Assembly of arenavirus envelope glycoprotein GPC in detergent-soluble membrane microdomains

Characterization of bi-segmented and tri-segmented recombinant Pichinde virus particles

Mammarenaviruses include several highly virulent pathogens (e.g., Lassa virus) capable of causing severe hemorrhagic fever diseases for which there are no approved vaccines and limited treatment options. Mammarenaviruses are enveloped, bi-segmented ambisense RNA viruses. There is limited knowledge about cellular proteins incorporated into progeny virion particles and their potential biological roles in viral infection. Pichinde virus (PICV) is a prototypic arenavirus used to characterize mammarenavirus replication and pathogenesis. We have developed a recombinant PICV with a tri-segmented RNA genome as a viral vector platform. Whether the tri-segmented virion differs from the wild-type bi-segmented one in viral particle morphology and protein composition has not been addressed. In this study, recombinant PICV (rPICV) virions with a bi-segmented (rP18bi) and a tri-segmented (rP18tri) genome were purified by density-gradient ultracentrifugation and analyzed by cryo-electron microscopy and mass spectrometry. Both virion types are pleomorphic with spherical morphology and have no significant difference in size despite rP18tri having denser particles. Both virion types also contain similar sets of cellular proteins. Among the highly enriched virion-associated cellular proteins are components of the endosomal sorting complex required for transport pathway and vesicle trafficking, such as ALIX, Tsg101, VPS, CHMP, and Ras-associated binding proteins, which have known functions in virus assembly and budding. Other enriched cellular proteins include peripheral and transmembrane proteins, chaperone proteins, and ribosomal proteins; their biological roles in viral infection warrant further analysis. Our study provides important insights into mammarenavirus particle formation and aids in the future development of viral vectors and antiviral discovery.IMPORTANCEMammarenaviruses, such as Lassa virus, are enveloped RNA viruses that can cause severe hemorrhagic fever diseases (Lassa fever) with no approved vaccine and limited therapeutic options. Cellular proteins incorporated into progeny virion particles and their biological roles in mammarenavirus infection have not been well characterized. Pichinde virus (PICV) is a prototypic mammarenavirus used as a surrogate model for Lassa fever. We used cryo-electron microscopy and proteomic analysis to characterize the morphology and protein contents of the purified PICV particles that package either two (bi-segmented) or three (tri-segmented) genomic RNA segments. Our results demonstrate a similar virion morphology but different particle density for the bi- and tri-segmented viral particles and reveal major virion-associated cellular proteins. This study provides important insights into the virus-host interactions that can be used for antiviral development and optimizing arenavirus-based vaccine vectors.

Depletion-Induced Self-Assembly of Colloidal Particles on a Solid Substrate

We investigate the depletion contributions to the self-assembly of microcolloids on solid substrates. The assembly is driven by the exclusion of nanoparticles and nonadsorbing polymers from the depletion zone between the microcolloids in the liquid and the underlying substrate. The model system consists of 1 μm polystyrene particles that we deposit on a flat glass slab in an electrolyte solution. Using polystyrene nanoparticles and poly(acrylic acid) polymers as depleting agents, we demonstrate in our experiments that nanoparticle concentrations of 0.5% (w/v) support well-ordered packing of microcolloids on glass, while the presence of polymers leads to irregular aggregate deposition structures. A mixture of nanoparticles and polymers enhances the formation of colloidal aggregate and particulate surface coverage compared to using the polymers alone as a depletion agent. Moreover, tuning the polymer ionization state from pH 4 to 9 modifies the polymer conformational state and radius of gyration, which in turn alters the microcolloid deposition from compact multilayers to flocculated structures. Our study provides entropic strategies for manipulating particulate assembly on substrates from dispersed to continuous coatings.

The Virus–Host Interplay in Junín Mammarenavirus Infection

Junín virus (JUNV) belongs to the Arenaviridae family and is the causative agent of Argentine hemorrhagic fever (AHF), a severe human disease endemic to agricultural areas in Argentina. At this moment, there are no effective antiviral therapeutics to battle pathogenic arenaviruses. Cumulative reports from recent years have widely provided information on cellular factors playing key roles during JUNV infection. In this review, we summarize research on host molecular determinants that intervene in the different stages of the viral life cycle: viral entry, replication, assembly and budding. Alongside, we describe JUNV tight interplay with the innate immune system. We also review the development of different reverse genetics systems and their use as tools to study JUNV biology and its close teamwork with the host. Elucidating relevant interactions of the virus with the host cell machinery is highly necessary to better understand the mechanistic basis beyond virus multiplication, disease pathogenesis and viral subversion of the immune response. Altogether, this knowledge becomes essential for identifying potential targets for the rational design of novel antiviral treatments to combat JUNV as well as other pathogenic arenaviruses.

