RESUMEN
Given the projected increase in multidrug-resistant HIV-1, there is an urgent need for development of antiretrovirals that act on virus life cycle stages not targeted by drugs currently in use. Host-targeting compounds are of particular interest because they can offer a high barrier to resistance. Here, we report identification of two related small molecules that inhibit HIV-1 late events, a part of the HIV-1 life cycle for which potent and specific inhibitors are lacking. This chemotype was discovered using cell-free protein synthesis and assembly systems that recapitulate intracellular host-catalyzed viral capsid assembly pathways. These compounds inhibit replication of HIV-1 in human T cell lines and peripheral blood mononuclear cells, and are effective against a primary isolate. They reduce virus production, likely by inhibiting a posttranslational step in HIV-1 Gag assembly. Notably, the compound colocalizes with HIV-1 Gag in situ; however, unexpectedly, selection experiments failed to identify compound-specific resistance mutations in gag or pol, even though known resistance mutations developed upon parallel nelfinavir selection. Thus, we hypothesized that instead of binding to Gag directly, these compounds localize to assembly intermediates, the intracellular multiprotein complexes containing Gag and host factors that form during immature HIV-1 capsid assembly. Indeed, imaging of infected cells shows compound colocalized with two host enzymes found in assembly intermediates, ABCE1 and DDX6, but not two host proteins found in other complexes. While the exact target and mechanism of action of this chemotype remain to be determined, our findings suggest that these compounds represent first-in-class, host-targeting inhibitors of intracellular events in HIV-1 assembly.IMPORTANCE The success of antiretroviral treatment for HIV-1 is at risk of being undermined by the growing problem of drug resistance. Thus, there is a need to identify antiretrovirals that act on viral life cycle stages not targeted by drugs in use, such as the events of HIV-1 Gag assembly. To address this gap, we developed a compound screen that recapitulates the intracellular events of HIV-1 assembly, including virus-host interactions that promote assembly. This effort led to the identification of a new chemotype that inhibits HIV-1 replication at nanomolar concentrations, likely by acting on assembly. This compound colocalized with Gag and two host enzymes that facilitate capsid assembly. However, resistance selection did not result in compound-specific mutations in gag, suggesting that the chemotype does not directly target Gag. We hypothesize that this chemotype represents a first-in-class inhibitor of virus production that acts by targeting a virus-host complex important for HIV-1 Gag assembly.
Asunto(s)
Antirretrovirales/farmacología , Infecciones por VIH/tratamiento farmacológico , VIH-1/efectos de los fármacos , Leucocitos Mononucleares/efectos de los fármacos , Bibliotecas de Moléculas Pequeñas/farmacología , Ensamble de Virus/efectos de los fármacos , Transportadoras de Casetes de Unión a ATP/metabolismo , ARN Helicasas DEAD-box/metabolismo , Infecciones por VIH/patología , Infecciones por VIH/virología , Humanos , Leucocitos Mononucleares/virología , Proteínas Proto-Oncogénicas/metabolismo , Productos del Gen gag del Virus de la Inmunodeficiencia Humana/metabolismoRESUMEN
During immature capsid assembly, HIV-1 genome packaging is initiated when Gag first associates with unspliced HIV-1 RNA by a poorly understood process. Previously, we defined a pathway of sequential intracellular HIV-1 capsid assembly intermediates; here we sought to identify the intermediate in which HIV-1 Gag first associates with unspliced HIV-1 RNA. In provirus-expressing cells, unspliced HIV-1 RNA was not found in the soluble fraction of the cytosol, but instead was largely in complexes ≥30S. We did not detect unspliced HIV-1 RNA associated with Gag in the first assembly intermediate, which consists of soluble Gag. Instead, the earliest assembly intermediate in which we detected Gag associated with unspliced HIV-1 RNA was the second assembly intermediate (~80S intermediate), which is derived from a host RNA granule containing two cellular facilitators of assembly, ABCE1 and the RNA granule protein DDX6. At steady-state, this RNA-granule-derived ~80S complex was the smallest assembly intermediate that contained Gag associated with unspliced viral RNA, regardless of whether lysates contained intact or disrupted ribosomes, or expressed WT or assembly-defective Gag. A similar complex was identified in HIV-1-infected T cells. RNA-granule-derived assembly intermediates were detected in situ as sites of Gag colocalization with ABCE1 and DDX6; moreover these granules were far more numerous and smaller than well-studied RNA granules termed P bodies. Finally, we identified two steps that lead to association of assembling Gag with unspliced HIV-1 RNA. Independent of viral-RNA-binding, Gag associates with a broad class of RNA granules that largely lacks unspliced viral RNA (step 1). If a viral-RNA-binding domain is present, Gag further localizes to a subset of these granules that contains unspliced viral RNA (step 2). Thus, our data raise the possibility that HIV-1 packaging is initiated not by soluble Gag, but by Gag targeted to a subset of host RNA granules containing unspliced HIV-1 RNA.
