The proline-rich domain of TonB possesses an extended polyproline II-like conformation of sufficient length to span the periplasm of Gram-negative bacteria.

TonB from Escherichia coli and its homologues are critical for the uptake of siderophores through the outer membrane of Gram-negative bacteria using chemiosmotic energy. When different models for the mechanism of TonB mediated energy transfer from the inner to the outer membrane are discussed, one of the key questions is whether TonB spans the periplasm. In this article, we use long range distance measurements by spin-label pulsed EPR (Double Electron-Electron Resonance, DEER) and CD spectroscopy to show that the proline-rich segment of TonB exists in a PPII-like conformation. The result implies that the proline-rich segment of TonB possesses a length of more than 15 nm, sufficient to span the periplasm of Gram-negative bacteria.