Dynamic fluorescent imaging of human immunodeficiency virus type 1 gag in live cells by biarsenical labeling

Double-Labeling Method for Visualization and Quantification of Membrane-Associated Proteins in Lactococcus lactis

Lactococcus lactis displaying recombinant proteins on its surface can be used as a potential drug delivery vector in prophylactic medication and therapeutic treatments for many diseases. These applications enable live-cell mucosal and oral administration, providing painless, needle-free solutions and triggering robust immune response at the site of pathogen entry. Immunization requires quantitative control of antigens and, ideally, a complete understanding of the bacterial processing mechanism applied to the target proteins. In this study, we propose a double-labeling method based on a conjugated dye specific for a recombinantly introduced polyhistidine tag (to visualize surface-exposed proteins) and a membrane-permeable dye specific for a tetra-cysteine tag (to visualize cytoplasmic proteins), combined with a method to block the labeling of surface-exposed tetra-cysteine tags, to clearly obtain location-specific signals of the two dyes. This allows simultaneous detection and quantification of targeted proteins on the cell surface and in the cytoplasm. Using this method, we were able to detect full-length peptide chains for the model proteins HtrA and BmpA in L. lactis, which are associated with the cell membrane by two different attachment modes, and thus confirm that membrane-associated proteins in L. lactis are secreted using the Sec-dependent post-translational pathway. We were able to quantitatively follow cytoplasmic protein production and accumulation and subsequent export and surface attachment, which provides a convenient tool for monitoring these processes for cell surface display applications.

Tracking the Replication-Competent Zika Virus with Tetracysteine-Tagged Capsid Protein in Living Cells

Zika virus (ZIKV) is the mosquito-borne enveloped flavivirus that causes microcephaly in neonates. While real-time imaging plays a critical role in dissecting viral biology, no fluorescent, genetically engineered ZIKV for single-particle tracking is currently available.

Imaging the Replication of Single Viruses: Lessons Learned from HIV and Future Challenges To Overcome

The molecular composition of viral particles indicates that a single virion is capable of initiating an infection. However, the majority of viruses that come into contact with cells fails to infect them. Understanding what makes one viral particle more successful than others requires visualizing the infection process directly in living cells, one virion at a time. In this Perspective, we explain how single-virus imaging using fluorescence microscopy can provide answers to unsolved questions in virology. We discuss fluorescent labeling of virus particles, resolution at the subviral and molecular levels, tracking in living cells, and imaging of interactions between viral and host proteins. We end this Perspective with a set of remaining questions in understanding the life cycle of retroviruses and how imaging a single virus can help researchers address these questions. Although we use examples from the HIV field, these methods are of value for the study of other viruses as well.

Single-Virus Tracking: From Imaging Methodologies to Virological Applications

Uncovering the mechanisms of virus infection and assembly is crucial for preventing the spread of viruses and treating viral disease. The technique of single-virus tracking (SVT), also known as single-virus tracing, allows one to follow individual viruses at different parts of their life cycle and thereby provides dynamic insights into fundamental processes of viruses occurring in live cells. SVT is typically based on fluorescence imaging and reveals insights into previously unreported infection mechanisms. In this review article, we provide the readers a broad overview of the SVT technique. We first summarize recent advances in SVT, from the choice of fluorescent labels and labeling strategies to imaging implementation and analytical methodologies. We then describe representative applications in detail to elucidate how SVT serves as a valuable tool in virological research. Finally, we present our perspectives regarding the future possibilities and challenges of SVT.

Monitoring HIV-1 Assembly in Living Cells: Insights from Dynamic and Single Molecule Microscopy

The HIV-1 assembly process is a multi-complex mechanism that takes place at the host cell plasma membrane. It requires a spatio-temporal coordination of events to end up with a full mature and infectious virus. The molecular mechanisms of HIV-1 assembly have been extensively studied during the past decades, in order to dissect the respective roles of the structural and non-structural viral proteins of the viral RNA genome and of some host cell factors. Nevertheless, the time course of HIV-1 assembly was observed in living cells only a decade ago. The very recent revolution of optical microscopy, combining high speed and high spatial resolution, in addition to improved fluorescent tags for proteins, now permits study of HIV-1 assembly at the single molecule level within living cells. In this review, after a short description of these new approaches, we will discuss how HIV-1 assembly at the cell plasma membrane has been revisited using advanced super resolution microscopy techniques and how it can bridge the study of viral assembly from the single molecule to the entire host cell.

Live-Cell Imaging of Early Steps of Single HIV-1 Infection

Live-cell imaging of single HIV-1 entry offers a unique opportunity to delineate the spatio-temporal regulation of infection. Novel virus labeling and imaging approaches enable the visualization of key steps of HIV-1 entry leading to nuclear import, integration into the host genome, and viral protein expression. Here, we discuss single virus imaging strategies, focusing on live-cell imaging of single virus fusion and productive uncoating that culminates in HIV-1 infection.

The NC domain of HIV-1 Gag contributes to the interaction of Gag with TSG101

BACKGROUND

HIV-1 Gag polyprotein orchestrates the assembly of viral particles. Its C-terminus consists of the nucleocapsid (NC) domain that interacts with RNA, and the p6 domain containing the PTAP motif that binds the cellular ESCRT factor TSG101 and ALIX. Deletion of the NC domain of Gag (GagNC) results in defective Gag assembly, a decrease in virus production and, thus probably affects recruitment of the ESCRT machinery. To investigate the role of GagNC in this recruitment, we analysed its impact on TSG101 and ALIX localisations and interactions in cells expressing Gag.

METHODS

Cells expressing mCherry-Gag or derivatives, alone or together with eGFP-TSG101 or eGFP-ALIX, were analysed by confocal microscopy and FLIM-FRET. Chemical shift mapping between TSG101-UEV motif and Gag C-terminus was performed by NMR.

RESULTS

We show that deletion of NC or of its two zinc fingers decreases the amount of Gag-TSG101 interacting complexes in cells. These findings are supported by NMR data showing chemical shift perturbations in the NC domain in- and outside - of the zinc finger elements upon TSG101 binding. The NMR data further identify a large stretch of amino acids within the p6 domain directly interacting with TSG101.

CONCLUSION

The NC zinc fingers and p6 domain of Gag participate in the formation of the Gag-TSG101 complex and in its cellular localisation.

GENERAL SIGNIFICANCE

This study illustrates that the NC and p6 domains cooperate in the interaction with TSG101 during HIV-1 budding. In addition, details on the Gag-TSG101 complex were obtained by combining two high resolution biophysical techniques.

