Cryo-EM structures of MERS-CoV and SARS-CoV spike glycoproteins reveal the dynamic receptor binding domains

Yuan, Yuan; Cao, Duanfang; Zhang, Yanfang; Ma, Jun; Qi, Jianxun; Wang, Qihui; Lu, Guangwen; Wu, Ying; Yan, Jinghua; Shi, Yi; Zhang, Xinzheng; Gao, George F.

For both MERS-CoV and SARS-CoV S proteins, the S2 subunit is mainly composed of a-helices and forms the stem region of the S protein (Fig. 2) . A long linker region connects the S2 cleavage site to the long upstream helix. The second S2 0 cleavage site is exposed at the peripheral after the long upstream helix, and is readily accessible by the endo-lysosomal proteases (Fig. 2 ). An exposed helical fusion peptide is also observed immediately downstream of the S2 0 cleavage site, and connects to the HR1 region by a long connecting region featuring three consecutive a-helices. Following the HR1 region, a long central helix stretches 70 Ã… along the three-fold axis towards the viral membrane (Fig. 2) . This central helix is tightly packed against the upstream helix via hydrophobic contacts, forming the central stem region of the S ectodomain trimer with equivalent contributions from the other two S ectodomain protomers. After the central helix, a b-hairpin structure is present at the bottom of the S trimer. The viral membrane proximal HR2 region is invisible due to poor density.