The Role of Fusion Protein F in the Virus Entry and Cell‐to‐Cell Spread for Nipah Virus

Abstract

Viruses are obligate parasites, where they require host cells to replicate and produce their progeny. Membrane and non‐segmented single‐stranded negative sense RNA genome are important signatures of paramyxoviruses. The paramyxovirus family contains many common human pathogenic viruses, including measles, mumps, respiratory syncytial virus, and Nipah virus (NiV) ‐ a deadly paramyxovirus with up to 75% mortality rate, with neither vaccine nor therapeutics available. Membrane fusion is necessary for entry of the membraned viruses like paramyxoviruses into their host cells (viral‐cell fusion), and for cell‐cell virus spread via syncytia formation (cell‐cell fusion). For most paramyxoviruses membrane fusion requires two viral glycoproteins – the attachment glycoprotein (HN/H/G) and fusion (F) glycoprotein. Structurally, type 1 fusion protein and a type 2 attachment protein is common to all paramyxoviruses. For virus entry into host cells during infection or cell‐cell spread, NiV has a unique pair of G and F proteins as its specific attachment glycoprotein and fusion glycoprotein respectively. NiV G protein made up of C‐terminal end globular head, stalk region, transmembrane (TM) domain and N‐terminal cytoplasmic tail. In contrast, NiV F protein consist of N‐terminal tail on the surface, hydrophobic fusion peptide (FP), single‐pass transmembrane (TM) domain, and C‐terminal cytoplasmic tail. It also has two heptad repeat domains (HR1/HRA [closer to FP] and HR2/HRB [closer to TM]). Both FP and HR domains play key roles in membrane fusion. NiV F proteins are synthesized as biologically inactive F0 precursors. To be active, F0 proteins need to be cleaved into metastable state of F1 and F2 fragments, covalently linked by disulphide bonds. Unlike other paramyxoviruses, where the cleaving of F0 into F1 + F2 is mediated by trypsin or trypsin‐like proteases, NiV F is cleaved by endocytosis and pH dependent process by endosomal proteases (cathepsin L/B) which exposes the FP as new N‐terminal of the F1 fragment. On the surface of NiV membrane, metastable structure of F1 + F2 makes the FP ‘hidden’ or ‘protected’ by protein‐protein interaction with G protein. Upon receptor (Ephrin B2/B3) binding, the NiV G protein alters its structure which then nudges and triggers the adjacent F protein to undergo conformational changes that causes the FP to stab into the host cell membrane, thus bridging between virus membrane and host cell membrane. On further conformational changes of F1+F2, the HRA/HR1 and HRB/HR2 coalesce or conjoin which then forms ‘fusion pores' allowing the NiV genome to enter the host cell cytoplasm and begin replication steps. When many similar NiV F and G mediated membrane fusion steps occur between a NiV‐infected cell with the non‐infected neighboring cells, it produces a multi‐nucleated giant syncytia formation, i.e. to facilitate cell‐cell virus spread. The Walton High School MSOE Center for BioMolecular Modeling SMART Team used 3‐D modeling and printing technology to examine structure‐function of NiV F and G proteins in membrane fusion.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.