Characterization of a Replicating Mammalian Orthoreovirus with Tetracysteine-Tagged μNS for Live-Cell Visualization of Viral Factories

Within infected host cells, mammalian orthoreovirus (MRV) forms viral factories (VFs), which are sites of viral transcription, translation, assembly, and replication. The MRV nonstructural protein μNS comprises the structural matrix of VFs and is involved in recruiting other viral proteins to VF structures. Previous attempts have been made to visualize VF dynamics in live cells, but due to current limitations in recovery of replicating reoviruses carrying large fluorescent protein tags, researchers have been unable to directly assess VF dynamics from virus-produced μNS. We set out to develop a method to overcome this obstacle by utilizing the 6-amino-acid (CCPGCC) tetracysteine (TC) tag and FlAsH-EDT2 reagent. The TC tag was introduced into eight sites throughout μNS, and the capacity of the TC-μNS fusion proteins to form virus factory-like (VFL) structures and colocalize with virus proteins was characterized. Insertion of the TC tag interfered with recombinant virus rescue in six of the eight mutants, likely as a result of loss of VF formation or important virus protein interactions. However, two recombinant (r)TC-μNS viruses were rescued and VF formation, colocalization with associating virus proteins, and characterization of virus replication were subsequently examined. Furthermore, the rTC-μNS viruses were utilized to infect cells and examine VF dynamics using live-cell microscopy. These experiments demonstrate active VF movement with fusion events as well as transient interactions between individual VFs and demonstrate the importance of microtubule stability for VF fusion during MRV infection. This work provides important groundwork for future in-depth studies of VF dynamics and host cell interactions.IMPORTANCE MRV has historically been used as a model to study the double-stranded RNA (dsRNA) Reoviridae family, the members of which infect and cause disease in humans, animals, and plants. During infection, MRV forms VFs that play a critical role in virus infection but remain to be fully characterized. To study VFs, researchers have focused on visualizing the nonstructural protein μNS, which forms the VF matrix. This work provides the first evidence of recovery of replicating reoviruses in which VFs can be labeled in live cells via introduction of a TC tag into the μNS open reading frame. Characterization of each recombinant reovirus sheds light on μNS interactions with viral proteins. Moreover, utilizing the TC-labeling FlAsH-EDT2 biarsenical reagent to visualize VFs, evidence is provided of dynamic VF movement and interactions at least partially dependent on intact microtubules.

The novel mitochondria localization of influenza A virus NS1 visualized by FlAsH labeling

The nonstructural protein 1 (NS1) of the influenza A virus (IAV) is a multifunctional protein that counteracts host cell antiviral responses and inhibits host cell pre‐mRNA processing. NS1 contains two nuclear localization signals that facilitate NS1 shuttling between cytoplasm and nucleus. In this study, we initially observed the novel mitochondria localization of NS1 in a subset of transfected cells. We then further monitored the localization dynamics of the NS1 protein in live cells infected with IAV expressing NS1 with insertion of a tetracysteine‐tag. The resulting mutant virus showed similar levels of infectivity and expression pattern of NS1 to those of wild‐type IAV. Pulse labeling using a biarsenical compound (fluorescein arsenical hairpin binder) allowed us to visualize the dynamic subcellular distribution of NS1 real time. We detected NS1 in mitochondria at a very early infection time point [1.5 h postinfection (hpi)] and observed the formation of a granular structure pattern in the nucleus at 4 hpi. This is the first identification of the novel mitochondria localization of NS1. The possible role of NS1 at an early infection time point is discussed.

Single particle labeling of RNA virus in live cells

Real-time and visual tracking of viral infection is crucial for elucidating the infectious and pathogenesis mechanisms. To track the virus successfully, an efficient labeling method is necessary. In this review, we first discuss the practical labeling techniques for virus tracking in live cells. We then describe the current knowledge of interactions between RNA viruses (especially influenza viruses, immunodeficiency viruses, and Flaviviruses) and host cellular structures, obtained using single particle labeling techniques combined with real-time fluorescence microscopy. Single particle labeling provides an easy system for understanding the RNA virus life cycle.

A Guide to Transient Expression of Membrane Proteins in HEK-293 Cells for Functional Characterization

The human embryonic kidney 293 (HEK-293) cells are commonly used as host for the heterologous expression of membrane proteins not least because they have a high transfection efficiency and faithfully translate and process proteins. In addition, their cell size, morphology and division rate, and low expression of native channels are traits that are particularly attractive for current-voltage measurements. Nevertheless, the heterologous expression of complex membrane proteins such as receptors and ion channels for biological characterization and in particular for single-cell applications such as electrophysiology remains a challenge. Expression of functional proteins depends largely on careful step-by-step optimization that includes the design of expression vectors with suitable identification tags, as well as the selection of transfection methods and detection parameters appropriate for the application. Here, we use the heterologous expression of a plant potassium channel, the Arabidopsis thaliana guard cell outward-rectifying K(+) channel, AtGORK (At5G37500) in HEK-293 cells as an example, to evaluate commonly used transfection reagents and fluorescent detection methods, and provide a detailed methodology for optimized transient transfection and expression of membrane proteins for in vivo studies in general and for single-cell applications in particular. This optimized protocol will facilitate the physiological and cellular characterization of complex membrane proteins.

Visualization of hepatitis B virus entry - novel tools and approaches to directly follow virus entry into hepatocytes

Hepatitis B virus (HBV) is a widespread human pathogen, responsible for chronic infections of ca. 240 million people worldwide. Until recently, the entry pathway of HBV into hepatocytes was only partially understood. The identification of human sodium taurocholate cotransporting polypeptide (NTCP) as a bona fide receptor of HBV has provided us with new tools to investigate this pathway in more details. Combined with advances in virus visualization techniques, approaches to directly visualize HBV cell attachment, NTCP interaction, virion internalization and intracellular transport are now becoming feasible. This review summarizes our current understanding of how HBV specifically enters hepatocytes, and describes possible visualization strategies applicable for a deeper understanding of the underlying cell biological processes.

