LC-MS/MS EThcD of MERS-CoV and SARS-CoV S N-linked glycopeptides from recombinant spike ectodomains and whole MERS virions

Recent outbreaks of severe acute respiratory syndrome and Middle-East respiratory syndrome along with the threat of a future coronavirus pandemic underscore the importance of finding ways to neutralize 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 using cryo electron microscopy and characterized the site-specific N-linked glycan profile of the recombinant S proteins with LC-MS/MS using EThcD fragmentation. Although the two antibodies studied blocked attachment to the host cell receptor, only the anti-SARS-CoV S antibody triggered premature fusogenic conformational changes via receptor functional mimicry. These results provide a structural framework for understanding coronavirus neutralization by human antibodies and shed light on the coronavirus fusion activation pathway which appears to take place through a receptor-driven ratcheting mechanism.