Biophysical Characterization of K33A Mutation to Stable Signal Peptide Reveals Insights into Lassa Virus Fusion Mechanism.

Lassa virus (LASV), a member of the arenavirus family, utilizes low-pH-dependent membrane fusion to enter host cells and elicit infection. LASV is a class I fusion virus, and thus its fusion protein, GP2, is functionally similar to other class I fusion proteins in that it follows the six-helix bundle fusion mechanism. LASV fusion is atypical in that it requires another viral protein, stable signal peptide (SSP), that does not undergo fusion itself but is required for the fusion process. However, the exact function of SSP in fusion is unknown. We aimed to understand the role of SSP in the fusion process by examining the structural changes that occur within SSP during the fusion process. We found that SSP undergoes two distinct structural changes; the first at pH 6 that is not associated with fusion, the second at below pH 5 that is associated with fusion activity. Furthermore, we found a mutation to a key residue of SSP, K33A, does not undergo these two structural changes; in fact, K33A is seen to undergo a different conformational change at pH 6. We believe this distinct conformation change is due to the loss of favorable interactions between the lysine at the end of the transmembrane helix of SSP. This disruption of favorable interaction then causes downstream effects to the SSP structure in the event of lowering pH. The difference in conformational changes between K33A and wildtype (WT) SSP is likely why K33A abrogates the fusion activity of GP2. We conclude that the conformational changes undergone by SSP are crucial for LASV fusion activity.