Common Features of Enveloped Viruses and Implications for Immunogen Design for Next-Generation Vaccines

Molecular organization and dynamics of the fusion protein Gc at the hantavirus surface

The hantavirus envelope glycoproteins Gn and Gc mediate virion assembly and cell entry, with Gc driving fusion of viral and endosomal membranes. Although the X-ray structures and overall arrangement of Gn and Gc on the hantavirus spikes are known, their detailed interactions are not. Here we show that the lateral contacts between spikes are mediated by the same 2-fold contacts observed in Gc crystals at neutral pH, allowing the engineering of disulfide bonds to cross-link spikes. Disrupting the observed dimer interface affects particle assembly and overall spike stability. We further show that the spikes display a temperature-dependent dynamic behavior at neutral pH, alternating between ‘open’ and ‘closed’ forms. We show that the open form exposes the Gc fusion loops but is off-pathway for productive Gc-induced membrane fusion and cell entry. These data also provide crucial new insights for the design of optimized Gn/Gc immunogens to elicit protective immune responses.

Hantaviruses infect rodents and other small mammals, but do not harm them. When transmitted to humans, often through rodent urine, feces or saliva, they can cause serious and even fatal diseases. Currently, there are no known methods that effectively prevent hantavirus infections or treat the diseases that they cause.

During an infection, viruses invade the cells of their host. A hantavirus interacts with target cells through proteins on its surface called Gn and Gc glycoproteins. Previous work has shown that these glycoproteins are organized in bundles of four Gn and four Gc proteins, termed spikes, which project from the membrane that surrounds the virus. The Gc protein changes shape when it is activated and exposes a hidden region that can insert into the membrane of the target cell. The Gc proteins then change shape again to force the cell to fuse with the viral membrane. This process allows the virus to be taken up into the cell, where it can replicate.

While the structures of each viral glycoprotein have been determined in isolation, it was not known how they interact within the Gn/Gc spike. Such information is crucial to understand how the viruses infect cells and which areas are exposed to the immune system of the host – and so could be targeted by antiviral treatments.

Bignon et al. have now identified the molecular contacts that occur between spikes and interconnect them into a grid-like lattice on the surface of the virus. Genetically altering specific sections of the Gc glycoprotein strengthened or weakened these contacts, which correspondingly increased or decreased how stable the spike was. Preventing the contacts from forming resulted in cells releasing fewer virus-like particles.

Bignon et al. also show that at the body temperature of mammals, the shape of the spike fluctuates between an ‘open’ form that exposes the region of Gc that inserts into the cell membrane, and a ‘closed’ form that hides this region. However, when Gc is activated, the open form becomes unable to cause the viral and cell membranes to fuse together.

Together, the results presented by Bignon et al. help us to understand how changes to the hantavirus surface enable the virus to infect cells. This knowledge will help researchers to design vaccines that protect against hantavirus infections.

A Bimolecular Multicellular Complementation System for the Detection of Syncytium Formation: A New Methodology for the Identification of Nipah Virus Entry Inhibitors

Fusion of viral and cellular membranes is a key step during the viral life cycle. Enveloped viruses trigger this process by means of specialized viral proteins expressed on their surface, the so-called viral fusion proteins. There are multiple assays to analyze the viral entry including those that focus on the cell-cell fusion induced by some viral proteins. These methods often rely on the identification of multinucleated cells (syncytium) as a result of cell membrane fusions. In this manuscript, we describe a novel methodology for the study of cell-cell fusion. Our approach, named Bimolecular Multicellular Complementation (BiMuC), provides an adjustable platform to qualitatively and quantitatively investigate the formation of a syncytium. Furthermore, we demonstrated that our procedure meets the requirements of a drug discovery approach and performed a proof of concept small molecule high-throughput screening to identify compounds that could block the entry of the emerging Nipah virus.

Emerging Diversity in Lipid-Protein Interactions

Membrane lipids interact with proteins in a variety of ways, ranging from providing a stable membrane environment for proteins to being embedded in to detailed roles in complicated and well-regulated protein functions. Experimental and computational advances are converging in a rapidly expanding research area of lipid–protein interactions. Experimentally, the database of high-resolution membrane protein structures is growing, as are capabilities to identify the complex lipid composition of different membranes, to probe the challenging time and length scales of lipid–protein interactions, and to link lipid–protein interactions to protein function in a variety of proteins. Computationally, more accurate membrane models and more powerful computers now enable a detailed look at lipid–protein interactions and increasing overlap with experimental observations for validation and joint interpretation of simulation and experiment. Here we review papers that use computational approaches to study detailed lipid–protein interactions, together with brief experimental and physiological contexts, aiming at comprehensive coverage of simulation papers in the last five years. Overall, a complex picture of lipid–protein interactions emerges, through a range of mechanisms including modulation of the physical properties of the lipid environment, detailed chemical interactions between lipids and proteins, and key functional roles of very specific lipids binding to well-defined binding sites on proteins. Computationally, despite important limitations, molecular dynamics simulations with current computer power and theoretical models are now in an excellent position to answer detailed questions about lipid–protein interactions.

Anthrax toxin requires ZDHHC5-mediated palmitoylation of its surface-processing host enzymes

Toxins exploit numerous pathways of their host cells to gain cellular entry and promote intoxication. Therefore, studying the action of toxins allows us to better understand basic mechanisms in cell biology. In this study, we found that ZDHHC5, an enzyme that adds a lipid posttranslational modification to cysteines of proteins, is responsible for allowing anthrax toxin to enter cells. This enzyme acts on proprotein convertases that are needed to cleave these toxins to their active forms. ZDHHC5 does not affect the enzymatic activity of these proteases, but allows them to encounter the toxin by favoring their partitioning in microdomains on the cell surface, domains where the toxin has previously been shown to preferentially reside.

The protein acyl transferase ZDHHC5 was recently proposed to regulate trafficking in the endocytic pathway. Therefore, we explored the function of this enzyme in controlling the action of bacterial toxins. We found that ZDHHC5 activity is required for two very different toxins: the anthrax lethal toxin and the pore-forming toxin aerolysin. Both of these toxins have precursor forms, the protoxins, which can use the proprotein convertases Furin and PC7 for activation. We show that ZDHHC5 indeed affects the processing of the protoxins to their active forms. We found that Furin and PC7 can both be S-palmitoylated and are substrates of ZDHHC5. The impact of ZDHHC5 on Furin/PC7-mediated anthrax toxin cleavage is dual, having an indirect and a direct component. First, ZDHHC5 affects the homeostasis and trafficking of a subset of cellular proteins, including Furin and PC7, presumably by affecting the endocytic/recycling pathway. Second, while not inhibiting the protease activity per se, ZDHHC5-mediated Furin/PC7 palmitoylation is required for the cleavage of the anthrax toxin. Finally, we show that palmitoylation of Furin and PC7 promotes their association with plasma membrane microdomains. Both the receptor-bound toxin and the convertases are of very low abundance at the cell surface. Their encounter is unlikely on reasonable time scales. This work indicates that palmitoylation drives their encounter in specific domains, allowing processing and thereby intoxication of the cell.