TonB-Dependent Transport Mechanisms in Gram-Negative Bacteria

Gram-negative bacteria maintain a robust outer membrane barrier that precludes passive uptake of many essential nutrients. To overcome this, they employ TonB-dependent transporters (TBDTs), specialised β-barrel proteins that bind scarce or bulky substrates—such as iron-chelating siderophores, cobalamin and certain carbohydrates—and import them into the periplasm. The energy to drive this uptake is harvested from the proton motive force across the inner membrane by the ExbB–ExbD motor complex and relayed via TonB, a periplasm-spanning protein anchored at its N-terminus in the inner membrane. Upon substrate engagement at the outer membrane, TonB interacts with a conserved motif in the transporter’s plug domain, inducing a conformational change that transiently opens a channel. The peptidoglycan layer provides anchorage and alignment for TonB and ExbD, ensuring efficient force transduction. This multi-protein machinery is vital for nutrient acquisition, pathogenesis and survival in iron-limited environments and represents a promising target for novel antimicrobial interventions.

Research from Nature Portfolio

Recent structural and functional studies have revealed a dynamic conformational switch in the periplasmic domain of ExbD, demonstrating its essential role in gating the proton channel. In its open state, ExbD promotes a disorder-to-order transition in TonB, enabling direct energy transfer to the outer membrane transporter. This work also highlights how peptidoglycan anchorage stabilises the ExbD–TonB interaction, providing a unified mechanistic model conserved across related bacterial motor systems.

Advances in cryo-electron microscopy have refined the architecture and stoichiometry of the inner membrane motor, showing that an ExbB pentamer encloses an ExbD dimer within its transmembrane pore. The revised stoichiometry has significant implications for understanding how proton-driven rotary motion is converted into mechanical force and clarifies the arrangement of motor subunits that is critical for energising TonB-dependent transport.

TonB-Dependent Transport Mechanisms in Gram-Negative Bacteria publication trend

The graph below shows the total number of articles in tonb-dependent transport mechanisms in gram-negative bacteria across all publications each year (not limited to Nature Index journals).

Technical terms

TonB-dependent transporter (TBDT): An outer membrane β-barrel protein that binds and imports scarce or bulky substrates by coupling to the Ton system.

Proton motive force (PMF): The electrochemical gradient of protons across the inner membrane that serves as the energy source for many bacterial transport and motility systems.

ExbB–ExbD motor complex: An inner membrane assembly that harvests PMF and transduces energy via TonB to outer membrane transporters.

Siderophore: A high-affinity iron-chelating molecule secreted by bacteria to scavenge ferric iron from the environment.

Peptidoglycan: The rigid polymeric mesh of sugars and amino acids in the bacterial cell wall that provides structural support and anchorage for periplasmic proteins.

References

1. Ton motor conformational switch and peptidoglycan role in bacterial nutrient uptake. Nature Communications (2024).
2. TonB-Dependent Transport Across the Bacterial Outer Membrane. Annual Review of Microbiology (2023).
3. Specificity and mechanism of TonB-dependent ferric catecholate uptake by Fiu. Frontiers in Microbiology (2024).
4. Discovery and structural characterization of the D-box, a conserved TonB motif that couples an inner-membrane motor to outer-membrane transport. Journal of Biological Chemistry (2024).
5. Cryo-EM structure of the bacterial Ton motor subcomplex ExbB–ExbD provides information on structure and stoichiometry. Communications Biology (2019).