Comparing capsid assembly of primate lentiviruses and hepatitis B virus using cell-free systems

A Pan-Respiratory Antiviral Chemotype Targeting a Host Multi-Protein Complex

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 virus in multiple cell culture models for all six families of viruses causing most respiratory disease in humans. In animals this chemotype has been demonstrated efficacious for Porcine Epidemic Diarrhea 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 lifecycles 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. 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.

Hepatitis B virus movement through the hepatocyte: An update

Viruses are obligate intracellular pathogens that utilize cellular machinery for many aspects of their propagation and effective egress of virus particles from host cells is one important determinant of virus infectivity. Hijacking host cell processes applies in particular to the hepatitis B virus (HBV), as its DNA genome with about 3 kb in size is one of the smallest viral genomes known. HBV is a leading cause of liver disease and still displays one of the most successful pathogens in human populations worldwide. The extremely successful spread of this virus is explained by its efficient transmission strategies and its versatile particle types, including virions, empty envelopes, naked capsids and others. HBV exploits distinct host trafficking machineries to assemble and release its particle types including nucleocytoplasmic shuttling transport, secretory and exocytic pathways, the Endosomal Sorting Complexes Required for Transport pathway, and the autophagy pathway. Understanding how HBV uses and subverts host membrane trafficking systems offers the chance of obtaining new mechanistic insights into the regulation and function of this essential cellular processes. It can also help to identify potential targets for antiviral interventions. Here, I will provide an overview of HBV maturation, assembly, and budding, with a focus on recent advances, and will point out areas where questions remain that can benefit from future studies. Unless otherwise indicated, almost all presented knowledge was gained from cell culture-based, HBV in vitro -replication and in vitro -infection systems. This article is protected by copyright. All rights reserved.

Easy Synthesis of Complex Biomolecular Assemblies: Wheat Germ Cell-Free Protein Expression in Structural Biology

Cell-free protein synthesis (CFPS) systems are gaining more importance as universal tools for basic research, applied sciences, and product development with new technologies emerging for their application. Huge progress was made in the field of synthetic biology using CFPS to develop new proteins for technical applications and therapy. Out of the available CFPS systems, wheat germ cell-free protein synthesis (WG-CFPS) merges the highest yields with the use of a eukaryotic ribosome, making it an excellent approach for the synthesis of complex eukaryotic proteins including, for example, protein complexes and membrane proteins. Separating the translation reaction from other cellular processes, CFPS offers a flexible means to adapt translation reactions to protein needs. There is a large demand for such potent, easy-to-use, rapid protein expression systems, which are optimally serving protein requirements to drive biochemical and structural biology research. We summarize here a general workflow for a wheat germ system providing examples from the literature, as well as applications used for our own studies in structural biology. With this review, we want to highlight the tremendous potential of the rapidly evolving and highly versatile CFPS systems, making them more widely used as common tools to recombinantly prepare particularly challenging recombinant eukaryotic proteins.

Addressing Antiretroviral Drug Resistance with Host-Targeting Drugs-First Steps towards Developing a Host-Targeting HIV-1 Assembly Inhibitor

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.

Identification of an Antiretroviral Small Molecule That Appears To Be a Host-Targeting Inhibitor of HIV-1 Assembly

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.

Synthesis and Assembly of Hepatitis B Virus-Like Particles in a Pichia pastoris Cell-Free System

Virus-like particles (VLPs) are supramolecular protein assemblies with the potential for unique and exciting applications in synthetic biology and medicine. Despite the attention VLPs have gained thus far, considerable limitations still persist in their production. Poorly scalable manufacturing technologies and inconsistent product architectures continue to restrict the full potential of VLPs. Cell-free protein synthesis (CFPS) offers an alternative approach to VLP production and has already proven to be successful, albeit using extracts from a limited number of organisms. Using a recently developed Pichia pastoris-based CFPS system, we have demonstrated the production of the model Hepatitis B core antigen VLP as a proof-of-concept. The VLPs produced in the CFPS system were found to have comparable characteristics to those previously produced in vivo and in vitro. Additionally, we have developed a facile and rapid synthesis, assembly and purification methodology that could be applied as a rapid prototyping platform for vaccine development or synthetic biology applications. Overall the CFPS methodology allows far greater throughput, which will expedite the screening of optimal assembly conditions for more robust and stable VLPs. This approach could therefore support the characterization of larger sample sets to improve vaccine development efficiency.

