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. 2020 May 6;117(21):11727–11734. doi: 10.1073/pnas.2003138117

Fig. 1.

Fig. 1.

PPC motif in SARS-CoV-2 spike protein. (A) Different stages of coronavirus entry where host cellular proteases may activate coronavirus spikes. (B) Schematic drawing of the three-dimensional (3D) structure of coronavirus spike. S1, receptor-binding subunit; S2, membrane fusion subunit; TM, transmembrane anchor; IC, intracellular tail. (C) Schematic drawing of the 1D structure of coronavirus spike. NTD, N-terminal domain. FP (fusion peptide), HR1 (heptad repeat 1), and HR2 (heptad repeat 2) are structural units in coronavirus S2 that function in membrane fusion. (D) Sequence comparison of the spike proteins from SARS-CoV-2, SARS-CoV, and two bat SARS-like coronaviruses in a region at the S1/S2 boundary. Only SARS-CoV-2 spike contains a putative PPC motif—RRAR (residues in the box). The assumed PPC cleavage site is in front of the arginine residue labeled in red. The spike region mutated from SARS-CoV-2 sequence (TNSPRRA) to SARS-CoV sequence (SLL) is labeled in blue. GenBank accession numbers are QHD43416.1 for SARS-CoV-2 spike, AFR58740.1 for SARS-CoV spike, MG916901.1 for bat Rs3367 spike, and QHR63300.2 for bat RaTG13 spike.