Investigating the cellular distribution and interactions of HIV-1 nucleocapsid protein by quantitative fluorescence microscopy

The nucleocapsid protein (NCp7) of the Human immunodeficiency virus type 1 (HIV-1) is a small basic protein containing two zinc fingers. About 2000 NCp7 molecules coat the genomic RNA in the HIV-1 virion. After infection of a target cell, the viral core enters into the cytoplasm, where NCp7 chaperones the reverse transcription of the genomic RNA into the proviral DNA. As a consequence of their much lower affinity for double-stranded DNA as compared to single-stranded RNAs, NCp7 molecules are thought to be released in the cytoplasm and the nucleus of infected cells in the late steps of reverse transcription. Yet, little is known on the cellular distribution of the released NCp7 molecules and on their possible interactions with cell components. Hence, the aim of this study was to identify potential cellular partners of NCp7 and to monitor its intracellular distribution and dynamics by means of confocal fluorescence microscopy, fluorescence lifetime imaging microscopy, fluorescence recovery after photobleaching, fluorescence correlation and cross-correlation spectroscopy, and raster imaging correlation spectroscopy. HeLa cells transfected with eGFP-labeled NCp7 were used as a model system. We found that NCp7-eGFP localizes mainly in the cytoplasm and the nucleoli, where it binds to cellular RNAs, and notably to ribosomal RNAs which are the most abundant. The binding of NCp7 to ribosomes was further substantiated by the intracellular co-diffusion of NCp7 with the ribosomal protein 26, a component of the large ribosomal subunit. Finally, gradient centrifugation experiments demonstrate a direct association of NCp7 with purified 80S ribosomes. Thus, our data suggest that NCp7 molecules released in newly infected cells may primarily bind to ribosomes, where they may exert a new potential role in HIV-1 infection.

Role of the nucleocapsid domain in HIV-1 Gag oligomerization and trafficking to the plasma membrane: a fluorescence lifetime imaging microscopy investigation

The Pr55 Gag of human immunodeficiency virus type 1 orchestrates viral particle assembly in producer cells, which requires the genomic RNA and a lipid membrane as scaffolding platforms. The nucleocapsid (NC) domain with its two invariant CCHC zinc fingers flanked by unfolded basic sequences is thought to direct genomic RNA selection, dimerization and packaging during virus assembly. To further investigate the role of NC domain, we analyzed the assembly of Gag with deletions in the NC domain in parallel with that of wild-type Gag using fluorescence lifetime imaging microscopy combined with Förster resonance energy transfer in HeLa cells. We found that, upon binding to nucleic acids, the NC domain promotes the formation of compact Gag oligomers in the cytoplasm. Moreover, the intracellular distribution of the population of oligomers further suggests that oligomers progressively assemble during their trafficking toward the plasma membrane (PM), but with no dramatic changes in their compact arrangement. This ultimately results in the accumulation at the PM of closely packed Gag oligomers that likely arrange in hexameric lattices, as revealed by the perfect match between the experimental Förster resonance energy transfer value and the one calculated from the structural model of Gag in immature viruses. The distal finger and flanking basic sequences, but not the proximal finger, appear to be essential for Gag oligomer compaction and membrane binding. Moreover, the full NC domain was found to be instrumental in the kinetics of Gag oligomerization and intracellular trafficking. These findings further highlight the key roles played by the NC domain in virus assembly.

Visualizing hepatitis B virus with biarsenical labelling in living cells

BACKGROUND

Study on viruses has greatly benefited from visualization of viruses tagged with green fluorescent protein (GFP) in living cells. But GFP tag, as a large inserted fragment, is not suitable for labelling Hepatitis B virus (HBV) that is a compact virion with limited internal space.

AIM

To visualize HBV in living cells, we constructed several recombinant HBV fluorescently labelled with biarsenical dye to track the behaviour of HBV in the cytoplasm of infected cells.

METHODS

By mutagenesis, a smaller size tetracysteine (TC) tag (C-C-P-G-C-C) that could be bound with a biarsenical fluorescent dye was genetically inserted at different cell epitopes of HBV core protein expressed in transfected cells.

RESULT

Confocal microscopy and transmission electron microscopy (TEM) observations showed that TC-tagged core proteins bound with biarsenical dye could specifically fluoresce in cells and be incorporated into nucleocapsid to form fluorescent virions. The recombinant fluorescent HBV virions retained their infectivity as wild-type ones. Moreover, tracking of fluorescent HBV particles in living cells reveals microtubule-dependent motility of the intracellular particles.

CONCLUSION

To the best of our knowledge, this is the first time to generate fluorescent HBV virions with biarsenical labelling and to visualize their trafficking in living cells. The fluorescent HBV may become one highly valuable tool for further studying detailed dynamic processes of HBV life cycle and interaction of HBV with host in live-imaging approach.

Role of the nucleocapsid region in HIV-1 Gag assembly as investigated by quantitative fluorescence-based microscopy

The Gag precursor of HIV-1, formed of the four proteic regions matrix (MA), capsid (CA), nucleocapsid (NC) and p6, orchestrates virus morphogenesis. This complex process relies on three major interactions, NC-RNA acting as a scaffold, CA-CA and MA-membrane that targets assembly to the plasma membrane (PM). The characterization of the molecular mechanism of retroviral assembly has extensively benefited from biochemical studies and more recently an important step forward was achieved with the use of fluorescence-based techniques and fluorescently labeled viral proteins. In this review, we summarize the findings obtained with such techniques, notably quantitative-based approaches, which highlight the role of the NC region in Gag assembly.

Interactions of AsCy3 with cysteine-rich peptides

There is great interest in fluorogenic compounds that tag biomolecules within cells. Biarsenicals are fluorogenic compounds that become fluorescent upon binding four proximal Cys thiols, a tetracysteine (Cys₄) motif. This work details interactions between the biarsenical AsCy3 and Cys₄ peptides. Maximal affinity was observed when two Cys-Cys pairs were separated by at least 8 amino acids; the highest affinity ligand bound in the nanomolar concentration range (K_app = 43 nM) and with a significant (3.2-fold) fluorescence enhancement.

Live Cell Imaging of Retroviral Entry

Cellular entry of retroviruses is the first critical stage of retroviral replication. Live cell imaging has been utilized to visualize the dynamics, localization, and kinetics of the viral fusion process. Here, we review the different methodologies used for live cell imaging and how the use of these techniques has better elucidated the viral entry process of avian sarcoma and leukosis virus (ASLV) and human immunodeficiency virus type 1 (HIV-1) as well as cell-to-cell transmission of retroviruses. Although some controversies remain, further development of these techniques will provide new insights into the process and dynamics of retroviral fusion in vivo.

Murine leukemia virus Gag localizes to the uropod of migrating primary lymphocytes

B and CD4(+) T lymphocytes are natural targets of murine leukemia virus (MLV). Migrating lymphocytes adopt a polarized morphology with a trailing edge designated the uropod. Here, we demonstrate that MLV Gag localizes to the uropod in polarized B cells and CD4(+) T cells. The uropod localization of MLV Gag was dependent on plasma membrane (PM) association and multimerization of Gag but independent of the viral glycoprotein Env. Basic residues in MA that are required for MLV Gag recruitment to virological synapses between HEK293 and XC cells were dispensable for uropod localization in migrating B cells. Ultrastructural studies indicated that both wild-type and basic-residue mutant Gag localized to the outer surface of the PM at the uropod. Late-domain mutant virus particles were seen at the uropod in form of budding-arrested intermediates. Finally, uropods mediated contact between MLV-infected B cells and uninfected T cells to form virological synapses. Our results suggest that MLV, not unlike HIV, accumulates at the uropod of primary lymphocytes to facilitate viral spreading through the formation of uropod-mediated cell-cell contacts.

