Fig. 5.

The polypeptide segment close to the THF. a Superposition of B. subtilis SecA (blue) in the current cryo-EM structure with T. maritima SecA (yellow) in the SecA-SecYEG crystal structure (PDB ID: 3DIN). SecYE and substrate density are from the current structure, while SecYEG and substrate from the crystal structure were deleted. The right panel shows a magnified view of the boxed area. b Close-up view of THF residues mediating substrate interaction (balls and sticks). c Top view of the polypeptide substrate close to the THF. Density for the substrate is shown in green and the surrounding TMs of SecY in ribbon representation. SecG, which is absent from the present structure, would be located at the position indicated by a dashed oval. d Superposition of G. thermodenitrificans SecYE in the current cryo-EM structure of the active channel with T. maritima SecYEG in a crystal structure of the SecA-primed channel (PDB ID: 3DIN). G. thermodenitrificans SecY is in salmon, T. maritima SecY is in purple, and T. maritima SecG is in gold. e Superposition of G. thermodenitrificans SecYE in the cryo-EM structure of the active channel with T. thermophilus SecYEG in a crystal structure of the idle channel (PDB ID:5AWW). T. thermophilus SecY is in purple and SecG is in magenta. The SecG loop clashes with the polypeptide density