the battle between viruses and hosts, focusing on how viruses hijack the ubiquitination process at different steps of the replication cycle, with a specific emphasis on viral entry. We discuss how ubiquitination of viral proteins may affect tropism and explore emerging therapeutics strategies targeting the ubiquitin system for antiviral drug discovery.

Chicken UFL1 Restricts Avian Influenza Virus Replication by Disrupting the Viral Polymerase Complex and Facilitating Type I IFN Production

During avian influenza virus (AIV) infection, host defensive proteins promote antiviral innate immunity or antagonize viral components to limit viral replication. UFM1-specific ligase 1 (UFL1) is involved in regulating innate immunity and DNA virus replication in mammals, but the molecular mechanism by which chicken (ch)UFL1 regulates AIV replication is unclear. In this study, we first identified chUFL1 as a negative regulator of AIV replication by enhancing innate immunity and disrupting the assembly of the viral polymerase complex. Mechanistically, chUFL1 interacted with chicken stimulator of IFN genes (chSTING) and contributed to chSTING dimerization and the formation of the STING-TBK1-IRF7 complex. We further demonstrated that chUFL1 promoted K63-linked polyubiquitination of chSTING at K308 to facilitate chSTING-mediated type I IFN production independent of UFMylation. Additionally, chUFL1 expression was upregulated in response to AIV infection. Importantly, chUFL1 also interacted with the AIV PA protein to inhibit viral polymerase activity. Furthermore, chUFL1 impeded the nuclear import of the AIV PA protein and the assembly of the viral polymerase complex to suppress AIV replication. Collectively, these findings demonstrate that chUFL1 restricts AIV replication by disrupting the viral polymerase complex and facilitating type I IFN production, which provides new insights into the regulation of AIV replication in chickens.

The ubiquitin codes in cellular stress responses

Ubiquitination/ubiquitylation, one of the most fundamental post-translational modifications, regulates almost every critical cellular process in eukaryotes. Emerging evidence has shown that essential components of numerous biological processes undergo ubiquitination in mammalian cells upon exposure to diverse stresses, from exogenous factors to cellular reactions, causing a dazzling variety of functional consequences. Various forms of ubiquitin signals generated by ubiquitylation events in specific milieus, known as ubiquitin codes, constitute an intrinsic part of myriad cellular stress responses. These ubiquitination events, leading to proteolytic turnover of the substrates or just switch in functionality, initiate, regulate, or supervise multiple cellular stress-associated responses, supporting adaptation, homeostasis recovery, and survival of the stressed cells. In this review, we attempted to summarize the crucial roles of ubiquitination in response to different environmental and intracellular stresses, while discussing how stresses modulate the ubiquitin system. This review also updates the most recent advances in understanding ubiquitination machinery as well as different stress responses and discusses some important questions that may warrant future investigation.

Role of protein Post-translational modifications in enterovirus infection

Enteroviruses (EVs) are the main cause of a number of neurological diseases. Growing evidence has revealed that successful infection with enteroviruses is highly dependent on the host machinery, therefore, host proteins play a pivotal role in viral infections. Both host and viral proteins can undergo post-translational modification (PTM) which can regulate protein activity, stability, solubility and interactions with other proteins; thereby influencing various biological processes, including cell metabolism, metabolic, signaling pathways, cell death, and cancer development. During viral infection, both host and viral proteins regulate the viral life cycle through various PTMs and different mechanisms, including the regulation of host cell entry, viral protein synthesis, genome replication, and the antiviral immune response. Therefore, protein PTMs play important roles in EV infections. Here, we review the role of various host- and virus-associated PTMs during enterovirus infection.

Snakehead vesiculovirus hijacks SH3RF1 for replication via mediating K63-linked ubiquitination at K264 of the phosphoprotein

