Interferon-stimulated gene 15 (ISG15) is a ubiquitin-like protein that is covalently conjugated to many substrate proteins in order to modulate their functions; this conjugation is called 'ISGylation'. Several groups reported that the ISGylation of hepatitis C virus (HCV) NS5A protein affects HCV replication. However, ISG15 conjugation sites on NS5A are not well determined, and it is unclear whether the role of NS5A-ISGylation in HCV replication is pro-viral or anti-viral. Here we investigated the role of NS5A-ISGylation in HCV replication by using HCV RNA replicons that have a mutation at each lysine (Lys) residue of NS5A protein. Immunoblot analyses revealed that five Lys residues (K44, K68, K166, K215, and K308) of 14 Lys residues within NS5A (1b, Con1) have the potential to accept ISGylation. We tested the NS5A-ISGylation among different HCV genotypes and observed that the NS5A of all of the HCV genotypes accept ISGylation at the multiple Lys residues. Using an HCV luciferase reporter replicon assay revealed that the residue K308 of NS5A is important for HCV (1b, Con1) RNA replication. We observed that K308, one of the Lys residues for NS5A-ISGylation, is located within the binding region of cyclophilin A (CypA), which is the critical host factor for HCV replication. We obtained evidence suggesting that NS5A-ISGylation derived from all of HCV genotypes enhances the interaction between NS5A and CypA. Taken together, these results suggest that NS5A-ISGylation functions as a pro-viral factor and promotes HCV replication via the recruitment of CypA. IMPORTANCE Host cells have evolved host defense machinery (such as innate immunity) to eliminate viral infections. Viruses have evolved several counteracting strategies for achieving an immune escape from host defense machinery, including type-I interferons (IFNs) and inflammatory cytokines. ISG15 is an IFN-inducible, ubiquitin-like protein that is covalently conjugated to the viral protein via specific Lys residues and suppresses viral functions and viral propagation. Here we demonstrate that HCV NS5A protein accepts ISG15-conjugation at specific Lys residues and that the HERC5 E3 ligase specifically promotes NS5A-ISGylation. We obtained evidence suggesting that NS5A-ISGylation facilitates the recruitment of CypA, which is the critical host factor for HCV replication, thereby promoting HCV replication. These findings indicate that E3 ligase HERC5 is a potential therapeutic target for HCV infection. We propose that HCV hijacks an intracellular ISG15 function to escape the host defense machinery in order to establish a persistent infection.
This study showed that the D allele in ACE I/D polymorphism is associated with worse functional outcomes. This highlights the possibility of further research to improve functional outcomes of ischaemic stroke by inhibiting the ACE system.
Ubiquitin and ubiquitin-like protein modification play important roles in modulating the functions of viral proteins in many viruses. Here we demonstrate that hepatitis B virus (HBV) X protein (HBx) is modified by ISG15, which is a type I IFN-inducible, ubiquitin-like protein; this modification is called ISGylation. Immunoblot analyses revealed that HBx proteins derived from four different HBV genotypes accepted ISGylation in cultured cells. Site-directed mutagenesis revealed that three lysine residues (K91, K95 and K140) on the HBx protein, which are well conserved among all the HBV genotypes, are involved in acceptance of ISGylation. Using expression plasmids encoding three known E3 ligases involved in the ISGylation to different substrates, we found that HERC5 functions as an E3 ligase for HBx-ISGylation. Treatment with type I and type III IFNs resulted in the limited suppression of HBV replication in Hep38.7-Tet cells. When cells were treated with IFN-α, silencing of ISG15 resulted in a marked reduction of HBV replication in Hep38.7-Tet cells, suggesting a role of ISG15 in the resistance to IFN-α. In contrast, the silencing of USP18 (an ISG15 de-conjugating enzyme) increased the HBV replication in Hep38.7-Tet cells. Taken together, these results suggest that the HERC5-mediated ISGylation of HBx protein confers pro-viral functions on HBV replication and participates in the resistance to IFN-α-mediated antiviral activity.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.