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Journal of Bacteriology, June 2008, p. 4342-4350, Vol. 190, No. 12
0021-9193/08/$08.00+0     doi:10.1128/JB.00047-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Investigating Early Events in Receptor Binding and Translocation of Colicin E9 Using Synchronized Cell Killing and Proteolytic Cleavage

Ying Zhang, Mireille N. Vankemmelbeke, Lisa E. Holland, David C. Walker, Richard James, and Christopher N. Penfold^*

School of Molecular Medical Sciences, Institute of Infection, Immunity and Inflammation, Centre for Biomolecular Sciences, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom

Received 10 January 2008/ Accepted 3 April 2008

Enzymatic colicins such as colicin E9 (ColE9) bind to BtuB on the cell surface of Escherichia coli and rapidly recruit a second coreceptor, either OmpF or OmpC, through which the N-terminal natively disordered region (NDR) of their translocation domain gains entry into the cell periplasm and interacts with TolB. Previously, we constructed an inactive disulfide-locked mutant ColE9 (ColE9^s-s) that binds to BtuB and can be reduced with dithiothreitol (DTT) to synchronize cell killing. By introducing unique enterokinase (EK) cleavage sites in ColE9^s-s, we showed that the first 61 residues of the NDR were inaccessible to cleavage when bound to BtuB, whereas an EK cleavage site inserted at residue 82 of the NDR remained accessible. This suggests that most of the NDR is occluded by OmpF shortly after binding to BtuB, whereas the extreme distal region of the NDR is surface exposed before unfolding of the receptor-binding domain occurs. EK cleavage of unique cleavage sites located in the ordered region of the translocation domain or in the distal region of the receptor-binding domain confirmed that these regions of ColE9 remained accessible at the E. coli cell surface. Lack of EK cleavage of the DNase domain of the cell-bound, oxidized ColE9/Im9 complex, and the rapid detection of Alexa Fluor 594-labeled Im9 (Im9^AF) in the cell supernatant following treatment of cells with DTT, suggested that immunity release occurred immediately after unfolding of the colicin and was not driven by binding to BtuB.

Published ahead of print on 11 April 2008.

Present address: CTSU, University of Oxford, Richard Doll Building, Old Road Campus, Roosevelt Drive, Oxford OX3 7LF, United Kingdom.

Present address: Merck Chemicals Ltd., Nottingham NG9 2JR, United Kingdom.