Ca2+ ions promote fusion of Middle East respiratory syndrome coronavirus with host cells and increase infectivity.

Straus, Marco R; Tang, Tiffany; Lai, Alex L; Flegel, Annkatrin; Bidon, Miya; Freed, Jack H; Daniel, Susan; Whittaker, Gary R

Fusion with, and subsequent entry into, the host cell is one of the critical steps in the life cycle of enveloped viruses. For Middle East respiratory syndrome coronavirus (MERS-CoV), the spike protein (S) is the main determinant of viral entry. Proteolytic cleavage of S exposes its fusion peptide (FP), which initiates the process of membrane fusion. Previous studies on the related severe acute respiratory syndrome coronavirus (SARS-CoV) FP have shown that calcium ions (Ca2+) play an important role for fusogenic activity via a Ca2+ binding pocket with conserved glutamic acid (E) and aspartic acid (D) residues. SARS-CoV and MERS-CoV FP share a high sequence homology and here, we investigated whether Ca2+ is required for MERS-CoV fusion by screening a mutant array in which E and D residues in the MERS-CoV FP were substituted with neutrally charged alanines (A). Upon verifying mutant cell surface expression and proteolytic cleavage, we tested their ability to mediate infection of pseudo-particles (PPs) on host cells in modulating Ca2+ environments. Our results demonstrate that intracellular Ca2+ enhances MERS-CoV WT PPs infection by approximately two-fold and that E891 is a crucial residue for Ca2+ interaction. Subsequent electron spin resonance (ESR) experiments revealed that this enhancement could be attributed to Ca2+ increasing MERS-CoV FP fusion-relevant membrane ordering. Intriguingly, isothermal calorimetry showed an approximate 1:1 MERS-CoV FP to Ca2+ ratio, as opposed to an 1:2 SARS-CoV FP to Ca2+ ratio, suggesting significant differences in FP Ca2+ interactions of MERS-CoV and SARS-CoV FP despite their high sequence similarity.SignificanceMERS-CoV is a major emerging infectious disease with zoonotic potential and a reservoir in dromedary camels and bats. Since its first outbreak in 2012, the virus has repeatedly transmitted from camels to humans with 2,468 confirmed cases causing 851 deaths. To date, there are no efficacious drugs and vaccines against MERS-CoV, increasing its potential to cause a public health emergency. In order to develop novel drugs and vaccines, it is important to understand the molecular mechanisms that enable the virus to infect host cells. Our data has found that calcium is an important regulator of viral fusion by interacting with negatively charged residues in the MERS-CoV FP region. This can guide therapeutic solutions to either block this calcium interaction and also repurpose already approved drugs for this use for a fast response to MERS-CoV outbreaks.