IMPORTANCE

Viruses have evolved mechanisms to coordinate their assembly and budding with cell polarity to facilitate their spreading. In this study, we demonstrated that the viral determinants for MLV Gag to localize to the uropod in polarized B cells are distinct from the requirements to localize to virological synapses in transformed cell lines. Basic residues in MA that are required for the Gag localization to virological synapses between HEK293 and XC cells are dispensable for Gag localization to the uropod in primary B cells. Rather, plasma membrane association and capsid-driven multimerization of Gag are sufficient to drive MLV Gag to the uropod. MLV-laden uropods also mediate contacts between MLV-infected B cells and uninfected T cells to form virological synapses. Our results indicate that MLV accumulates at the uropod of primary lymphocytes to facilitate viral spreading through the formation of uropod-mediated cell-cell contacts.

Retrospective on the all-in-one retroviral nucleocapsid protein

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This retrospective reviews 30 years of research on the retroviral nucleocapsid protein (NC) focusing on HIV-1 NC.

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Originally considered as a non-specific nucleic-acid binding protein, NC has seminal functions in virus replication.

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Indeed NC turns out to be a all-in-one viral protein that chaperones viral DNA synthesis and integration, and virus formation.

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As a chaperone NC provides assistance to genetic recombination thus allowing the virus to escape the immune response and antiretroviral therapies against HIV-1.

This review aims at briefly presenting a retrospect on the retroviral nucleocapsid protein (NC), from an unspecific nucleic acid binding protein (NABP) to an all-in-one viral protein with multiple key functions in the early and late phases of the retrovirus replication cycle, notably reverse transcription of the genomic RNA and viral DNA integration into the host genome, and selection of the genomic RNA together with the initial steps of virus morphogenesis. In this context we will discuss the notion that NC protein has a flexible conformation and is thus a member of the growing family of intrinsically disordered proteins (IDPs) where disorder may account, at least in part, for its function as a nucleic acid (NA) chaperone and possibly as a protein chaperone vis-à-vis the viral DNA polymerase during reverse transcription. Lastly, we will briefly review the development of new anti-retroviral/AIDS compounds targeting HIV-1 NC because it represents an ideal target due to its multiple roles in the early and late phases of virus replication and its high degree of conservation.

Rabies virus envelope glycoprotein targets lentiviral vectors to the axonal retrograde pathway in motor neurons

Rabies pseudotyped lentiviral vectors have great potential in gene therapy, not least because of their ability to transduce neurons following their distal axonal application. However, very little is known about the molecular processes that underlie their retrograde transport and cell transduction. Using multiple labeling techniques and confocal microscopy, we demonstrated that pseudotyping with rabies virus envelope glycoprotein (RV-G) enabled the axonal retrograde transport of two distinct subtypes of lentiviral vector in motor neuron cultures. Analysis of this process revealed that these vectors trafficked through Rab5-positive endosomes and accumulated within a non-acidic Rab7 compartment. RV-G pseudotyped vectors were co-transported with both the tetanus neurotoxin-binding fragment and the membrane proteins thought to mediate rabies virus endocytosis (neural cell adhesion molecule, nicotinic acetylcholine receptor, and p75 neurotrophin receptor), thus demonstrating that pseudotyping with RV-G targets lentiviral vectors for transport along the same pathway exploited by several toxins and viruses. Using motor neurons cultured in compartmentalized chambers, we demonstrated that axonal retrograde transport of these vectors was rapid and efficient; however, it was not able to transduce the targeted neurons efficiently, suggesting that impairment in processes occurring after arrival of the viral vector in the soma is responsible for the low transduction efficiency seen in vivo, which suggests a novel area for improvement of gene therapy vectors.

Progress on the labeling and single-particle tracking technologies of viruses

We review recent advances in virus labeling and the emerging fluorescence imaging technologies used in the imaging and tracking of viruses.

Imaging of HIV assembly and release

Assembly, release and maturation of HIV-1 particles comprise a highly dynamic sequence of events, characterized by a series of dramatic rearrangements of the viral structural proteins and overall virion architecture. HIV-1 morphogenesis is a relatively rapid and asynchronous process, showing high variability between cells and individual virions. Therefore, bulk biochemical methods are not ideally suited to study specific aspects of this process in detail. In contrast, imaging represents a direct approach to analyze individual particles and events. While live-cell imaging can reveal the dynamics of intracellular events with high temporal resolution, it falls short in revealing ultra-structural details. Thus, live-cell fluorescence microscopy and electron microscopy (EM) can complement each other to gain insight into both the dynamics of assembly and the structures detected at HIV-1 assembly sites. In this chapter we describe microscopic setups, tools, and methods for live-cell fluorescence microscopy as well as for different EM techniques, which have been successfully used by us and others to study HIV-1 assembly at the host cell plasma membrane. These methods can be used in a complementary manner to investigate the effects of cellular factors, mutations in the viral genome or antiviral drugs on dynamic and structural aspects of HIV-1 morphogenesis.

Analysis of borna disease virus trafficking in live infected cells by using a virus encoding a tetracysteine-tagged p protein

Borna disease virus (BDV) is a nonsegmented, negative-stranded RNA virus characterized by noncytolytic persistent infection and replication in the nuclei of infected cells. To gain further insight on the intracellular trafficking of BDV components during infection, we sought to generate recombinant BDV (rBDV) encoding fluorescent fusion viral proteins. We successfully rescued a virus bearing a tetracysteine tag fused to BDV-P protein, which allowed assessment of the intracellular distribution and dynamics of BDV using real-time live imaging. In persistently infected cells, viral nuclear inclusions, representing viral factories tethered to chromatin, appeared to be extremely static and stable, contrasting with a very rapid and active trafficking of BDV components in the cytoplasm. Photobleaching (fluorescence recovery after photobleaching [FRAP] and fluorescence loss in photobleaching [FLIP]) imaging approaches revealed that BDV components were permanently and actively exchanged between cellular compartments, including within viral inclusions, albeit with a fraction of BDV-P protein not mobile in these structures, presumably due to its association with viral and/or cellular proteins. We also obtained evidence for transfer of viral material between persistently infected cells, with routing of the transferred components toward the cell nucleus. Finally, coculture experiments with noninfected cells allowed visualization of cell-to-cell BDV transmission and movement of the incoming viral material toward the nucleus. Our data demonstrate the potential of tetracysteine-tagged recombinant BDV for virus tracking during infection, which may provide novel information on the BDV life cycle and on the modalities of its interaction with the nuclear environment during viral persistence.