Combining Cell-Free Protein Synthesis and NMR Into a Tool to Study Capsid Assembly Modulation

Modulation of capsid assembly by small molecules has become a central concept in the fight against viral infection. Proper capsid assembly is crucial to form the high molecular weight structures that protect the viral genome and that, often in concert with the envelope, allow for cell entry and fusion. Atomic details underlying assembly modulation are generally studied using preassembled protein complexes, while the activity of assembly modulators during assembly remains largely open and poorly understood, as necessary tools are lacking. We here use the full-length hepatitis B virus (HBV) capsid protein (Cp183) as a model to present a combination of cell-free protein synthesis and solid-state NMR as an approach which shall open the possibility to produce and analyze the formation of higher-order complexes directly on exit from the ribosome. We demonstrate that assembled capsids can be synthesized in amounts sufficient for structural studies, and show that addition of assembly modulators to the cell-free reaction produces objects similar to those obtained by addition of the compounds to preformed Cp183 capsids. These results establish the cell-free system as a tool for the study of capsid assembly modulation directly after synthesis by the ribosome, and they open the perspective of assessing the impact of natural or synthetic compounds, or even enzymes that perform post-translational modifications, on capsids structures.

Host Cell Rab GTPases in Hepatitis B Virus Infection

Hepatitis B virus (HBV) is a leading cause of liver disease and is presently estimated to infect more than 250 million humans. The extremely successful spread of this virus among the human population is explained by its effective transmission strategies and its manifold particle types, including virions, empty envelopes and naked capsids. Due to its tiny genome, HBV depends on cellular machineries to thrive in infected hepatocytes. To enter, traverse and exit the cell, HBV exploits host membrane trafficking pathways, including intracellular highways directed by Rab GTPases. Here, we review recent discoveries focused on how HBV co-opts and perturbs host Rab GTPase functions with an emphasis on Rab7A- and Rab33B-mediated trafficking pathways. Rab7A plays bidirectional roles in the viral life cycle, as it promotes the endocytic uptake of HBV in early stages, but restricts exocytic virion release in late stages. In intermediate stages of HBV propagation, Rab33B is needed to guide the assembly of replicative progeny nucleocapsids. Rab33B acts together with its Atg5-12/16L1 effector, a protein complex required for autophagosome formation, suggesting the concept that HBV exploits this Rab/effector complex as an assembly scaffold and machine. We also discuss whether Rab-directed trafficking pathways engaged by HBV may be applicable to other virus families. Identification of overlapping Rab functions may offer new chances to develop broad-spectrum host-targeted antiviral strategies.

Hepatitis B Virus Subverts the Autophagy Elongation Complex Atg5-12/16L1 and Does Not Require Atg8/LC3 Lipidation for Viral Maturation

