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. The proline-rich segment of TonB exists in a PPII-like conformation [1]. 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.

1: The proline-rich domain of TonB possesses an extended polyproline II-like conformation of sufficient length to span the periplasm of Gram-negative bacteria. Kohler SD, Weber A, Howard SP, Welte W, Drescher M; Protein Sci. 2010;19:625-630. PMID:20095050