Cryo-EM structures of HKU2 and SADS-CoV spike glycoproteins and insights into coronavirus evolution

Jinfang Yu; Shuyuan Qiao; Runyu Guo; Xinquan Wang

The NTDs of β-coronaviruses including BCoV, HCoV-HKU1, HCoV-OC43, 218 MERS-CoV, SARS-CoV and MHV resemble the subtype I, rather than the subtype II 219 NTD in the topology and distribution of the disulfide bonds (Fig. 2c) . These β-220 coronavirus NTDs have additional loops in the N-terminus, between β1 and β2 strands, 221 and between β6 and β7 stands (numbered in HKU2 structure), forming an extensive The CTD of HKU2 has a twisted five-stranded antiparallel β sheet as the core with 234 connecting loops between the stands (Fig. 3a) . It contains four disulfide bonds: C 277 -235 C 300 and C 285 -C 290 at the N-terminus, C 341 -C 397 at the C-terminus and the last one C 331 -236 C 369 connecting the β2 and β5 strands in the core β sheet (Fig. 3a) . Interestingly, the 237 CTD core of HKU2 is of high structural similarity with the conserved CTD core of β-238 coronaviruses and the disulfide bonds in the CTD of HKU2 except for C 285 -C 290 are 239 also detected in all β-coronavirus CTDs (Fig. 3b) . These CTDs have the core of one 240 twisted β-sheet and here we name them as one-layer CTD subtype ( Fig. 3a and Fig. 3b ).