Previous studies indicated that hepatitis B virus (HBV) stimulates autophagy to favor its production. To understand how HBV co-opts autophagy as a proviral machinery, we studied the roles of key autophagy proteins in HBV-replicating liver cell cultures. RNA interference-mediated silencing of Atg5, Atg12, and Atg16L1, which promote autophagophore expansion and LC3 membrane conjugation, interfered with viral core/nucleocapsid (NC) formation/stability and strongly diminished virus yields. Concomitantly, the core/NC membrane association and their sorting to envelope-positive compartments were perturbed. A close inspection of the HBV/autophagy cross talk revealed that the virus depended on Atg12 covalently conjugated to Atg5. In support of this finding, HBV required the E2-like enzymes Atg10 and Atg3, which catalyze or facilitate Atg5-12 conjugation, respectively. Atg10 and Atg3 knockdowns decreased HBV production, while Atg3 overexpression increased virus yields. Mapping analyses demonstrated that the HBV core protein encountered the Atg5-12/16L1 complex via interaction with the intrinsically disordered region of the Atg12 moiety that is dispensable for autophagy function. The role of Atg12 in HBV replication was confirmed by its incorporation into virions. Although the Atg5-12/16L1 complex and Atg3 are essential for LC3 lipidation and, thus, for autophagosome maturation and closure, HBV propagation did not require LC3. Silencing of LC3B, the most abundant LC3 isoform, did not inhibit but rather augmented virus production. Similar augmenting effects were obtained upon overexpression of a dominant negative mutant of Atg4B that blocked the lipid conjugation of the LC3 isoforms and their GABARAP paralogues. Together, our data indicate that HBV subverts early, nondegradative autophagy components as assembly scaffolds, thereby concurrently avoiding autophagosomal destruction.IMPORTANCE Infections with the hepatitis B virus (HBV), an enveloped pararetrovirus, cause about 1 million deaths per year, as current therapies rarely achieve a cure. Understanding the HBV life cycle and concomitant host cell interactions is instrumental to develop new antiviral concepts. Here, we proceeded to dissect the roles of the autophagy machinery in virus propagation. By using RNA interference and overexpression studies in HBV-replicating cell lines, we identified the autophagic Atg5-12/16L1 elongation complex along with Atg10 and Atg3 to be an essential scaffold for HBV nucleocapsid assembly/stability. Deficits in Atg5-12/16L1 and Atg10/Atg3, which normally drive autophagophore membrane expansion, strongly impaired progeny virus yields. HBV gained access to Atg5-12/16L1 via interaction of its core protein with the Atg12 moiety of the complex. In contrast, subsequent autophagosome maturation and closure events were unnecessary for HBV replication, as evidenced by inhibition of Atg8/LC3 conjugation. Interfering with the HBV/Atg12 cross talk may be a tool for virus control.

Design of virus-based nanomaterials for medicine, biotechnology, and energy

This review provides an overview of recent developments in "chemical virology." Viruses, as materials, provide unique nanoscale scaffolds that have relevance in chemical biology and nanotechnology, with diverse areas of applications. Some fundamental advantages of viruses, compared to synthetically programmed materials, include the highly precise spatial arrangement of their subunits into a diverse array of shapes and sizes and many available avenues for easy and reproducible modification. Here, we will first survey the broad distribution of viruses and various methods for producing virus-based nanoparticles, as well as engineering principles used to impart new functionalities. We will then examine the broad range of applications and implications of virus-based materials, focusing on the medical, biotechnology, and energy sectors. We anticipate that this field will continue to evolve and grow, with exciting new possibilities stemming from advancements in the rational design of virus-based nanomaterials.

Rab33B and its autophagic Atg5/12/16L1 effector assist in hepatitis B virus naked capsid formation and release

Hepatitis B virus morphogenesis is accompanied by the production and release of non-enveloped capsids/nucleocapsids. Capsid particles are formed inside the cell cytosol by multimerization of core protein subunits and ultimately exported in an uncommon coatless state. Here, we investigated potential roles of Rab GTPases in capsid formation and trafficking by using RNA interference and overexpression studies. Naked capsid release does not require functions of the endosome-associated Rab5, Rab7 and Rab27 proteins, but depends on functional Rab33B, a GTPase participating in autophagosome formation via interaction with the Atg5-Atg12/Atg16L1 complex. During capsid formation, Rab33B acts in conjunction with its effector, as silencing of Atg5, Atg12 and Atg16L1 also impaired capsid egress. Analysis of capsid maturation steps revealed that Rab33B and Atg5/12/16L1 are required for proper particle assembly and/or stability. In support, the capsid protein was found to interact with Atg5 and colocalize with Atg5/12/16L1, implicating that autophagy pathway functions are involved in capsid biogenesis. However, a complete and functional autophagy pathway is dispensable for capsid release, as judged by knockdown analysis of Atg8/LC3 family members and pharmaceutical ablation of canonical autophagy. Experiments aimed at analysing the capsid release-stimulating activity of the Alix protein provide further evidence for a link between capsid formation and autophagy.