Asunto(s)
Infecciones por VIH/virología , VIH-1/genética , Empalme del ARN , ARN Viral/metabolismo , Ensamble de Virus , Productos del Gen gag del Virus de la Inmunodeficiencia Humana/metabolismo , Transportadoras de Casetes de Unión a ATP/genética , Transportadoras de Casetes de Unión a ATP/metabolismo , Animales , Células COS , Membrana Celular/metabolismo , Chlorocebus aethiops , Citoplasma/metabolismo , ARN Helicasas DEAD-box/genética , ARN Helicasas DEAD-box/metabolismo , Infecciones por VIH/genética , Humanos , Proteínas Proto-Oncogénicas/genética , Proteínas Proto-Oncogénicas/metabolismo , ARN Viral/genética , Virión , Productos del Gen gag del Virus de la Inmunodeficiencia Humana/genéticaRESUMEN
During immature capsid assembly in cells, human immunodeficiency virus type 1 (HIV-1) Gag co-opts a host RNA granule, forming a pathway of intracellular assembly intermediates containing host components, including two cellular facilitators of assembly, ABCE1 and DDX6. A similar assembly pathway has been observed for other primate lentiviruses. Here we asked whether feline immunodeficiency virus (FIV), a nonprimate lentivirus, also forms RNA granule-derived capsid assembly intermediates. First, we showed that the released FIV immature capsid and a large FIV Gag-containing intracellular complex are unstable during analysis, unlike for HIV-1. We identified harvest conditions, including in situ cross-linking, that overcame this problem, revealing a series of FIV Gag-containing complexes corresponding in size to HIV-1 assembly intermediates. Previously, we showed that assembly-defective HIV-1 Gag mutants are arrested at specific assembly intermediates; here we identified four assembly-defective FIV Gag mutants, including three not previously studied, and demonstrated that they appear to be arrested at the same intermediate as the cognate HIV-1 mutants. Further evidence that these FIV Gag-containing complexes correspond to assembly intermediates came from coimmunoprecipitations demonstrating that endogenous ABCE1 and the RNA granule protein DDX6 are associated with FIV Gag, as shown previously for HIV-1 Gag, but are not associated with a ribosomal protein, at steady state. Additionally, we showed that FIV Gag associates with another RNA granule protein, DCP2. Finally, we validated the FIV Gag-ABCE1 and FIV Gag-DCP2 interactions with proximity ligation assays demonstrating colocalization in situ Together, these data support a model in which primate and nonprimate lentiviruses form intracellular capsid assembly intermediates derived from nontranslating host RNA granules.IMPORTANCE Like HIV-1 Gag, FIV Gag assembles into immature capsids; however, it is not known whether FIV Gag progresses through a pathway of immature capsid assembly intermediates derived from host RNA granules, as shown for HIV-1 Gag. Here we showed that FIV Gag forms complexes that resemble HIV-1 capsid assembly intermediates in size and in their association with ABCE1 and DDX6, two host facilitators of HIV-1 immature capsid assembly that are found in HIV-1 assembly intermediates. Our studies also showed that known and novel assembly-defective FIV Gag mutants fail to progress past putative intermediates in a pattern resembling that observed for HIV-1 Gag mutants. Finally, we used imaging to demonstrate colocalization of FIV Gag with ABCE1 and with the RNA granule protein DCP2. Thus, we conclude that formation of assembly intermediates derived from host RNA granules is likely conserved between primate and nonprimate lentiviruses and could provide targets for future antiviral strategies.
Asunto(s)
Proteínas de la Cápside/metabolismo , Cápside/metabolismo , Productos del Gen gag/genética , VIH-1/metabolismo , Virus de la Inmunodeficiencia Felina/metabolismo , Ensamble de Virus/genética , Transportadoras de Casetes de Unión a ATP/metabolismo , Animales , Células COS , Proteínas de la Cápside/genética , Gatos , Línea Celular , Chlorocebus aethiops , ARN Helicasas DEAD-box/metabolismo , Endorribonucleasas/metabolismo , VIH-1/genética , Virus de la Inmunodeficiencia Felina/genética , Proteínas de Unión al ARN/biosíntesisRESUMEN
UNLABELLED: The major homology region (MHR) is a highly conserved motif that is found within the Gag protein of all orthoretroviruses and some retrotransposons. While it is widely accepted that the MHR is critical for assembly of HIV-1 and other retroviruses, how the MHR functions and why it is so highly conserved are not understood. Moreover, consensus is lacking on when HIV-1 MHR residues function during assembly. Here, we first addressed previous conflicting reports by confirming that MHR deletion, like conserved MHR residue substitution, leads to a dramatic reduction in particle production in human and nonhuman primate cells expressing HIV-1 proviruses. Next, we used biochemical analyses and immunoelectron microscopy to demonstrate that conserved residues in the MHR are required after assembling Gag has associated with genomic RNA, recruited critical host factors involved in assembly, and targeted to the plasma membrane. The exact point of inhibition at the plasma membrane differed depending on the specific mutation, with one MHR mutant arrested as a membrane-associated intermediate that is stable upon high-salt treatment and other MHR mutants arrested as labile, membrane-associated intermediates. Finally, we observed the same assembly-defective phenotypes when the MHR deletion or conserved MHR residue substitutions were engineered into Gag from a subtype B, lab-adapted provirus or Gag from a subtype C primary isolate that was codon optimized. Together, our data support a model in which MHR residues act just after membrane targeting, with some MHR residues promoting stability and another promoting multimerization of the membrane-targeted assembling Gag oligomer. IMPORTANCE: The retroviral Gag protein exhibits extensive amino acid sequence variation overall; however, one region of Gag, termed the major homology region, is conserved among all retroviruses and even some yeast retrotransposons, although the reason for this conservation remains poorly understood. Highly conserved residues in the major homology region are required for assembly of retroviruses; however, when these residues are required during assembly is not clear. Here, we used biochemical and electron microscopic analyses to demonstrate that these conserved residues function after assembling HIV-1 Gag has associated with genomic RNA, recruited critical host factors involved in assembly, and targeted to the plasma membrane but before Gag has completed the assembly process. By revealing precisely when conserved residues in the major homology region are required during assembly, these studies resolve existing controversies and set the stage for future experiments aimed at a more complete understanding of how the major homology region functions.
