Computational investigations on the dynamic binding effect of molecular tweezer CLR01 toward intrinsically disordered HIV-1 Nef.

Intrinsically disordered proteins (IDPs) are highly flexible molecules that undergo disorder to order transition through their interaction with other molecules. IDPs play a vital role in several biological processes ranging from molecular recognition to several human diseases through the protein-protein interaction. The dynamic flexibility of IDPs and their implications in several human diseases enable these molecules to serve as novel therapeutic targets. However, the challenging task is to develop novel drugs against IDPs because of their lack of stable structures and the nature of high conformational flexibility. In this study, we have calculated the dynamic binding effect of the supramolecular tweezer CLR01 against the intrinsically disordered HIV-1 Nef by employing molecular docking and dynamics simulation approaches. From docking results, we predicted the strong binding affinity of the tweezer with the target residues of Nef. The docking results were further validated from the molecular dynamics simulation studies confirming the conformational stability of Nef upon tweezer binding. These findings provide useful insights into the development of potent inhibitors for targeting Nef protein functions.

HIV-I Nef inhibitors: a novel class of HIV-specific immune adjuvants in support of a cure.

The success of many current vaccines relies on a formulation that incorporates an immune activating adjuvant. This will hold true for the design of a successful therapeutic HIV vaccine targeted at controlling reactivated virus following cessation of combined antiretroviral therapy (cART). The HIV accessory protein Nef functions by interfering with HIV antigen presentation through the major histocompatibility complex I (MHC-I) pathway thereby suppressing CD8+ cytotoxic T cell (CTL)-mediated killing of HIV infected cells. Thus, this important impediment to HIV vaccine success must be circumvented. This review covers our current knowledge of Nef inhibitors that may serve as immune adjuvants that will specifically restore and enhance CTL-mediated killing of reactivated HIV infected cells as part of an overall vaccine strategy to affect a cure for HIV infection.

Microglia-derived HIV Nef+ exosome impairment of the blood-brain barrier is treatable by nanomedicine-based delivery of Nef peptides.

The negative factor (Nef) of human immunodeficiency virus (HIV) is an accessory protein that is thought to be integral to HIV-associated immune- and neuroimmune pathogenesis. Here, we show that nef-transfected microglia-released Nef+ exosome (exNef) disrupts the apical blood-brain barrier (BBB) and that only nef-transfected microglia release Nef in exosomes. nef-gfp-transduced neurons and astrocytes release exosomes but did not release exNef in the extracellular space. Apical administration of exNef derived from nef-transfected 293T cells reduced transendothelial electrical resistance (TEER) and increased permeability of the BBB. Microglia-derived exNef applied to either the apical/basal BBB significantly reduced expression of the tight junction protein, ZO-1, suggesting a mechanism of exNef-mediated neuropathogenesis. Microglia exposed to exNef release elevated levels of Toll-like receptor-induced cytokines and chemokines IL-12, IL-8, IL-6, RANTES, and IL-17A. Magnetic nanoparticle delivery of Nef peptides containing the Nef myrisolation site across an in vitro BBB ultimately reduced nef-transfected microglia release of Nef exosomes and prevented the loss of BBB integrity and permeability as measured by TEER and dextran-FITC transport studies, respectively. Overall, we show that exNef is released from nef-gfp-transfected microglia; exNef disrupts integrity and permeability, and tight junctions of the BBB, and induces microglial cytokine/chemokine secretion. These exNef-mediated effects were significantly restricted by Nef peptides. Taken together, this study provides preliminary evidence of the role of exNef in HIV neuroimmune pathogenesis and the feasibility of a nanomedicine-based therapeutics targeting exNef to treat HIV-associated neuropathogenesis.

Synthesis and evaluation of orally active small molecule HIV-1 Nef antagonists.

