CHANNELS, RECEPTORS, AND ELECTRICAL SIGNALING |
Mechanics of force propagation in TonB-dependent outer
membrane transport
James C. Gumbart 1, Michael C. Wiener 2 and Emad Tajkhorshid 3*
1 University of Illinois, Urbana-Champaign
2 University of Virginia
3 University of Illinois at Urbana-Champaign
* To whom correspondence should be addressed. E-mail: emad{at}ks.uiuc.edu.
Submitted on January 9, 2007
Revised on February 3, 2007
Accepted on 20 March 2007
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Abstract |
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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 in order 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.
Key Words:
BtuB, Gram-negative bacteria, steered molecular dynamics
