Figure 1.

Domain organization and protease cleavage sites of MERS-S. MERS-S possesses two subunits, a surface subunit (S1) and a membrane-anchored subunit (S2). The S1 subunit harbors an N-terminal signal peptide (SP) and the receptor binding domain (RBD) while the S2 subunit contains domains required for membrane fusion, the fusion peptide (FP), two heptad repeats (HR1, HR2), and a transmembrane domain (TM). Moreover, the S2 subunit contains a cytoplasmic tail (CT). The S1/S2 cleavage site is located at the border between the S1 and S2 subunits while the S2′ site is located at the N-terminus of the FP. A proposed cleavage site for cathepsin L is located between the S1/S2 and S2′ sites (ECP, endosomal cysteine protease). The amino acid residues of the S1/S2, cathepsin L and S2′ sites are printed in bold and the mutations introduced into the cleavage sites are indicated.