The HIV-1 Nef accessory factor enhances viral replication and promotes immune system evasion of HIV-infected cells, making it an attractive target for drug discovery. Recently we described a novel class of diphenylpyrazolodiazene compounds that bind directly to Nef in vitro and inhibit Nef-dependent HIV-1 infectivity and replication in cell culture. However, these first-generation Nef antagonists have several structural liabilities, including an azo linkage that led to poor oral bioavailability. The azo group was therefore replaced with either a one- or two-carbon linker. The resulting set of non-azo analogs retained nanomolar binding affinity for Nef by surface plasmon resonance, while inhibiting HIV-1 replication with micromolar potency in cell-based assays without cytotoxicity. Computational docking studies show that these non-azo analogs occupy the same predicted binding site within the HIV-1 Nef dimer interface as the original azo compound. Computational methods also identified a hot spot for inhibitor binding within this site that is defined by conserved HIV-1 Nef residues Asp108, Leu112, and Pro122. Pharmacokinetic evaluation of the non-azo B9 analogs in mice showed that replacement of the azo linkage dramatically enhanced oral bioavailability without substantially affecting plasma half-life or clearance. The improved oral bioavailability of non-azo diphenylpyrazolo Nef antagonists provides a starting point for further drug lead optimization in support of future efficacy testing in animal models of HIV/AIDS.

Subtle Dynamic Changes Accompany Hck Activation by HIV-1 Nef and are Reversed by an Antiretroviral Kinase Inhibitor.

The HIV-1 virulence factor Nef interacts with the macrophage Src-family kinase Hck, resulting in constitutive kinase activation that contributes to viral replication and immune escape. Previous chemical library screens identified the diphenylfuranopyrimdine kinase inhibitor DFP-4AB, which selectively inhibits Nef-dependent Hck activity in biochemical assays and potently blocks HIV replication in vitro. In the present study, hydrogen exchange mass spectrometry (HX MS) was used to study conformational changes in downregulated Hck that result from Nef binding, as well as the impact of DFP-4AB on these changes. Remarkably, interaction with Nef induced only subtle changes in deuterium uptake by Hck, with the most significant changes in the N-lobe of the kinase domain adjacent to the docking site for Nef on the SH3 domain. No changes in hydrogen exchange were observed in the Hck SH2 domain or C-terminal tail, indicating that this regulatory interaction is unaffected by Nef binding. When HX MS was performed in the presence of DFP-4AB, the effect of Nef on Hck N-lobe dynamics was completely reversed. These results show that constitutive activation of Hck by HIV-1 Nef requires only modest changes to the conformational dynamics of the overall kinase structure. DFP-4AB reverses these effects, consistent with its activity against this Nef-induced signaling event in HIV-infected cells.

Dynamic features of apo and bound HIV-Nef protein reveal the anti-HIV dimerization inhibition mechanism.

The first account on the dynamic features of Nef or negative factor, a small myristoylated protein located in the cytoplasm believes to increase HIV-1 viral titer level, is reported herein. Due to its major role in HIV-1 pathogenicity, Nef protein is considered an emerging target in anti-HIV drug design and discovery process. In this study, comparative long-range all-atom molecular dynamics simulations were employed for apo and bound protein to unveil molecular mechanism of HIV-Nef dimerization and inhibition. Results clearly revealed that B9, a newly discovered Nef inhibitor, binds at the dimeric interface of Nef protein and caused significant separation between orthogonally opposed residues, namely Asp108, Leu112 and Gln104. Large differences in magnitudes were observed in the radius of gyration (∼1.5 Å), per-residue fluctuation (∼2 Å), C-alpha deviations (∼2 Å) which confirm a comparatively more flexible nature of apo conformation due to rapid dimeric association. Compared to the bound conformer, a more globally correlated motion in case of apo structure of HIV-Nef confirms the process of dimeric association. This clearly highlights the process of inhibition as a result of ligand binding. The difference in principal component analysis (PCA) scatter plot and per-residue mobility plot across first two normal modes further justifies the same findings. The in-depth dynamic analyses of Nef protein presented in this report would serve crucial in understanding its function and inhibition mechanisms. Information on inhibitor binding mode would also assist in designing of potential inhibitors against this important HIV target.

