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Mechanics of Force Propagation in TonB-Dependent Outer Membrane Transport

James Gumbart ^*, Michael C. Wiener  and Emad Tajkhorshid 

^* Department of Physics and Beckman Institute, and Department of Biochemistry and Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, Illinois; and Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, Virginia

Correspondence: Address reprint requests to Emad Tajkhorshid, Tel.: 217-244-6914; Fax: 217-244-6078; E-mail: emad{at}life.uiuc.edu.

For the uptake of scarce yet essential organometallic compounds, outer membrane transporters of Gram-negative bacteria work in concert with an energy-generating inner membrane complex, thus spanning the periplasmic space to drive active transport. Here, we examine the interaction of TonB, an inner membrane protein, with an outer membrane transporter based upon a recent crystal structure of a TonB-transporter complex to characterize two largely unknown steps of the transport cycle: how energy is transmitted from TonB to the transporter and how energy transduction initiates transport. Simulations of TonB in complex with BtuB reveal that force applied to TonB is transmitted to BtuB without disruption of the very small connection between the two, supporting a mechanical mode of coupling. Based on the results of different pulling simulations, we propose that the force transduction instigates a partial unfolding of the pore-occluding luminal domain of the transporter, a potential step in the transport cycle. Furthermore, analysis of the electrostatic potentials and salt bridge interactions between the two proteins during the simulations hints at involvement of electrostatic forces in long-range interaction and binding of TonB and BtuB.

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