Modeling suggests that virion production cycles within individual cells is key to understanding acute hepatitis B virus infection kinetics.

Hepatitis B virus (HBV) infection kinetics in immunodeficient mice reconstituted with humanized livers from inoculation to steady state is highly dynamic despite the absence of an adaptive immune response. To recapitulate the multiphasic viral kinetic patterns, we developed an agent-based model that includes intracellular virion production cycles reflecting the cyclic nature of each individual virus lifecycle. The model fits the data well predicting an increase in production cycles initially starting with a long production cycle of 1 virion per 20 hours that gradually reaches 1 virion per hour after approximately 3-4 days before virion production increases dramatically to reach to a steady state rate of 4 virions per hour per cell. Together, modeling suggests that it is the cyclic nature of the virus lifecycle combined with an initial slow but increasing rate of HBV production from each cell that plays a role in generating the observed multiphasic HBV kinetic patterns in humanized mice.

Epstein-Barr Virus BBLF1 Mediates Secretory Vesicle Transport to Facilitate Mature Virion Release.

Enveloped viruses undergo a complex multistep process of assembly, maturation, and release into the extracellular space utilizing host secretory machinery. Several studies of the herpesvirus subfamily have shown that secretory vesicles derived from the trans-Golgi network (TGN) or endosomes transport virions into the extracellular space. However, the regulatory mechanism underlying the release of Epstein-Barr virus, a human oncovirus, remains unclear. We demonstrate that disruption of BBLF1, a tegument component, suppressed viral release and resulted in the accumulation of viral particles on the inner side of the vesicular membrane. Organelle separation revealed the accumulation of infectious viruses in fractions containing vesicles derived from the TGN and late endosomes. Deficiency of an acidic amino acid cluster in BBLF1 reduced viral secretion. Moreover, truncational deletion of the C-terminal region of BBLF1 increased infectious virus production. These findings suggest that BBLF1 regulates the viral release pathway and reveal a new aspect of tegument protein function. IMPORTANCE Several viruses have been linked to the development of cancer in humans. Epstein-Barr virus (EBV), the first identified human oncovirus, causes a wide range of cancers. Accumulating literature has demonstrated the role of viral reactivation in tumorigenesis. Elucidating the functions of viral lytic genes induced by reactivation, and the mechanisms of lytic infection, is essential to understanding pathogenesis. Progeny viral particles synthesized during lytic infection are released outside the cell after the assembly, maturation, and release steps, leading to further infection. Through functional analysis using BBLF1-knockout viruses, we demonstrated that BBLF1 promotes viral release. The acidic amino acid cluster in BBLF1 was also important for viral release. Conversely, mutants lacking the C terminus exhibited more efficient virus production, suggesting that BBLF1 is involved in the fine-tuning of progeny release during the EBV life cycle.

Kinesin-1 transports morphologically distinct intracellular virions during vaccinia infection.

Intracellular mature viruses (IMVs) are the first and most abundant infectious form of vaccinia virus to assemble during its replication cycle. IMVs can undergo microtubule-based motility, but their directionality and the motor involved in their transport remain unknown. Here, we demonstrate that IMVs, like intracellular enveloped viruses (IEVs), the second form of vaccinia that are wrapped in Golgi-derived membranes, recruit kinesin-1 and undergo anterograde transport. In vitro reconstitution of virion transport in infected cell extracts revealed that IMVs and IEVs move toward microtubule plus ends with respective velocities of 0.66 and 0.56 µm/s. Quantitative imaging established that IMVs and IEVs recruit an average of 139 and 320 kinesin-1 motor complexes, respectively. In the absence of kinesin-1, there was a near-complete loss of in vitro motility and reduction in the intracellular spread of both types of virions. Our observations demonstrate that kinesin-1 transports two morphologically distinct forms of vaccinia. Reconstitution of vaccinia-based microtubule motility in vitro provides a new model to elucidate how motor number and regulation impacts transport of a bona fide kinesin-1 cargo.

Liquid-liquid phase separation mediates the formation of herpesvirus assembly compartments.