Discovery of a diaminoquinoxaline benzenesulfonamide antagonist of HIV-1 Nef function using a yeast-based phenotypic screen.

BACKGROUND: HIV-1 Nef is a viral accessory protein critical for AIDS progression. Nef lacks intrinsic catalytic activity and binds multiple host cell signaling proteins, including Hck and other Src-family tyrosine kinases. Nef binding induces constitutive Hck activation that may contribute to HIV pathogenesis by promoting viral infectivity, replication and downregulation of cell-surface MHC-I molecules. In this study, we developed a yeast-based phenotypic screen to identify small molecules that inhibit the Nef-Hck complex. RESULTS: Nef-Hck interaction was faithfully reconstituted in yeast cells, resulting in kinase activation and growth arrest. Yeast cells expressing the Nef-Hck complex were used to screen a library of small heterocyclic compounds for their ability to rescue growth inhibition. The screen identified a dihydrobenzo-1,4-dioxin-substituted analog of 2-quinoxalinyl-3-aminobenzene-sulfonamide (DQBS) as a potent inhibitor of Nef-dependent HIV-1 replication and MHC-I downregulation in T-cells. Docking studies predicted direct binding of DQBS to Nef which was confirmed in differential scanning fluorimetry assays with recombinant purified Nef protein. DQBS also potently inhibited the replication of HIV-1 NL4-3 chimeras expressing Nef alleles representative of all M-group HIV-1 clades. CONCLUSIONS: Our findings demonstrate the utility of a yeast-based growth reversion assay for the identification of small molecule Nef antagonists. Inhibitors of Nef function discovered with this assay, such as DQBS, may complement the activity of current antiretroviral therapies by enabling immune recognition of HIV-infected cells through the rescue of cell surface MHC-I.

Single-domain antibody-SH3 fusions for efficient neutralization of HIV-1 Nef functions.

HIV-1 Nef is essential for AIDS pathogenesis, but this viral protein is not targeted by antiviral strategies. The functions of Nef are largely related to perturbations of intracellular trafficking and signaling pathways through leucine-based and polyproline motifs that are required for interactions with clathrin-associated adaptor protein complexes and SH3 domain-containing proteins, such as the phagocyte-specific kinase Hck. We previously described a single-domain antibody (sdAb) targeting Nef and inhibiting many, but not all, of its biological activities. We now report a further development of this anti-Nef strategy through the demonstration of the remarkable inhibitory activity of artificial Nef ligands, called Neffins, comprised of the anti-Nef sdAb fused to modified SH3 domains. The Neffins inhibited all key activities of Nef, including Nef-mediated CD4 and major histocompatibility complex class I (MHC-I) cell surface downregulation and enhancement of virus infectivity. When expressed in T lymphocytes, Neffins specifically inhibited the Nef-induced mislocalization of the Lck kinase, which contributes to the alteration of the formation of the immunological synapse. In macrophages, Neffins inhibited the Nef-induced formation of multinucleated giant cells and podosome rosettes, and it counteracted the inhibitory activity of Nef on phagocytosis. Since we show here that these effects of Nef on macrophage and T cell functions were both dependent on the leucine-based and polyproline motifs, we confirmed that Neffins disrupted interactions of Nef with both AP complexes and Hck. These results demonstrate that it is possible to inhibit all functions of Nef, both in T lymphocytes and macrophages, with a single ligand that represents an efficient tool to develop new antiviral strategies targeting Nef.

Inhibition of the Nef regulatory protein of HIV-1 by a single-domain antibody.

