PREVENTING THE NEXT PANDEMIC: ELUCIDATING THE ROLE OF THE LASSA VIRUS FUSION DOMAIN IN INFECTIONS

Abstract

Infection with Lassa virus (LASV), an arenavirus endemic to West Africa, results in Lassa fever (LF) – a severe hemorrhagic fever with high morbidity and mortality. There are presently no FDA-approved therapeutic options for the explicit treatment of LF, largely due to a lack of knowledge surrounding LASV’s lifecycle. A notable component of LASV’s lifecycle is the delivery of its genetic material into the host cell. This is accomplished via membrane fusion, a process initiated by a hydrophobic sequence known as the fusion domain (FD). The LASV FD (G260 – N295) consists of two structurally distinct regions, an N-terminal fusion peptide (FP, G260 – T274) and an internal fusion loop (FL, C279 – N295), which are conjoined by a short linker region (P275 – Y278). However, the molecular mechanisms underlying LASV FD-initiated fusion remain unclear, limiting the ability to develop antiviral therapies. Here, we provide critical insights into how the LASV FD initiates membrane fusion. We demonstrate that the LASV FD exists in a nonfusogenic, random coil conformation until a pH akin to the lysosomal compartment is achieved, at which point it adopts a fusogenic, helical conformation. This was followed by the discovery that acidic bicelles are the optimal membrane mimic for the stabilization of the FD for structural studies. In turn, we were able to elucidate the pH-dependent structural changes undergone by the FP and FL throughout the fusion process. We provide evidence that the entire LASV FD samples alternative conformations before fusion occurs, but only the FP continues to do so after membrane association. This is because the helix adopted by the LASV FD at a low pH occurs within the FL, particularly from R282 – L290, which inserts itself into the lipid head group of the host cell to mediate fusion, whereas the FP remains solvent-exposed to interact with the environment. Moreover, we demonstrate that LASV FD-initiated fusion is specifically influenced by bis(monoacylglycero)phosphate (BMP), an anionic lipid abundant in the lysosomal membrane, but this preference is not mediated by ionic interactions. In conclusion, our findings provide details into the mechanistic details underlying LASV FD-initiated fusion and, hopefully, can be employed to develop targeted therapeutics.