Cryo-electron microscopy structure of a coronavirus spike glycoprotein trimer

Abstract

The high-resolution cryo-electron microscopy structure of a pre-fusion coronavirus spike trimer from mouse hepatitis virus is presented; the structure reveals architectural similarities to paramyxovirus F proteins, suggesting that these fusion proteins may have evolved from a distant common ancestor. Coronaviruses are responsible for respiratory infections worldwide, many of them mild, but also including severe pneumonia and the recent SARS and MERS outbreaks. The entry of coronaviruses into cells is mediated by the virus glycoprotein spike trimer, which contains the receptor-binding domain, as well as membrane fusion domains. Two papers published in this issue of Nature provide high-resolution (4Å) cryo-electron microscopy structures of pre-fusion coronavirus spike trimers. David Veesler and colleagues studied the trimer from murine hepatitis virus; Andrew Ward and colleagues used the human betacoronavirus HKU1, a cause of mild respiratory disease. The structures reveal mechanistic insights into the viral fusion process and architectural similarities to paramyxovirus F proteins, suggesting that these fusion proteins may have evolved from a distant common ancestor. The tremendous pandemic potential of coronaviruses was demonstrated twice in the past few decades by two global outbreaks of deadly pneumonia. Entry of coronaviruses into cells is mediated by the transmembrane spike glycoprotein S, which forms a trimer carrying receptor-binding and membrane fusion functions1. S also contains the principal antigenic determinants and is the target of neutralizing antibodies. Here we present the structure of a mouse coronavirus S trimer ectodomain determined at 4.0 Å resolution by single particle cryo-electron microscopy. It reveals the metastable pre-fusion architecture of S and highlights key interactions stabilizing it. The structure shares a common core with paramyxovirus F proteins2,3, implicating mechanistic similarities and an evolutionary connection between these viral fusion proteins. The accessibility of the highly conserved fusion peptide at the periphery of the trimer indicates potential vaccinology strategies to elicit broadly neutralizing antibodies against coronaviruses. Finally, comparison with crystal structures of human coronavirus S domains allows rationalization of the molecular basis for species specificity based on the use of spatially contiguous but distinct domains.