BPS2026 – Understanding the role of stable signal peptide in Lassa virus fusion and the process of its inhibition

Lassa virus (LASV), the foremost member of the arenavirus family, is the causative agent of hemorrhagic fever in infected humans. Infection begins with the entry of LASV into a host cell, a process that requires membrane fusion mediated by the glycoprotein complex (GPC). The LASV GPC consists of the stable signal peptide (SSP) and two glycoproteins (GP1 and GP2). Uniquely, SSP is retained following maturation and is associated with GP2, the fusion subunit of GPC, where it plays a crucial yet not fully understood role in the low pH-dependent membrane fusion process of LASV. To better understand this unique role, we investigated key structural traits of SSP at different pH levels to recreate the cellular environment where LASV initiates fusion. We found that SSP undergoes a structural rearrangement at low pH, which may act as a molecular trigger for LASV fusion. Furthermore, we explored the mechanism of inhibition for a class of small molecules that target LASV fusion. We found ST-193, one of the more potent LASV fusion inhibitor, binds to GPC near the transmembrane helices of SSP, potentially preventing SSP from undergoing its fusion-relevant structural transition. In addition, we probed the structural implications of K33, which is fully conserved throughout the arenavirus family and is crucial for fusion. We found that the K33A mutant lowered fusion efficiency but did not alter the prefusion conformation of SSP, indicating its perturbation of fusion comes during the fusion process. Taken together, we believe the structural alteration undergone by SSP at low pH is a keystone step in the LASV fusion process and is prevented by small molecule inhibitors and the K33A mutation.