Pore-Forming Colicins and Their Relatives
Amongst the variety of toxins produced by bacteria, few act against other bacteria. Those that do go under the umbrella term of bacteriocins and consist of a varied collection of molecules ranging from short peptides to large 80-kDa proteins (James et al.
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LAKEyl
and
S.L. SLATIN2
I 1.1 1.2 1.3 1.4
Introduction............. The Structures of Pore-Forming Colicins The Colicin Receptors. . . . . . . .. The Filamentous Phage Connection . . . Structures of the Tol Proteins . . . . . . .
131 135 138 140 141
2 2.1 2.2 2.3
Colicin Interactions with the Cytoplasmic Membrane . Studies with Colicin A. Studies with Colicin El . pH Dependence . . ...
142 142 143 143
3.1 3.2 3.3 3.4 3.5
The Central Conundrum: The Structure of the Open Channel Channel Properties. The Molecularity . . . . . . . The Lumen Size . . . . . . . The Pore-Forming Domain. Disquieting Implications ...
144 144 145 146 146 147
4 4.1 4.2 4.3 4.4
Voltage Dependence and Translocation . Translocation. . . . . . . . . .... The Mechanism of Voltage Dependence. The Role of the Hydrophobic Segment in the Channel What Forms the Channel? . . . . . . ...... .
148 148 lSI 152 153
5
Comment on the Relationship of Colicins to Similar Channels.
154
6
Summary
ISS
References ..
155
1 Introduction Amongst the variety of toxins produced by bacteria, few act against other bacteria. Those that do go under the umbrella term of bacteriocins and consist of a varied collection of molecules ranging from short peptides to large 80-kDa proteins (JAMES et al. 1992). They differ in one clear respect from the toxins that are directed 1 School
of Biochemistry and Genetics, The Medical School, University of Newcastle, NE2 4HH, UK of Physiology and Biophysics, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, NY 10461, USA
2 Department
F. G. van der Goot (ed.), Pore-Forming Toxins © Springer-Verlag Berlin Heidelberg 2001
132
J.H. Lakey and S.L. Slatin
against eukaryotic cells in that they are generally directed against similar species and thus are involved in competition for resources rather than the provision of the resources themselves (RILEY 1998). As with most toxins, all of these molecules share as a common feature an ability to bind to and penetrate biological membranes. It is arguable that one group of these proteins, the pore-forming colicins, exhibit the most developed and complex membrane interactions of any bacterial toxin. Here we hope to give the reader an insight into this unique series of toxin-membrane interactions. Colicins take their name from their "host" organism Escherichia coli and very similar bacteriocins are to be found in other gram-negative bacteria (pyocin = Pseudomonas (KAGEYAMA et al. 1996); pesticin = Yersinia pestis (RAKIN et al. 1996; VOLLMER et al. 1997); etc.). The colicins consist of a family of proteins ranging in size from colicin N [387 amino acid residues; PUGSLEY (1987)] to colicin Ia [626 amino acids; WIENER et al. (1997)]. They are generally carried on plasmids, although "colicin" 28b from Serratia marcescens is an unusual example of a chromosomally coded form (GUASCH et al. 1995). The plasmid origin may explain the curious biology of the colicins, since their expression is toxic to the producing cell. It would appear that the advantage conferred upon o
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