As cell-invading molecular machinery, coronavirus spike proteins pose an evolutionary conundrum due to their high divergence. In this study, we determined the cryo-EM structure of avian infectious bronchitis coronavirus (IBV) spike protein from the γ-genus. The trimeric IBV spike ectodomain contains three receptor-binding S1 heads and a trimeric membrane-fusion S2 stalk. While IBV S2 is structurally similar to those from the other genera, IBV S1 possesses structural features that are unique to different other genera, thereby bridging these diverse spikes into an evolutionary spectrum. Specifically, among different genera, the two domains of S1, the N-terminal domain (S1-NTD) and C-terminal domain (S1-CTD), diverge from simpler tertiary structures and quaternary packing to more complex ones, leading to different functions of the spikes in receptor usage and membrane fusion. Based on the above structural and functional comparisons, we propose that the evolutionary spectrum of coronavirus spikes follows the order of α-, δ-, γ-, and β-genus. This study has provided insight into the evolutionary relationships among coronavirus spikes and deepened our understanding of their structural and functional diversity.
Bibliographical noteFunding Information:
This work was supported by National Institute of Allergy and Infectious Diseases grants R01AI089728 and R01AI110700 (to FL) and University of Minnesota funding (to WZ). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Initial cryo-EM images were collected using FEI Tecnai TEMs maintained at the Characterization Facility of the University of Minnesota. Final cryo-EM data were collected at the John M. Cowley Center for High Resolution Electron Microscopy of Arizona State University. We thank Dr. Dewight Williams for helping us prepare grids and collect data. Initial image analysis and computation work were performed using the workstations at the Basic Sciences Computing Laboratory of the University of Minnesota Supercomputing Institute.