S protein of SARS-CoV is composed of two functionally distinct subunits: the globular S1 subunit (~aa 12–680) allows receptor recognition, whereas the S2 subunit (~aa 681–1255) facilitates membrane fusion and anchors S into the viral membrane. S1 is organized in four distinct domains A–D. Domains S1A and S1B may be used as a receptor-binding domain (RBD, aa 318–510) containing the highly conserved receptor-binding motif (RBM, aa 424–494) [21]. Moreover, RBD contains 3 functional glycosylation sites located at amino acids 318, 330 and 357, which are necessary for S expression but do not affect ACE2 binding [22]. S1B forms an extended loop on the viral membrane-distal side and is a hypervariable region [20]. S2 contains the fusion peptides (FP1 and FP2) [23], two heptad repeat regions (HR1, aa ~889–972 and HR2, aa ~1142–1193) and the well conserved transmembrane domain [24].