Asunto(s)
VIH-1/fisiología , Ensamble de Virus , Productos del Gen gag del Virus de la Inmunodeficiencia Humana/genética , Productos del Gen gag del Virus de la Inmunodeficiencia Humana/metabolismo , Animales , Línea Celular , Membrana Celular/metabolismo , Membrana Celular/virología , VIH-1/genética , Interacciones Huésped-Patógeno , Humanos , Modelos Biológicos , Mutación Missense , Unión Proteica , Multimerización de Proteína , Eliminación de SecuenciaRESUMEN
We present an unconventional approach to antiviral drug discovery, which is used to identify potent small molecules against rabies virus. First, we conceptualized viral capsid assembly as occurring via a host-catalyzed biochemical pathway, in contrast to the classical view of capsid formation by self-assembly. This suggested opportunities for antiviral intervention by targeting previously unappreciated catalytic host proteins, which were pursued. Second, we hypothesized these host proteins to be components of heterogeneous, labile, and dynamic multi-subunit assembly machines, not easily isolated by specific target protein-focused methods. This suggested the need to identify active compounds before knowing the precise protein target. A cell-free translation-based small molecule screen was established to recreate the hypothesized interactions involving newly synthesized capsid proteins as host assembly machine substrates. Hits from the screen were validated by efficacy against infectious rabies virus in mammalian cell culture. Used as affinity ligands, advanced analogs were shown to bind a set of proteins that effectively reconstituted drug sensitivity in the cell-free screen and included a small but discrete subfraction of cellular ATP-binding cassette family E1 (ABCE1), a host protein previously found essential for HIV capsid formation. Taken together, these studies advance an alternate view of capsid formation (as a host-catalyzed biochemical pathway), a different paradigm for drug discovery (whole pathway screening without knowledge of the target), and suggest the existence of labile assembly machines that can be rendered accessible as next-generation drug targets by the means described.
Asunto(s)
Antivirales/farmacología , Interacciones Huésped-Patógeno/efectos de los fármacos , Virus de la Rabia/efectos de los fármacos , Virus de la Rabia/fisiología , Proteínas Virales/fisiología , Secuencia de Aminoácidos , Animales , Sistema Libre de Células , Chlorocebus aethiops , Descubrimiento de Drogas , Interacciones Huésped-Patógeno/fisiología , Humanos , Pruebas de Sensibilidad Microbiana , Datos de Secuencia Molecular , Proteínas de la Nucleocápside/química , Proteínas de la Nucleocápside/genética , Proteínas de la Nucleocápside/fisiología , Dominios y Motivos de Interacción de Proteínas , Virus de la Rabia/genética , Células Vero , Proteínas Virales/química , Proteínas Virales/genética , Ensamble de Virus/efectos de los fármacosRESUMEN
UNLABELLED: During HIV-1 assembly, Gag polypeptides target to the plasma membrane, where they multimerize to form immature capsids that undergo budding and maturation. Previous mutational analyses identified residues within the Gag matrix (MA) and capsid (CA) domains that are required for immature capsid assembly, and structural studies showed that these residues are clustered on four exposed surfaces in Gag. Exactly when and where the three critical surfaces in CA function during assembly are not known. Here, we analyzed how mutations in these four critical surfaces affect the formation and stability of assembly intermediates in cells expressing the HIV-1 provirus. The resulting temporospatial map reveals that critical MA residues act during membrane targeting, residues in the C-terminal CA subdomain (CA-CTD) dimer interface are needed for the stability of the first membrane-bound assembly intermediate, CA-CTD base residues are necessary for progression past the first membrane-bound intermediate, and residues in the N-terminal CA subdomain (CA-NTD) stabilize the last membrane-bound intermediate. Importantly, we found that all four critical surfaces act while Gag is associated with the cellular facilitators of assembly ABCE1 and DDX6. When correlated with existing structural data, our findings suggest the following model: Gag dimerizes via the CA-CTD dimer interface just before or during membrane targeting, individual CA-CTD hexamers form soon after membrane targeting, and the CA-NTD hexameric lattice forms just prior to capsid release. This model adds an important new dimension to current structural models by proposing the potential order in which key contacts within the immature capsid lattice are made during assembly in cells. IMPORTANCE: While much is known about the structure of the completed HIV-1 immature capsid and domains of its component Gag proteins, less is known about the sequence of events leading to formation of the HIV-1 immature capsid. Here we used biochemical and ultrastructural analyses to generate a temporospatial map showing the precise order in which four critical surfaces in Gag act during immature capsid formation in provirus-expressing cells. Because three of these surfaces make important contacts in the hexameric lattices that are found in the completed immature capsid, these data allow us to propose a model for the sequence of events leading to formation of the hexameric lattices. By providing a dynamic view of when and where critical Gag-Gag contacts form during the assembly process and how those contacts function in the nascent capsid, our study provides novel insights into how an immature capsid is built in infected cells.
