5.A.1.2.8
Cytochrome c-type biogenesis protein (CcdA) of 190 aas and 6 TMSs. The NMR structure of a reduced-state mimic of CcdA that transfers electrons across the inner membrane has been determined (Williamson et al. 2015).  The two cysteine positions in CcdA are separated by 20 Å. Whereas one is accessible to the cytoplasm, the other resides in the protein core, thus implying that conformational exchange is required for periplasmic accessibility, confirmed in vitro. The existence of multiple conformational states was demonstrated, suggesting a four-state model for relaying electrons from cytosolic to periplasmic redox substrates (Williamson et al. 2015).