Visualizing HIV-1 Assembly at the T-Cell Plasma Membrane Using Single-Molecule Localization Microscopy.

The 20-year revolution in optical fluorescence microscopy, supported by the optimization of both spatial resolution and timely acquisition, allows the visualization of nanoscaled objects in cell biology. Currently, the use of a recent generation of super-resolution fluorescence microscope coupled with improved fluorescent probes gives the possibility to study the replicative cycle of viruses in living cells, at the single-virus particle or protein level. Here, we highlight the protocol for visualizing HIV-1 Gag assembly at the host T-cell plasma membrane using super-resolution light microscopy. Total internal reflection fluorescence microscopy (TIRF-M) coupled with single-molecule localization microscopy (SMLM) enables the detection and characterization of the assembly of viral proteins at the plasma membrane of infected host cells at the single protein level. Here, we describe the TIRF equipment, the T-cell culture for HIV-1, the sample preparation for single-molecule localization microscopies such as PALM and STORM, acquisition protocols, and Gag assembling cluster analysis.

Sequestration of membrane cholesterol by cholesterol-binding proteins inhibits SARS-CoV-2 entry into Vero E6 cells.

Membrane lipids and proteins form dynamic domains crucial for physiological and pathophysiological processes, including viral infection. Many plasma membrane proteins, residing within membrane domains enriched with cholesterol (CHOL) and sphingomyelin (SM), serve as receptors for attachment and entry of viruses into the host cell. Among these, human coronaviruses, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), use proteins associated with membrane domains for initial binding and internalization. We hypothesized that the interaction of lipid-binding proteins with CHOL in plasma membrane could sequestrate lipids and thus affect the efficiency of virus entry into host cells, preventing the initial steps of viral infection. We have prepared CHOL-binding proteins with high affinities for lipids in the plasma membrane of mammalian cells. Binding of the perfringolysin O domain four (D4) and its variant D4E458L to membrane CHOL impaired the internalization of the receptor-binding domain of the SARS-CoV-2 spike protein and the pseudovirus complemented with the SARS-CoV-2 spike protein. SARS-CoV-2 replication in Vero E6 cells was also decreased. Overall, our results demonstrate that the integrity of CHOL-rich membrane domains and the accessibility of CHOL in the membrane play an essential role in SARS-CoV-2 cell entry.

Ebola Virus Glycoprotein Strongly Binds to Membranes in the Absence of Receptor Engagement.

Ebola virus (EBOV) is an enveloped virus that must fuse with the host cell membrane in order to release its genome and initiate infection. This process requires the action of the EBOV envelope glycoprotein (GP), encoded by the virus, which resides in the viral envelope and consists of a receptor binding subunit, GP1, and a membrane fusion subunit, GP2. Despite extensive research, a mechanistic understanding of the viral fusion process is incomplete. To investigate GP-membrane association, a key step in the fusion process, we used two approaches: high-throughput measurements of single-particle diffusion and single-molecule measurements with optical tweezers. Using these methods, we show that the presence of the endosomal Niemann-Pick C1 (NPC1) receptor is not required for primed GP-membrane binding. In addition, we demonstrate this binding is very strong, likely attributed to the interaction between the GP fusion loop and the membrane's hydrophobic core. Our results also align with previously reported findings, emphasizing the significance of acidic pH in the protein-membrane interaction. Beyond Ebola virus research, our approach provides a powerful toolkit for studying other protein-membrane interactions, opening new avenues for a better understanding of protein-mediated membrane fusion events.

HCMV US2 co-opts TRC8 to degrade the endoplasmic reticulum-resident protein LMAN2L.