Imaging of the alphavirus capsid protein during virus replication

Alphaviruses are enveloped viruses with highly organized structures. The nucleocapsid (NC) core contains a capsid protein lattice enclosing the plus-sense RNA genome, and it is surrounded by a lipid bilayer containing a lattice of the E1 and E2 envelope glycoproteins. Capsid protein is synthesized in the cytoplasm and particle budding occurs at the plasma membrane (PM), but the traffic and assembly of viral components and the exit of virions from host cells are not well understood. To visualize the dynamics of capsid protein during infection, we developed a Sindbis virus infectious clone tagged with a tetracysteine motif. Tagged capsid protein could be fluorescently labeled with biarsenical dyes in living cells without effects on virus growth, morphology, or protein distribution. Live cell imaging and colocalization experiments defined distinct groups of capsid foci in infected cells. We observed highly motile internal puncta that colocalized with E2 protein, which may represent the transport machinery that capsid protein uses to reach the PM. Capsid was also found in larger nonmotile internal structures that colocalized with cellular G3BP and viral nsP3. Thus, capsid may play an unforeseen role in these previously observed G3BP-positive foci, such as regulation of cellular stress granules. Capsid puncta were also observed at the PM. These puncta colocalized with E2 and recruited newly synthesized capsid protein; thus, they may be sites of virus assembly and egress. Together, our studies provide the first dynamic views of the alphavirus capsid protein in living cells and a system to define detailed mechanisms during alphavirus infection.

Fluorosomes: fluorescent virus-like nanoparticles that represent a convenient tool to visualize receptor-ligand interactions

Viruses are the smallest life forms and parasitize on many eukaryotic organisms, including humans. Consequently, the study of viruses and viral diseases has had an enormous impact on diverse fields of biology and medicine. Due to their often pathogenic properties, viruses have not only had a strong impact on the development of immune cells but also on shaping entire immune mechanisms in their hosts. In order to better characterize virus-specific surface receptors, pathways of virus entry and the mechanisms of virus assembly, diverse methods to visualize virus particles themselves have been developed in the past decades. Apart from characterization of virus-specific mechanisms, fluorescent virus particles also serve as valuable platforms to study receptor-ligand interactions. Along those lines the authors have developed non-infectious virus-like nanoparticles (VNP), which can be decorated with immune receptors of choice and used for probing receptor-ligand interactions, an especially interesting application in the field of basic but also applied immunology research. To be able to better trace receptor-decorated VNP the authors have developed technology to introduce fluorescent proteins into such particles and henceforth termed them fluorosomes (FS). Since VNP are assembled in a simple expression system relying on HEK-293 cells, gene-products of interest can be assembled in a simple and straightforward fashion—one of the reasons why the authors like to call fluorosomes ‘the poor-man's staining tool’. Within this review article an overview on virus particle assembly, chemical and recombinant methods of virus particle labeling and examples on how FS can be applied as sensors to monitor receptor-ligand interactions on leukocytes are given.

APOBEC3G impairs the multimerization of the HIV-1 Vif protein in living cells

The HIV-1 viral infectivity factor (Vif) is a small basic protein essential for viral fitness and pathogenicity. Vif allows productive infection in nonpermissive cells, including most natural HIV-1 target cells, by counteracting the cellular cytosine deaminases APOBEC3G (apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3G [A3G]) and A3F. Vif is also associated with the viral assembly complex and packaged into viral particles through interactions with the viral genomic RNA and the nucleocapsid domain of Pr55(Gag). Recently, we showed that oligomerization of Vif into high-molecular-mass complexes induces Vif folding and influences its binding to high-affinity RNA binding sites present in the HIV genomic RNA. To get further insight into the role of Vif multimerization in viral assembly and A3G repression, we used fluorescence lifetime imaging microscopy (FLIM)- and fluorescence resonance energy transfer (FRET)-based assays to investigate Vif-Vif interactions in living cells. By using two N-terminally tagged Vif proteins, we show that Vif-Vif interactions occur in living cells. This oligomerization is strongly reduced when the putative Vif multimerization domain ((161)PPLP(164)) is mutated, indicating that this domain is crucial, but that regions outside this motif also participate in Vif oligomerization. When coexpressed together with Pr55(Gag), Vif is largely relocated to the cell membrane, where Vif oligomerization also occurs. Interestingly, wild-type A3G strongly interferes with Vif multimerization, contrary to an A3G mutant that does not bind to Vif. These findings confirm that Vif oligomerization occurs in living cells partly through its C-terminal motif and suggest that A3G may target and perturb the Vif oligomerization state to limit its functions in the cell.

Single-cell imaging of HIV-1 provirus (SCIP)

Recent advances in fluorescence microscopy provided tools for the investigation and the analysis of the viral replication steps in the cellular context. In the HIV field, the current visualization systems successfully achieve the fluorescent labeling of the viral envelope and proteins, but not the genome. Here, we developed a system able to visualize the proviral DNA of HIV-1 through immunofluorescence detection of repair foci for DNA double-strand breaks specifically induced in the viral genome by the heterologous expression of the I-SceI endonuclease. The system for Single-Cell Imaging of HIV-1 Provirus, named SCIP, provides the possibility to individually track integrated-viral DNA within the nuclei of infected cells. In particular, SCIP allowed us to perform a topological analysis of integrated viral DNA revealing that HIV-1 preferentially integrates in the chromatin localized at the periphery of the nuclei.

Major histocompatibility complex class-II molecules promote targeting of human immunodeficiency virus type 1 virions in late endosomes by enhancing internalization of nascent particles from the plasma membrane

Productive assembly of human immunodeficiency virus type 1 (HIV-1) takes place, primarily, at the plasma membrane. However, depending on the cell types, a significant proportion of nascent virus particles are internalized and routed to late endosomes. We previously reported that expression of human leucocyte antigen (HLA)-DR promoted a redistribution of Gag in late endosomes and an increased detection of mature virions in these compartments in HeLa and human embryonic kidney 293T model cell lines. Although this redistribution of Gag resulted in a marked decrease of HIV-1 release, the underlying mechanism remained undefined. Here, we provide evidence that expression of HLA-DR at the cell surface induces a redistribution of mature Gag products into late endosomes by enhancing nascent HIV-1 particle internalization from the plasma membrane through a process that relies on the presence of intact HLA-DR α and β-chain cytosolic tails. These findings raise the possibility that major histocompatibility complex class-II molecules might influence endocytic events at the plasma membrane and as a result promote endocytosis of progeny HIV-1 particles.

