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Cell entry mechanism of enzymatic bacterial colicins: Porin recruitment and the thermodynamics of receptor binding

Housden, Nicholas G., Loftus, Steven R., Moore, Geoffrey R., James, Richard and Kleanthous, Colin (2005) Cell entry mechanism of enzymatic bacterial colicins: Porin recruitment and the thermodynamics of receptor binding. Proceedings of the National Academy of Sciences of the United States of America. pp. 13849-13854. ISSN 1091-6490

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Binding of enzymatic E colicins to the vitamin B-12 receptor, BtuB, is the first stage in a cascade of events that culminate in the translocation of the cytotoxic nuclease into the Escherichia coli cytoplasm and release of its tightly bound immunity protein. A dogma of colicin biology is that the toxin coiled-coil connecting its functional domains must unfold or unfurl to span the periplasm, with recent reports claiming this reaction is initiated by receptor binding. We report isothermal titration calorimetry data of BtuB binding the endonuclease toxin ColE9 and a disulfide form (CoIE9(S-S)) where unfolding of the coiled-coil is prevented and, as a consequence, the toxin is biologically inactive. Contrary to expectation, the thermodynamics of receptor binding, characterized by large negative values for T Delta S, are identical for the two colicins, arguing against any form of BtuB-induced unfolding. We go on to delineate key features of the "colicin translocon" that assembles at the cell surface after BtuB binding by using a complex of histidine-tagged Im9 bound to ColE9(S-S). First, we show that the porin OmpF is recruited directly to the BtuB-colicin complex to form the translocon. Second, recruitment is through the natively unfolded region of the colicin translocation domain, with this domain likely having two contact points for OmpF. Finally, the immunity protein is not released during its assembly. Our study demonstrates that although colicin unfolding is undoubtedly a prerequisite for E. coli cell death, it must occur after assembly of the translocon.

Item Type: Article
Copyright, Publisher and Additional Information: © 2005 by The National Academy of Sciences of the USA
Institution: The University of York
Academic Units: The University of York > Biology (York)
Depositing User: Christina Hudson
Date Deposited: 23 Feb 2006
Last Modified: 15 May 2016 17:19
Published Version: http://dx.doi.org/10.1073/pnas.0503567102
Status: Published
Refereed: Yes
URI: http://eprints.whiterose.ac.uk/id/eprint/1065

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