Toxins channel-forming

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... 

Parker MW, van der Goot FG, Buckley JT (1996) Aerolysin - the ins and outs of a model channel-forming toxin. In Mol Microbiol 19 205-212. 

Van der Goot FG, Lakey J, Pattus F et al. (1992) Spectroscopic study of the activation and oligomerization of the channel-forming toxin aerolysin identification of the site of proteolytic activation. In Biochemistry 31 8566—8570. 

Studies of single channels formed in lipid bilayers by Staphylococcus aureus alpha toxin showed that fluctuations in the open-channel current are pH-dependent (47). The phenomenon was attributed to conductance noise that arises from reversible ionization of residues in the channel-forming molecule. The pH-dependent spectral density of the noise, shown in Figure 6, is well described by a simple model based on a first-order ionization reaction that permits evaluation of the reaction parameters. This study demonstrates the use of noise analysis to measure the rate constants of rapid and reversible reactions that occur within the lumen of an ion channel. 

Figure 6. Noise of protonation in the current through an open alpha-toxin channel as a function of pH (47). Spectral density is white at low frequencies and is represented by values averaged over the 200-2000-Hz range. The data were obtained in 1-M NaCl solutions at 150 mV of membrane voltage. The solid line is a two-parameter fit to the first-order ionization reaction that describes a reversible protonation of residues in the channel-forming molecule.

Menestrina, G. 1986, Ionic channel formed by Staphylococcus aureus cx -toxin voltage-dependent inhibition by divalent and trivalent cations. Journal of Membrane Biology 90, 177-190. 

One of the most famous examples are probably the ion channels, such as the Cl channel or the Na, or Ca " channels that allow ion flux across a membrane through water-filled pores, or the aquaporins that allow the passage of water molecules across biological membranes. a-Hemolysin, a bacterial toxin, is another example of a channel-forming protein assembly that can form pores in the membrane of a cell through which ions can leak and eventually kill the cell. Another example are the porins OmpF and PhoE, tubular protein assemblies in the outer membranes of Gram-negative bacteria that allow the transport of nutrients and waste. 

Gouaux, E. Alpha-hemolysin from Staphylococcus aureus An archetype of beta-barrel, channel-forming toxins. J Struct Biol 1998,121, 110-122. 

Varanda, W., and A. Finkelstein Ion and Nonelectrolyte Permeability Properties at Channels Formed in Planar Lipid Bilayer Membranes by the Cytolytic Toxin from the Sea Anemone Stoichactis helianthus. J. Membrane Biol. 55, 203 (1980). 

Nelson, K.L., Raja, S.M. and Buckley, J.T. (1997) The glycosylphosphatidylinositol-anchored surface glycoprotein Thy-1 is a receptor for the channel-forming toxin aerolysin. J. Biol Chem. 272, 12170-12174. 

Schwartz, J.L., L. Potvin, X.J. Chen, R. Brousseau, R. Laprade, and D.H. Dean. 1997. Single-site mutations in the conserved alternating-arginine region affect ionic channels formed by CrylAa, a Bacillus thuringiensis toxin. Appl. Environ. Microbiol. 63 3978-3984. 

Boquet, P. and Duflot, E., 1982, Tetanus toxin fragment forms channels in lipid vesicles at low pH. Proc. Natl Acad. Set USA 79 7614-7618. 

Hoch, D. H., Romero-Mira, M., Ehlrich, B. E., Finkelstein, A., DasGupta, B. R. and Simpson, L. L., 1985, Channels formed by botulinum, tetanus and diphtheria toxins in planer lipid bilayers relevance to translocation of proteins across membranes. Proc. Natl. Acad. Sci. USA 82 1692-1696. 

B. thurigiensis is a common Gram-positive, spore-forming soil bacterium that produces inclusion bodies, microcrystalline clusters of many different proteins. These crystalline proteins, called 5-endotoxins, are the ion channel toxins that are sold commercially for pest control. Most such endotoxins are protoxins, which are inactive until cleaved to smaller, active proteins by proteases in the gut of a susceptible insect. One such crystalline protoxin. 

Knowles, B. H., Blatt, M. R., Tester, M., et al., 1989. A cytosolic 5-endo-toxin from Bacillus thurigiensis var. israelensis forms cation-selective channels in planar lipid bilayers. FEES Letters 244 259-262. 

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... 

FIGURE 10.32 The structures of (a) S-eudotoxiu (two views) from Bacillus thuringiensis and (b) diphtheria toxin from Cmynehacterium diphtheriae. Each of these toxins possesses a bundle of a-hehces which is presumed to form the trausmembraue channel when the toxin Is Inserted across the host membrane. In S-endotoxln, helix 5 (white) Is surrounded by 6 helices (red) In a 7-hellx bundle. In diphtheria toxin, three hydrophobic helices (white) lie at the center of the transmembrane domain (red). 

Recently, a variety of natural peptides that form transmembrane channels have been identified and characterized. Melittin (Figure 10.35) is a bee venom toxin peptide of 26 residues. The cecropins are peptides induced in Hyalophora cecropia (Figure 10.36) and other related silkworms when challenged by bacterial infections. These peptides are thought to form m-helical aggregates in mem-... 

All of the transport systems examined thus far are relatively large proteins. Several small molecule toxins produced by microorganisms facilitate ion transport across membranes. Due to their relative simplicity, these molecules, the lonophore antibiotics, represent paradigms of the mobile carrier and pore or charmel models for membrane transport. Mobile carriers are molecules that form complexes with particular ions and diffuse freely across a lipid membrane (Figure 10.38). Pores or channels, on the other hand, adopt a fixed orientation in a membrane, creating a hole that permits the transmembrane movement of ions. These pores or channels may be formed from monomeric or (more often) multimeric structures in the membrane. 

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