Lassa virus glycoprotein complex review: insights into its unique fusion machinery

Lassa virus (LASV), an arenavirus endemic to West Africa, causes Lassa fever-a lethal hemorrhagic fever. Entry of LASV into the host cell is mediated by the glycoprotein complex (GPC), which is the only protein located on the viral surface and comprises three subunits: glycoprotein 1 (GP1), glycoprotein 2 (GP2), and a stable signal peptide (SSP). The LASV GPC is a class one viral fusion protein, akin to those found in viruses such as human immunodeficiency virus (HIV), influenza, Ebola virus (EBOV), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). These viruses are enveloped and utilize membrane fusion to deliver their genetic material to the host cell. Like other class one fusion proteins, LASV-mediated membrane fusion occurs through an orchestrated sequence of conformational changes in its GPC. The receptor-binding subunit, GP1, first engages with a host cell receptor then undergoes a unique receptor switch upon delivery to the late endosome. The acidic pH and change in receptor result in the dissociation of GP1, exposing the fusion subunit, GP2, such that fusion can occur. These events ultimately lead to the formation of a fusion pore so that the LASV genetic material is released into the host cell. Interestingly, the mature GPC retains its SSP as a third subunit-a feature that is unique to arenaviruses. Additionally, the fusion domain contains two separate fusion peptides, instead of a standard singular fusion peptide. Here, we give a comprehensive review of the LASV GPC components and their unusual features.

Myristoylation of the Arenavirus Envelope Glycoprotein Stable Signal Peptide Is Critical for Membrane Fusion but Dispensable for Virion Morphogenesis

Arenaviruses are responsible for severe and often fatal hemorrhagic disease. In the absence of effective antiviral therapies and vaccines, these viruses pose serious threats to public health and biodefense. Arenaviruses enter the host cell by fusion of the viral and endosomal membranes, a process mediated by the virus envelope glycoprotein GPC. Unlike other class I viral fusion proteins, GPC retains its stable signal peptide (SSP) as an essential third subunit in the mature complex. SSP spans the membrane twice and is myristoylated at its cytoplasmic N terminus. Mutations that abolish SSP myristoylation have been shown to reduce pH-induced cell-cell fusion activity of ectopically expressed GPC to ∼20% of wild-type levels. In order to examine the role of SSP myristoylation in the context of the intact virus, we used reverse genetics to generate Junín viruses (Candid #1 isolate) in which the critical glycine-2 residue in SSP was either replaced by alanine (G2A) or deleted (ΔG2). These mutant viruses produced smaller foci of infection in Vero cells and showed an ∼5-fold reduction in specific infectivity, commensurate with the defect in cell-cell fusion. However, virus assembly and GPC incorporation into budded virions were unaffected. Our findings suggest that the myristate moiety is cryptically disposed in the prefusion GPC complex and may function late in the fusion process to promote merging of the viral and cellular membranes.

IMPORTANCE

Hemorrhagic fever arenaviruses pose significant threats to public health and biodefense. Arenavirus entry into the host cell is promoted by the virus envelope glycoprotein GPC. Unlike other viral envelope glycoproteins, GPC contains a myristoylated stable signal peptide (SSP) as an essential third subunit. Myristoylation has been shown to be important for the membrane fusion activity of recombinantly expressed GPC. Here, we use reverse genetics to study the role of SSP myristoylation in the context of the intact virion. We find that nonmyristoylated GPC mutants of the Candid #1 strain of Junín virus display a commensurate deficiency in their infectivity, albeit without additional defects in virion assembly and budding. These results suggest that SSP myristoylation may function late in the fusion process to facilitate merging of the viral and cellular membranes. Antiviral agents that target this novel aspect of GPC membrane fusion may be useful in the treatment of arenavirus hemorrhagic fevers.

Cis- and cell-type-dependent trans-requirements for Lassa virus-like particle production

Lassa virus (LASV) small zinc-finger protein (Z), which contains two L-domain motifs, plays a central role in virus budding. Here, we report that co-expression of glycoprotein (GPC) altered the requirements for cholesterol but not the L-domains and host factor, Tsg101, for Z-induced virus-like particle (VLP) production. In particular, the cholesterol requirement for VLP production was cell-type-dependent. In addition, GPC was found to be important for co-localization of Z with CD63, a late endosomal marker. We also found that the N-terminal region (aa 3-10) of Z was critical for its myristoylation and VLP production. These findings will contribute to our understanding of LASV assembly and budding.

Sorting of small infectious virus particles by flow virometry reveals distinct infectivity profiles

The nature and concentration of lipids and proteins at the surface of viruses are essential parameters for determining particle infectiveness. Historically, averaged bulk analysis of viral particles has been the primary method to quantitatively investigate these parameters, though this neglects heterogeneity within populations. Here we analyze the properties of Junin virus particles using a sensitive flow virometry assay and further sort virions, while conserving their infectivness. This method allows us to characterize the relationship between infectivity, virus size, and RNA content and to compare particles secreted by Vero cells with those from physiologically relevant human primary macrophages. Our study highlights significant differences in particle infectivity according to its nature, the type of producer cells and the lipid membrane composition at the budding site. Together, our results present the flow virometry assay as a powerful and versatile tool to define virus particle profiles.