Asunto(s)
Antígenos VIH/metabolismo , Proteína p24 del Núcleo del VIH/metabolismo , VIH-1/fisiología , Ensamble de Virus , Productos del Gen gag del Virus de la Inmunodeficiencia Humana/metabolismo , Transportadoras de Casetes de Unión a ATP/metabolismo , Cápside/metabolismo , ARN Helicasas DEAD-box/metabolismo , Análisis Mutacional de ADN , Antígenos VIH/genética , Proteína p24 del Núcleo del VIH/genética , VIH-1/genética , Modelos Biológicos , Proteínas Mutantes/genética , Proteínas Mutantes/metabolismo , Unión Proteica , Proteínas Proto-Oncogénicas/metabolismo , Factores de Tiempo , Productos del Gen gag del Virus de la Inmunodeficiencia Humana/genéticaRESUMEN
The cervical sympathetic trunks (CSTs) contain axons of preganglionic neurons that innervate the superior cervical ganglia (SCGs). Because regeneration of CST fibers can be extensive and can reestablish certain specific patterns of SCG connections, restoration of end organ function would be expected. This expectation was examined with respect to the pineal gland, an organ innervated by the two SCGs. The activity of pineal serotonin N-acetyltransferase (NAT) exhibits a large circadian rhythm that is dependent on the sympathetic input of the gland, with high activity at night. Thirty-six hours after the CSTs were crushed bilaterally, nocturnal NAT was decreased by 99%. Three months later, enzyme activity had recovered only to 15% of control values, a recovery dependent on regeneration of CST fibers. Nevertheless, a small day/night rhythm was present in lesioned animals. Neither the density of the adrenergic innervation of the gland nor the ability of an adrenergic agonist to stimulate NAT activity was reduced in rats with regenerated CSTs. In addition, stimulation of the regenerated CST at a variety of frequencies was at least as effective in increasing NAT activity as seen with control nerves. These data suggest that the failure of pineal function to recover is not attributable to a quantitative deficit in the extent of reinnervation or synaptic efficacy. Rather, we suggest that there is some loss of specificity in the synaptic connections made in the SCG during reinnervation, resulting in a loss of the central neuronal information necessary for directing a normal NAT rhythm and thus normal pineal function.
Asunto(s)
Enfermedades del Sistema Nervioso Autónomo/patología , Enfermedades del Sistema Nervioso Autónomo/fisiopatología , Axones/patología , Regeneración Nerviosa/fisiología , Glándula Pineal/fisiopatología , Ganglio Cervical Superior/patología , Animales , N-Acetiltransferasa de Arilalquilamina/metabolismo , Axones/efectos de los fármacos , Biofisica , Brocresina/farmacología , Ritmo Circadiano/fisiología , Modelos Animales de Enfermedad , Relación Dosis-Respuesta a Droga , Estimulación Eléctrica , Inhibidores Enzimáticos/farmacología , Regulación Enzimológica de la Expresión Génica/fisiología , Isoproterenol/farmacología , Masculino , Nordefrin/farmacología , Glándula Pineal/metabolismo , Ratas , Ratas Sprague-Dawley , Ganglio Cervical Superior/efectos de los fármacos , Simpatomiméticos/farmacología , Factores de Tiempo , Tirosina 3-MonooxigenasaRESUMEN
We present a novel small molecule antiviral chemotype that was identified by an unconventional cell-free protein synthesis and assembly-based phenotypic screen for modulation of viral capsid assembly. Activity of PAV-431, a representative compound from the series, has been validated against infectious viruses in multiple cell culture models for all six families of viruses causing most respiratory diseases in humans. In animals, this chemotype has been demonstrated efficacious for porcine epidemic diarrhoea virus (a coronavirus) and respiratory syncytial virus (a paramyxovirus). PAV-431 is shown to bind to the protein 14-3-3, a known allosteric modulator. However, it only appears to target the small subset of 14-3-3 which is present in a dynamic multi-protein complex whose components include proteins implicated in viral life cycles and in innate immunity. The composition of this target multi-protein complex appears to be modified upon viral infection and largely restored by PAV-431 treatment. An advanced analog, PAV-104, is shown to be selective for the virally modified target, thereby avoiding host toxicity. Our findings suggest a new paradigm for understanding, and drugging, the host-virus interface, which leads to a new clinical therapeutic strategy for treatment of respiratory viral disease.