The human cytomegalovirus (HCMV) pUS2 glycoprotein exploits the host's endoplasmic reticulum (ER)-associated degradation (ERAD) pathway to degrade major histocompatibility complex class I (MHC-I) and prevent antigen presentation. Beyond MHC-I, pUS2 has been shown to target a range of cellular proteins for degradation, preventing their cell surface expression. Here we have identified a novel pUS2 target, ER-resident protein lectin mannose binding 2 like (LMAN2L). pUS2 expression was both necessary and sufficient for the downregulation of LMAN2L, which was dependent on the cellular E3 ligase TRC8. Given the hypothesized role of LMAN2L in the trafficking of glycoproteins, we employed proteomic plasma membrane profiling to measure LMAN2L-dependent changes at the cell surface. A known pUS2 target, integrin alpha-6 (ITGA6), was downregulated from the surface of LMAN2L-deficient cells, but not other integrins. Overall, these results suggest a novel strategy of pUS2-mediated protein degradation whereby pUS2 targets LMAN2L to impair trafficking of ITGA6. Given that pUS2 can directly target other integrins, we propose that this single viral protein may exhibit both direct and indirect mechanisms to downregulate key cell surface molecules.

Phospho-eIF4E stimulation regulates coronavirus entry by selective expression of cell membrane-residential factors.

The eukaryotic translation initiation factor eIF4E can regulate cellular translation via phosphorylation on serine 209. In a recent study, by two rounds of TMT relative quantitative proteomics, we found that phosphorylated eIF4E (p-eIF4E) favors the translation of selected mRNAs, and the encoded proteins are mainly involved in ECM-receptor, focal adhesion, and PI3K-Akt signaling. The current paper is focused on the relationship between p-eIF4E and the downstream host cell proteins, and their presumed effect on efficient entry of PEDV. We found that the depletion of membrane-residential factor TSPAN3, CD63, and ITGB2 significantly inhibited viral invasion of PEDV, and reduced the entry of pseudotyped particles PEDV-pp, SARS-CoV-pp, and SARS-CoV-2-pp. The specific antibodies of TSPAN3, CD63, and ITGB2 blocked the adsorption of PEDV into host cells. Moreover, we detected that eIF4E phosphorylation was increased at 1 h after PEDV infection, in accordance with the expression of TSPAN3, CD63, and ITGB2. Similar trends appeared in the intestines of piglets in the early stage of PEDV challenge. Compared with Vero cells, S209A-Vero cells in which eIF4E cannot be phosphorylated showed a decrease of invading PEDV virions. MNK kinase inhibitor blocked PEDV invasion, as well as reduced the accumulation of TSPAN3, CD63, and ITGB2. Further study showed that the ERK-MNK pathway was responsible for the regulation of PEDV-induced early phosphorylation of eIF4E. This paper demonstrates for the first time the connections among p-eIF4E stimulation and membrane-residential host factors. Our findings also enrich the understanding of the biological function of phosphorylated eIF4E during the viral life cycle.IMPORTANCEThe eukaryotic translation initiation factor eIF4E can regulate cellular translation via phosphorylation. In our previous study, several host factors susceptible to a high level of p-eIF4E were found to be conducive to viral infection by coronavirus PEDV. The current paper is focused on cell membrane-residential factors, which are involved in signal pathways that are sensitive to phosphorylated eIF4E. We found that the ERK-MNK pathway was activated, which resulted in the stimulation of phosphorylation of eIF4E in early PEDV infection. Phospho-eIF4E promoted the viral invasion of PEDV by upregulating the expression of host factors TSPAN3, CD63, and ITGB2 at the translation level rather than at the transcription level. Moreover, TSPAN3, CD63, or ITGB2 facilitates the efficient entry of coronavirus SARS-CoV, SARS-CoV-2, and HCoV-OC43. Our findings broaden our insights into the dynamic phosphorylation of eIF4E during the viral life cycle, and provide further evidence that phosphorylated eIF4E regulates selective translation of host mRNA.

CLIC and membrane wound repair pathways enable pandemic norovirus entry and infection.

Globally, most cases of gastroenteritis are caused by pandemic GII.4 human norovirus (HuNoV) strains with no approved therapies or vaccines available. The cellular pathways that these strains exploit for cell entry and internalization are unknown. Here, using nontransformed human jejunal enteroids (HIEs) that recapitulate the physiology of the gastrointestinal tract, we show that infectious GII.4 virions and virus-like particles are endocytosed using a unique combination of endosomal acidification-dependent clathrin-independent carriers (CLIC), acid sphingomyelinase (ASM)-mediated lysosomal exocytosis, and membrane wound repair pathways. We found that besides the known interaction of the viral capsid Protruding (P) domain with host glycans, the Shell (S) domain interacts with both galectin-3 (gal-3) and apoptosis-linked gene 2-interacting protein X (ALIX), to orchestrate GII.4 cell entry. Recognition of the viral and cellular determinants regulating HuNoV entry provides insight into the infection process of a non-enveloped virus highlighting unique pathways and targets for developing effective therapeutics.