The Nef protein of HIV-1 is important for AIDS pathogenesis, but it is not targeted by current antiviral strategies. Here, we describe a single-domain antibody (sdAb) that binds to HIV-1 Nef with a high affinity (K(d) = 2 × 10(-9)M) and inhibited critical biologic activities of Nef both in vitro and in vivo. First, it interfered with the CD4 down-regulation activity of a broad panel of nef alleles through inhibition of the Nef effects on CD4 internalization from the cell surface. Second, it was able to interfere with the association of Nef with the cellular p21-activated kinase 2 as well as with the resulting inhibitory effect of Nef on actin remodeling. Third, it counteracted the Nef-dependent enhancement of virion infectivity and inhibited the positive effect of Nef on virus replication in peripheral blood mononuclear cells. Fourth, anti-Nef sdAb rescued Nef-mediated thymic CD4(+) T-cell maturation defects and peripheral CD4(+) T-cell activation in the CD4C/HIV-1(Nef) transgenic mouse model. Because all these Nef functions have been implicated in Nef effects on pathogenesis, this anti-Nef sdAb may represent an efficient tool to elucidate the molecular functions of Nef in the virus life cycle and could now help to develop new strategies for the control of AIDS.

Small molecule inhibitors of the HIV-1 virulence factor, Nef.

Although antiretroviral therapy has revolutionized the clinical management of AIDS, life-long treatment is required because these drugs do not eradicate HIV- infected cells. Chronic antiretroviral therapy may not protect AIDS patients from cognitive impairment, raising important quality of life issues. Because of the rise of HIV strains resistant to current drugs and uncertain vaccine prospects, an urgent need exists for the discovery and development of new therapeutic approaches. This review is focused on one such approach, which involves targeting HIV-1 Nef, a viral accessory protein essential for AIDS pathogenesis.

A specific protein disorder catalyzer of HIV-1 Nef.

The HIV-1 auxiliary protein Nef is required for the onset and progression of AIDS in HIV-1-infected persons. Here, we have deciphered the mode of action of a second-generation inhibitor of Nef, DLC27-14, presenting a competitive IC(50) of ∼16 µM measured by MALDI-TOF experiments. Thermal protein denaturation experiments revealed a negative effect on stability of Nef in the presence of a saturating concentration of the inhibitor. The destabilizing action of DLC27-14 was confirmed by a HIV protease-based experiment, in which the protease sensitivity of DLC27-14-bound Nef was three times as high as that of apo Nef. The only compatible docking modes of action for DLC27-14 suggest that DLC27-14 promotes an opening of two α-helices that would destabilize the Nef core domain. DLC27-14 thus acts as a specific protein disorder catalyzer that destabilizes the folded conformation of the protein. Our results open novel avenues toward the development of next-generation Nef inhibitors.

Protein protein interaction inhibition (2P2I) combining high throughput and virtual screening: Application to the HIV-1 Nef protein.

Protein-protein recognition is the cornerstone of multiple cellular and pathological functions. Therefore, protein-protein interaction inhibition (2P2I) is endowed with great therapeutic potential despite the initial belief that 2P2I was refractory to small-molecule intervention. Improved knowledge of complex molecular binding surfaces has recently stimulated renewed interest for 2P2I, especially after identification of "hot spots" and first inhibitory compounds. However, the combination of target complexity and lack of starting compound has thwarted experimental results and created intellectual barriers. Here we combined virtual and experimental screening when no previously known inhibitors can be used as starting point in a structure-based research program that targets an SH3 binding surface of the HIV type I Nef protein. High-throughput docking and application of a pharmacophoric filter on one hand and search for analogy on the other hand identified drug-like compounds that were further confirmed to bind Nef in the micromolar range (isothermal titration calorimetry), to target the Nef SH3 binding surface (NMR experiments), and to efficiently compete for Nef-SH3 interactions (cell-based assay, GST pull-down). Initial identification of these compounds by virtual screening was validated by screening of the very same library of compounds in the cell-based assay, demonstrating that a significant enrichment factor was attained by the in silico screening. To our knowledge, our results identify the first set of drug-like compounds that functionally target the HIV-1 Nef SH3 binding surface and provide the basis for a powerful discovery process that should help to speed up 2P2I strategies and open avenues for new class of antiviral molecules.