Asunto(s)
Antivirales , Antivirales/farmacología , Antivirales/química , Humanos , Animales , Proteínas 14-3-3/metabolismo , Complejos Multiproteicos/metabolismo , Interacciones Huésped-Patógeno/efectos de los fármacos , Línea CelularRESUMEN
During HIV-1 assembly, Gag polypeptides multimerize to form an immature capsid and also package HIV-1 genomic RNA. Assembling Gag forms immature capsids by progressing through a stepwise pathway of assembly intermediates containing the cellular ATPase ABCE1, which facilitates capsid formation. The NC domain of Gag is required for ABCE1 binding, acting either directly or indirectly. NC is also critical for Gag multimerization and RNA binding. Previous studies of GagZip chimeric proteins in which NC was replaced with a heterologous leucine zipper that promotes protein dimerization but not RNA binding established that the RNA binding properties of NC are dispensable for capsid formation per se. Here we utilized GagZip proteins to address the question of whether the RNA binding properties of NC are required for ABCE1 binding and for the formation of ABCE1-containing capsid assembly intermediates. We found that assembly-competent HIV-1 GagZip proteins formed ABCE1-containing intermediates, while assembly-incompetent HIV-1 GagZip proteins harboring mutations in residues critical for leucine zipper dimerization did not. Thus, these data suggest that ABCE1 does not bind to NC directly or through an RNA bridge, and they support a model in which dimerization of Gag, mediated by NC or a zipper, results in exposure of an ABCE1-binding domain located elsewhere in Gag, outside NC. Additionally, we demonstrated that immature capsids formed by GagZip proteins are insensitive to RNase A, as expected. However, unexpectedly, immature HIV-1 capsids were almost as insensitive to RNase A as GagZip capsids, suggesting that RNA is not a structural element holding together immature wild-type HIV-1 capsids.
Asunto(s)
Cápside , Productos del Gen gag/metabolismo , VIH-1/fisiología , Leucina Zippers , Ribonucleasas/metabolismo , Secuencia de Aminoácidos , Productos del Gen gag/química , Datos de Secuencia Molecular , Homología de Secuencia de AminoácidoRESUMEN
We present a small molecule chemotype, identified by an orthogonal drug screen, exhibiting nanomolar activity against members of all the six viral families causing most human respiratory viral disease, with a demonstrated barrier to resistance development. Antiviral activity is shown in mammalian cells, including human primary bronchial epithelial cells cultured to an air-liquid interface and infected with SARS-CoV-2. In animals, efficacy of early compounds in the lead series is shown by survival (for a coronavirus) and viral load (for a paramyxovirus). The drug target is shown to include a subset of the protein 14-3-3 within a transient host multi-protein complex containing components implicated in viral lifecycles and in innate immunity. This multi-protein complex is modified upon viral infection and largely restored by drug treatment. Our findings suggest a new clinical therapeutic strategy for early treatment upon upper respiratory viral infection to prevent progression to lower respiratory tract or systemic disease. One Sentence Summary: A host-targeted drug to treat all respiratory viruses without viral resistance development.
RESUMEN
In HIV-1-infected individuals, G-to-A hypermutation is found in HIV-1 DNA isolated from peripheral blood mononuclear cells (PBMCs). These mutations are thought to result from editing by one or more host enzymes in the APOBEC3 (A3) family of cytidine deaminases, which act on CC (APOBEC3G) and TC (other A3 proteins) dinucleotide motifs in DNA (edited cytidine underlined). Although many A3 proteins display high levels of deaminase activity in model systems, only low levels of A3 deaminase activity have been found in primary cells examined to date. In contrast, here we report high levels of deaminase activity at TC motifs when whole PBMCs or isolated primary monocyte-derived cells were treated with interferon-alpha (IFNalpha) or IFNalpha-inducing toll-like receptor ligands. Induction of TC-specific deaminase activity required new transcription and translation and correlated with the appearance of two APOBEC3A (A3A) isoforms. Knockdown of A3A in monocytes with siRNA abolished TC-specific deaminase activity, confirming that A3A isoforms are responsible for all TC-specific deaminase activity observed. Both A3A isoforms appear to be enzymatically active; moreover, our mutational studies raise the possibility that the smaller isoform results from internal translational initiation. In contrast to the high levels of TC-specific activity observed in IFNalpha-treated monocytes, CC-specific activity remained low in PBMCs, suggesting that A3G deaminase activity is relatively inhibited, unlike that of A3A. Together, these findings suggest that deaminase activity of A3A isoforms in monocytes and macrophages may play an important role in host defense against viruses.
