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.

Hepatitis C virus (HCV) infection often causes extrahepatic manifestations, such as type 2 diabetes. We previously reported that HCV infection induces the lysosomal degradation of the transcription factor HNF-1a via an interaction with viral NS5A, thereby suppressing GLUT2 gene expression. However, the molecular mechanism of NS5A-induced degradation of HNF-1a is largely unknown. We aimed to identify the determinants necessary for the degradation of HNF-1a induced by NS5A. Coimmunoprecipitation analysis revealed that the POU specific (POU s ) domain spanning from aa 91 to 181 of HNF-1a is responsible for the interaction of NS5A. We also found that the region from aa 121 to 126 of NS5A, which is known as the binding motif of the HCV replication factor FKBP8, is important for the degradation of HNF-1a. A NS5A V121A mutation disrupted the NS5A-HNF-1a interaction as well as the degradation of HNF-1a. Our findings suggest that NS5A Val 121 is crucial for viral pathogenesis.

Hepatitis C virus (HCV) infection is closely associated with type 2 diabetes. We reported that HCV infection induces the lysosomal degradation of hepatocyte nuclear factor 1 alpha (HNF-1α) via interaction with HCV nonstructural protein 5A (NS5A) protein, thereby suppressing GLUT2 gene expression. The molecular mechanisms of selective degradation of HNF-1α caused by NS5A are largely unknown. Chaperone-mediated autophagy (CMA) is a selective lysosomal degradation pathway. Here, we investigated whether CMA is involved in the selective degradation of HNF-1α in HCV-infected cells and observed that the pentapeptide spanning from amino acid (aa) 130 to aa 134 of HNF-1α matches the rule for the CMA-targeting motif, also known as KFERQ motif. A cytosolic chaperone protein, heat shock cognate protein of 70 kDa (HSC70), and a lysosomal membrane protein, lysosome-associated membrane protein type 2A (LAMP-2A), are key components of CMA. Immunoprecipitation analysis revealed that HNF-1α was coimmunoprecipitated with HSC70, whereas the Q130A mutation (mutation of Q to A at position 130) of HNF-1α disrupted the interaction with HSC70, indicating that the CMA-targeting motif of HNF-1α is important for the association with HSC70. Immunoprecipitation analysis revealed that increasing amounts of NS5A enhanced the association of HNF-1α with HSC70. To determine whether LAMP-2A plays a role in the degradation of HNF-1α protein, we knocked down LAMP-2A mRNA by RNA interference; this knockdown by small interfering RNA (siRNA) recovered the level of HNF-1α protein in HCV J6/JFH1-infected cells. This result suggests that LAMP-2A is required for the degradation of HNF-1α. Immunofluorescence study revealed colocalization of NS5A and HNF-1α in the lysosome. Based on our findings, we propose that HCV NS5A interacts with HSC70 and recruits HSC70 to HNF-1α, thereby promoting the lysosomal degradation of HNF-1α via CMA. Many viruses use a protein degradation system, such as the ubiquitin-proteasome pathway or the autophagy pathway, for facilitating viral propagation and viral pathogenesis. We investigated the mechanistic details of the selective lysosomal degradation of hepatocyte nuclear factor 1 alpha (HNF-1α) induced by hepatitis C virus (HCV) NS5A protein. Using site-directed mutagenesis, we demonstrated that HNF-1α contains a pentapeptide chaperone-mediated autophagy (CMA)-targeting motif within the POU-specific domain of HNF-1α. The CMA-targeting motif is important for the association with HSC70. LAMP-2A is required for degradation of HNF-1α caused by NS5A. We propose that HCV NS5A interacts with HSC70, a key component of the CMA machinery, and recruits HSC70 to HNF-1α to target HNF-1α for CMA-mediated lysosomal degradation, thereby facilitating HCV pathogenesis. We discovered a role of HCV NS5A in CMA-dependent degradation of HNF-1α. Our results may lead to a better understanding of the role of CMA in the pathogenesis of HCV.

Hepatitis C virus (HCV) NS5A protein plays crucial roles in viral RNA replication, virus assembly, and viral pathogenesis. Although NS5A has no known enzymatic activity, it modulates various cellular pathways through interaction with cellular proteins. HCV NS5A (and other HCV proteins) are reportedly degraded through the ubiquitin-proteasome pathway; however, the physiological roles of ubiquitylation and deubiquitylation in HCV infection are largely unknown. To elucidate the role of deubiquitylation in HCV infection, an attempt was made to identify a deubiquitinase (DUB) that can interact with NS5A protein. An ovarian tumor protein (OTU), deubiquitinase 7B (OTUD7B), was identified as a novel NS5A-binding protein. Co-immunoprecipitation analyses showed that NS5A interacts with OTUD7B in both Huh-7 and HCV RNA replicon cells. Immunofluorescence staining revealed that HCV NS5A protein colocalizes with OTUD7B in the cytoplasm. Moreover, HCV infection was found to enhance the nuclear localization of OTUD7B. The OTUD7B-binding domain on NS5A was mapped using a series of NS5A deletion mutants. The present findings suggest that the domain I of NS5A is important and the region from amino acid 121 to 126 of NS5A essential for the interaction. Either V121A or V124A mutation in NS5A disrupts the NS5A-OTUD7B interaction. The results of this in vivo ubiquitylation assay suggest that HCV NS5A enhances OTUD7B DUB activity. Taken together, these results suggest that HCV NS5A protein interacts with OTUD7B, thereby modulating its DUB activity.

Interferon-stimulated gene 15 (ISG15), a ubiquitin-like protein, is induced by type I INF. Although several groups have reported ISGylation of the HCV NS5A protein, it is still unclear whether ISGylation of NS5A has anti- or pro-viral effects in hepatitis C virus (HCV) infection. In the present study, the role of ISGylation-independent, unconjugated ISG15 in HCV infection was examined. Immunoprecipitation analyses revealed that ISG15 interacts specifically with NS5A domain I. ISG15 mutants lacking the C-terminal glycine residue that is essential for ISGylation still interacted with NS5A protein. Taken together, these results suggest that unconjugated ISG15 affects the functions of HCV NS5A through protein-protein interaction.