Snakehead vesiculovirus (SHVV) is a type of rhabdovirus that causes serious economic losses in snakehead fish culture in China. However, no specific antiviral drugs or vaccines are currently available for SHVV infection. In this study, 4D label-free ubiquitome analysis of SHVV-infected cells revealed dozens of ubiquitinated sites on the five SHVV proteins. We focused on investigating the ubiquitination of phosphoprotein (P), a viral polymerase co-factor involved in viral replication. SHVV-P was proved to be ubiquitinated via K63-linked ubiquitination at lysine 264 (K264). Overexpression of wild-type P, but not its K264R mutant, facilitated SHVV replication, indicating that K264 ubiquitination of the P protein is critical for SHVV replication. RNAi screening of 26 cellular E3 ubiquitin ligases identified five pro-viral factors for SHVV replication, including macrophage erythroblast attacher (MAEA), TNF receptor-associated factor 7 (TRAF7), and SH3 domain-containing ring finger protein 1 (SH3RF1), which interacted with and mediated ubiquitination of SHVV P. TRAF7 and SH3RF1, but not MAEA, mediated K63-linked ubiquitination of SHVV P, while only SH3RF1 mediated K264 ubiquitination of SHVV P. Besides, overexpression of SH3RF1 promoted SHVV replication and maintained the stability of SHVV P. In summary, SH3RF1 mediated K63-linked ubiquitination of SHVV P at K264 to facilitate SHVV replication, providing targets for developing anti-SHVV drugs and live-attenuated SHVV vaccines. Our study provides novel insights into the role of P protein in the replication of single-stranded, negative-sense RNA viruses.

Usp25-Erlin1/2 activity limits cholesterol flux to restrict virus infection

Reprogramming lipid metabolic pathways is a critical feature of activating immune responses to infection. However, how these reconfigurations occur is poorly understood. Our previous screen to identify cellular deubiquitylases (DUBs) activated during influenza virus infection revealed Usp25 as a prominent hit. Here, we show that Usp25-deleted human lung epithelial A549 cells display a >10-fold increase in pathogenic influenza virus production, which was rescued upon reconstitution with the wild type but not the catalytically deficient (C178S) variant. Proteomic analysis of Usp25 interactors revealed a strong association with Erlin1/2, which we confirmed as its substrate. Newly synthesized Erlin1/2 were degraded in Usp25-/- or Usp25C178S cells, activating Srebp2, with increased cholesterol flux and attenuated TLR3-dependent responses. Our study therefore defines the function of a deubiquitylase that serves to restrict a range of viruses by reprogramming lipid biosynthetic flux to install appropriate inflammatory responses.

USP39-mediated deubiquitination of Cyclin B1 promotes tumor cell proliferation and glioma progression

BACKGROUND

The elevated Cyclin B1 expression contributes to various tumorigenesis and poor prognosis. Cyclin B1 expression could be regulated by ubiquitination and deubiquitination. However, the mechanism of how Cyclin B1 is deubiquitinated and its roles in human glioma remain unclear.

METHODS

Co-immunoprecipitation and other assays were performed to detect the interacting of Cyclin B1 and USP39. A series of in vitro and in vivo experiments were performed to investigate the effect of USP39 on the tumorigenicity of tumor cells.

RESULTS

USP39 interacts with Cyclin B1 and stabilizes its expression by deubiquitinating Cyclin B1. Notably, USP39 cleaves the K29-linked polyubiquitin chain on Cyclin B1 at Lys242. Additionally, overexpression of Cyclin B1 rescues the arrested cell cycle at G2/M transition and the suppressed proliferation of glioma cells caused by USP39 knockdown in vitro. Furthermore, USP39 promotes the growth of glioma xenograft in subcutaneous and in situ of nude mice. Finally, in human tumor specimens, the expression levels of USP39 and Cyclin B1 are positively relevant.

CONCLUSION

Our data support the evidence that USP39 acts a novel deubiquitinating enzyme of Cyclin B1 and promoted tumor cell proliferation at least in part through Cyclin B1 stabilization, represents a promising therapeutic strategy for tumor patients.

Phosphorylation of the PA subunit of influenza polymerase at Y393 prevents binding of the 5'-termini of RNA and polymerase function

The influenza A virus (IAV) polymerase is a multifunctional machine that can adopt alternative configurations to perform transcription and replication of the viral RNA genome in a temporally ordered manner. Although the structure of polymerase is well understood, our knowledge of its regulation by phosphorylation is still incomplete. The heterotrimeric polymerase can be regulated by posttranslational modifications, but the endogenously occurring phosphorylations at the PA and PB2 subunits of the IAV polymerase have not been studied. Mutation of phosphosites in PB2 and PA subunits revealed that PA mutants resembling constitutive phosphorylation have a partial (S395) or complete (Y393) defect in the ability to synthesize mRNA and cRNA. As PA phosphorylation at Y393 prevents binding of the 5' promoter of the genomic RNA, recombinant viruses harboring such a mutation could not be rescued. These data show the functional relevance of PA phosphorylations to control the activity of viral polymerase during the influenza infectious cycle.