Assembly and budding of negative-strand RNA viruses

Assembly of negative-strand RNA viruses occurs by budding from host plasma membranes. The budding process involves association of the viral core or nucleocapsid with a region of cellular membrane that will become the virus budding site, which contains the envelope glycoproteins and matrix protein. This region of membrane then buds out and pinches off to become the virus envelope. This review will address the questions of what are the mechanisms that bring the nucleocapsid and envelope glycoproteins together to form the virus budding site, and how does this lead to release of progeny virions? Recent evidence supports the idea that viral envelope glycoproteins and matrix proteins are organized into membrane microdomains that coalesce to form virus budding sites. There has also been substantial progress in understanding the last step in virus release, referred to as the "late budding function," which often involves host proteins of the vacuolar protein sorting apparatus. Key questions are raised as to the mechanism of the initial steps in formation of virus budding sites: How are membrane microdomains brought together and how are nucleocapsids selected for incorporation into these budding sites, particularly in the case of viruses for which genome RNA sequences are important for envelopment of nucleocapsids?

The role of oligomerization for the biological functions of the arenavirus nucleoprotein

The Lassa virus nucleoprotein (NP) is a multifunctional protein that plays an essential role in many aspects of the viral life cycle, including RNA encapsidation, viral transcription and replication, recruitment of ribonucleoprotein complexes to viral budding sites, and inhibition of the host cell interferon response. While it is known that NP is capable of forming oligomers, both the oligomeric state of NP in mammalian cells and the significance of NP oligomerization for its various functions remain unclear. Here, we demonstrate that Lassa virus NP solely forms trimers upon expression in mammalian cells. Using a minigenome assay we show that mutants that are not able to form stable trimers are no longer functional during transcription and/or replication of the minigenome, indicating that NP trimerization is essential for transcription and/or replication of the viral genome. However, mutations leading to destabilization of the NP trimer did not impact the incorporation of NP into virus-like particles or its ability to suppress interferon-induced gene expression, two important functions of arenavirus NP.

LCMV glycosylation modulates viral fitness and cell tropism

The glycoprotein (GP) of arenaviruses is glycosylated at 11 conserved N-glycosylation sites. We constructed recombinant lymphocytic choriomeningitis virus (rLCMV) featuring either additions or deletions of these N-glycans to investigate their role in the viral life cycle. N-glycosylation at two sites, T87 and S97, were found to be necessary to rescue rLCMV. Three of nine successfully rescued mutants, S116A, T234A, and S373A, under selective pressures in either epithelial, neuronal, or macrophage cells reverted to WT sequence. Of the seven stable N-glycan deletion mutants, five of these led to altered viral fitness and cell tropism, assessed as growth in either mouse primary cortical neurons or bone marrow derived macrophages. These results demonstrate that the deletion of N-glycans in LCMV GP may confer an advantage to the virus for infection of neurons but a disadvantage in macrophages.

Membrane localization of Junín virus glycoproteins requires cholesterol and cholesterol rich membranes

Arenavirus morphogenesis and budding occurs at cellular plasma membrane; however, the nature of membrane assembly sites remains poorly understood. In this study we examined the effect of different cholesterol-lowering agents on Junín virus (JUNV) multiplication. We found that cholesterol cell depletion reduced JUNV glycoproteins (GPs) membrane expression and virus budding. Analysis of membrane protein insolubility in Triton X-100 suggested that JUNV GPs associate with cholesterol enriched membranes. Rafts dissociation conditions as warm detergent extraction and cholesterol removal by methyl-β-cyclodextrin compound showed to impair GPs cholesterol enriched membrane association. Analysis of GPs transfected cells showed similar results suggesting that membrane raft association is independent of other viral proteins.

Multifunctional nature of the arenavirus RING finger protein Z

Arenaviruses are a family of enveloped negative-stranded RNA viruses that can cause severe human disease ranging from encephalitis symptoms to fulminant hemorrhagic fever. The bi‑segmented RNA genome encodes four polypeptides: the nucleoprotein NP, the surface glycoprotein GP, the polymerase L, and the RING finger protein Z. Although it is the smallest arenavirus protein with a length of 90 to 99 amino acids and a molecular weight of approx. 11 kDa, the Z protein has multiple functions in the viral life cycle including (i) regulation of viral RNA synthesis, (ii) orchestration of viral assembly and budding, (iii) interaction with host cell proteins, and (iv) interferon antagonism. In this review, we summarize our current understanding of the structural and functional role of the Z protein in the arenavirus replication cycle.

