Human immunodeficiency virus 1 (HIV-1) viral protease (PR) is one of the most studied viral enzymes and a crucial antiviral target. Despite its well-characterized role in virion maturation, an increasing body of research is starting to focus on its ability to cleave host cell proteins. Such findings are apparently in contrast with the dogma of HIV-1 PR activity being restricted to the interior of nascent virions and suggest catalytic activity within the host cell environment. Given the limited amount of PR present in the virion at the time of infection, such events mainly occur during late viral gene expression, mediated by newly synthesized Gag-Pol polyprotein precursors, rather than before proviral integration. HIV-1 PR mainly targets proteins involved in three different processes: those involved in translation, those controlling cell survival, and restriction factors responsible for innate/intrinsic antiviral responses. Indeed, by cleaving host cell translation initiation factors, HIV-1 PR can impair cap-dependent translation, thus promoting IRES-mediated translation of late viral transcripts and viral production. By targeting several apoptotic factors, it modulates cell survival, thus promoting immune evasion and viral dissemination. Additionally, HIV-1 PR counteracts restriction factors incorporated in the virion that would otherwise interfere with nascent virus vitality. Thus, HIV-1 PR appears to modulate host cell function at different times and locations during its life cycle, thereby ensuring efficient viral persistency and propagation. However, we are far from having a complete picture of PR-mediated host cell modulation, which is emerging as a field that needs further investigation.

The human immunodeficiency virus 1 (HIV-1) viral protease (PR) is one of the most studied viral enzymes, and approval of drugs targeting its catalytic activity opened the door to the develop-ment of highly active antiretroviral therapy (HAART). Despite the fact that its crucial role in viri-on maturation is well characterized, an increasing body of research is starting to focus on its abil-ity to cleave host cell proteins, based on recent advances in proteomics and genomics technologies. Such findings are apparently in contrast with the dogma of HIV-1 PR activity being restricted to the interior of nascent virions, and suggest catalytic activity within the host cell environment. Given the limited amount of PR present in the virion at the time of infection, it is tempting to specu-late that such events mainly occur during viral late gene expression, mediated by newly synthe-sized Gag-Pol polyprotein precursors, rather than at a very early stage of infection, before pro-viral integration. Among cellular targets of HIV-1 PR, three major clusters can be identified: pro-teins involved in viral and cellular translation, those controlling cell survival, and restriction fac-tors responsible for innate/intrinsic antiviral responses. Indeed, by cleaving host cell translation initiation factors HIV-1 PR can impair cap-dependent translation, thus promoting IRES-mediated translation of late viral transcripts and viral production, while by targeting several apoptotic fac-tors it modulates cell survival, thus promoting immune evasion and viral dissemination. Addi-tionally, HIV-1 PR counteracts restriction factors incorporated in the virion that would otherwise interfere with nascent virus vitality. Thus, HIV-1 PR appears to modulate host cell function at dif-ferent times and locations during its life cycle, to ensure efficient viral persistency and propaga-tion. This kind of PR-mediated host cell modulation is found in a plethora of different viruses and HIV-1 is no exception, and although we are far from having a complete picture, it is clear that the PR has a multifaceted role in interfering with host machineries to better suit viral replication, and is a field that needs to be explored further.

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.