Asunto(s)
Citidina Desaminasa/metabolismo , Regulación Enzimológica de la Expresión Génica/inmunología , Inmunidad Innata/genética , Monocitos/enzimología , Monocitos/inmunología , Proteínas/metabolismo , Secuencias Repetitivas de Ácidos Nucleicos , Transducción de Señal/genética , Desaminasa APOBEC-3G , Secuencia de Aminoácidos , Animales , Secuencia de Bases , Bovinos , Línea Celular , Citidina Desaminasa/química , Citidina Desaminasa/deficiencia , Citidina Desaminasa/genética , Técnicas de Silenciamiento del Gen , Humanos , Interferón-alfa/metabolismo , Macrófagos/citología , Macrófagos/enzimología , Macrófagos/inmunología , Macrófagos/metabolismo , Datos de Secuencia Molecular , Monocitos/citología , Monocitos/metabolismo , Biosíntesis de Proteínas/inmunología , Proteínas/química , Proteínas/genética , ARN Interferente Pequeño/genética , Especificidad por Sustrato , Receptores Toll-Like/metabolismo , Transcripción Genética/inmunologíaRESUMEN
The concerning increase in HIV-1 resistance argues for prioritizing the development of host-targeting antiviral drugs because such drugs can offer high genetic barriers to the selection of drug-resistant viral variants. Targeting host proteins could also yield drugs that act on viral life cycle events that have proven elusive to inhibition, such as intracellular events of HIV-1 immature capsid assembly. Here, we review small molecule inhibitors identified primarily through HIV-1 self-assembly screens and describe how all act either narrowly post-entry or broadly on early and late events of the HIV-1 life cycle. We propose that a different screening approach could identify compounds that specifically inhibit HIV-1 Gag assembly, as was observed when a potent rabies virus inhibitor was identified using a host-catalyzed rabies assembly screen. As an example of this possibility, we discuss an antiretroviral small molecule recently identified using a screen that recapitulates the host-catalyzed HIV-1 capsid assembly pathway. This chemotype potently blocks HIV-1 replication in T cells by specifically inhibiting immature HIV-1 capsid assembly but fails to select for resistant viral variants over 37 passages, suggesting a host protein target. Development of such small molecules could yield novel host-targeting antiretroviral drugs and provide insight into chronic diseases resulting from dysregulation of host machinery targeted by these drugs.
Asunto(s)
Antirretrovirales/farmacología , Farmacorresistencia Viral , VIH-1/efectos de los fármacos , Interacciones Microbiota-Huesped/efectos de los fármacos , Ensamble de Virus/efectos de los fármacos , Antirretrovirales/aislamiento & purificación , Cápside/metabolismo , Seropositividad para VIH , VIH-1/fisiología , Humanos , Linfocitos T/efectos de los fármacos , Linfocitos T/virologíaRESUMEN
The deoxycytidine deaminase APOBEC3G (A3G) is expressed in human T cells and inhibits HIV-1 replication. When transfected into A3G-deficient epithelial cell lines, A3G induces catastrophic hypermutation by deaminating the HIV-1 genome. Interestingly, studies suggest that endogenous A3G in T cells induces less hypermutation than would be expected. However, to date, the specific deaminase activity of endogenous A3G in human CD4+ T cells has not been examined directly. Here, we compared deaminase activity of endogenous and exogenous A3G in various human cell lines using a standard assay and a novel, quantitative, high-throughput assay. Exogenous A3G in epithelial cell lysates displayed deaminase activity only following RNase treatment, as expected given that A3G is known to form an enzymatically inactive RNA-containing complex. Surprisingly, comparable amounts of endogenous A3G from T cell lines or from resting or activated primary CD4+ T cells exhibited minimal deaminase activity, despite RNase treatment. Specific deaminase activity of endogenous A3G in H9, CEM, and other T cell lines was up to 36-fold lower than specific activity of exogenous A3G in epithelial-derived cell lines. Furthermore, RNase-treated T cell lysates conferred a dose-dependent inhibition to epithelial cell lysates expressing enzymatically active A3G. These studies suggest that T cells, unlike epithelial-derived cell lines, express an unidentified RNase-resistant factor that inhibits A3G deaminase activity. This factor could be responsible for reduced levels of hypermutation in T cells, and its identification and blockade could offer a means for increasing antiretroviral intrinsic immunity of T cells.
Asunto(s)
Linfocitos T CD4-Positivos/enzimología , Citidina Desaminasa/metabolismo , Desaminasa APOBEC-3G , Línea Celular , Citidina Desaminasa/antagonistas & inhibidores , Citidina Desaminasa/genética , Regulación Enzimológica de la Expresión Génica/efectos de los fármacos , Humanos , Ribonucleasas/metabolismo , TransfecciónRESUMEN
For many years it has been known that viral capsid proteins are capable of self-assembly, but increasing evidence over the past decade indicates that in cells HIV-1 capsid assembly occurs via a complex but transient series of steps requiring multiple viral-host interactions. To better understand the biochemistry of HIV assembly, our group established a cell-free system that faithfully reconstitutes HIV-1 Gag synthesis and post-translational events of capsid assembly using cellular extracts, albeit more slowly and less efficiently. This system allowed initial identification of interactions that occur very transiently in cells but can be tracked in the cell-free system. Analysis of the cell-free system revealed that Gag progresses sequentially through a step-wise, energy-dependent series of assembly intermediates containing cellular proteins. One of these cellular proteins, the ATPase ABCE1, has been shown to play a critical role in the assembly process. The existence of this energy-dependent assembly pathway was subsequently confirmed in cellular systems, further validating the cell-free HIV-1 capsid assembly system as an excellent tool for identifying mechanisms underlying HIV-1 capsid formation. Here we describe how to assemble immature HIV-1 capsids in a cell-free system and separate assembly intermediates by velocity sedimentation.