Virus assembly, which takes place during the late stage of viral replication, is essential for virus propagation. However, the underlying mechanisms remain poorly understood, especially for viruses with complicated structures. Here, we use correlative light and electron microscopy to examine the formation of cytoplasmic virion assembly compartments (cVACs) during infection by a γ-herpesvirus. These cVACs are membraneless organelles with liquid-like properties. Formation of cVACs during virus infection is mediated by ORF52, an abundant tegument protein. ORF52 undergoes liquid-liquid phase separation (LLPS), which is promoted by both DNA and RNA. Disrupting ORF52 phase separation blocks cVACs formation and virion production. These results demonstrate that phase separation of ORF52 is critical for cVACs formation. Our work defines herpesvirus cVACs as membraneless compartments that are generated through a process of LLPS mediated by a tegument protein and adds to the cellular processes that are facilitated by phase separation.

The Interaction between Tegument Proteins ORF33 and ORF45 Plays an Essential Role in Cytoplasmic Virion Maturation of a Gammaherpesvirus.

Tegument, which occupies the space between the nucleocapsid and the envelope, is a unique structure of a herpesvirion. Tegument proteins are major components of tegument and play critical roles in virus life cycle. Murine gammaherpesvirus 68 (MHV-68), a member of the gammaherpesvirus subfamily, is closely related to two human herpesviruses, Kaposi's sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV). We have previously shown that MHV-68 ORF33, conserved among all herpesviruses, encodes a tegument protein that is associated with intranuclear capsids and is essential for virion morphogenesis and egress. Another tegument protein ORF45, which is conserved only among gammaherpesviruses, also plays an essential role in virion morphogenesis of MHV-68. In this study, we investigated the underlying mechanism and showed that these two proteins colocalize and interact with each other during virus infection. We mapped the ORF33-interacting domain to the conserved carboxyl-terminal 23 amino acids (C23) of ORF45. Deletion of the C23 coding sequence in the context of viral genome abolished the production of infectious virions. Transmission electron microscopy results demonstrated that C23 of ORF45 are essential for virion tegumentation in the cytoplasm. We further mapped the ORF45-interacting domain to the N-terminal 17 amino acids (N17) of ORF33. Deletion of the N17 coding sequence in the context of viral genome also abolished production of infectious virions, and N17 of ORF33 are also essential for virion tegumentation in the cytoplasm. Taken together, our data strongly indicate that the interaction between ORF45 and ORF33 plays an essential role in cytoplasmic maturation of MHV-68 virions. IMPORTANCE A critical step in viral lytic replication is the assembly of progeny viral particles. Herpesviruses are important pathogens. A herpesvirus particle comprises, from inside to outside, four layers: DNA core, capsid, tegument, and envelope. The tegument layer contains dozens of virally encoded tegument proteins, which play critical roles in virus assembly. Murine gammaherpesvirus 68 (MHV-68) is a tumor-associated herpesvirus and is closely related to Kaposi's sarcoma-associated herpesvirus and Epstein-Barr virus. We previously found that the absence of either tegument protein ORF33 or ORF45 inhibits the translocation of nucleocapsids to the cytoplasm and blocks virion maturation, but the underlying mechanism remained unclear. Here, we showed that ORF33 interacts with ORF45. We mapped their interaction domains and constructed viral mutants with defects in ORF33-ORF45 interaction. Transmission electron microscopy data demonstrated that the assembly of these viral mutants in the cytoplasm is blocked. Our results indicate that ORF33-ORF45 interaction is essential for gammaherpesvirus replication.

SARS-CoV-2 ORF7a potently inhibits the antiviral effect of the host factor SERINC5.

Serine Incorporator 5 (SERINC5), a cellular multipass transmembrane protein that is involved in sphingolipid and phosphatydilserine biogenesis, potently restricts a number of retroviruses, including Human Immunodeficiency Virus (HIV). SERINC5 is incorporated in the budding virions leading to the inhibition of virus infectivity. In turn, retroviruses, including HIV, encode factors that counteract the antiviral effect of SERINC5. While SERINC5 has been well studied in retroviruses, little is known about its role in other viral families. Due to the paucity of information regarding host factors targeting Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), we evaluated the effect of SERINC proteins on SARS-CoV-2 infection. Here, we show SERINC5 inhibits SARS-CoV-2 entry by blocking virus-cell fusion, and SARS-CoV-2 ORF7a counteracts the antiviral effect of SERINC5 by blocking the incorporation of over expressed SERINC5 in budding virions.

