Human parvovirus B19 (B19V) is a major human pathogen in which the ssDNA genome is replicated within the nucleus of infected human erythroid progenitor cells (EPCs) through a process involving both cellular and viral proteins, including the non-structural protein (NS)1. We previously characterized the interaction between NS1 classical nuclear localization signal (cNLS: GACHAKKPRIT-182) and host cell importin (IMP)α and proposed it as a potential target for antiviral drug development. Here, we further extend on such findings. First, we demonstrate that NS1 nuclear localization is required for viral production since introducing the K177T substitution in a cloned, infectious viral genome resulted in a non-viable virus. Secondly, we demonstrate that the antiparasitic drug ivermectin (IVM), known to inhibit the IMPα/β dependent nuclear import pathway, could impair the NS1-NLS:IMPα interaction and suppress viral replication in UT7/EpoS1 cells in a dose-dependent manner. We also show that a panel of structurally related avermectins (AVMs) can dissociate the NS1-NLS:IMPα complex with half-maximal inhibitory concentrations in the nanomolar range. Among them, Eprinomectin emerged as the most selective inhibitor of B19V replication, with a selectivity index of c. 5.0. However, when tested in EPCs generated from peripheral blood mononuclear cells, which constitute a cellular population close to the natural target cells in bone marrow, the inhibitory effect of IVM and Eprinomectin was demonstrated to a lesser extent, and both compounds exhibited high toxicity, thus highlighting the need for more specific inhibitors of the NS1-NLS:IMPα interaction.

Given their role in viral polyprotein processing, viral proteases (PRs) are excellent targets for antiviral therapy. Most assays developed for screening PR inhibitors are in vitro assays, and therefore have several limitations, including the inability to account for cell permeability, toxicity and the need for compounds activation within cells. The development of cellular reporters overcoming these limitations is therefore highly desirable. In this study, we developed two different Bioluminescence Resonance Energy Transfer (BRET)-based reporters for Human Immunodeficiency virus-1 (HIV-1) PR, allowing the simultaneous monitoring of cell viability and HIV-1 PR activity. The reporters employ two different BRET pairs as donor and acceptor moieties: Renilla luciferase (RLuc) with Yellow Fluorescent Protein (YFP), and Nano luciferase (NLuc) with mNeonGreen (mNG), both linked by the HIV-1 p2/p7 cleavage site. While both reporters specifically detected HIV-1 protease activity, mNG-p2/p7-NLuc exhibited higher sensitivity, increased energy transfer and better spectral separation between donor and acceptor emissions, resulting in a significantly higher BRET ratio. mNG-p2/p7-NLuc was used to quantify the effect of a panel of protease inhibitors in living cells, assessing simultaneously cell viability and HIV-1 PR activity. Additionally, it was employed to measure the potency of well-known HIV-1 PR inhibitors. Together, these findings demonstrate the utility of the mNG-p2/p7-NLuc reporter as a cell-based tool for the evaluation of HIV-1 PR activity and inhibitor efficacy. Its dual-readout capability provides a valuable platform for antiviral drug screening in physiologically relevant conditions.

Ovine gammaherpesviruses 2 (OvGHV2), a member of the Macavirus genus within the Orthoherpesviridae family, causes lymphoproliferative diseases in susceptible species, most notably sheep-associated malignant catarrhal fever. Research on OvGHV2 has been hindered by the absence of a permissive cell culture system, limiting investigations into viral replication, entry mechanisms and cell tropism. This challenge constrains progress towards understanding OvGHV2 pathogenesis and developing effective vaccines or therapeutics. We investigated the molecular mechanisms underlying OvGHV2 infection, focusing on the nuclear trafficking pathways of the latency-associated nuclear antigen (LANA), encoded by ORF73. In other gammaherpesviruses, LANA is known to mediate viral episome maintenance, chromatin tethering and latency through its nuclear localization. We identified and functionally characterized a novel bipartite nuclear localization signal (NLS) within the C-terminal region of OvGHV2 LANA and elucidated its interactions with host nuclear import receptors. Using high-resolution crystallography and quantitative binding assays, we mapped the key residues responsible for binding to importin alpha (IMP α ) and demonstrated isoform-specific variations in binding affinity. Confocal microscopy revealed that the OvGHV2 LANA predominantly localizes to the nucleus through an IMP α / β 1-dependent pathway, as mutation or inhibition of the NLS significantly reduced nuclear accumulation. Interestingly, partial nuclear localization under these conditions suggests an additional IMP α / β 1-independent nuclear import mechanism. Collectively, our biochemical and structural analyses confirm that the identified NLS is essential for IMP α -mediated nuclear import of OvGHV2 LANA. These findings provide new insights into OvGHV2 host interactions and establish a molecular basis for developing targeted antiviral strategies against ovine gammaherpesvirus 2 infection.