| Colicin | |||||||
|---|---|---|---|---|---|---|---|
 Structure ofTolB in complex with a peptide of the colicin e9 t-domain | |||||||
| Identifiers | |||||||
| Symbol | Colicin | ||||||
| Pfam | PF03515 | ||||||
| Pfam clan | CL0446 | ||||||
| InterPro | IPR003058 | ||||||
| SCOP2 | 1jch /SCOPe /SUPFAM | ||||||
| |||||||
Acolicin is a type ofbacteriocin produced by and toxic to some strains ofEscherichia coli.[1] Colicins are released into the environment to reduce competition from otherbacterial strains. Colicins bind toouter membrane receptors, using them totranslocate to the cytoplasm or cytoplasmic membrane, where they exert their cytotoxic effect, including depolarisation of the cytoplasmic membrane,DNase activity,RNase activity, or inhibition ofmurein synthesis.
Channel-forming colicins (colicins A, B, E1, Ia, Ib, and N) aretransmembrane proteins that depolarize the cytoplasmic membrane, leading to dissipation of cellular energy.[2] These colicins contain at least three domains: an N-terminal translocation domain responsible for movement across theouter membrane andperiplasmic space (T domain); a central domain responsible for receptor recognition (R domain); and a C-terminalcytotoxic domain responsible for channel formation in the cytoplasmic membrane (C domain).[3][4][5] R domain regulates the target and binds to the receptor on the sensitive cell. T domain is involved in translocation, co-opting the machinery of the target cell. The C domain is the 'killing' domain and may produce a pore in the targetcell membrane, or act as anuclease to chop up theDNA orRNA of the target cell.[5]
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 tocell surface receptors such as the outer membrane proteins OmpF, FepA, BtuB, Cir and FhuA; colicins have been classified according to which receptors they bind to. The presence of specific periplasmic proteins, such asTolA, TolB, TolC, or TonB, are required for translocation across the membrane.[6]Cloacin[7] DF13 is a bacteriocin that inactivates ribosomes by hydrolysing 16S RNA in 30S ribosomes at a specific site.[8]
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 asiron or aB vitamin), but benefit by not being killed. Colicins exhibit a '1-hit killing kinetic'[citation needed] which does not necessarily mean a single molecule is sufficient to kill, but certainly that it only takes a small number. In his 1969 Nobel Laureate speech,Salvador E. Luria speculated that colicins could only be this toxic by causing adomino effect that destabilized the cell membrane.[9] He was not entirely correct, but pore-forming colicins do depolarize the membrane and thus eliminate the energy source for the cell. The colicins are highly effectivetoxins.[citation needed]
Virtually all colicins are carried onplasmids. The two general classes ofcolicinogenic plasmids are 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 majorgenes. These include a colicin structural gene, an immunity gene, and abacteriocin release protein (BRP), orlysis, gene. The immunity gene is often produced constitutively, while the BRP is generally produced only as a read-through of thestop codon on the colicin structural gene. The colicin itself is repressed by theSOS response and may be regulated in other ways as well.[10]
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 theSOS response, which responds to significantDNA damage. In short, colicin production may only occur in terminally ill cells. The Professor Kleanthous Research Group at theUniversity of Oxford study colicins extensively as a model system for characterising and investigating protein-protein interactions and recognition.[11]
BACTIBASE[12][13] database is an open-access database for bacteriocins including colicins (view complete list).