Advances in deciphering the interactions between viral proteins of influenza A virus and host cellular proteins

Influenza A virus (IAV) poses a severe threat to the health of animals and humans. The genome of IAV consists of eight single-stranded negative-sense RNA segments, encoding ten essential proteins as well as certain accessory proteins. In the process of virus replication, amino acid substitutions continuously accumulate, and genetic reassortment between virus strains readily occurs. Due to this high genetic variability, new viruses that threaten animal and human health can emerge at any time. Therefore, the study on IAV has always been a focus of veterinary medicine and public health. The replication, pathogenesis, and transmission of IAV involve intricate interplay between the virus and host. On one hand, the entire replication cycle of IAV relies on numerous proviral host proteins that effectively allow the virus to adapt to its host and support its replication. On the other hand, some host proteins play restricting roles at different stages of the viral replication cycle. The mechanisms of interaction between viral proteins and host cellular proteins are currently receiving particular interest in IAV research. In this review, we briefly summarize the current advances in our understanding of the mechanisms by which host proteins affect virus replication, pathogenesis, or transmission by interacting with viral proteins. Such information about the interplay between IAV and host proteins could provide insights into how IAV causes disease and spreads, and might help support the development of antiviral drugs or therapeutic approaches.

UBL7 enhances antiviral innate immunity by promoting Lys27-linked polyubiquitination of MAVS

RNA virus infection usually triggers a range of host immune responses, including the induction of proinflammatory cytokines, interferons, and interferon-stimulated genes (ISGs). Here, we report that UBL7, a ubiquitin-like protein, is upregulated during RNA virus infection and induced by type I interferon as an ISG. UBL7-deficient mice exhibit increased susceptibility to viral infection due to attenuated antiviral innate immunity. UBL7 enhances innate immune response to viral infection by promoting the K27-linked polyubiquitination of MAVS. UBL7 interacts with TRIM21, an E3 ubiquitin ligase of MAVS, and promotes the combination of TRIM21 with MAVS in a dose-dependent manner, facilitating the K27-linked polyubiquitination of MAVS and recruiting of TBK1 to enhance the IFN signaling pathway. Moreover, UBL7 has a broad-spectrum antiviral function as an immunomodulatory adaptor protein. Therefore, UBL7 positively regulates innate antiviral signaling and promotes positive feedback to enhance and amplify the antiviral response.

The ubiquitination landscape of the influenza A virus polymerase

During influenza A virus (IAV) infections, viral proteins are targeted by cellular E3 ligases for modification with ubiquitin. Here, we decipher and functionally explore the ubiquitination landscape of the IAV polymerase proteins during infection of human alveolar epithelial cells by applying mass spectrometry analysis of immuno-purified K-ε-GG (di-glycyl)-remnant-bearing peptides. We have identified 59 modified lysines across the three subunits, PB2, PB1 and PA of the viral polymerase of which 17 distinctively affect mRNA transcription, vRNA replication and the generation of recombinant viruses via non-proteolytic mechanisms. Moreover, further functional and in silico analysis indicate that ubiquitination at K578 in the PB1 thumb domain is mechanistically linked to dynamic structural transitions of the viral polymerase that are required for vRNA replication. Mutations K578A and K578R differentially affect the generation of recombinant viruses by impeding cRNA and vRNA synthesis, NP binding as well as polymerase dimerization. Collectively, our results demonstrate that the ubiquitin-mediated charge neutralization at PB1-K578 disrupts the interaction to an unstructured loop in the PB2 N-terminus that is required to coordinate polymerase dimerization and facilitate vRNA replication. This provides evidence that IAV exploits the cellular ubiquitin system to modulate the activity of the viral polymerase for viral replication.

