BIOCHEMISTRY: Pulling on a Trailor Hitch

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Science  25 May 2007:
Vol. 316, Issue 5828, pp. 1101a
DOI: 10.1126/science.316.5828.1101a

Building transmembrane gradients of small molecules, such as protons and alkali metal ions, is the job of ion-transporting enzymes, which convert adenosine triphosphate into an electrochemical potential; this stored energy is then used by coupled transporters for the import of nutrients and the export of waste materials. Gram-negative bacteria feature inner and outer membranes. Members of the porin family of membrane proteins reside in the outer membrane and allow small molecules to pass across freely, but this makes it impossible to establish electrochemical gradients. How then do these bacteria transmit power to the outer-membrane transporters (which collect essential substances such as vitamin B12 and chelated iron)?

Using steered molecular dynamics (for more, see Sotomayor and Schulten, Reviews, this issue, p. 1144), Gumbart et al. have looked at the interaction between the barrel-like outer-membrane protein BtuB (the vitamin B12 transporter) and the inner-membrane protein TonB, which is known to provide the energy that drives the inward transport of many substrates across the outer membrane. They start from a configuration that is based on the crystal structure of a TonB fragment in complex with the TonB-binding region of BtuB, and find that pulling (computationally) on the TonB portion does not distort it. Furthermore, through a network of hydrogen bonds oriented perpendicularly to the direction of applied force, TonB holds tightly to one end of the BtuB lumenal domain, which plugs the barrel. Pulling harder begins to unfold the plug and loosens it enough to allow vitamin B12 to squeeze by, but the simulated forces are somewhat higher than experimental measurements of what it takes to unravel a protein. — GJC

Biophys. J. 92, 10.1529/biophysj.107.104158 (2007).

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