| The SARS‑CoV and SARS‑CoV‑2 spike (S) glycoprotein is a trimeric class I fusion protein embedded in the viral envelope that mediates host cell recognition and membrane fusion through a coordinated series of conformational changes centered on its S1 and S2 subunits, with S1 harboring the N‑terminal domain and receptor‑binding domain that engage host receptors such as ACE2 and S2 containing the fusion peptide, heptad‑repeat regions, transmembrane anchor, and cytoplasmic tail that drive fusion after proteolytic activation. The prefusion spike samples “closed” and “open” states in which the receptor‑binding domains are down or up, and ACE2 binding is favored by the up conformation, which stabilizes specific RBD loops and a surface lined by residues that differ subtly between SARS‑CoV and SARS‑CoV‑2, explaining the higher ACE2 affinity of the latter and its sensitivity to mutations at key positions within the receptor‑binding motif. Host proteases including furin, TMPRSS2, and endosomal cathepsins cleave the S1/S2 and S2′ sites to prime S2, expose the fusion peptide, and allow the heptad‑repeat 1 and 2 segments to refold into a six‑helix bundle that juxtaposes viral and cellular membranes and completes fusion, a process modulated by extensive N‑linked glycosylation that shields large regions of the spike surface while leaving immunodominant epitopes and the ACE2‑binding interface relatively accessible. The spike trimer decorates the virion surface at variable densities and tilt angles, with individual protomers adopting different RBD conformations within the same trimer, creating a dynamic antigenic landscape that shapes neutralizing antibody binding and underlies the mechanism of many monoclonal antibodies that either block ACE2 engagement directly at the RBD or stabilize nonfunctional prefusion or post‑fusion states. Many variants of concern accumulate substitutions, deletions, or insertions in the N‑terminal domain antigenic supersite, the receptor‑binding motif, and the S1/S2 region, altering local charge distribution, glycan positioning, and inter‑protomer contacts to tune ACE2 affinity, fusion activation thresholds, and escape from classes of neutralizing antibodies, while often preserving the core fusion machinery required for viral entry. The spike cytoplasmic tail contains signals for incorporation into budding virions and interacts with other structural proteins, linking entry functions to virion assembly, and engineered stabilizing mutations and proline insertions in S2 have been used to lock the spike in its prefusion form for structural studies and for vaccine immunogens, taking advantage of the conformational states defined in high‑resolution cryo‑EM and cryo‑ET analyses. |