Density Analysis of Enterovirus D68 Shows Viral Particles Can Associate with Exosomes.

Enterovirus D68 (EV-D68) is an emerging pathogen which causes respiratory disease and is associated with an acute flaccid myelitis that predominately affects children. EV-D68 can infect motor neurons, causing cell death and a loss of motor control leading to flaccid paralysis. However, it remains unknown how viral particles gain entry into the central nervous system (CNS). Here, we show that three distinct densities of EV-D68 particle can be isolated from infected muscle and neural cell lines (RD and SH-SY5Y) using high-speed density centrifugation to separate cell supernatant. The lowest-density peak is composed of viral particles, which have adhered to the exterior surface of a small extracellular vesicle called an exosome. Analysis of prototypic (historic) and contemporary EV-D68 strains suggests that binding to exosomes is a ubiquitous characteristic of EV-D68. We further show that interaction with exosomes increases viral infectivity in a neural cell line. Analysis of the two higher-density peaks, which are not associated with exosomes, revealed that a significant amount of viral titer in the modern (2014) EV-D68 strains is found at 1.20 g/cm3, whereas this density has a very low viral titer in the prototypic Fermon strain. IMPORTANCE Despite the strong causal link between enterovirus D68 (EV-D68) and acute flaccid myelitis (AFM), it remains unclear how EV-D68 gains entry into the central nervous system and what receptors enable it to infect motor neurons. We show that EV-D68 particles can adhere to exosomes, placing EV-D68 among a handful of other picornaviruses which are known to interact with extracellular vesicles. Uptake and infection of permissive cells by virally contaminated exosomes would have major implications in the search for the EV-D68 receptor, as well as providing a possible route for viral entry into motor neurons. This work identifies a novel cellular entry route for EV-D68 and may facilitate the identification of genetic risk factors for development of AFM.

Structural Assembly of Qß Virion and Its Diverse Forms of Virus-like Particles.

The coat proteins (CPs) of single-stranded RNA bacteriophages (ssRNA phages) directly assemble around the genomic RNA (gRNA) to form a near-icosahedral capsid with a single maturation protein (Mat) that binds the gRNA and interacts with the retractile pilus during infection of the host. Understanding the assembly of ssRNA phages is essential for their use in biotechnology, such as RNA protection and delivery. Here, we present the complete gRNA model of the ssRNA phage Qß, revealing that the 3' untranslated region binds to the Mat and the 4127 nucleotides fold domain-by-domain, and is connected through long-range RNA-RNA interactions, such as kissing loops. Thirty-three operator-like RNA stem-loops are located and primarily interact with the asymmetric A/B CP-dimers, suggesting a pathway for the assembly of the virions. Additionally, we have discovered various forms of the virus-like particles (VLPs), including the canonical T = 3 icosahedral, larger T = 4 icosahedral, prolate, oblate forms, and a small prolate form elongated along the 3-fold axis. These particles are all produced during a normal infection, as well as when overexpressing the CPs. When overexpressing the shorter RNA fragments encoding only the CPs, we observed an increased percentage of the smaller VLPs, which may be sufficient to encapsidate a shorter RNA.

Vectorial Release of Human RNA Viruses from Epithelial Cells.

Epithelial cells are apico-basolateral polarized cells that line all tubular organs and are often targets for infectious agents. This review focuses on the release of human RNA virus particles from both sides of polarized human cells grown on transwells. Most viruses that infect the mucosa leave their host cells mainly via the apical side while basolateral release is linked to virus propagation within the host. Viruses do this by hijacking the cellular factors involved in polarization and trafficking. Thus, understanding epithelial polarization is essential for a clear understanding of virus pathophysiology.

Cooperative Chikungunya Virus Membrane Fusion and Its Substoichiometric Inhibition by CHK-152 Antibody.

