TonB

TonB is a protein complex found in the membranes of Gram-negative bacteria and archaea. It plays a crucial role in the uptake of iron-containing compounds and other essential nutrients from the environment. These nutrients are often bound to specific outer membrane receptors that are only accessible to TonB.

The TonB complex is composed of three main components: TonB itself, ExbB, and ExbD. TonB is an inner membrane protein that spans the periplasm. ExbB and ExbD are also inner membrane proteins that form a proton channel. Together, these proteins harness the energy of the proton motive force across the inner membrane.

When a nutrient-binding receptor on the outer membrane interacts with its ligand, it also binds to the TonB complex. This interaction triggers a conformational change in the receptor, which is then energized by the proton motive force. This energy allows the receptor to pull the bound nutrient across the outer membrane into the periplasm. Once in the periplasm, other transport systems can further move the nutrient into the cytoplasm.

The TonB system is essential for bacterial survival, particularly in environments where iron is scarce. It is a target for research in the development of new antimicrobial agents, as inhibiting its function can starve bacteria of essential nutrients. The precise mechanism by which TonB energizes outer membrane receptors is still an active area of investigation.