Abstract
ABSTRACTHepatitis C virus (HCV) nonstructural protein 5A (NS5A) is a phosphoprotein that plays key, yet poorly defined, roles in both virus genome replication and virion assembly/release. It has been proposed that differential phosphorylation could act as a switch to regulate the various functions of NS5A; however, the mechanistic details of the role of this posttranslational modification in the virus life cycle remain obscure. We previously reported (D. Ross-Thriepland, J. Mankouri, and M. Harris, J Virol 89:3123–3135, 2015, doi:10.1128/JVI.02995-14) a role for phosphorylation at serine 225 (S225) of NS5A in the regulation of JFH-1 (genotype 2a) genome replication. A phosphoablatant (S225A) mutation resulted in a 10-fold reduction in replication and a perinuclear restricted distribution of NS5A, whereas the corresponding phosphomimetic mutation (S225D) had no phenotype. To determine the molecular mechanisms underpinning this phenotype we conducted a label-free proteomics approach to identify cellular NS5A interaction partners. This analysis revealed that the S225A mutation disrupted the interactions of NS5A with a number of cellular proteins, in particular the nucleosome assembly protein 1-like protein 1 (NAP1L1), bridging integrator 1 (Bin1, also known as amphiphysin II), and vesicle-associated membrane protein-associated protein A (VAP-A). These interactions were validated by immunoprecipitation/Western blotting, immunofluorescence, and proximity ligation assay. Importantly, small interfering RNA (siRNA)-mediated knockdown of NAP1L1, Bin1 or VAP-A impaired viral genome replication and recapitulated the perinuclear redistribution of NS5A seen in the S225A mutant. These results demonstrate that S225 phosphorylation regulates the interactions of NS5A with a defined subset of cellular proteins. Furthermore, these interactions regulate both HCV genome replication and the subcellular localization of replication complexes.IMPORTANCE Hepatitis C virus is an important human pathogen. The viral nonstructural 5A protein (NS5A) is the target for new antiviral drugs. NS5A has multiple functions during the virus life cycle, but the biochemical details of these roles remain obscure. NS5A is known to be phosphorylated by cellular protein kinases, and in this study, we set out to determine whether this modification is required for the binding of NS5A to other cellular proteins. We identified 3 such proteins and show that they interacted only with NS5A that was phosphorylated on a specific residue. Furthermore, these proteins were required for efficient virus replication and the ability of NS5A to spread throughout the cytoplasm of the cell. Our results help to define the function of NS5A and may contribute to an understanding of the mode of action of the highly potent antiviral drugs that are targeted to NS5A.
Highlights
IMPORTANCE Hepatitis C virus is an important human pathogen
S225A and S225D mutations were cloned into pSGR-neomycin phosphotransferase (Neo)-JFH-1-5A-One-Strep tag (OST), which contained the OST cloned into a welltolerated insertion site near the C terminus of nonstructural 5A protein (NS5A) domain III [25] (Fig. 1a and b)
As expected [24], the S225A mutation resulted in a reduction in hyperphosphorylation (Fig. 1c) and in a distribution of the protein that was restricted to the perinuclear region (Fig. 1d)
Summary
IMPORTANCE Hepatitis C virus is an important human pathogen. The viral nonstructural 5A protein (NS5A) is the target for new antiviral drugs. NS5A is thought to be involved in delivery of nascent virus genomes from the MW to sites of assembly While the latter are yet to be unambiguously defined, it is accepted that an association of both NS5A and the HCV capsid (core) protein with lipid droplets (LDs; a host organelle responsible for storage of neutral lipids) is required during this process. Alanine substitution (S225A) resulted in a 10-fold reduction in genome replication and was concomitant with a restricted distribution of NS5A, and other factors known to participate in genome replication (NS3 and PI4P lipids), to a perinuclear region [24] This restriction was dramatic compared to the extensive distribution of these components throughout the cytoplasm in wild-type (WT)-infected cells
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