Research Progress of HIV-1 Nef Inhibitors.

HIV-1 infection poses a major threat to the public health worldwide. The antiretroviral agents that are currently used to treat HIV-1 infection target viral reverse transcriptase, integrase and protease, or block the fusion of viral envelop and cell membrane. Studies have shown that the HIV-1 encoded protein Nef plays an important role in the pathogenesis of viral infection. Nef ensures efficient counterattack against host immune responses as well as long-term evasion of immune surveillance. In addition, Nef, expressing at a high level early in the viral life cycle, is required for maintaining a high viral load in the persistent infection in vivo and for full pathologic potential. Therefore, Nef may be an excellent target to treat HIV-1 infection. In this manuscript, we reviewed five potential Nef inhibitors, namely, DLC27-14, t ightly bound hydroxypyrazole HIV-1 Nef inhibitor B9, 2c-like inhibitors, N-(3-aminoquinoxalin-2-yl)-4-chlorobenzenesulfonamide and compound 1[(7-oxo-7H-benzo[anthracene]-3-yl)amino]anthraquinone, and their working mechanisms. These drugs may be further developed into new regimens for the treatment of HIV-1 infection.

Tight-Binding Hydroxypyrazole HIV-1 Nef Inhibitors Suppress Viral Replication in Donor Mononuclear Cells and Reverse Nef-Mediated MHC-I Downregulation.

The HIV-1 Nef accessory factor is critical to the viral life cycle in vivo and promotes immune escape of infected cells via downregulation of cell-surface MHC-I. Previously, we discovered small molecules that bind directly to Nef and block many of its functions, including enhancement of viral infectivity and replication in T cell lines. These compounds also restore cell-surface MHC-I expression in HIV-infected CD4 T cells from AIDS patients, enabling recognition and killing by autologous cytotoxic T lymphocytes (CTLs). In this study, we describe the synthesis and evaluation of a diverse set of analogs based on the original hydroxypyrazole Nef inhibitor core. All analogs were screened for the interaction with recombinant HIV-1 Nef by surface plasmon resonance (SPR) and for antiretroviral activity in TZM-bl reporter cells infected with HIV-1. Active analogs were ranked on the basis of an activity score that integrates three aspects of the SPR data (affinity, residence time, and extent of binding) with antiretroviral activity. The top scoring compounds bound tightly to Nef by SPR, with KD values in the low nM to pM range, and displayed very slow dissociation from their Nef target. These analogs also suppressed HIV-1 replication in donor peripheral blood mononuclear cells (PBMCs) with IC50 values in the 1-10 nM range without cytotoxicity, inhibited Nef-mediated IL-2-inducible tyrosine kinase (Itk) and hematopoietic cell kinase (Hck) activation, and rescued MHC-I downregulation in a Nef-transfected T cell line. The development of Nef inhibitors based on the structure-activity relationships defined here has promise as a new approach to antiretroviral therapy that includes a path to eradication of HIV-infected cells via the adaptive immune response.

Strategies for inhibiting function of HIV-1 accessory proteins: a necessary route to AIDS therapy?

The Human Immunodeficiency Virus (HIV) genome encodes three major structural proteins common to all retroviruses (Gag, Pol and Env), two regulatory proteins (Tat and Rev) that are essential for viral replication, and four accessory proteins (Nef, Vif, Vpu, Vpr). While accessory proteins were initially reported to be unnecessary for viral growth, their importance as virulence factors is now being more and more appreciated: they can dramatically alter the course and severity of viral infection, replication and disease progression. None of the HIV accessory proteins display enzymatic activity: they rather act altering cellular pathways via multiple protein-protein interactions with a number of host cell factors. All currently approved anti-HIV drugs target pol and env encoded proteins. Therefore, widening the molecular targets of HIV therapy by additionally targeting accessory proteins may expand treatment options, resulting in high impact effective new therapy. In this review we present the state of the art of compounds that target HIV accessory proteins. Most of the research has focused on the inhibition of specific accessory proteins/host cell partner interactions. Promising compounds have been found within different classes of molecules: small natural and synthetic molecules, peptides and proteins, oligonucleotides, in particular those used as RNA interference (RNAi) tools. With the assortment of compounds available, especially against Nef and Vif functions, the demonstration of the clinical efficacy of the new anti-HIV-1 drugs targeting accessory proteins is next challenge.