Chikungunya virus (CHIKV) presents a major burden on healthcare systems worldwide, but specific treatment remains unavailable. Attachment and fusion of CHIKV to the host cell membrane is mediated by the E1/E2 protein spikes. We used an in vitro single-particle fusion assay to study the effect of the potent, neutralizing antibody CHK-152 on CHIKV binding and fusion. We find that CHK-152 shields the virions, inhibiting interaction with the target membrane and inhibiting fusion. The analysis of the ratio of bound antibodies to epitopes implied that CHIKV fusion is a highly cooperative process. Further, dissociation of the antibody at lower pH results in a finely balanced kinetic competition between inhibition and fusion, suggesting a window of opportunity for the spike proteins to act and mediate fusion, even in the presence of the antibody.

Salmon Erythrocytes Sequester Active Virus Particles in Infectious Salmon Anaemia.

Infectious salmon anaemia virus (ISAV) binds circulating Atlantic salmon erythrocytes, but the relevance of this interaction for the course of infection and development of disease remains unclear. We here characterise ISAV-erythrocyte interactions in experimentally infected Atlantic salmon and show that ISAV-binding to erythrocytes is common and precedes the development of disease. Viral RNA and infective particles were enriched in the cellular fraction of blood. While erythrocyte-associated ISAV remained infectious, erythrocytes dose-dependently limited the infection of cultured cells. Surprisingly, immunostaining of blood smears revealed expression of ISAV proteins in a small fraction of erythrocytes in one of the examined trials, confirming that ISAV can be internalised in this cell type and engage the cellular machinery in transcription and translation. However, viral protein expression in erythrocytes was rare and not required for development of disease and mortality. Furthermore, active transcription of ISAV mRNA was higher in tissues than in blood, supporting the assumption that ISAV replication predominantly takes place in endothelial cells. In conclusion, Atlantic salmon erythrocytes bind ISAV and sequester infective virus particles during infection, but do not appear to significantly contribute to ISAV replication. We discuss the implications of our findings for infection dynamics and pathogenesis of infectious salmon anaemia.

Structural Insights into Alphavirus Assembly Revealed by the Cryo-EM Structure of Getah Virus.

Getah virus (GETV) is a member of the alphavirus genus, and it infects a variety of animal species, including horses, pigs, cattle, and foxes. Human infection with this virus has also been reported. The structure of GETV has not yet been determined. In this study, we report the cryo-EM structure of GETV at a resolution of 3.5 Å. This structure reveals conformational polymorphism of the envelope glycoproteins E1 and E2 at icosahedral 3-fold and quasi-3-fold axes, which is believed to be a necessary organization in forming a curvature surface of virions. In our density map, three extra densities are identified, one of which is believed a "pocket factor"; the other two are located by domain D of E2, and they may maintain the stability of E1/E2 heterodimers. We also identify three N-glycosylations at E1 N141, E2 N200, and E2 N262, which might be associated with receptor binding and membrane fusion. The resolving of the structure of GETV provides new insights into the structure and assembly of alphaviruses and lays a basis for studying the differences of biology and pathogenicity between arthritogenic and encephalitic alphaviruses.

Visualization of Retroviral Gag-Genomic RNA Cellular Interactions Leading to Genome Encapsidation and Viral Assembly: An Overview.

Retroviruses must selectively recognize their unspliced RNA genome (gRNA) among abundant cellular and spliced viral RNAs to assemble into newly formed viral particles. Retroviral gRNA packaging is governed by Gag precursors that also orchestrate all the aspects of viral assembly. Retroviral life cycles, and especially the HIV-1 one, have been previously extensively analyzed by several methods, most of them based on molecular biology and biochemistry approaches. Despite these efforts, the spatio-temporal mechanisms leading to gRNA packaging and viral assembly are only partially understood. Nevertheless, in these last decades, progress in novel bioimaging microscopic approaches (as FFS, FRAP, TIRF, and wide-field microscopy) have allowed for the tracking of retroviral Gag and gRNA in living cells, thus providing important insights at high spatial and temporal resolution of the events regulating the late phases of the retroviral life cycle. Here, the implementation of these recent bioimaging tools based on highly performing strategies to label fluorescent macromolecules is described. This report also summarizes recent gains in the current understanding of the mechanisms employed by retroviral Gag polyproteins to regulate molecular mechanisms enabling gRNA packaging and the formation of retroviral particles, highlighting variations and similarities among the different retroviruses.

