Structure of a phleboviral envelope glycoprotein reveals a consolidated model of membrane fusion

Abstract

An emergent viral pathogen termed severe fever with thrombocytopenia syndrome virus (SFTSV) is responsible for thousands of clinical cases and associated fatalities in China, Japan, and South Korea. Akin to other phleboviruses, SFTSV relies on a viral glycoprotein, Gc, to catalyze the merger of endosomal host and viral membranes during cell entry. Here, we describe the postfusion structure of SFTSV Gc, revealing that the molecular transformations the phleboviral Gc undergoes upon host cell entry are conserved with otherwise unrelated alpha-and flaviviruses. By comparison of SFTSV Gc with that of the prefusion structure of the related Rift Valley fever virus, we show that these changes involve refolding of the protein into a trimeric state. Reverse genetics and rescue of site-directed histidine mutants enabled localization of histidines likely to be important for triggering this pH-dependent process. These data provide structural and functional evidence that the mechanism of phlebovirushost cell fusion is conserved among genetically and pathophysiologically distinct viral pathogens.emerging virus | phlebovirus | viral membrane fusion | bunyavirus | structure

Establishment of Subclones of the Severe Fever with Thrombocytopenia Syndrome Virus YG1 Strain Selected Using Low pH-Dependent Cell Fusion Activity

“…The presence of plaques of different sizes cannot be explained at present. The crystal structure of the SFTSV Gc was recently reported, and it was shown to have some surface-exposed histidines that are important for triggering the pH-dependent entry process via glycoproteins (20). These findings indicate that several amino acids played critical roles in cell fusion and plaque formation.…”

<b>The amino acid at position 624 in the glycoprotein of SFTSV (severe fever with thrombocytopenia virus) plays a critical role in low-pH-dependent cell fusion </b><b>activity </b>

“…Structural analysis of SFTSV and other phleboviruses demonstrated that Gc is a class II viral fusion protein and that the membrane fusion process is induced by structural rearrangement triggered by acidic conditions (2, 5,11,17). This fusion process could be evaluated as low-pH-dependent cell fusion caused by Gn and Gc expressed on the cell surface (9,21).…”

“…De Boer et al reported that mutation of a single histidine (857H) in RVFV Gc completely abrogated virus entry as well as acid-induced Gc oligomerization (4). The histidine residue at position 940 (940H) of SFTSV Gc is also important for virus replication (11). Conversely, the structure and function of SFTSV Gn during virus entry remains unclear.…”

“…SFTSV YG1 (GP-Y/R) showed little expression of GP on the cell surface, and did not induce cell fusion in the transfected cells. However, GP possessing an amino acid substitution at position SFTSV Gc was determined in a recent study (11). Using this structure, we generated pre-and post-fusion structural models of SFTSV Gc in an effort to identify the structural contribution made by the amino acid at position 624 (Fig.…”

“…Cells fixed with 10% formalin were permeabilized with 1% Triton X-100 in PBS for 10 min. Gn and Gc expressed in the cells were viral propagation (11,17). Nevertheless, the molecular mechanism of cell fusion and the function of Gn and Gc of SFTSV remain to be elucidated.…”