The trajectory patterns of single HIV-1 virus-like particle in live CD4 cells: A real time three-dimensional multi-resolution microscopy study using encapsulated nonblinking giant quantum dot.

BACKGROUND: The exploration of virology knowledge was limited by the optical technology for the observation of virus. Previously, a three-dimensional multi-resolution real-time microscope system (3D-MRM) was developed to observe the uptake of HIV-1-tat peptide-modified nanoparticles in cell membrane. In this study, we labeled HIV-1 virus-like particles (VLPs) with passivated giant quantum dots (gQDs) and recorded their interactive trajectories with human Jurkat CD4 cells through 3D-MRM. METHODS: The labeled of gQDs of the HIV-1 VLPs in sucrose-gradient purified viral lysates was first confirmed by Cryo-electronic microscopy and Western blot assay. After the infection with CD4 cells, the gQD-labeled VLPs were visualized and their extracellular and intracellular trajectories were recorded by 3D-MRM. RESULTS: A total of 208 prime trajectories was identified and classified into three distinct patterns: cell-free random diffusion pattern, directional movement pattern and cell-associated movement pattern, with distributions and mean durations were 72.6%/87.6 s, 9.1%/402.7 s and 18.3%/68.7 s, respectively. Further analysis of the spatial-temporal relationship between VLP trajectories and CD4 cells revealed the three stages of interactions: (1) cell-associated (extracellular) diffusion stage, (2) cell membrane surfing stage and (3) intracellular directional movement stage. CONCLUSION: A complete trajectory of HIV-1 VLP interacting with CD4 cells was presented in animation. This encapsulating method could increase the accuracy for the observation of HIV-1-CD4 cell interaction in real time and three dimensions.

Identification of Cellular Genes Involved in Baculovirus GP64 Trafficking to the Plasma Membrane.

The baculovirus envelope protein GP64 is an essential component of the budded virus and is necessary for efficient virion assembly. Little is known regarding intracellular trafficking of GP64 to the plasma membrane, where it is incorporated into budding virions during egress. To identify host proteins and potential cellular trafficking pathways that are involved in delivery of GP64 to the plasma membrane, we developed and characterized a stable Drosophila cell line that inducibly expresses the AcMNPV GP64 protein and used that cell line in combination with a targeted RNA interference (RNAi) screen of vesicular protein trafficking pathway genes. Of the 37 initial hits from the screen, we validated and examined six host genes that were important for trafficking of GP64 to the cell surface. Validated hits included Rab GTPases Rab1 and Rab4, Clathrin heavy chain, clathrin adaptor protein genes AP-1-2ß and AP-2µ, and Snap29. Two gene knockdowns (Rab5 and Exo84) caused substantial increases (up to 2.5-fold) of GP64 on the plasma membrane. We found that a small amount of GP64 is released from cells in exosomes and that some portion of cell surface GP64 is endocytosed, suggesting that recycling helps to maintain GP64 at the cell surface. IMPORTANCE While much is known regarding trafficking of viral envelope proteins in mammalian cells, little is known about this process in insect cells. To begin to understand which factors and pathways are needed for trafficking of insect virus envelope proteins, we engineered a Drosophila melanogaster cell line and implemented an RNAi screen to identify cellular proteins that aid transport of the model baculovirus envelope protein (GP64) to the cell surface. For this we developed an experimental system that leverages the large array of tools available for Drosophila and performed a targeted RNAi screen to identify cellular proteins involved in GP64 trafficking to the cell surface. Since viral envelope proteins are often critical for production of infectious progeny virions, these studies lay the foundation for understanding how either pathogenic insect viruses (baculoviruses) or insect-vectored viruses (e.g., flaviviruses, alphaviruses) egress from cells in tissues such as the midgut to enable systemic virus infection.

