Unexpected receptor functional mimicry elucidates activation of coronavirus fusion

Walls, Alexandra C.; Xiong, Xiaoli; Park, Young-Jun; Tortorici, M. Alejandra; Snijder, Joost; Quispe, Joel; Cameroni, Elisabetta; Gopal, Robin; Dai, Mian; Lanzavecchia, Antonio; Zambon, Maria; Rey, Félix A.; Corti, Davide; Veesler, David

Recent outbreaks of severe acute respiratory syndrome and Middle-East respiratory syndrome along with the threat of a future coronavirus-mediated pandemic underscore the importance of finding ways to combat these viruses. The trimeric spike transmembrane glycoprotein S mediates entry into host cells and is the major target of neutralizing antibodies. To understand the humoral immune response elicited upon natural infections with coronaviruses, we structurally characterized the SARS-CoV and MERS-CoV S glycoproteins in complex with neutralizing antibodies isolated from human survivors. Although the two antibodies studied blocked attachment to the host cell receptor, only the anti-SARS-CoV S antibody triggered fusogenic conformational changes via receptor functional mimicry. These results provide a structural framework for understanding coronavirus neutralization by human antibodies and shed light on activation of coronavirus membrane fusion, which takes place through a receptor-driven ratcheting mechanism.