SARS-CoV-2 virions are surrounded by a lipid bilayer from which spike (S) protein trimers protrude. Heavily glycosylated S trimers bind the ACE2 receptor and mediate entry of virions into target cells. S exhibits extensive conformational flexibility: it modulates exposure of its receptor binding site and later undergoes complete structural rearrangement to drive fusion of viral and cellular membranes.

The paper reports the application of cryo-electron microscopy and tomography to image intact SARS-CoV-2 virions, determining the high-resolution structure, conformational flexibility and distribution of S trimers in situ on the virion surface. These results reveal the conformations of S present on the virion, and provide a basis from which to understand interactions between S and neutralizing antibodies during infection or vaccination.