Neddylation of Enterovirus 71 VP2 Protein Reduces Its Stability and Restricts Viral Replication

Posttranslational modifications (PTMs) of viral proteins play critical roles in virus infection. The role of neddylation in enterovirus 71 (EV71) replication remains poorly defined. Here, we showed that the structural protein VP2 of EV71 can be modified by neural precursor cell-expressed developmentally downregulated protein 8 (NEDD8) in an E3 ligase X-linked inhibitor of apoptosis protein (XIAP)-dependent manner. Mutagenesis and biochemical analyses mapped the neddylation site at lysine 69 (K69) of VP2 and demonstrated that neddylation reduced the stability of VP2. In agreement with the essential role of VP2 in viral replication, studies with EV71 reporter viruses with wild-type VP2 (enhanced green fluorescent protein [EGFP]-EV71) and a K69R mutant VP2 (EGFP-EV71-VP2 K69R) showed that abolishment of VP2 neddylation increased EV71 replication. In support of this finding, overexpression of NEDD8 significantly inhibited the replication of wild-type EV71 and EGFP-EV71, but not EGFP-EV71-VP2 K69R, whereas pharmacologic inhibition of neddylation with the NEDD8-activating enzyme inhibitor MLN4924 promoted the replication of EV71 in biologically relevant cell types. Our results thus support the notion that EV71 replication can be negatively regulated by host cellular and pathobiological cues through neddylation of VP2 protein. IMPORTANCE Neddylation is a ubiquitin-like posttranslational modification by conjugation of neural precursor cell-expressed developmentally downregulated protein 8 (NEDD8) to specific proteins for regulation of their metabolism and biological activities. In this study, we demonstrated for the first time that EV71 VP2 protein is neddylated at K69 residue to promote viral protein degradation and consequentially suppress multiplication of the virus. Our findings advance knowledge related to the roles of VP2 in EV71 virulence and the neddylation pathway in the host restriction of EV71 infection.

The Roles of Ubiquitination in Pathogenesis of Influenza Virus Infection

The ubiquitin system denotes a potent post-translational modification machinery that is capable of activation or deactivation of target proteins through reversible linkage of a single ubiquitin or ubiquitin chains. Ubiquitination regulates major cellular functions such as protein degradation, trafficking and signaling pathways, innate immune response, antiviral defense, and virus replication. The RNA sensor RIG-I ubiquitination is specifically induced by influenza A virus (IAV) to activate type I IFN production. Influenza virus modulates the activity of major antiviral proteins in the host cell to complete its full life cycle. Its structural and non-structural proteins, matrix proteins and the polymerase complex can regulate host immunity and antiviral response. The polymerase PB1-F2 of mutated 1918 IAV, adapts a novel IFN antagonist function by sending the DDX3 into proteasomal degradation. Ultimately the fate of virus is determined by the outcome of interplay between viral components and host antiviral proteins and ubiquitination has a central role in the encounter of virus and its host cell.

Non-proteolytic ubiquitylation in cellular signaling and human disease

Ubiquitylation is one of the most common post-translational modifications (PTMs) of proteins that frequently targets substrates for proteasomal degradation. However it can also result in non-proteolytic events which play important functions in cellular processes such as intracellular signaling, membrane trafficking, DNA repair and cell cycle. Emerging evidence demonstrates that dysfunction of non-proteolytic ubiquitylation is associated with the development of multiple human diseases. In this review, we summarize the current knowledge and the latest concepts on how non-proteolytic ubiquitylation pathways are involved in cellular signaling and in disease-mediating processes. Our review, may advance our understanding of the non-degradative ubiquitylation process.

Evanthia Pangou and co-authors review recent insights into the important roles of non-proteolytic ubiquitylation in cellular signaling as well as in physiology and disease.

[Induced degradation of proteins by PROTACs and other strategies: towards promising drugs]

Targeted protein degradation (TPD), discovered twenty years ago through the PROTAC technology, is rapidly developing thanks to the implication of many scientists from industry and academia. PROTAC chimeras are heterobifunctional molecules able to link simultaneously a protein to be degraded and an E3 ubiquitin ligase. This allows the protein ubiquitination and its degradation by 26S proteasome. PROTACs have evolved from small peptide molecules to small non-peptide and orally available molecules. It was shown that PROTACs are capable to degrade proteins considered as "undruggable" i.e. devoid of well-defined pockets and deep grooves possibly occupied by small molecules. Among these "hard to drug" proteins, several can be degraded by PROTACs: scaffold proteins, BAF complex, transcription factors, Ras family proteins. Two PROTACs are clinically tested for breast (ARV471) and prostate (ARV110) cancers. The protein degradation by proteasome is also induced by other types of molecules: molecular glues, hydrophobic tagging (HyT), HaloPROTACs and homo-PROTACs. Other cellular constituents are eligible to induced degradation: RNA-PROTACs for RNA binding proteins and RIBOTACs for degradation of RNA itself (SARS-CoV-2 RNA). TPD has recently moved beyond the proteasome with LYTACs (lysosome targeting chimeras) and MADTACs (macroautophagy degradation targeting chimeras). Several techniques such as screening platforms together with mathematical modeling and computational design are now used to improve the discovery of new efficient PROTACs.

