Unlocking the Significance of the Stable Signal Peptide: A Vital Component of Lassa Virus Membrane Fusion

Abstract

Lassa virus (LASV) is an arenavirus prevalent in West Africa that causes a severe hemorrhagic fever in infected humans with no approved therapeutics. One of the key steps in the LASV lifecycle is low pH-dependent membrane fusion, which is the process by which the viral and host membranes are merged to form a fusion pore. LASV fusion is carried out by the glycoprotein 2 (GP2) subunit of the glycoprotein complex (GPC). GP2 is a class I viral fusion protein and thus follows the six-helix bundle fusion mechanism used by other viruses like Influenza and HIV. However, unlike other viruses in this class, LASV contains a stable signal peptide (SSP) that forms a part of the mature GPC and assists GP2 in carrying out fusion. While the presence of SSP has previously been shown to be critical for fusion, how it influences fusion is currently unknown. Here, we address SSP’s role in the LASV fusion process. First, we developed a platform of incorporating the membrane spanning SSP into several distinct membrane mimetic systems for in vitro characterization. We discovered that SSP structure is composed of three helices in its prefusion state (pH7). SSP then undergoes two distinct conformational changes as it is exposed to a low pH environment that triggers fusion. We found that a mutation to a keystone residue, K33A, causes SSP to undergo an off-pathway conformational change that leads to loss of fusion activity. We demonstrate that interactions made between SSP and GP2 are the main driver of SSP’s role in fusion. Furthermore, we provide evidence that SSP can oligomerize at low pH, which may be associated with its role in fusion. Finally, we demonstrate a small-molecule fusion inhibitor binds to SSP, indicating SSP may be a suitable target for therapeutic intervention against LASV.