Superresolution imaging of HIV in infected cells with FlAsH-PALM

Imaging protein assemblies at molecular resolution without affecting biological function is a long-standing goal. The diffraction-limited resolution of conventional light microscopy (∼200-300 nm) has been overcome by recent superresolution (SR) methods including techniques based on accurate localization of molecules exhibiting stochastic fluorescence; however, SR methods still suffer important restrictions inherent to the protein labeling strategies. Antibody labels are encumbered by variable specificity, limited commercial availability and affinity, and are mostly restricted to fixed cells. Fluorescent protein fusions, though compatible with live cell imaging, substantially increase protein size and can interfere with their biological activity. We demonstrate SR imaging of proteins tagged with small tetracysteine motifs and the fluorescein arsenical helix binder (FlAsH-PALM). We applied FlAsH-PALM to image the integrase enzyme (IN) of HIV in fixed and living cells under experimental conditions that fully preserved HIV infectivity. The obtained resolution (∼30 nm) allowed us to characterize the distribution of IN within virions and intracellular complexes and to distinguish different HIV structural populations based on their morphology. We could thus discriminate ∼100 nm long mature conical cores from immature Gag shells and observe that in infected cells cytoplasmic (but not nuclear) IN complexes display a morphology similar to the conical capsid. Together with the presence of capsid proteins, our data suggest that cytoplasmic IN is largely present in intact capsids and that these can be found deep within the cytoplasm. FlAsH-PALM opens the door to in vivo SR studies of microbial complexes within host cells and may help achieve truly molecular resolution.

Picking faces out of a crowd: genetic labels for identification of proteins in correlated light and electron microscopy imaging

Correlated light and electron microscopic (CLEM) imaging is a powerful method for dissecting cell and tissue function at high resolution. Each imaging mode provides unique information, and the combination of the two can contribute to a better understanding of the spatiotemporal patterns of protein expression, trafficking, and function. Critical to these methods is the use of genetically appended tags that highlight specific proteins of interest in order to be able to pick them out of their complex cellular environment. Here we review and discuss the current generation of genetic labels for direct protein identification by CLEM, addressing their relative strengths and weaknesses and in what experiments they would be most useful.

Surveying protein structure and function using bis-arsenical small molecules

Exploration across the fields of biology, chemical biology, and medicine has led to an increasingly complex, albeit incomplete, view of the interactions that drive life's processes. The ability to monitor and track the movement, activity, and interactions of biomolecules in living cells is an essential part of this investigation. In our laboratory, we have endeavored to develop tools that are capable not only of monitoring protein localization but also reporting on protein structure and function. Central to our efforts is a new strategy, bipartite tetracysteine display, that relies on the specific and high-affinity interaction between a fluorogenic, bis-arsenical small molecule and a unique protein sequence, conformation, or assembly. In 1998, a small-molecule analogue of fluorescein with two arsenic atoms, FlAsH, was shown by Tsien and coworkers to fluoresce upon binding to a linear amino acid sequence, Cys-Cys-Arg-Glu-Cys-Cys. Later work demonstrated that substituting Pro-Gly for Arg-Glu optimized both binding and fluorescence yield. Our strategy of bipartite tetracysteine display emanated from the idea that it would be possible to replace the intervening Pro-Gly dipeptide in this sequence with a protein or protein partnership, provided the assembled protein fold successfully reproduced the approximate placement of the two Cys-Cys pairs. In this Account, we describe our recent progress in this area, with an emphasis on the fundamental concepts that underlie the successful use of bis-arsenicals such as FlAsH and the related ReAsH for bipartite display experiments. In particular, we highlight studies that have explored how broadly bipartite tetracysteine display can be employed and that have navigated the conformational boundary conditions favoring success. To emphasize the utility of these principles, we outline two recently reported applications of bipartite tetracysteine display. The first is a novel, encodable, selective, Src kinase sensor that lacks fluorescent proteins but possesses a fluorescent readout exceeding that of most sensors based on Förster resonance energy transfer (FRET). The second is a unique method, called complex-edited electron microscopy (CE-EM), that facilitates visualization of protein-protein complexes with electron microscopy. Exciting as these applications may be, the continued development of small-molecule tools with improved utility in living cells, let alone in vivo, will demand a more nuanced understanding of the fundamental photophysics that lead to fluorogenicity, as well as creative approaches toward the synthesis and identification of new and orthogonal dye-tag pairs that can be applied facilely in tandem. We describe one example of a dye-sequence tag pair that is chemically distinct from bis-arsenical chemistry. Through further effort, we expect that that bipartite tetracysteine display will find successful use in the study of sophisticated biological questions that are essential to the fields of biochemistry and biology as well as to our progressive understanding of human disease.

Visualizing HIV-1 assembly

The assembly of an HIV-1 particle is a complex, multistep process involving several viral and cellular proteins, RNAs and lipids. While many macroscopic and fixed-cell microscopic techniques have provided important insights into the structure of HIV-1 particles and the mechanisms by which they assemble, analysis of individual particles and their assembly in living cells offers the potential of surmounting many of the limitations inherent in other approaches. In this review, we discuss how the recent application of live-cell microscopic imaging techniques has increased our understanding of the process of HIV-1 particle assembly. In particular, we focus on recent studies that have employed total internal reflection fluorescence microscopy and other single-virion imaging techniques in live cells. These approaches have illuminated the dynamics of Gag protein assembly, viral RNA packaging and ESCRT (endosomal sorting complex required for transport) protein recruitment at the level of individual viral particles. Overall, the particular advantages of individual particle imaging in living cells have yielded findings that would have been difficult or impossible to obtain using macroscopic or fixed-cell microscopic techniques.

Utilization of fluorescently-labeled tetracysteine-tagged proteins to study virus entry by live cell microscopy

Viruses exploit cellular machinery to gain entry and initiate their replication cycle within host cells. The development of methods to visualize virus entry in live cells has provided new insights to the cellular processes involved in virus entry and the intracellular locations where viral payloads are deposited. The use of fluorescently labeled virus and high-resolution microscopy is currently the method of choice to study virus entry in live cells. While fluorescent protein fusions (e.g. viral proteins fused to GFP) have been used, the labeling of viral proteins that contain a small tetracysteine (tc) tag with biarsenical fluorescent compounds (e.g. FlAsH, ReAsH, Lumio-x) offers several advantages over conventional xFP-fusion constructs. This article describes methods for generating fluorescently labeled viruses encoding tc-tagged proteins that are suitable for the study of virus entry in live cells by fluorescence microscopy. Critical parameters required to quantify fluorescence signals from the labeled, tc-tagged proteins in individual virus particles during the entry process and the subsequent fate of the labeled viral proteins after virus uncoating are also described.