How HIV-1 Gag assembles in cells: Putting together pieces of the puzzle

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.

Host-rabies virus protein-protein interactions as druggable antiviral targets

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.

Chapter 2. Development of key technologies for high-throughput cell-free protein production with the extract from wheat embryos

The cell-free translation system from wheat embryos had been considered to be inefficient as compared with the E. coli cell-based and cell-free protein production methods. However, it was revealed that the extract from extensively washed wheat embryo particles can provide a very productive cell-free protein synthesis system. Since then, the method has been improved, so that it fits the postgenomic researches. New mRNA configurations enabled us to synthesize many different proteins in parallel and to prepare large amounts of proteins, which fits the need for screening of suitable proteins for structural and functional analyses before large-scale production. The new reaction formats promoted the developments of new machines that perform highly parallel and highly productive protein synthesis reactions automatically. It was revealed that, by parallel synthesis of many proteins, much more multidomain proteins are produced in soluble forms in the wheat system than in the prokaryotic systems. The wheat system provides a rapid and cost-effective method for stable isotope labeling of proteins for NMR analyses. Selenomethionine substitution of proteins for X-ray crystallography through the cell-free synthesis was also achieved. Synthesis of some families of proteins that were difficult to be produced by conventional methods has been tested. At least, cytotoxic restriction enzymes were readily produced in a large amount. Some multisubunit proteins and cofactor-binding proteins could be synthesized by the method and were characterized successfully. Membrane proteins have also been tested, and a transporter was synthesized in an active form. Although some issues remains to be solved, we expect that the wheat cell-free protein synthesis system can contribute to the structural and functional genomics and to the future understanding of life.

Testing an electrostatic interaction hypothesis of hepatitis B virus capsid stability by using an in vitro capsid disassembly/reassembly system

To test a previously coined "charge balance hypothesis" of human hepatitis B virus (HBV) capsid stability, we established an in vitro disassembly and reassembly system using bacterially expressed HBV capsids. Capsid disassembly can be induced by micrococcal nuclease digestion of encapsidated RNA. HBV core protein (HBc) mutants containing various amounts of arginine were constructed by serial truncations at the C terminus. Capsids containing smaller amounts of arginine (HBc 149, 154, and 157) remained intact after micrococcal nuclease digestion by native gel electrophoresis. Capsids containing larger amounts of arginine (HBc 159, 164, 169, and 171) exhibited reduced and more diffuse banding intensity and slightly upshifted mobility (HBc 159 and 164). Capsids containing the largest amounts of arginine (HBc 173, 175, and 183), as well as HBc 167, exhibited no detectable banding signal, indicating loss of capsid integrity or stability. Interestingly, capsid reassembly can be induced by polyanions, including oligonucleotides, poly-glutamic acid, and nonbiological polymer (polyacrylic acid). In contrast, polycations (polylysine and polyethylenimine) and low-molecular-weight anions (inositol triphosphate) induced no capsid reassembly. Results obtained by gel assay were confirmed by electron microscopy. Reassembled capsids comigrated with undigested parental capsids on agarose gels and cosedimented with undigested capsids by sucrose gradient ultracentrifugation. Taken together, the results indicate that HBV capsid assembly and integrity depend on polyanions, which probably can help minimize intersubunit charge repulsion caused mainly by arginine-rich domain III or IV in close contact. The exact structure of polyanions is not important for in vitro capsid reassembly. A large amount of independent experimental evidence for this newly coined "electrostatic interaction hypothesis" is discussed.

Assembly of immature HIV-1 capsids using a cell-free system

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.