Modelling interaction between HIV-1 Nef and calnexin.

BACKGROUND: HIV-associated atherosclerosis is a major comorbidity due, in part, to systemic effects of the virus on cholesterol metabolism. HIV protein Nef plays an important role in this pathology by impairing maturation of the main cellular cholesterol transporter ATP-Binding Cassette (ABCA) 1. ABCA1 maturation critically depends on calnexin, an integral endoplasmic reticulum membrane chaperone, and Nef binds to the cytoplasmic domain of calnexin and impairs interaction of calnexin with ABCA1. Overarching goal of the present study was to model Nef-calnexin interaction interface, and identify small molecule compounds potentially inhibiting this interaction. METHODS: Molecular dynamics was utilized to build structure model of calnexin cytoplasmic domain, followed by global docking combined with application of QASDOM software developed by us for efficient analysis of receptor-ligand complexes. Structure-based virtual screening was performed for all sites identified by docking. A soluble analogue of a compound from the screening results list was tested for ability to down-regulate ABCA1. RESULTS: We identified major interaction sites in calnexin and reciprocal sites in Nef. Virtual screening yielded a number of small-molecule compounds potentially blocking a calnexin site. Interestingly, one of the compounds, NSC13987, was previously identified by us as an inhibitor targeting a Nef site. An analogue of NSC13987, AMS-55, potently reversed the negative effect of Nef on ABCA1 abundance. CONCLUSIONS: We have modelled Nef-calnexin interaction, predicted small molecule compounds that can potentially inhibit this interaction, and experimentally tested one of these compounds, confirming its effectiveness. These findings provide a platform for searching for new therapeutic agents to treat HIV-associated comorbidities.

Study of non-covalent interactions on dendriplex formation: Influence of hydrophobic, electrostatic and hydrogen bonds interactions.

The interaction of a double stranded small interference RNA (siRNA Nef) with cationic carbosilane dendrimers of generations 1-3 with two different ammonium functions at the periphery ([-NMe2R]+, R=Me, (CH2)2OH) has been studied by experimental techniques (zeta potential, electrophoresis, single molecule pulling experiments) and molecular dynamic calculations. These studies state the presence of different forces on dendriplex formation, depending on generation and type of ammonium group. Whilst for higher dendrimers electrostatic forces mainly drive the stability of dendriplexes, first generation compounds can penetrate into siRNA strands due to the establishment of hydrophobic interactions. Finally, in the particular case of first generation dendrimer [G1O3(NMe2(CH2)2OH))6]6+; the presence of hydroxyl groups reinforces dendriplex stability by hydrogen bonds formation. However, since these small dendrimers do not cover the RNA, only higher generation derivatives protect RNA from degradation.

Identification of Binding Mode and Prospective Structural Features of Novel Nef Protein Inhibitors as Potential Anti-HIV Drugs.