The curious case of arenavirus entry, and its inhibition

Arenaviruses comprise a diverse family of enveloped negative-strand RNA viruses that are endemic to specific rodent hosts worldwide. Several arenaviruses cause severe hemorrhagic fevers in humans, including Junín and Machupo viruses in South America and Lassa fever virus in western Africa. Arenavirus entry into the host cell is mediated by the envelope glycoprotein complex, GPC. The virion is endocytosed on binding to a cell-surface receptor, and membrane fusion is initiated in response to physiological acidification of the endosome. As with other class I virus fusion proteins, GPC-mediated membrane fusion is promoted through a regulated sequence of conformational changes leading to formation of the classical postfusion trimer-of-hairpins structure. GPC is, however, unique among the class I fusion proteins in that the mature complex retains a stable signal peptide (SSP) as a third subunit, in addition to the canonical receptor-binding and fusion proteins. We will review the curious properties of the tripartite GPC complex and describe evidence that SSP interacts with the fusion subunit to modulate pH-induced activation of membrane fusion. This unusual solution to maintaining the metastable prefusion state of GPC on the virion and activating the class I fusion cascade at acidic pH provides novel targets for antiviral intervention.

Glycosylation modulates arenavirus glycoprotein expression and function

The glycoprotein of lymphocytic choriomeningitis virus (LCMV) contains nine potential N-linked glycosylation sites. We investigated the function of these N-glycosylations by using alanine-scanning mutagenesis. All the available sites were occupied on GP1 and two of three on GP2. N-linked glycan mutations at positions 87 and 97 on GP1 resulted in reduction of expression and absence of cleavage and were necessary for downstream functions, as confirmed by the loss of GP-mediated fusion activity with T87A and S97A mutants. In contrast, T234A and E379N/A381T mutants impaired GP-mediated cell fusion without altered expression or processing. Infectivity via virus-like particles required glycans and a cleaved glycoprotein. Glycosylation at the first site within GP2, not normally utilized by LCMV, exhibited increased VLP infectivity. We also confirmed the role of the N-linked glycan at position 173 in the masking of the neutralizing epitope GP-1D. Taken together, our results indicated a strong relationship between fusion and infectivity.

Viral protein determinants of Lassa virus entry and release from polarized epithelial cells

The epithelium plays a key role in the spread of Lassa virus. Transmission from rodents to humans occurs mainly via inhalation or ingestion of droplets, dust, or food contaminated with rodent urine. Here, we investigated Lassa virus infection in cultured epithelial cells and subsequent release of progeny viruses. We show that Lassa virus enters polarized Madin-Darby canine kidney (MDCK) cells mainly via the basolateral route, consistent with the basolateral localization of the cellular Lassa virus receptor alpha-dystroglycan. In contrast, progeny virus was efficiently released from the apical cell surface. Further, we determined the roles of the glycoprotein, matrix protein, and nucleoprotein in directed release of nascent virus. To do this, a virus-like-particle assay was developed in polarized MDCK cells based on the finding that, when expressed individually, both the glycoprotein GP and matrix protein Z form virus-like particles. We show that GP determines the apical release of Lassa virus from epithelial cells, presumably by recruiting the matrix protein Z to the site of virus assembly, which is in turn essential for nucleocapsid incorporation into virions.

Characterization of Lassa virus glycoprotein oligomerization and influence of cholesterol on virus replication

Mature glycoprotein spikes are inserted in the Lassa virus envelope and consist of the distal subunit GP-1, the transmembrane-spanning subunit GP-2, and the signal peptide, which originate from the precursor glycoprotein pre-GP-C by proteolytic processing. In this study, we analyzed the oligomeric structure of the viral surface glycoprotein. Chemical cross-linking studies of mature glycoprotein spikes from purified virus revealed the formation of trimers. Interestingly, sucrose density gradient analysis of cellularly expressed glycoprotein showed that in contrast to trimeric mature glycoprotein complexes, the noncleaved glycoprotein forms monomers and oligomers spanning a wide size range, indicating that maturation cleavage of GP by the cellular subtilase SKI-1/S1P is critical for formation of the correct oligomeric state. To shed light on a potential relation between cholesterol and GP trimer stability, we performed cholesterol depletion experiments. Although depletion of cholesterol had no effect on trimerization of the glycoprotein spike complex, our studies revealed that the cholesterol content of the viral envelope is important for the infectivity of Lassa virus. Analyses of the distribution of viral proteins in cholesterol-rich detergent-resistant membrane areas showed that Lassa virus buds from membrane areas other than those responsible for impaired infectivity due to cholesterol depletion of lipid rafts. Thus, derivation of the viral envelope from cholesterol-rich membrane areas is not a prerequisite for the impact of cholesterol on virus infectivity.