The Conserved Fusion Peptide Sequence of SARS-CoVs is Insensitive to Ca²⁺ in Inducing Lipid Mixing in Model Membranes

The viral envelope fuses with the host cell membrane to initiate viral infection. In the case of SARS-CoVs, the fusion process is catalyzed by the spike (S) protein. The S1 subunit interacts with the host cell receptor and assists in docking the viral particle at the cell surface, whereas the fusion process is induced by the insertion of a conserved hydrophobic motif at the N-terminal of the S2 subunit, known as a fusion peptide, into the host cell. Current experimental findings suggest that the fusion peptide of SARS-CoVs displays a strong Ca²⁺-dependent membrane binding and fusogenic activity, indicating that Ca²⁺ functions as a critical cofactor during viral entry. Nevertheless, many earlier studies fail to clearly differentiate between fusion driven directly by Ca²⁺ and that mediated by the fusion peptide, often due to the absence of appropriate controls for different Ca²⁺ concentrations. In this work, we have assessed the impact of the consensus fusion peptide of SARS-CoVs in polyethylene glycol (PEG)-mediated fusion of small unilamellar vesicles with varying Ca²⁺ concentration, where appropriate control experiments have been carried out in the absence of the peptide. Our results show that the peptide has no significant effect on Ca²⁺ in inducing lipid mixing of model membranes.