Colicin N

Colicin N is a type of Colicin, a bacteriocin made by E. coli which acts against other nearby E. coli to kill them by forming a pore in the membrane, leading to depolarisation of the membrane which kills the cell. This structure shows both the receptor binding and pore forming domains.

Synthesis and release

The plasmid encoding the c. 42kDa^[1] ColN also encodes its corresponding Colicin Immunity Protein, Cni, in order to protect the colicinergic cell from the cytotoxic activity of the colicin. Two types of ColN plasmid have been discovered^[2].

Mechanism of uptake

Colicin N parasitizes the outer membrane protein F (OmpF) as both its receptor and its translocator^[3], alongside the Tol proteins TolQRA. There is evidence to show that the translocation may occur at the protein-lipid interface. The receptor binding domain contains a six-stranded antiparallel β sheet wrapped around the N-terminal α helix of the pore-forming domain^[4]. ColN binding displaces OmpF-bound lipopolysaccharide, the major lipid in the outer leaflet of the outer membrane. The N terminal region of ColN then rearranges and binds to OmpF^[5]. As only OmpF is required, this may reveal how a general translocation route for a number of colicins may function^[6]. However there is no evidence that this simply forms a pore for translocation, more work is needed to establish the translocation mechanism^[7]. The primary reason for continuing to look for another mechanism is that the diameter of the pore formed by OmpF is not large enough to translocate even an unfolded colicin molecule^[8].

The model currently defined has the translocation domain inserting itself into the porin pore. The receptor binding domain remains outside the targeted cell and the pore-forming domain translocates along the outer wall of the trimeric porin channel^[9].

Killing Activities

Colicin N depolarises the E. coli membrane with its Pore Formation activity, leading to lysis of the cell, and the cytotoxic domain that initiates this reaction is highly homologous to the C-terminal domain of ColA^[10]. The pore-forming domain structure is a ten α helix bundle also observed in Colicin A, Colicin Ia and Colicin E1.

References

1. ↑ Pugsley AP. Genetic analysis of ColN plasmid determinants for colicin production, release, and immunity. J Bacteriol. 1984 May;158(2):523-9. PMID:6327609
2. ↑ Pugsley AP. Nucleotide sequencing of the structural gene for colicin N reveals homology between the catalytic, C-terminal domains of colicins A and N. Mol Microbiol. 1987 Nov;1(3):317-25. PMID:2834623
3. ↑ el Kouhen R, Fierobe HP, Scianimanico S, Steiert M, Pattus F, Pages JM. Characterization of the receptor and translocator domains of colicin N. Eur J Biochem. 1993 Jun 15;214(3):635-9. PMID:8319674
4. ↑ Vetter IR, Parker MW, Tucker AD, Lakey JH, Pattus F, Tsernoglou D. Crystal structure of a colicin N fragment suggests a model for toxicity. Structure. 1998 Jul 15;6(7):863-74. PMID:9687368
5. ↑ Baboolal TG, Conroy MJ, Gill K, Ridley H, Visudtiphole V, Bullough PA, Lakey JH. Colicin N binds to the periphery of its receptor and translocator, outer membrane protein F. Structure. 2008 Mar;16(3):371-9. PMID:18334212 doi:10.1016/j.str.2007.12.023
6. ↑ Vetter IR, Parker MW, Tucker AD, Lakey JH, Pattus F, Tsernoglou D. Crystal structure of a colicin N fragment suggests a model for toxicity. Structure. 1998 Jul 15;6(7):863-74. PMID:9687368
7. ↑ Sharma O, Yamashita E, Zhalnina MV, Zakharov SD, Datsenko KA, Wanner BL, Cramer WA. Structure of the complex of the colicin E2 R-domain and its BtuB receptor. The outer membrane colicin translocon. J Biol Chem. 2007 Aug 10;282(32):23163-70. Epub 2007 Jun 4. PMID:17548346 doi:10.1074/jbc.M703004200
8. ↑ Bainbridge G, Armstrong GA, Dover LG, Whelan KF, Lakey JH. Displacement of OmpF loop 3 is not required for the membrane translocation of colicins N and A in vivo. FEBS Lett. 1998 Aug 7;432(3):117-22. PMID:9720908
9. ↑ Vetter IR, Parker MW, Tucker AD, Lakey JH, Pattus F, Tsernoglou D. Crystal structure of a colicin N fragment suggests a model for toxicity. Structure. 1998 Jul 15;6(7):863-74. PMID:9687368
10. ↑ Pugsley AP. Nucleotide sequencing of the structural gene for colicin N reveals homology between the catalytic, C-terminal domains of colicins A and N. Mol Microbiol. 1987 Nov;1(3):317-25. PMID:2834623