Energy-coupled transport through the outer membrane of Escherichia coli small deletions in the gating loop convert the FhuA transport protein into a diffusion channel.

Active transport of Fe3+ as ferrichrome complex through the outer membrane of Escherichia coli is mediated by the FhuA outer membrane protein and the TonB-ExbB-ExbD protein complex in the cytoplasmic membrane. The required energy is provided by the electrochemical potential of the cytoplasmic membrane which is assumed to induce a conformation of the TonB protein that causes a conformational change in FhuA so that bound ferrichrome is released into the periplasmic space located between the outer and the cytoplasmic membrane. Excision of segments as small as 12 amino acids in the largest surface loop of FhuA converted FhuA into an open channel through which ferrichrome and antibiotics diffused independent of TonB-ExbB-ExbD. It is proposed that FhuA forms a closed channel which is opened by movement of the gating loop through a kind of allosteric interaction with TonB. The gating loop is also involved in binding of all FhuA ligands which in addition to ferrichrome are the phages T1, T5, phi 80, colicin M and the antibiotic albomycin.