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首页> 外文期刊>Proceedings of the National Academy of Sciences of the United States of America >Outer-membrane translocation of bulky small molecules by passive diffusion
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Outer-membrane translocation of bulky small molecules by passive diffusion

机译：Outer-membrane translocation of bulky small molecules by passive diffusion

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The outer membrane (OM) of gram-negative bacteria forms a protective layer around the cell that serves as a permeability barrier to prevent unrestricted access of noxious substances. The permeability barrier of the OM results partly from the limited pore diameters of OM diffusion channels. As a consequence, there is an "OM size-exclusion limit," and the uptake of bulky molecules with molecular masses of more than similar to 600 Da is thought to be mediated by TonB-dependent, active transporters. Intriguingly, the OM protein CymA from Klebsiella oxytoca does not depend on TonB but nevertheless mediates efficient OM passage of cyclodextrins with diameters of up to similar to 15 angstrom. Here we show, by using X-ray crystallography, molecular dynamics simulations, and single-channel electrophysiology, that CymA forms a monomeric 14-stranded beta-barrel with a large pore that is occluded on the periplasmic side by the N-terminal 15 residues of the protein. Representing a previously unidentified paradigm in OM transport, CymA mediates the passive diffusion of bulky molecules via an elegant transport mechanism in which a mobile element formed by the N terminus acts as a ligand-expelled gate to preserve the permeability barrier of the OM.

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