Energy-coupled Transport Across the Outer-membrane of Gram-negative Bacteria
from: Bacterial Membranes: Structural and Molecular Biology (Edited by: Han Remaut and Rémi Fronzes). Caister Academic Press, U.K. (2014)
Active, energy-coupled transport across the Outer-membrane (OM) of Gram-negative bacteria is intriguing. Because there is no energy source in the periplasm, the energy required for transport is instead provided by the cytoplasmic membrane proton-motive force, to which a particular class of transport proteins responds. Such an energy-coupled transporter in the OM forms a β-barrel that is tightly closed on the periplasmic side by the N-terminal domain plug. This plug domain distinguishes active transporters from porins-other β-barrel proteins that form permanently open pores, through which substrates passively diffuse. The energy from the cytoplasmic membrane is transferred to the OM transporters by the TonB-ExbB-ExbD protein complex (Ton system). Crystal structures of transporters and of transporters with bound TonB fragments provide clues as to how OM transport is energized, but experimental evidence of how the Ton system responds to the proton motive force, how it transfers energy from the cytoplasmic membrane to the OM, and how the transporters react to energy input is largely lacking. Substrates actively taken up across the OM include heme, Fe3+ incorporated into siderophores, transferrin, lactoferrin, haptoglobin, hemopexin, lipocalin, vitamin B12, Ni2+, Zn2+, various oligosaccharides, and aromatic compounds. OM transporters also regulate transcription of genes from the cell surface and serve as receptors for toxic peptides and protein toxins. The transporters are particularly abundant in environmental and gut bacteria (up to 140 per strain), which use a large variety of substrates, but are absent in intracellular pathogens and obligate parasites read more ...