Hijacking of Cellular Functions by Severe Acute Respiratory Syndrome Coronavirus-2. Permeabilization and Polarization of the Host Lipid Membrane by Viroporins.

Like all viral infections, SARS-CoV-2 acts at multiple levels, hijacking fundamental cellular functions and assuring its replication and immune system evasion. In particular, the viral 3' Open Reading Frame (ORF3a) codes for a hydrophobic protein, which embeds in the cellular membrane, where it acts as an ion viroporin and is related to strong inflammatory response. Here we report equilibrium and enhanced sampling molecular dynamic simulation of the SARS-CoV-2 ORF3a in a model lipid bilayer, showing how the protein permeabilizes the lipid membrane, via the formation of a water channel, which in turn assures ion transport. We report the free energy profile for both K+ and Cl- transfer from the cytosol to the extracellular domain. The important role of ORF3a in the viral cycle and its high conservation among coronaviruses may also make it a target of choice for future antiviral development, further justifying the elucidation of its mechanism at the atomistic level.

Theta-Defensins to Counter COVID-19 as Furin Inhibitors: In Silico Efficiency Prediction and Novel Compound Design.

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), was characterized as a pandemic by the World Health Organization (WHO) in Dec. 2019. SARS-CoV-2 binds to the cell membrane through spike proteins on its surface and infects the cell. Furin, a host-cell enzyme, possesses a binding site for the spike protein. Thus, molecules that block furin could potentially be a therapeutic solution. Defensins are antimicrobial peptides that can hypothetically inhibit furin because of their arginine-rich structure. Theta-defensins, a subclass of defensins, have attracted attention as drug candidates due to their small size, unique structure, and involvement in several defense mechanisms. Theta-defensins could be a potential treatment for COVID-19 through furin inhibition and an anti-inflammatory mechanism. Note that inflammatory events are a significant and deadly condition that could happen at the later stages of COVID-19 infection. Here, the potential of theta-defensins against SARS-CoV-2 infection was investigated through in silico approaches. Based on docking analysis results, theta-defensins can function as furin inhibitors. Additionally, a novel candidate peptide against COVID-19 with optimal properties regarding antigenicity, stability, electrostatic potential, and binding strength was proposed. Further in vitro/in vivo investigations could verify the efficiency of the designed novel peptide.

Altered TMPRSS2 usage by SARS-CoV-2 Omicron impacts infectivity and fusogenicity.

The SARS-CoV-2 Omicron BA.1 variant emerged in 20211 and has multiple mutations in its spike protein2. Here we show that the spike protein of Omicron has a higher affinity for ACE2 compared with Delta, and a marked change in its antigenicity increases Omicron's evasion of therapeutic monoclonal and vaccine-elicited polyclonal neutralizing antibodies after two doses. mRNA vaccination as a third vaccine dose rescues and broadens neutralization. Importantly, the antiviral drugs remdesivir and molnupiravir retain efficacy against Omicron BA.1. Replication was similar for Omicron and Delta virus isolates in human nasal epithelial cultures. However, in lung cells and gut cells, Omicron demonstrated lower replication. Omicron spike protein was less efficiently cleaved compared with Delta. The differences in replication were mapped to the entry efficiency of the virus on the basis of spike-pseudotyped virus assays. The defect in entry of Omicron pseudotyped virus to specific cell types effectively correlated with higher cellular RNA expression of TMPRSS2, and deletion of TMPRSS2 affected Delta entry to a greater extent than Omicron. Furthermore, drug inhibitors targeting specific entry pathways3 demonstrated that the Omicron spike inefficiently uses the cellular protease TMPRSS2, which promotes cell entry through plasma membrane fusion, with greater dependency on cell entry through the endocytic pathway. Consistent with suboptimal S1/S2 cleavage and inability to use TMPRSS2, syncytium formation by the Omicron spike was substantially impaired compared with the Delta spike. The less efficient spike cleavage of Omicron at S1/S2 is associated with a shift in cellular tropism away from TMPRSS2-expressing cells, with implications for altered pathogenesis.

Investigating interactions between chloroquine/hydroxychloroquine and their single enantiomers and angiotensin-converting enzyme 2 by a cell membrane chromatography method.

