structure of tolb in complex with a peptide of the colicin e9 t-domain
A colicin is a type of bacteriocin produced by and toxic to some strains of Escherichia coli. Colicins are released into the environment to reduce competition from other bacterial strains. Colicins bind to outer membrane receptors, using them to translocate to the cytoplasm or cytoplasmic membrane, where they exert their cytotoxic effect, including depolarisation of the cytoplasmic membrane, DNase activity, RNase activity, or inhibition of murein synthesis.
Channel-forming colicins (colicins A, B, E1, Ia, Ib, and N) are transmembrane proteins that depolarize the cytoplasmic membrane, leading to dissipation of cellular energy. These colicins contain at least three domains: an N-terminal translocation domain responsible for movement across the outer membrane and periplasmic space; a central domain responsible for receptor recognition; and a C-terminal cytotoxic domain responsible for channel formation in the cytoplasmic membrane. One domain regulates the target and binds to the receptor on the sensitive cell. The second is involved with translocation, co-opting the machinery of the target cell. The third is the 'killing' domain and may produce a pore in the target cell membrane, or act as a nuclease to chop up the DNA or RNA of the target cell.
Most colicins are able to translocate the outer membrane by a two-receptor system, where one receptor is used for the initial binding and the second for translocation. The initial binding is to cell surface receptors such as the porins OmpF, FepA, BtuB, Cir and FhuA; colicins have been classified according which receptors they bind to. The presence of specific periplasmic proteins, such as TolA, TolB, TolC, or TonB, are required for translocation across the membrane. Cloacin DF13 is a bacteriocin that inactivates ribosomes by hydrolysing 16S RNA in 30S ribosomes at a specific site.
Because they target specific receptors and use specific translocation machinery, cells can make themselves resistant to the colicin by repressing or deleting the genes for these proteins. Such resistant cells may suffer the lack of a key nutrient (such as iron or vitamin B) but benefit by not being killed. Colicins exhibit a '1-hit killing kinetic' which doesn't necessarily mean a single molecule is sufficient to kill, but certainly that it only takes a small number. In his Nobel Laureate speech, Salvador E. Luria, 1969, speculated that colicins could only be this toxic by causing a domino effect that destabilized the cell membrane. He was not entirely correct, but pore-forming colicins do de-polarize the membrane and thus eliminate the energy source for the cell. The colicins are highly effective toxins.
Virtually all colicins are carried on plasmids. There are two general classes of colicinogenic plasmids, large, low-copy number plasmids, and small high copy number plasmids. The larger plasmids carry other genes as well as the colicin operon. The colicin operons are generally organized with several major genes. These include an immunity gene, a colicin structural gene, and a BRP (bacteriocin release protein), or lysis, gene. The immunity gene is often produced constitutively, while the BRP is generally produced only as a read-through of the stop codon on the colicin structural gene. The colicin itself is repressed by the SOS response and may be regulated in other ways as well.
Research indicates that retaining the colicin plasmid is very important for cells that live with their relatives, because if a cell loses the immunity gene, it quickly becomes subject to destruction by circulating colicin. At the same time, colicin is only released from a producing cell by the use of the lysis protein, which results in that cell's death. This suicidal production mechanism would appear to be very costly, except for the fact that it is regulated by the SOS response, which responds to significant DNA damage. In short, colicin production may only occur in terminally-ill cells. Still these matters require further research. The Professor Kleanthous Research Group at the University of York study colicins extensively as a model system for characterising and investigating protein-protein interactions and recognition.
- * Feldgarden, M., & Riley, M. A. (August 1999). The phenotypic and fitness effects of colicin resistance in Escherichia coli K-12" Evolution 53(4), 1019-27. 
- Kang C, Postle K, Chen G, Park H, Youn B, Hilsenbeck JL (2004). "Crystal structure of the cytotoxic bacterial protein colicin B at 2.5 A resolution". Mol. Microbiol. 51 (3): –. doi:10.1111/j.1365-2958.2003.03884.x. PMID 14731273.
- Cramer WA, Zakharov SD, Antonenko YN, Kotova EA (2004). "On the role of lipid in colicin pore formation". Biochim. Biophys. Acta 1666 (1): –. doi:10.1016/j.bbamem.2004.07.001. PMID 15519318.
- Cascales et al. (2007). Colicin Biology. Microbio. and Mol. Bio. Rev. 71(1), 158-229. Abstractpdf
- Cao Z, Klebba PE (2002). "Mechanisms of colicin binding and transport through outer membrane porins". Biochimie 84 (5-6): 399–412. doi:10.1016/S0300-9084(02)01455-4. PMID 12423783.
- van den Elzen PJ, Veltkamp E, Nijkamp HJ, Walters HH (1983). "Molecular structure and function of the bacteriocin gene and bacteriocin protein of plasmid Clo DF13". Nucleic Acids Res. 11 (8): 2465–2477. doi:10.1093/nar/11.8.2465. PMC 325896. PMID 6344017.
- Luria, S. E. (1969) Nobel Lecture
- "Kleanthous Research Group". University of York. Retrieved 11 April 2011.
- Hammami R, Zouhir A, Ben Hamida J, Fliss I (2007). "BACTIBASE: a new web-accessible database for bacteriocin characterization". BMC Microbiology 7: 89. doi:10.1186/1471-2180-7-89. PMC 2211298. PMID 17941971.
- Hammami R, Zouhir A, Le Lay C, Ben Hamida J, Fliss I (2010). "BACTIBASE second release: a database and tool platform for bacteriocin characterization.". BMC Microbiology 10: 22. doi:10.1186/1471-2180-10-22. PMC 2824694. PMID 20105292.
- Molecular mechanisms of colicin evolution pdf
- The newly characterized colicin Y provides evidence of positive selection in pore-former colicin diversification
- Colicin OPM database
- 3D interactive pages about colicins
- Transport Classification Database listing for colicin
- Protein Data Bank colicin listing