Fig. 3: The S301P mutation locks the C-terminal tail of 3CLpro in the post-cleavage state.

a, b In the crystal structure of the WT 3CLpro in the mature state (PDB code: 6M03), the C-terminal tail (residues 301–306, shown in sticks and colored orange) of one 3CLpro protomer binds to the other 3CLpro protomer (the protein contact potential was calculated using PyMOL) within the same 3CLpro homodimer. c, d In the crystal structure of the M49K/S301P double mutant, the C-terminal tail (colored cyan) of one 3CLpro protomer turns away from the other 3CLpro protomer within the same 3CLpro homodimer. e Alignment of the C-terminal tails of the two 3CLpro protomers (colored purple and salmon, respectively) within the same 3CLpro homodimer in the crystal structure of the WT 3CLpro in the post-cleavage state (PDB code: 7E5X). The two C-terminal tails have distinct orientations. Rotation of the Φ angle of S301 switches the C-terminal tail from the post-cleavage state (chain A) to the mature state (chain B). f Alignment of the C-terminal tails of 3CLpro in the M49K/S301P structure (colored cyan) with that in the mature WT 3CLpro structure (colored orange). In the M49K/S301P mutant, the Φ angle of P301 is fixed, and therefore, the C-terminal tail of 3CLpro is locked at the post-cleavage state.