Pore Colicins

Colicins are pore-forming proteins, produced by certain strains of E. coli, that kill or inhibit the growth of other, competing bacteria and even other strains of E. coli (a process known as allelopathy). Channel-forming colicins are released as soluble monomers. Upon encountering a host cell, the colicin molecule traverses the bacterial outer membrane and periplasm, then inserts itself... 

Interestingly, certain other pore-forming toxins possess helix-bundle motifs that may participate in channel formation, in a manner similar to that proposed for colicin la. For example, the S-endotoxui produced by Bacillus thuringiensis is toxic to Coleoptera insects (beetles) and is composed of three domains, including a seven-helix bundle, a three-sheet domain, and a /3-sandwich. In the seven-helix bundle, helix 5 is highly hydrophobic, and the other six helices are amphipathic. In solution (Figure 10.32), the six amphipathic... 

Lazdunski, C. J. (1995). Colicin import and pore formation a system for studying protein transport across membranes Mol. Microbiol., 16, 1059-1066. 

The VP1 helices could then insert into the membrane and form a pore, which could allow the passage of RNA through the lipid bilayer into the cytosol. This is reminiscent of the pore formation by colicin [51]. 

Channel-forming toxins and antibiotics. Some of the bacterial toxins known as colicins (Box 8-D) kill susceptible bacteria by creating pores that allow K+ to leak out of the cells. One part of the complement system of blood (Chapter 31) uses specific proteins to literally punch holes in foreign cell membranes. Mel-litin, a 26-residue peptide of bee venom,372 373 as well as other hemolytic toxins and antibiotic peptides of insects, amphibians, and mammals (Chapter 31) form amphip-athic helices which associate to form voltage-dependent anion-selective channels in membranes.374-377... 

The N-terminal portion of the 522-residue polypeptide chain of colicin El appears to be required for transport into the membrane and the central part for binding to the receptor the channelforming property is characteristic of the C-terminal region.k A similar organization has been established for the smaller colicin N translocation domain, (residues 1-66), receptor domain, (residues 67-182), and pore-forming domain (residues 183-387). 

An outer membrane pore protein of molecular weight 25,000 is required for nucleoside uptake as well as for T6 and colicin K resistance. It is believed to be directed by the tsx-nup gene locus at 9-10 min on the E. coli chromosomal map (98, 99). 

The lipopolysaccharide, which exists exclusively in the outer membrane, has been shown not to provide hydrophilic diffusion pores. Another possible function of the outer membrane is that it determines the shape of the bacterium. Furthermore, the outer membrane is supposed to provide receptors for phages and colicins. 

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