Asunto(s)
Cápside/metabolismo , VIH-1/fisiología , Ensamble de Virus , Transportadoras de Casetes de Unión a ATP/metabolismo , Sistema Libre de Células , Chaperoninas/metabolismo , Interacciones Huésped-Patógeno , Humanos , Sustancias Macromoleculares/aislamiento & purificación , Ultracentrifugación/métodosRESUMEN
The antiretroviral activity of the cellular enzyme APOBEC3G has been attributed to the excessive deamination of cytidine (C) to uridine (U) in minus strand reverse transcripts, a process resulting in guanosine (G) to adenosine (A) hypermutation of plus strand DNAs. The HIV-1 Vif protein counteracts APOBEC3G by inducing proteasomal degradation and exclusion from virions through recruitment of a cullin5 ECS E3 ubiquitin ligase complex. APOBEC3G belongs to the APOBEC protein family, members of which possess consensus (H/C)-(A/V)-E-(X)24-30-P-C-(X)2-C cytidine deaminase motifs. Earlier analyses of APOBEC-1 have defined specific residues that are important for zinc coordination, proton transfer, and, therefore, catalysis within this motif. Because APOBEC3G contains two such motifs, we used site-directed mutagenesis of conserved residues to assess each region's contribution to anti-HIV-1 activity. Surprisingly, whereas either the N- or C-terminal domain could confer antiviral function in tissue culture-based infectivity assays, only an intact C-terminal motif was essential for DNA mutator activity. These findings reveal the nonequivalency of APOBEC3G's N- and C-terminal domains and imply that APOBEC3G-mediated DNA editing may not always be necessary for antiviral activity. Accordingly, we propose that APOBEC3G can achieve an anti-HIV-1 effect through an undescribed mechanism that is distinct from cytidine deamination.
Asunto(s)
Antivirales/fisiología , Citidina Desaminasa/metabolismo , Productos del Gen vif/metabolismo , VIH-1 , Mutación/fisiología , Proteínas/fisiología , Desaminasa APOBEC-3G , Secuencias de Aminoácidos , Células Cultivadas , Ensayo de Inmunoadsorción Enzimática , Humanos , Mutagénesis Sitio-Dirigida , Mutación/genética , Nucleósido Desaminasas , Estructura Terciaria de Proteína , Proteínas Represoras , Virión/metabolismo , Productos del Gen vif del Virus de la Inmunodeficiencia HumanaRESUMEN
The HIV-1 Gag protein assembles into immature capsids when expressed in human cells. Although self-assembly of Gag was once thought to be sufficient to explain capsid formation, in the past decade it has become increasingly apparent that in cells, the pathway from Gag synthesis to assembled capsids is coordinated and facilitated by host factors. These cellular factors likely direct the trafficking, membrane targeting, and multimerization of Gag, and could also assist with encapsidation of viral RNA. While some of these factors have been identified, much remains to be learned about the mechanisms by which they act to promote capsid formation. Moreover, studies suggest that the amount of intracellular Gag undergoing assembly per se at any given time may be quite low, with the majority of Gag in some cell types undergoing degradation or representing Gag that remains cell-associated after assembly. If this model holds true, then defining the Gag subpopulations on which individual cellular factors act will be important for understanding the rqle of host factors. Towards this end, it will be important to find markers and features that can distinguish subpopulations of Gag destined for different outcomes so that these populations can be quantified and tracked separately both at the biochemical and microscopic level. Thus, the challenge for the future will be to understand which cellular factors act during the pathway from Gag synthesis to assembly, and exactly where and how they act in this pathway.
Asunto(s)
Endocitosis , Productos del Gen gag/metabolismo , Infecciones por VIH/virología , VIH-1/fisiología , Animales , Cápside/fisiología , Membrana Celular/metabolismo , Productos del Gen gag/química , VIH-1/genética , Humanos , Estructura Terciaria de Proteína , ARN Viral/genética , ARN Viral/metabolismo , Ensamble de VirusRESUMEN
We review the important role that cell-free protein-synthesizing systems (CFPSS) have played in the history of modern biology, and highlight two recent applications that illustrate their continued utility for the exploration of otherwise intractable aspects of gene expression and its regulation. Viral capsid assembly recreated in CFPSS reveals a catalyzed biochemical pathway involving transient, energy-dependent action of host proteins and discrete assembly intermediates, rather than the classical notion of self-assembly that was expected for capsid formation. Study of prion protein biogenesis reveals a new conformation critical for disease pathogenesis and advances the paradigm of protein bioconformatics, by which cells may productively regulate the folding of various proteins. In each example, the CFPSS made it easier to analyze biochemical mechanism than is possible in other currently available whole cell systems, illustrating why this approach is likely to be a continuing source of insight into important features of biological regulation.