Escherichia coli-based cell-free synthesis of virus-like particles

Virus-like particles (VLP) have received considerable attention for vaccine, drug delivery, gene therapy and material science applications. Although the number of unique VLP and their applications are rapidly growing, the positive impact of VLP applications is limited by the current diverse, expensive, and typically low-yielding production technologies available. These technologies, when scaled, often result in structurally and compositionally inconsistent products. We present Escherichia coli-based cell-free protein synthesis as a production technology to overcome many of the limitations of current VLP production processes. Using this technique, the MS2 bacteriophage coat protein VLP was produced at a yield 14 times the best published production yield. Also, a C-terminally truncated Hepatitis B core protein VLP was produced at similarly high yields (6 x 10(13) VLP/mL). These VLP were found to have comparable characteristics to those produced in vivo. The scalability of this technology was tested without loss in production yields. To our knowledge, this is the first time a prokaryote-based in vitro transcription/translation system has generated a virus-like particle.

Tumor necrosis factor activates a conserved innate antiviral response to hepatitis B virus that destabilizes nucleocapsids and reduces nuclear viral DNA

Tumor necrosis factor (TNF) is critical for the control of hepatitis B virus (HBV) in the clinical setting and in model systems. TNF induces noncytopathic suppression and clearance of HBV in animal models, possibly through reduction of viral nucleocapsids, but the mechanism is not well described. Here, we demonstrate the molecular mechanism and broad host range for TNF action against HBV. We show that TNF rapidly blocks HBV replication by promoting destabilization of preexisting cytoplasmic viral nucleocapsids containing viral RNA and DNA, as well as empty nucleocapsids. TNF destabilized human HBV nucleocapsids in a variety of human hepatocytic cell lines and in primary rat hepatocytes and also destabilized duck HBV (DHBV) nucleocapsids in chicken hepatocytic cells. Lysates from TNF-treated uninfected cells also destabilized HBV nucleocapsids in vitro. Moreover, inhibition of DHBV DNA replication by TNF blocks nuclear accumulation of the viral transcription template, maintenance of which is essential for the establishment and maintenance of chronic infection. We show that TNF destabilization of HBV nucleocapsids does not involve ubiquitination or methylation of the viral core protein and is not mediated by the nitric oxide free radical arm of the TNF pathway. These results define a novel antiviral mechanism mediated by TNF against multiple types of HBVs in different species.

Basic residues in the nucleocapsid domain of Gag are required for interaction of HIV-1 gag with ABCE1 (HP68), a cellular protein important for HIV-1 capsid assembly

During human immunodeficiency virus, type 1 (HIV-1) assembly, Gag polypeptides multimerize into immature HIV-1 capsids. The cellular ATP-binding protein ABCE1 (also called HP68 or RNase L inhibitor) appears to be critical for proper assembly of the HIV-1 capsid. In primate cells, ABCE1 associates with Gag polypeptides present in immature capsid assembly intermediates. Here we demonstrate that the NC domain of Gag is critical for interaction with endogenous primate ABCE1, whereas other domains in Gag can be deleted without eliminating the association of Gag with ABCE1. NC contains two Cys-His boxes that form zinc finger motifs and are responsible for encapsidation of HIV-1 genomic RNA. In addition, NC contains basic residues known to play a critical role in nonspecific RNA binding, Gag-Gag interactions, and particle formation. We demonstrate that basic residues in NC are needed for the Gag-ABCE1 interaction, whereas the cysteine and histidine residues in the zinc fingers are dispensable. Constructs that fail to interact with primate ABCE1 or interact poorly also fail to form capsids and are arrested at an early point in the immature capsid assembly pathway. Whereas others have shown that basic residues in NC bind nonspecifically to RNA, which in turn scaffolds or nucleates assembly, our data demonstrate that the same basic residues in NC act either directly or indirectly to recruit a cellular protein that also promotes capsid formation. Thus, in cells, basic residues in NC appear to act by two mechanisms, recruiting both RNA and a cellular ATPase in order to facilitate efficient assembly of HIV-1 capsids.