UNCOVERING A REGION IN THE NIPAH VIRUS ATTACHMENT PROTEIN IMPORTANT FOR TRIGGERING MEMBRANE FUSION
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Membrane fusion is essential for entry of the biomedically-important Nipah virus (NiV) into their host cells (viral-cell fusion), and for syncytia formation (cell-cell fusion), often induced by NiV infections. For NiV, membrane fusion requires two viral glycoproteins. Upon receptor binding, the attachment glycoprotein (G) triggers the fusion glycoprotein (F) to undergo conformational changes that merge viral and/or cell membranes. The objectives of this study were to identify the region in NiV-G that is important for triggering F, to elucidate the mechanism by which NiV-G triggers F, and to explore potential strategies for preventing NiV infections. The NiV-G glycoprotein is consisted of a cytoplasmic tail, transmembrane domain, stalk and head from the N- to C-terminus. Results in chapter one show that a NiV-G headless mutant 167, lacking the head domain, was able to trigger membrane fusion without receptor binding, suggesting the NiV-G stalk harbors an important region for F triggering. Additionally, NiV-G head prevents premature triggering of NiV-F by concealing the F-triggering stalk domain until the correct time and place: receptor-binding. Therefore, these results reveal a comprehensive mechanism by which NiV-G couples receptor binding to F triggering: Receptor binding causes conformational changes in the G head, leading to the exposure of a F-triggering domain at the C-terminus of the G stalk, which interacts and triggers F to undergo a conformational cascade that merges the viral/cell membranes. As a follow-up study, we identified a microdomain (aa 159-163) at the C-terminus of the NiV-G stalk that is important for multiple G functions including G tetramerization, conformational integrity, F/G interaction, receptor-induced conformational changes in G, and F triggering. Based on these results, we propose that the NiV-G stalk C-terminal region 159-163 serves as an important structural and functional linker between the NiV-G head and the rest of the stalk, and is critical in propagating the F-triggering signal via specific conformational changes that open a concealed F-triggering domain(s) in the G stalk. In chapter three, we synthesized eight NiV-G stalk-derived peptides starting from residue 167 towards the N-terminus of the stalk, expecting these peptides to inhibit viral entry by prematurely triggering F in virions, or to induce membrane fusion independent of G in cells. Unfortunately, these peptides were unable to mediate membrane fusion in virions or cells.