Cryo-EM structure of infectious bronchitis coronavirus spike protein reveals structural and functional evolution of coronavirus spike proteins
(A) Schematic drawing of IBV spike ectodomain. S1: receptor-binding subunit. S2: membrane-fusion subunit. GCN4-His6: GCN4 trimerization tag followed by His6 tag. S1-NTD: N-terminal domain of S1. S1-CTD: C-terminal domain of S1. CH: central helix. FP: fusion peptide. HR1 and HR2: heptad repeats 1 and 2. Residues in shaded regions (N-terminus, HR2, GCN4 tag, and His6 tag) were not included in the structural model. Question mark indicates that the exact location of FP is uncertain; the range of FP used for making figures is consistent with a previous structural study on β-genus mouse hepatitis coronavirus spike . (B) Cryo-EM density maps of IBV spike ectodomain with atomic model fitted in. The maps have a contour of 7.5 σ. (C) Cryo-EM structure of IBV spike ectodomain in the pre-fusion conformation. Each of the monomeric subunits is colored differently. (D) Structure of a monomeric subunit in the pre-fusion conformation. The structural elements are colored in the same way as in panel (A). Dotted line indicates residues 702–710 that are missing in the structural model. All structures are viewed from the side.