Chloroquine and hydroxychloroquine have been studied since the early clinical treatment of SARS-CoV-2 outbreak. Considering these two chiral drugs are currently in use as the racemate, high-expression angiotensin-converting enzyme 2 cell membrane chromatography was established for investigating the differences of two paired enantiomers binding to angiotensin-converting enzyme 2 receptor. Molecular docking assay and detection of SARS-CoV-2 spike pseudotyped virus entry into angiotensin-converting enzyme 2-HEK293T cells were also conducted for further investigation. Results showed that each single enantiomer could bind well to angiotensin-converting enzyme 2, but there were differences between the paired enantiomers and corresponding racemate in frontal analysis. R-Chloroquine showed better angiotensin-converting enzyme 2 receptor binding ability compared to S-chloroquine/chloroquine (racemate). S-Hydroxychloroquine showed better angiotensin-converting enzyme 2 receptor binding ability than R-hydroxychloroquine/hydroxychloroquine. Moreover, each single enantiomer was proved effective compared with the control group; compared with S-chloroquine or the racemate, R-chloroquine showed better inhibitory effects at the same concentration. As for hydroxychloroquine, R-hydroxychloroquine showed better inhibitory effects than S-hydroxychloroquine, but it slightly worse than the racemate. In conclusion, R-chloroquine showed better angiotensin-converting enzyme 2 receptor binding ability and inhibitory effects compared to S-chloroquine/chloroquine (racemate). S-Hydroxychloroquine showed better angiotensin-converting enzyme 2 receptor binding ability than R-hydroxychloroquine/hydroxychloroquine (racemate), while the effect of preventing SARS-CoV-2 pseudovirus from entering cells was weaker than R-hydroxychloroquine/hydroxychloroquine (racemate).

Phytosterol metabolism in plant positive-strand RNA virus replication.

The genome of most plant viruses consists of a single positive-strand of RNA (+ ssRNA). Successful replication of these viruses is fully dependent on the endomembrane system of the infected cells, which experiences a massive proliferation and a profound reshaping that enables assembly of the macromolecular complexes where virus genome replication occurs. Assembly of these viral replicase complexes (VRCs) requires a highly orchestrated interplay of multiple virus and co-opted host cell factors to create an optimal microenvironment for efficient assembly and functioning of the virus genome replication machinery. It is now widely accepted that VRC formation involves the recruitment of high levels of sterols, but the specific role of these essential components of cell membranes and the precise molecular mechanisms underlying sterol enrichment at VRCs are still poorly known. In this review, we intend to summarize the most relevant knowledge on the role of sterols in ( +)ssRNA virus replication and discuss the potential of manipulating the plant sterol pathway to help plants fight these infectious agents.

Porcine Deltacoronavirus Utilizes Sialic Acid as an Attachment Receptor and Trypsin Can Influence the Binding Activity.

Porcine deltacoronavirus (PDCoV) is a novel coronavirus that causes diarrhea in nursing piglets. Studies showed that PDCoV uses porcine aminopeptidase N (pAPN) as an entry receptor, but the infection of pAPN-knockout cells or pigs with PDCoV revealed that pAPN might be not a critical functional receptor, implying there exists an unidentified receptor involved in PDCoV infection. Herein, we report that sialic acid (SA) can act as an attachment receptor for PDCoV invasion and facilitate its infection. We first demonstrated that the carbohydrates destroyed on the cell membrane using NaIO4 can alleviate the susceptibility of cells to PDCoV. Further study showed that the removal of SA, a typical cell-surface carbohydrate, could influence the PDCoV infectivity to the cells significantly, suggesting that SA was involved in the infection. The results of plaque assay and Western blotting revealed that SA promoted PDCoV infection by increasing the number of viruses binding to SA on the cell surface during the adsorption phase, which was also confirmed by atomic force microscopy at the microscopic level. In in vivo experiments, we found that the distribution levels of PDCoV and SA were closely relevant in the swine intestine, which contains huge amount of trypsin. We further confirmed that SA-binding capacity to PDCoV is related to the pre-treatment of PDCoV with trypsin. In conclusion, SA is a novel attachment receptor for PDCoV infection to enhance its attachment to cells, which is dependent on the pre-treatment of trypsin on PDCoV. This study paves the way for dissecting the mechanisms of PDCoV-host interactions and provides new strategies to control PDCoV infection.