New insights into HIV assembly and trafficking

Assembly and release of human immunodeficiency virus type 1 (HIV-1) particles is mediated by the viral Gag polyprotein precursor. Gag is synthesized in the cytosol and rapidly translocates to membrane to orchestrate particle production. The cell biology of HIV-1 Gag trafficking is currently one of the least understood aspects of HIV-1 replication. In this review, we highlight the current understanding of the cellular machinery involved in Gag trafficking and virus assembly.

Novel imaging technologies in the study of HIV

The replication cycle of HIV proceeds within an infected cell and imaging techniques allow us to focus on the pathogen in this cellular environment. During recent years, both electron microscopy and fluorescence microscopy have evolved from methods providing two-dimensional still images to techniques that can resolve native, three-dimensional structures at resolutions down to approximately 20 Å, or allow direct real-time observation of dynamic intracellular events, respectively, thereby yielding numerous novel insights into HIV biology. Future technological developments are expected to narrow the gap between electron microscopy (high spatial and structural resolution, but no information about dynamics) and fluorescence microscopy (high temporal resolution and high throughput, but low spatial resolution), providing detailed views that will deepen our understanding of HIV–cell interactions.

A SNAP-tagged derivative of HIV-1--a versatile tool to study virus-cell interactions

Fluorescently labeled human immunodeficiency virus (HIV) derivatives, combined with the use of advanced fluorescence microscopy techniques, allow the direct visualization of dynamic events and individual steps in the viral life cycle. HIV proteins tagged with fluorescent proteins (FPs) have been successfully used for live-cell imaging analyses of HIV-cell interactions. However, FPs display limitations with respect to their physicochemical properties, and their maturation kinetics. Furthermore, several independent FP-tagged constructs have to be cloned and characterized in order to obtain spectral variations suitable for multi-color imaging setups. In contrast, the so-called SNAP-tag represents a genetically encoded non-fluorescent tag which mediates specific covalent coupling to fluorescent substrate molecules in a self-labeling reaction. Fusion of the SNAP-tag to the protein of interest allows specific labeling of the fusion protein with a variety of synthetic dyes, thereby offering enhanced flexibility for fluorescence imaging approaches.

Here we describe the construction and characterization of the HIV derivative HIV^SNAP, which carries the SNAP-tag as an additional domain within the viral structural polyprotein Gag. Introduction of the tag close to the C-terminus of the matrix domain of Gag did not interfere with particle assembly, release or proteolytic virus maturation. The modified virions were infectious and could be propagated in tissue culture, albeit with reduced replication capacity. Insertion of the SNAP domain within Gag allowed specific staining of the viral polyprotein in the context of virus producing cells using a SNAP reactive dye as well as the visualization of individual virions and viral budding sites by stochastic optical reconstruction microscopy. Thus, HIV^SNAP represents a versatile tool which expands the possibilities for the analysis of HIV-cell interactions using live cell imaging and sub-diffraction fluorescence microscopy.

Evidence of a role for soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) machinery in HIV-1 assembly and release

Retrovirus assembly is a complex process that requires the orchestrated participation of viral components and host-cell factors. The concerted movement of different viral proteins to specific sites in the plasma membrane allows for virus particle assembly and ultimately budding and maturation of infectious virions. The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins constitute the minimal machinery that catalyzes the fusion of intracellular vesicles with the plasma membrane, thus regulating protein trafficking. Using siRNA and dominant negative approaches we demonstrate here that generalized disruption of the host SNARE machinery results in a significant reduction in human immunodeficiency virus type 1 (HIV-1) and equine infectious anemia virus particle production. Further analysis of the mechanism involved revealed a defect at the level of HIV-1 Gag localization to the plasma membrane. Our findings demonstrate for the first time a role of SNARE proteins in HIV-1 assembly and release, likely by affecting cellular trafficking pathways required for Gag transport and association with the plasma membrane.

Chemical tags: applications in live cell fluorescence imaging

Technologies to visualize cellular structures and dynamics enable cell biologists to gain insight into complex biological processes. Currently, fluorescent proteins are used routinely to investigate the behavior of proteins in live cells. Chemical biology techniques for selective labeling of proteins with fluorescent labels have become an attractive alternative to fluorescent protein labeling. In the last ten years the progress in the development of chemical tagging methods have been substantial offering a broad palette of applications for live cell fluorescent microscopy. Several methods for protein labeling have been established, using protein tags, peptide tags and enzyme mediated tagging. This review focuses on the different strategies to achieve the attachment of fluorophores to proteins in live cells and cast light on the advantages and disadvantages of each individual method. Selected experiments in which chemical tags have been successfully applied to live cell imaging will be discussed and evaluated.

Exploration of biarsenical chemistry--challenges in protein research

The fluorescent modification of proteins (with genetically encoded low-molecular-mass fluorophores, affinity probes, or other chemically active species) is extraordinarily useful for monitoring and controlling protein functions in vitro, as well as in cell cultures and tissues. The large sizes of some fluorescent tags, such as fluorescent proteins, often perturb normal activity and localization of the protein of interest, as well as other effects. Of the many fluorescent-labeling strategies applied to in vitro and in vivo studies, one is very promising. This requires a very short (6- to 12-residue), appropriately spaced, tetracysteine sequence (-CCXXCC-); this is either placed at a protein terminus, within flexible loops, or incorporated into secondary structure elements. Proteins that contain the tetracysteine motif become highly fluorescent upon labeling with a nonluminescent biarsenical probe, and form very stable covalent complexes. We focus on the development, growth, and multiple applications of this protein research methodology, both in vitro and in vivo. Its application is not limited to intact-cell protein visualization; it has tremendous potential in other protein research disciplines, such as protein purification and activity control, electron microscopy imaging of cells or tissue, protein-protein interaction studies, protein stability, and aggregation studies.

Detecting RNA viruses in living mammalian cells by fluorescence microscopy

Traditional methods that rely on viral isolation and culture techniques continue to be the gold standards used for detection of infectious viral particles. However, new techniques that rely on visualization of live cells can shed light on understanding virus-host interaction for early stage detection and potential drug discovery. Live-cell imaging techniques that incorporate fluorescent probes into viral components provide opportunities for understanding mRNA expression, interaction, and virus movement and localization. Other viral replication events inside a host cell can be exploited for non-invasive detection, such as single-virus tracking, which does not inhibit viral infectivity or cellular function. This review highlights some of the recent advances made using these novel approaches for visualization of viral entry and replication in live cells.

