a, Rigid body alignment of the Wzm–WztN transporter halves with the corresponding NBDs of the closed WztN dimer structure. The closed WztN dimer is shown in grey, and Wzm–WztN is coloured in red and green for Wzm and cyan and blue for WztN. Residues replaced with Cys are shown with spheres at their Cα carbons. Observed disulfide cross-links are indicated with a dashed line. b, Cartoon illustration of the open to closed transition of the transporter. c, Disulfide cross-linking of Wzm protomers. Purified Wzm–WztN transporters harbouring the indicated Cys mutations were oxidized with either copper phenanthroline (Co-Phen) or sodium tetrathionate (STT), blocked with N-ethylmaleimide (NEM), and analysed by western blotting against the N-terminal Wzm Flag-tag. Experiments were repeated three times with similar results. M and D, Wzm monomer and dimer.