Exogenous Vimentin Supplementation Transiently Affects Early Steps during HPV16 Pseudovirus Infection.

Understanding and modulating the early steps in oncogenic Human Papillomavirus (HPV) infection has great cancer-preventative potential, as this virus is the etiological agent of virtually all cervical cancer cases and is associated with many other anogenital and oropharyngeal cancers. Previous work from our laboratory has identified cell-surface-expressed vimentin as a novel HPV16 pseudovirus (HPV16-PsVs)-binding molecule modulating its infectious potential. To further explore its mode of inhibiting HPV16-PsVs internalisation, we supplemented it with exogenous recombinant human vimentin and show that only the globular form of the molecule (as opposed to the filamentous form) inhibited HPV16-PsVs internalisation in vitro. Further, this inhibitory effect was only transient and not sustained over prolonged incubation times, as demonstrated in vitro and in vivo, possibly due to full-entry molecule engagement by the virions once saturation levels have been reached. The vimentin-mediated delay of HPV16-PsVs internalisation could be narrowed down to affecting multiple steps during the virus' interaction with the host cell and was found to affect both heparan sulphate proteoglycan (HSPG) binding as well as the subsequent entry receptor complex engagement. Interestingly, decreased pseudovirus internalisation (but not infection) in the presence of vimentin was also demonstrated for oncogenic HPV types 18, 31 and 45. Together, these data demonstrate the potential of vimentin as a modulator of HPV infection which can be used as a tool to study early mechanisms in infectious internalisation. However, further refinement is needed with regard to vimentin's stabilisation and formulation before its development as an alternative prophylactic means.

Plasma Membrane Anchoring and Gag:Gag Multimerization on Viral RNA Are Critical Properties of HIV-1 Gag Required To Mediate Efficient Genome Packaging.

Human immunodeficiency virus type 1 (HIV-1) Gag selects and packages the HIV RNA genome during virus assembly. However, HIV-1 RNA constitutes only a small fraction of the cellular RNA. Although Gag exhibits a slight preference to viral RNA, most of the cytoplasmic Gag proteins are associated with cellular RNAs. Thus, it is not understood how HIV-1 achieves highly efficient genome packaging. We hypothesize that besides RNA binding, other properties of Gag are important for genome packaging. Many Gag mutants have assembly defects that preclude analysis of their effects on genome packaging. To bypass this challenge, we established complementation systems that separate the particle-assembling and RNA-binding functions of Gag: we used a set of Gag proteins to drive particle assembly and an RNA-binding Gag to package HIV-1 RNA. We have developed two types of RNA-binding Gag in which packaging is mediated by the authentic nucleocapsid (NC) domain or by a nonviral RNA-binding domain. We found that in both cases, mutations that affect the multimerization or plasma membrane anchoring properties of Gag reduce or abolish RNA packaging. These mutant Gag can coassemble into particles but cannot package the RNA genome efficiently. Our findings indicate that HIV-1 RNA packaging occurs at the plasma membrane and RNA-binding Gag needs to multimerize on RNA to encapsidate the viral genome. IMPORTANCE To generate infectious virions, HIV-1 must package its full-length RNA as the genome during particle assembly. HIV-1 Gag:RNA interactions mediate genome packaging, but the mechanism remains unclear. Only a minor portion of the cellular RNA is HIV-1 RNA, and most of the RNAs associated with cytoplasmic Gag are cellular RNAs. However, >94% of the HIV-1 virions contain viral RNA genome. We posited that, besides RNA binding, other properties of Gag contribute to genome packaging. Using two complementation systems, we examined features of Gag that are important for genome packaging. We found that the capacities for Gag to multimerize and to anchor at the plasma membrane are critical for genome packaging. Our results revealed that Gag needs to multimerize on viral RNA at the plasma membrane in order to package RNA genome.

M-Sec induced by HTLV-1 mediates an efficient viral transmission.

