Possible transport mechanism of ECF transporters (Type III importers) based on the available structures (A) and in schematic representation (B). Structures have been determined of the nucleotide- and substrate-free transporter (ECF-HmpT, Protein Data Bank accession no. 4HZU, is shown here; see Table 1 for details of all available structures of ABC transporters). The S-component (orange) is predicted to rotate in the membrane (“topple over”) to bring the substrate from the trans-side to the cis-side.