Asunto(s)
Proteínas de la Cápside/genética , Sistema Libre de Células , Biosíntesis de Proteínas , Ensamble de Virus/genética , Cápside , Expresión Génica , Modelos Moleculares , Priones , Conformación Proteica , Transporte de Proteínas/genéticaRESUMEN
During the late stage of the viral life cycle, HIV-1 Gag assembles into a spherical immature capsid, and undergoes budding, release, and maturation. Here we review events involved in immature capsid assembly from the perspective of five different approaches used to study this process: mutational analysis, structural studies, assembly of purified recombinant Gag, assembly of newly translated Gag in a cell-free system, and studies in cells using biochemical and imaging techniques. We summarize key findings obtained using each approach, point out where there is consensus, and highlight unanswered questions. Particular emphasis is placed on reconciling data suggesting that Gag assembles by two different paths, depending on the assembly environment. Specifically, in assembly systems that lack cellular proteins, high concentrations of Gag can spontaneously assemble using purified nucleic acid as a scaffold. However, in the more complex intracellular environment, barriers that limit self-assembly are present in the form of cellular proteins, organelles, host defenses, and the absence of free nucleic acid. To overcome these barriers and promote efficient immature capsid formation in an unfavorable environment, Gag appears to utilize an energy-dependent, host-catalyzed, pathway of assembly intermediates in cells. Overall, we show how data obtained using a variety of techniques has led to our current understanding of HIV assembly.
Asunto(s)
VIH-1/fisiología , Ensamble de Virus , Productos del Gen gag del Virus de la Inmunodeficiencia Humana/metabolismo , Proteínas de la Cápside/química , Proteínas de la Cápside/genética , Proteínas de la Cápside/metabolismo , Sistema Libre de Células , Técnicas de Silenciamiento del Gen , Humanos , Técnicas In Vitro , Imagen Molecular , Mutación , Dominios y Motivos de Interacción de Proteínas , Interferencia de ARN , Relación Estructura-Actividad , Replicación Viral , Productos del Gen gag del Virus de la Inmunodeficiencia Humana/química , Productos del Gen gag del Virus de la Inmunodeficiencia Humana/genéticaRESUMEN
To produce progeny virus, human immunodeficiency virus type I (HIV-1) Gag assembles into capsids that package the viral genome and bud from the infected cell. During assembly of immature capsids, Gag traffics through a pathway of assembly intermediates (AIs) that contain the cellular adenosine triphosphatase ABCE1 (ATP-binding cassette protein E1). In this paper, we showed by coimmunoprecipitation and immunoelectron microscopy (IEM) that these Gag-containing AIs also contain endogenous processing body (PB)-related proteins, including AGO2 and the ribonucleic acid (RNA) helicase DDX6. Moreover, we found a similar complex containing ABCE1 and PB proteins in uninfected cells. Additionally, knockdown and rescue studies demonstrated that the RNA helicase DDX6 acts enzymatically to facilitate capsid assembly independent of RNA packaging. Using IEM, we localized the defect in DDX6-depleted cells to Gag multimerization at the plasma membrane. We also confirmed that DDX6 depletion reduces production of infectious HIV-1 from primary human T cells. Thus, we propose that assembling HIV-1 co-opts a preexisting host complex containing cellular facilitators such as DDX6, which the virus uses to catalyze capsid assembly.
Asunto(s)
ARN Helicasas DEAD-box/metabolismo , Productos del Gen gag/genética , VIH-1/metabolismo , Proteínas Proto-Oncogénicas/metabolismo , Transportadoras de Casetes de Unión a ATP/metabolismo , Animales , Proteínas Argonautas/metabolismo , Células COS , Cápside/metabolismo , Membrana Celular/metabolismo , Chlorocebus aethiops , Productos del Gen gag/metabolismo , Humanos , Microscopía Inmunoelectrónica/métodos , Mutación , Linfocitos T/enzimologíaRESUMEN
In primate cells, assembly of a single HIV-1 capsid involves multimerization of thousands of Gag polypeptides, typically at the plasma membrane. Although studies support a model in which HIV-1 assembly proceeds through complexes containing Gag and the cellular adenosine triphosphatase ABCE1 (also termed HP68 or ribonuclease L inhibitor), whether these complexes constitute true assembly intermediates remains controversial. Here we demonstrate by pulse labeling in primate cells that a population of Gag associates with endogenous ABCE1 within minutes of translation. In the next approximately 2 h, Gag-ABCE1 complexes increase in size to approximately that of immature capsids. Dissociation of ABCE1 from Gag correlates closely with Gag processing during virion maturation and occurs much less efficiently when the HIV-1 protease is inactivated. Finally, quantitative double-label immunogold electron microscopy reveals that ABCE1 is recruited to sites of assembling wild-type Gag at the plasma membrane but not to sites of an assembly-defective Gag mutant at the plasma membrane. Together these findings demonstrate that a population of Gag present at plasma membrane sites of assembly associates with ABCE1 throughout capsid formation until the onset of virus maturation, which is then followed by virus release. Moreover, the data suggest a linkage between Gag-ABCE1 dissociation and subsequent events of virion production.