Fig. 1.

Proteolytic activation of coronavirus S proteins. (A) Schematic of the MHV S glycoprotein organization with emphasis on the S₂ subunit. The dashed black box shows the region of the S₂ polypeptide chain that is unresolved in the map. Gray dashed boxes show regions that were not part of the construct. BH, beta hairpin; CD, connector domain; CH, central helix; CT, cytoplasmic tail; FP, fusion peptide; HR1, heptad repeat 1; HR2, heptad repeat 2; SH, stem helix; TM, transmembrane domain; UH, upstream helix. (B–D) 2D class averages of negatively stained MHV S (B), SARS-CoV S (C), and MERS-CoV S (D) trimers in the prefusion state (Left) and in the trypsin-cleaved postfusion state (Right). (Scale bar, 10 nm.)