Human immunodeficiency virus (HIV)-negative factor (Nef) protein is an accessory pathogenic factor, which plays a significant role in acquired immune deficiency syndrome (AIDS). Nef deficient HIV virus took a longer time to progress into AIDS. Therefore, targeting Nef protein is considered as a key strategy towards HIV/AIDS treatment. Up-to-date, only few compounds were reported as Nef inhibitors. This has prompted us to provide a first account of an integrated computational framework in order to identify more potential Nef inhibitors. Herein, using a hybrid ligand (shape similarity and pharmacophore) and structure-(molecular docking) based virtual screening approaches combined with molecular dynamics as well as post dynamics analysis, potential new hits were identified as HIV-Nef inhibitors. The top ranked compounds of molecular docking from the shape similarity-based library (ZINC04177596, ∆ G bind= -28.7482 kcal/mol) and pharmacophore-based library (ZINC36617540, ∆ G bind= -20.2271 kcal/mol) possess comparatively better binding affinities than the reference molecule, B9 (∆ G bind = -18.0694 kcal/mol). Both these hits (ZINC04177596 and ZINC36617540) showed similar binding mode at the binding site as like the prototype, B9. Hydrophobic and electrostatic interactions seemed to be the prominent binding forces that hold these ligands at the dimer interface of Nef protein. Finally, a set of chemical structural features that can be used as a guide in the design of novel potential Nef inhibitors is also highlighted herein. We believe that the information gained from this study would be of great importance in the discovery and design of potential small molecules targeting HIV-Nef.

Development and validation of a high-content bimolecular fluorescence complementation assay for small-molecule inhibitors of HIV-1 Nef dimerization.

Nef is a human immunodeficiency virus 1 (HIV-1) accessory factor essential for viral pathogenesis and AIDS progression. Many Nef functions require dimerization, and small molecules that block Nef dimerization may represent antiretroviral drug leads. Here we describe a cell-based assay for Nef dimerization inhibitors based on bimolecular fluorescence complementation (BiFC). Nef was fused to nonfluorescent, complementary fragments of yellow fluorescent protein (YFP) and coexpressed in the same cell population. Dimerization of Nef resulted in juxtaposition of the YFP fragments and reconstitution of the fluorophore. For automation, the Nef-YFP fusion proteins plus a monomeric red fluorescent protein (mRFP) reporter were expressed from a single vector, separated by picornavirus "2A" linker peptides to permit equivalent translation of all three proteins. Validation studies revealed a critical role for gating on the mRFP-positive subpopulation of transfected cells, as well as use of the mRFP signal to normalize the Nef-BiFC signal. Nef-BiFC/mRFP ratios resulting from cells expressing wild-type versus dimerization-defective Nef were very clearly separated, with Z factors consistently in the 0.6 to 0.7 range. A fully automated pilot screen of the National Cancer Institute Diversity Set III identified several hit compounds that reproducibly blocked Nef dimerization in the low micromolar range. This BiFC-based assay has the potential to identify cell-active small molecules that directly interfere with Nef dimerization and function.

Characteristics of IFITM, the newly identified IFN-inducible anti-HIV-1 family proteins.

IFN-inducible IFITM proteins (IFITM1, 2, and 3) inhibit the replication of various viruses including HIV-1 through poorly understood mechanisms. Here, we further analyzed characteristics of these newly identified HIV-1 restriction factors. Firstly, in contrast to other anti-HIV-1 proteins, such as tetherin and APOBEC3G, IFITMs were resistant to a down-regulation of surface expression or degradation by HIV-1 proteins. Secondly, the enforced expression of IFITMs reduced the production of HIV-1 viruses from cells transfected with proviral plasmids containing whole viral sequences. Although their inhibitory activities were modest when compared to that of tetherin, IFITMs, but not tetherin, directly reduced the expression of HIV-1 proteins including Gag, Vif and Nef. Of importance, however, IFITMs had no inhibitory effect when these viral proteins were expressed by codon-optimized cDNAs that bypassed the viral-specific expression machinery. Indeed, our results supported the idea that IFITMs interfere with viral protein expression mediated by double-stranded viral RNAs, such as RRE and TAR. Finally, the S-palmitoylation of IFITMs, which is crucial for their anti-influenza virus activity, was not required for their anti-HIV-1 activity, indicating that IFITMs restrict these viruses at different steps. These characteristics lead to a better understanding of the mechanism by which IFITMs restrict HIV-1 and other viruses.