High-Throughput Platform for Detection of Neutralizing Antibodies Using Flavivirus Reporter Replicon Particles.

Flavivirus outbreaks require fast and reliable diagnostics that can be easily adapted to newly emerging and re-emerging flaviviruses. Due to the serological cross-reactivity among flavivirus antibodies, neutralization tests (NT) are considered the gold standard for sero-diagnostics. Here, we first established wild-type single-round infectious virus replicon particles (VRPs) by packaging a yellow fever virus (YFV) replicon expressing Gaussia luciferase (Gluc) with YFV structural proteins in trans using a double subgenomic Sindbis virus (SINV) replicon. The latter expressed the YFV envelope proteins prME via the first SINV subgenomic promoter and the capsid protein via a second subgenomic SINV promoter. VRPs were produced upon co-electroporation of replicon and packaging RNA. Introduction of single restriction enzyme sites in the packaging construct flanking the prME sequence easily allowed to exchange the prME moiety resulting in chimeric VRPs that have the surface proteins of other flaviviruses including dengue virus 1--4, Zika virus, West Nile virus, and tick-borne encephalitis virus. Besides comparing the YF-VRP based NT assay to a YF reporter virus NT assay, we analyzed the neutralization efficiencies of different human anti-flavivirus sera or a monoclonal antibody against all established VRPs. The assays were performed in a 96-well high-throughput format setting with Gluc as readout in comparison to classical plaque reduction NTs indicating that the VRP-based NT assays are suitable for high-throughput analyses of neutralizing flavivirus antibodies.

Virus-like Particles: Measures and Biological Functions.

Virus-like particles resemble infectious virus particles in size, shape, and molecular composition; however, they fail to productively infect host cells. Historically, the presence of virus-like particles has been inferred from total particle counts by microscopy, and infectious particle counts or plaque-forming-units (PFUs) by plaque assay; the resulting ratio of particles-to-PFUs is often greater than one, easily 10 or 100, indicating that most particles are non-infectious. Despite their inability to hijack cells for their reproduction, virus-like particles and the defective genomes they carry can exhibit a broad range of behaviors: interference with normal virus growth during co-infections, cell killing, and activation or inhibition of innate immune signaling. In addition, some virus-like particles become productive as their multiplicities of infection increase, a sign of cooperation between particles. Here, we review established and emerging methods to count virus-like particles and characterize their biological functions. We take a critical look at evidence for defective interfering virus genomes in natural and clinical isolates, and we review their potential as antiviral therapeutics. In short, we highlight an urgent need to better understand how virus-like genomes and particles interact with intact functional viruses during co-infection of their hosts, and their impacts on the transmission, severity, and persistence of virus-associated diseases.

[Plants and animals biological functions obtained from viruses]. / Des virus bénéfiques pour les plantes et les animaux.

Viruses can provide new biological functions to plants and animals. Some viruses persisting at low levels in plants might confer resistance to stress and parasites. In animals, more numerous examples of genes originating from viruses and used by different organisms have been described. For examples these genes might contribute to protect from new infections, or to ensure communication between neurons or to enable placenta development. In parasitic wasps, a complex viral machinery has been conserved as an endogenous virus dispersed in the wasp genome, which produces virions. These virions infect the parasitized host resulting in the production of virulence factors that inhibit defense mechanisms against the parasite. Different organisms have used the same viral functions repeatedly during animal evolution.

Title: Des virus bénéfiques pour les plantes et les animaux. Abstract: Les virus peuvent apporter de nouvelles fonctions aux organismes qui les portent. Chez les plantes, des virus, présents à des niveaux d'infection faibles, confèrent des propriétés de résistance aux stress et aux parasites. Chez les animaux, de plus nombreux exemples d'appropriation de gènes viraux, qui participent en particulier à la protection contre de nouvelles infections, à la communication entre les neurones, ou à la morphogenèse du placenta, ont été décrits. Chez les guêpes parasites, une machinerie virale complexe est conservée sous la forme d'un virus endogène dispersé dans le génome, leur permettant d'infecter l'hôte parasité et de lui faire exprimer des protéines inhibant ses propres mécanismes de défense. Les processus d'appropriation des mêmes fonctions virales se sont souvent répétés au cours de l'évolution. Cette revue aborde des exemples de symbioses virales (c'est-à-dire, des cas où le virus exploite un organisme-hôte en lui étant par ailleurs bénéfique), où l'apport positif des virus est bien documenté.