Human T-cell leukemia virus type 1 (HTLV-1) infects target cells primarily through cell-to-cell routes. Here, we provide evidence that cellular protein M-Sec plays a critical role in this process. When purified and briefly cultured, CD4+ T cells of HTLV-1 carriers, but not of HTLV-1- individuals, expressed M-Sec. The viral protein Tax was revealed to mediate M-Sec induction. Knockdown or pharmacological inhibition of M-Sec reduced viral infection in multiple co-culture conditions. Furthermore, M-Sec knockdown reduced the number of proviral copies in the tissues of a mouse model of HTLV-1 infection. Phenotypically, M-Sec knockdown or inhibition reduced not only plasma membrane protrusions and migratory activity of cells, but also large clusters of Gag, a viral structural protein required for the formation of viral particles. Taken together, these results suggest that M-Sec induced by Tax mediates an efficient cell-to-cell viral infection, which is likely due to enhanced membrane protrusions, cell migration, and the clustering of Gag.

A Comprehensive Analysis of Human Endogenous Retroviruses HERV-K (HML.2) from Teratocarcinoma Cell Lines and Detection of Viral Cargo in Microvesicles.

About 8% of our genome is composed of sequences from Human Endogenous Retroviruses (HERVs). The HERV-K (HML.2) family, here abbreviated HML.2, is able to produce virus particles that were detected in cell lines, malignant tumors and in autoimmune diseases. Parameters and properties of HML.2 released from teratocarcinoma cell lines GH and Tera-1 were investigated in detail. In most experiments, analyzed viruses were purified by density gradient centrifugation. HML.2 structural proteins, reverse transcriptase (RT) activity, viral RNA (vRNA) and particle morphology were analyzed. The HML.2 markers were predominantly detected in fractions with a buoyant density of 1.16 g/cm3. Deglycosylation of TM revealed truncated forms of transmembrane (TM) protein. Free virions and extracellular vesicles (presumably microvesicles-MVs) with HML.2 elements, including budding intermediates, were detected by electron microscopy. Viral elements and assembled virions captured and exported by MVs can boost specific immune responses and trigger immunomodulation in recipient cells. Sequencing of cDNA clones demonstrated exclusive presence of HERV-K108 env in HML.2 from Tera-1 cells. Not counting two recombinant variants, four known env sequences were found in HML.2 from GH cells. Obtained results shed light on parameters and morphology of HML.2. A possible mechanism of HML.2-induced diseases is discussed.

Effector-mediated membrane disruption controls cell death in CBASS antiphage defense.

Cyclic oligonucleotide-based antiphage signaling systems (CBASS) are antiviral defense operons that protect bacteria from phage replication. Here, we discover a widespread class of CBASS transmembrane (TM) effector proteins that respond to antiviral nucleotide signals and limit phage propagation through direct membrane disruption. Crystal structures of the Yersinia TM effector Cap15 reveal a compact 8-stranded ß-barrel scaffold that forms a cyclic dinucleotide receptor domain that oligomerizes upon activation. We demonstrate that activated Cap15 relocalizes throughout the cell and specifically induces rupture of the inner membrane. Screening for active effectors, we identify the function of distinct families of CBASS TM effectors and demonstrate that cell death via disruption of inner-membrane integrity is a common mechanism of defense. Our results reveal the function of the most prominent class of effector protein in CBASS immunity and define disruption of the inner membrane as a widespread strategy of abortive infection in bacterial phage defense.

How Viruses Hijack and Modify the Secretory Transport Pathway.

Eukaryotic cells contain dynamic membrane-bound organelles that are constantly remodeled in response to physiological and environmental cues. Key organelles are the endoplasmic reticulum, the Golgi apparatus and the plasma membrane, which are interconnected by vesicular traffic through the secretory transport route. Numerous viruses, especially enveloped viruses, use and modify compartments of the secretory pathway to promote their replication, assembly and cell egression by hijacking the host cell machinery. In some cases, the subversion mechanism has been uncovered. In this review, we summarize our current understanding of how the secretory pathway is subverted and exploited by viruses belonging to Picornaviridae, Coronaviridae, Flaviviridae,Poxviridae, Parvoviridae and Herpesviridae families.