Jun 30, 2022 · The epitope that 76E1 binds is partially buried in the structure of the SARS-CoV-2 spike trimer in the prefusion state, but is exposed when the spike protein binds to ACE2.. Such antibodies must target the spike protein in its aptly named prefusion conformation. Unfortunately for vaccine developers, spike proteins are liable to spring from their stubby prefusion shape into their elongated postfusion form on a hair trigger. Here we present the 4.0 Å resolution structure of the trimeric HKU1 S protein determined using single-particle cryo-electron microscopy. In the pre-fusion conformation, the.