Fluorescent labeling of tetracysteine-tagged proteins in intact cells

In this paper, we provide a general protocol for labeling proteins with the membrane-permeant fluorogenic biarsenical dye fluorescein arsenical hairpin binder-ethanedithiol (FlAsH-EDT₂). Generation of the tetracysteine-tagged protein construct by itself is not described, as this is a protein-specific process. This method allows site-selective labeling of proteins in living cells and has been applied to a wide variety of proteins and biological problems. We provide here a generally applicable labeling procedure and discuss the problems that can occur as well as general considerations that must be taken into account when designing and implementing the procedure. The method can even be applied to proteins with expression below 1 pmol mg⁻¹ of protein, such as G protein-coupled receptors, and it can be used to study the intracellular localization of proteins as well as functional interactions in fluorescence resonance energy transfer experiments. The labeling procedure using FlAsH-EDT₂ as described takes 2-3 h, depending on the number of samples to be processed.

Introduction of tag epitopes in the inter-AUG region of foot and mouth disease virus: effect on the L protein

Foot-and-mouth disease virus (FMDV) initiates translation from two in-frame AUG codons producing two forms of the leader (L) proteinase, Lab (starting at the first AUG) and Lb (starting at second AUG). In a previous study, we have demonstrated that a cDNA-derived mutant FMDV (A24-L1123) containing a 57-nucleotide transposon (tn) insertion between the two AUG initiation codons (inter-AUG region) was completely attenuated in cattle, suggesting that this region is involved in viral pathogenesis. To investigate the potential role of the Lab protein in attenuation, we have introduced two epitope tags (Flag: DYKDDDK and HA: YPYDVPDYA) or a small tetracysteine motif (tc: CCGPCC) into the pA24-L1123 infectious DNA clone. Mutant viruses with a small plaque phenotype similar to the parental A24-L1123 were recovered after transfection of constructs encoding the Flag tag and the tc motif. However, expression of the Flag- or tc-tagged Lab protein was abolished or greatly diminished in these viruses. Interestingly, the A24-L1123/Flag virus acquired an extra base in the inter-AUG region that resulted in new AUG codons in-frame with the second AUG, and produced a larger Lb protein. This N terminal extension of the Lb protein in mutant A24-L1123/Flag did not affect virus viability or L functions in cell culture.

HIV-1 viral protein r: from structure to function

The viral protein r (Vpr) of HIV-1 binds several host proteins leading to pleiotropic functions, such as G2/M cell cycle arrest, apoptosis induction and gene transactivation. Vpr is encapsidated through the Gag C-terminus into the nascent viral particles, suggesting that Vpr plays several important functions in the early stages of the viral lifecycle. In this regard, Vpr interacts with nucleic acids and membranes to facilitate the preintegration complex migration and incorporation into the nucleus of nondividing cells. Thus, Vpr has to recruit several host and viral factors to promote its functions during HIV-1 pathogenesis. This article focuses on its interacting partners by giving an overview of the functional outcome of the different Vpr complexes, as well as the structural determinants of Vpr required for its binding properties.

Live-cell coimaging of the genomic RNAs and Gag proteins of two lentiviruses

Human immunodeficiency virus type 1 (HIV-1) Gag and genomic RNA determinants required for encapsidation are well established, but where and when encapsidation occurs in the cell is unknown. We constructed MS2 phage coat protein labeling systems to track spatial dynamics of primate and nonprimate lentiviral genomic RNAs (HIV-1 and feline immunodeficiency virus [FIV]) vis-à-vis their Gag proteins in live cells. Genomic RNAs of both lentiviral genera were observed to traffic into the cytoplasm, and this was Rev dependent. In transit, FIV Gag and genomic RNA accumulated independently of each other at the nuclear envelope, and focal colocalizations of genomic RNA with an intact packaging signal (psi) and Gag were observed to extend outward from the cytoplasmic face. In contrast, although HIV-1 genomic RNA was detected at the nuclear envelope, HIV-1 Gag was not. For both lentiviruses, genomic RNAs were seen at the plasma membrane if and only if Gag was present and psi was intact. In addition, HIV-1 and FIV genomes accumulated with Gag in late endosomal foci, again, only psi dependently. Thus, lentiviral genomic RNAs require specific Gag binding to accumulate at the plasma membrane, packaged genomes cointernalize with Gag into the endosomal pathway, and plasma membrane RNA incorporation by Gag does not trigger committed lentiviral particle egress from the cell. Based on the FIV results, we hypothesize that the Gag-genome association may initiate at the nuclear envelope.

Tetracysteine-tagged prion protein allows discrimination between the native and converted forms

The conformational conversion of prion protein (PrP) from a native conformation to the amyloid form is a hallmark of transmissible spongiform encephalopathies. Conversion is usually monitored by fluorescent dyes, which bind generic amyloids and are less suited for living cell imaging. We report a new method for the synthesis of membrane-permeable and membrane-impermeable biarsenical reagents, which are then used to monitor murine PrP (mPrP) misfolding. We introduced tetracysteine (TC) tags into three different positions of mPrP, which folded into a native-like structure. Whereas mPrPs with a TC tag inserted at the N-terminus or C-terminus supported fibril formation, insertion into the helix 2-helix 3 loop inhibited conversion. We devised a quantitative protease-free method to determine the fraction of converted PrP, based on the ability of the fluorescein arsenical helix binder reagent to differentiate between the monomeric and fibrilized form of TC-tagged PrP, and showed that TC-tagged mPrP could be detected on transfected cells, thereby expanding the potential use of this method for the detection and study of conformational diseases.

HIV-1 Vpr oligomerization but not that of Gag directs the interaction between Vpr and Gag

During HIV-1 assembly, the viral protein R (Vpr) is incorporated into newly made viral particles via an interaction with the C-terminal domain of the Gag polyprotein precursor Pr55(Gag). Vpr has been implicated in the nuclear import of newly made viral DNA and subsequently in its transcription. In addition, Vpr can affect the cell physiology by causing G(2)/M cell cycle arrest and apoptosis. Vpr can form oligomers, but their roles have not yet been investigated. We have developed fluorescence lifetime imaging microscopy-fluorescence resonance energy transfer-based assays to monitor the interaction between Pr55(Gag) and Vpr in HeLa cells. To that end, we used enhanced green fluorescent protein-Vpr that can be incorporated into the virus and tetracysteine (TC)-tagged Pr55(Gag)-TC. This TC motif is tethered to the C terminus of Pr55(Gag) and does not interfere with Pr55(Gag) trafficking and the assembly of virus-like particles (VLPs). Results show that the Pr55(Gag)-Vpr complexes accumulated mainly at the plasma membrane. In addition, results with Pr55(Gag)-TC mutants confirm that the (41)LXXLF domain of Gag-p6 is essential for Pr55(Gag)-Vpr interaction. We also report that Vpr oligomerization is crucial for Pr55(Gag) recognition and its accumulation at the plasma membrane. On the other hand, Pr55(Gag)-Vpr complexes are still formed when Pr55(Gag) carries mutations impairing its multimerization. These findings suggest that Pr55(Gag)-Vpr recognition and complex formation occur early during Pr55(Gag) assembly.