TCBG Seminar

TonB-Dependent Outer Membrane Transport: Structural Answers and Mechanistic Questions

Professor Michael Wiener
Department of Molecular Physiology and Biological Physics
University of Virginia
Charlottesville, VA

Monday, December 3, 2007
3:00 pm (CT)
3269 Beckman Institute


The outer membrane of Gram-negative bacteria contains transporters specific for uptake of essential organometallic compounds; in pathogenic bacteria, these transporters are implicated as virulence factors. As the outer membrane does not maintain an electrochemical gradient (pmf), energy-dependent steps in transport across the outer membrane are mediated through a coupling protein, TonB, that links the pmf-utilizing ExbB-ExbD protein complex (‘the motor’) of the inner membrane to the outer-membrane transporter. This ‘action-at-a-distance’, where the transporter may be several hundred Ångströms away from the motor that drives it, is a unique feature of this transport system. The common architecture of TonB-dependent transporters (TBDTs) is a twenty-two stranded β-barrel situated in the membrane, long extracellular loops, short periplasmic turns and a distinctive luminal domain. This globular-like luminal domain, comprised of the amino-terminal portion of the transporter, occludes the barrel. TBDT structures are mechanistically provocative: how does substrate binding on the ‘outside’ signal across the transporter to recruit TonB? what conformational changes does the luminal domain undergo to permit substrate permeation? We have extensively studied, primarily by x-ray crystallography, the TonB-dependent cobalamin transporter BtuB. We are also now utilizing additional experimental and computational approaches, along with structural methods, to characterize this unique active transport system and to access central mechanistic questions. These studies may, in the (more distant) future, lead to development of novel anti-bacterial drugs.

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