PROS1 and infection: The fusion of the virus S-protein and host receptor ACE2, which occurs within acidified endosomes (plasma membrane in MHV), is of pivotal importance for the entry of the virus into cells and for inducing the humoral immune response during infection (Fehr and Perlman, 2015; Kirchdoerfer et al., 2016; Yang X. et al., 2020).