K29-linked ubiquitin signaling regulates proteotoxic stress response and cell cycle

Protein ubiquitination shows remarkable topological and functional diversity through the polymerization of ubiquitin via different linkages. Deciphering the cellular ubiquitin code is of central importance to understand the physiology of the cell. However, our understanding of its function is rather limited due to the lack of specific binders as tools to detect K29-linked polyubiquitin. In this study, we screened and characterized a synthetic antigen-binding fragment, termed sAB-K29, that can specifically recognize K29-linked polyubiquitin using chemically synthesized K29-linked diubiquitin. We further determined the crystal structure of this fragment bound to the K29-linked diubiquitin, which revealed the molecular basis of specificity. Using sAB-K29 as a tool, we uncovered that K29-linked ubiquitination is involved in different kinds of cellular proteotoxic stress response as well as cell cycle regulation. In particular, we showed that K29-linked ubiquitination is enriched in the midbody and downregulation of the K29-linked ubiquitination signal arrests cells in G1/S phase.

MEKK1-dependent activation of the CRL4 complex is important for DNA damage-induced degradation of p21 and DDB2 and cell survival

Cullin-4 ubiquitin ligase (CRL4) complexes are differentially composed and highly dynamic protein assemblies that control many biological processes including the global genome nucleotide excision repair (GG-NER) pathway. Here we identified the kinase mitogen-activated protein kinase kinase kinase 1 (MEKK1) as a novel constitutive interactor of a cytosolic CRL4 complex that disassembles after DNA damage due to the Caspase-mediated cleavage of MEKK1. The kinase activity of MEKK1 was important to trigger auto-ubiquitination of the CRL4 complex by K48- and K63-linked ubiquitin chains. MEKK1 knockdown prohibited DNA damage-induced degradation of the CRL4 component DNA-damage binding protein 2 (DDB2) and the CRL4 substrate p21 and also cell recovery and survival. A ubiquitin replacement strategy revealed a contribution of K63-branched ubiquitin chains for DNA damage-induced DDB2/p21 decay, cell cycle regulation and cell survival. These data might have also implications for cancer, as frequently occurring mutations of MEKK1 might have an impact on genome stability and the therapeutic efficacy of CRL4-dependent immunomodulatory drugs such as thalidomide-derivatives.

Influenza Virus RNA-Dependent RNA Polymerase and the Host Transcriptional Apparatus

Influenza virus RNA-dependent RNA polymerase (FluPol) transcribes the viral RNA genome in the infected cell nucleus. In the 1970s, researchers showed that viral transcription depends on host RNA polymerase II (RNAP II) activity and subsequently that FluPol snatches capped oligomers from nascent RNAP II transcripts to prime its own transcription. Exactly how this occurs remains elusive. Here, we review recent advances in the mechanistic understanding of FluPol transcription and early events in RNAP II transcription that are relevant to cap-snatching. We describe the known direct interactions between FluPol and the RNAP II C-terminal domain and summarize the transcription-related host factors that have been found to interact with FluPol. We also discuss open questions regarding how FluPol may be targeted to actively transcribing RNAP II and the exact context and timing of cap-snatching, which is presumed to occur after cap completion but before the cap is sequestered by the nuclear cap-binding complex.

