Understanding coronavirus fusion through structure

Abstract

Two recent coronavirus epidemics highlight the need for vaccine development since no therapeutic currently exists. There are six known human-infecting coronaviruses and many others that can infect livestock and household pets. Viral surveys in bat populations suggest many coronaviruses are poised to cross the species barrier, suggesting future outbreaks are likely. Coronaviruses are enveloped viruses and infection is mediated by a trimeric spike glycoprotein present on the virion surface. The spike proteins are class I fusion proteins, similar to HIV envelope or influenza hemagglutinin, and are the main target of neutralizing antibodies during infection. The viral tropism and ability for the virus to enter cells is determined by the spike protein which undergoes a large conformational change from its pre-fusion state on the viral surface to the post-fusion state following viral and host membrane fusion. The structure and organization of the coronavirus spike protein had not previously been established. In this dissertation, I begin by investigating the architecture of the coronavirus spike protein from various different genus’s and infection hosts. Using state of the art single particle cryo-electron microscopy we were able to solve two near atomic resolution structures of the first beta-coronavirus and alpha-coronavirus spike proteins and elucidate their commonalities and differences. In order to better understand the fusion transition, we also solved the first structure of a beta-coronavirus spike protein in the post-fusion state. Moving forward from these snapshots of coronavirus fusion proteins we wanted to start understanding both how fusion is prevented by the human immune system and also how receptor binding can trigger fusion. We solved the structures of two deadly beta-coronaviruses in complex with potent, neutralizing human antibodies where we learned the interactions necessary for blocking receptor binding and where we also identified a functional mimicking antibody that triggered protein refolding similarly to the native receptor. Finally, we have also been working to understand how glycan (co-)receptors work with coronaviruses, adding another layer of specificity to the challenges of entry. Through this work and the work done by others in the field, we now have a clear picture of coronavirus spike protein structure and are beginning to uncover the mechanisms of coronavirus entry and membrane fusion.