ding to the host-cell receptor and facilitating membrane fusion, respectively9–11. MERS S binds to dipeptidyl-peptidase 4 (DPP4)12, whereas SARS S13 and SARS-CoV-214,15 utilize angiotensin-converting enzyme 2 (ACE2) as a host cellular receptor. CoV S proteins are the largest class I viral fusion proteins known9, and are