Role of Host-Mediated Post-Translational Modifications (PTMs) in RNA Virus Pathogenesis

Being opportunistic intracellular pathogens, viruses are dependent on the host for their replication. They hijack host cellular machinery for their replication and survival by targeting crucial cellular physiological pathways, including transcription, translation, immune pathways, and apoptosis. Immediately after translation, the host and viral proteins undergo a process called post-translational modification (PTM). PTMs of proteins involves the attachment of small proteins, carbohydrates/lipids, or chemical groups to the proteins and are crucial for the proteins’ functioning. During viral infection, host proteins utilize PTMs to control the virus replication, using strategies like activating immune response pathways, inhibiting viral protein synthesis, and ultimately eliminating the virus from the host. PTM of viral proteins increases solubility, enhances antigenicity and virulence properties. However, RNA viruses are devoid of enzymes capable of introducing PTMs to their proteins. Hence, they utilize the host PTM machinery to promote their survival. Proteins from viruses belonging to the family: Togaviridae, Flaviviridae, Retroviridae, and Coronaviridae such as chikungunya, dengue, zika, HIV, and coronavirus are a few that are well-known to be modified. This review discusses various host and virus-mediated PTMs that play a role in the outcome during the infection.

Hepatitis B Core Protein Is Post-Translationally Modified through K29-Linked Ubiquitination

Hepatitis B virus (HBV) core protein (HBc) plays many roles in the HBV life cycle, such as regulation of transcription, RNA encapsidation, reverse transcription, and viral release. To accomplish these functions, HBc interacts with many host proteins and undergoes different post-translational modifications (PTMs). One of the most common PTMs is ubiquitination, which was shown to change the function, stability, and intracellular localization of different viral proteins, but the role of HBc ubiquitination in the HBV life cycle remains unknown. Here, we found that HBc protein is post-translationally modified through K29-linked ubiquitination. We performed a series of co-immunoprecipitation experiments with wild-type HBc, lysine to arginine HBc mutants and wild-type ubiquitin, single lysine to arginine ubiquitin mutants, or single ubiquitin-accepting lysine constructs. We observed that HBc protein could be modified by ubiquitination in transfected as well as infected hepatoma cells. In addition, ubiquitination predominantly occurred on HBc lysine 7 and the preferred ubiquitin chain linkage was through ubiquitin-K29. Mass spectrometry (MS) analyses detected ubiquitin protein ligase E3 component N-recognin 5 (UBR5) as a potential E3 ubiquitin ligase involved in K29-linked ubiquitination. These findings emphasize that ubiquitination of HBc may play an important role in HBV life cycle.

Molecular Evolution, Neurodevelopmental Roles and Clinical Significance of HECT-Type UBE3 E3 Ubiquitin Ligases

Protein ubiquitination belongs to the best characterized pathways of protein degradation in the cell; however, our current knowledge on its physiological consequences is just the tip of an iceberg. The divergence of enzymatic executors of ubiquitination led to some 600–700 E3 ubiquitin ligases embedded in the human genome. Notably, mutations in around 13% of these genes are causative of severe neurological diseases. Despite this, molecular and cellular context of ubiquitination remains poorly characterized, especially in the developing brain. In this review article, we summarize recent findings on brain-expressed HECT-type E3 UBE3 ligases and their murine orthologues, comprising Angelman syndrome UBE3A, Kaufman oculocerebrofacial syndrome UBE3B and autism spectrum disorder-associated UBE3C. We summarize evolutionary emergence of three UBE3 genes, the biochemistry of UBE3 enzymes, their biology and clinical relevance in brain disorders. Particularly, we highlight that uninterrupted action of UBE3 ligases is a sine qua non for cortical circuit assembly and higher cognitive functions of the neocortex.

Specificity in Ubiquitination Triggered by Virus Infection

Ubiquitination is a prominent posttranslational modification, in which the ubiquitin moiety is covalently attached to a target protein to influence protein stability, interaction partner and biological function. All seven lysine residues of ubiquitin, along with the N-terminal methionine, can each serve as a substrate for further ubiquitination, which effectuates a diverse combination of mono- or poly-ubiquitinated proteins with linear or branched ubiquitin chains. The intricately composed ubiquitin codes are then recognized by a large variety of ubiquitin binding domain (UBD)-containing proteins to participate in the regulation of various pathways to modulate the cell behavior. Viruses, as obligate parasites, involve many aspects of the cell pathways to overcome host defenses and subjugate cellular machineries. In the virus-host interactions, both the virus and the host tap into the rich source of versatile ubiquitination code in order to compete, combat, and co-evolve. Here, we review the recent literature to discuss the role of ubiquitin system as the infection progresses in virus life cycle and the importance of ubiquitin specificity in the regulation of virus-host relation.