Mechanical Capsid Maturation Facilitates the Resolution of Conflicting Requirements for Herpesvirus Assembly.

Most viruses undergo a maturation process from a weakly self-assembled, noninfectious particle to a stable, infectious virion. For herpesviruses, this maturation process resolves several conflicting requirements: (i) assembly must be driven by weak, reversible interactions between viral particle subunits to reduce errors and minimize the energy of self-assembly, and (ii) the viral particle must be stable enough to withstand tens of atmospheres of DNA pressure resulting from its strong confinement in the capsid. With herpes simplex virus 1 (HSV-1) as a prototype of human herpesviruses, we demonstrated that this mechanical capsid maturation is mainly facilitated through capsid binding auxiliary protein UL25, orthologs of which are present in all herpesviruses. Through genetic manipulation of UL25 mutants of HSV-1 combined with the interrogation of capsid mechanics with atomic force microscopy nano-indentation, we suggested the mechanism of stepwise binding of distinct UL25 domains correlated with capsid maturation and DNA packaging. These findings demonstrate another paradigm of viruses as elegantly programmed nano-machines where an intimate relationship between mechanical and genetic information is preserved in UL25 architecture. IMPORTANCE The minor capsid protein UL25 plays a critical role in the mechanical maturation of the HSV-1 capsid during virus assembly and is required for stable DNA packaging. We modulated the UL25 capsid interactions by genetically deleting different UL25 regions and quantifying the effect on mechanical capsid stability using an atomic force microscopy (AFM) nanoindentation approach. This approach revealed how UL25 regions reinforced the herpesvirus capsid to stably package and retain pressurized DNA. Our data suggest a mechanism of stepwise binding of two main UL25 domains timed with DNA packaging.

CRISPR-Cas9 mediated knockout of AnxA6 gene enhances influenza A virus replication in low-permissive HEK293FT cell line.

Using cell cultures of human origin for the propagation of influenza virus is an attractive way to preserve its glycosylation profile and antigenic properties, which is essential in influenza surveillance and vaccine production. However, only few cell lines are highly permissive to influenza virus, and none of them are of human origin. The barrier might be associated with host restriction factors inhibiting influenza growth, such as AnxA6 protein counteracting the process of influenza virion packaging. In the presented work we explore the CRISPR-Cas9 mediated knockout of ANXA6 gene as a way to overcome the host restriction barrier and increase the susceptibility of human cell line to influenza infection. By CRISPR-Cas9 genome editing we modified HEK293FT cells and obtained several clones defective in the ANXA6 gene. The replication of the influenza A virus in original HEK293FT cells and the HEK293FT-ANXA6-/- mutant cells was compared in growth curve experiments. By combination of methods including TCID assay and flow cytometry we showed that accumulation of influenza A virus in the mutant HEK293FT-ANXA6-/- cells significantly exceeded the virus titer in the original HEK293FT cells.

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.

A plant virus satellite RNA directly accelerates wing formation in its insect vector for spread.

Cucumber mosaic virus (CMV) often accompanies a short RNA molecule called a satellite RNA (satRNA). When infected with CMV in the presence of Y-satellite RNA (Y-sat), tobacco leaves develop a green mosaic, then turn yellow. Y-sat has been identified in the fields in Japan. Here, we show that the yellow leaf colour preferentially attracts aphids, and that the aphids fed on yellow plants, which harbour Y-sat-derived small RNAs (sRNAs), turn red and subsequently develop wings. In addition, we found that leaf yellowing did not necessarily reduce photosynthesis, and that viral transmission was not greatly affected despite the low viral titer in the Y-sat-infected plants. Y-sat-infected plants can therefore support a sufficient number of aphids to allow for efficient virus transmission. Our results demonstrate that Y-sat directly alters aphid physiology via Y-sat sRNAs to promote wing formation, an unprecedented survival strategy that enables outward spread via the winged insect vector.