FIG 5.
TcdC topology models. TcdC is located in the cell membrane with an extracellular C-terminal region. The localization of the 50-amino-acid N-terminal domain of TcdC is unknown. In topology model 1, the N terminus can cross the cell membrane, exposing the N-terminal domain to the extracellular environment. Another possibility, topology model 2, has the N terminus present in the cell membrane, where it is not accessible from the extracellular or intracellular milieu. Finally, in topology model 3, the N terminus is present in the intracellular environment of the cell. The cysteine residues used for the cysteine accessibility analysis (red dots) and the C-terminal location of the HiBiTopt tag (orange line) are indicated.