Use of lipid bilayer membranes to detect pore formation by toxins

The geometry of diphtheria toxoid CRM197 channel assessed by thiazolium salts and nonelectrolytes

The geometry of the channel formed by nontoxic derivative of diphtheria toxin CRM197 in lipid bilayer was determined using the dependence of single-channel conductance upon the hydrodynamic radii of different nonelectrolytes. It was found that the cis entrance of CRM197 channel on the side of membrane to which the toxoid was added at pH 4.8 and the trans entrance on the opposite side at pH 6.0 had effective radii of 3.90 and 3.48 Å, respectively. The 3-alkyloxycarbonylmethyl-5-(2-hydroxyethyl)-4-methyl-1,3-thiazolium salts reversibly reduced current via CRM197 channels. The potency of the blockers increased with increasing length of alkyl chain at symmetric pH 6.0 and remained high and stable at pH 4.8 on the cis side. Comparative analysis of CRM197 and amphotericin B pore size with the inhibitory action of thiazolium salts revealed a significant increase in CRM197 pore dimension at pH 6.0. Addition of thiazolium salt with nine carbons alkyl tail increased by ∼30% the viability of human carcinoma cells A431 treated with diphtheria toxin.

Effects of antimicrobial peptides on methanogenic archaea

As members of the indigenous human microbiota found on several mucosal tissues, Methanobrevibacter smithii and Methanosphaera stadtmanae are exposed to the effects of antimicrobial peptides (AMPs) secreted by these epithelia. Although antimicrobial and molecular effects of AMPs on bacteria are well described, data for archaea are not available yet. Besides, it is not clear whether AMPs affect them as the archaeal cell envelope differs profoundly in terms of chemical composition and structure from that of bacteria. The effects of different synthetic AMPs on growth of M. smithii, M. stadtmanae, and Methanosarcina mazei were tested using a microtiter plate assay adapted to their anaerobic growth requirements. All three tested methanoarchaea were highly sensitive against derivatives of human cathelicidin, of porcine lysin, and a synthetic antilipopolysaccharide peptide (Lpep); however, sensitivities differed markedly among the methanoarchaeal strains. The potent AMP concentrations affecting growth were below 10 μM, whereas growth of Escherichia coli WBB01 was not affected at peptide concentrations up to 10 μM under the same anaerobic growth conditions. Atomic force microscopy and transmission electron microscopy revealed that the structural integrity of the methanoarchaeal cells is destroyed within 4 h after incubation with AMPs. The disruption of the cell envelope of M. smithii, M. stadtmanae, and M. mazei within a few minutes of exposure was verified by using LIVE/DEAD staining. Our results strongly suggest that the release of AMPs by eukaryotic epithelial cells is a potent defense mechanism targeting not only bacteria, but also methanoarchaea.

Blepharismins, produced by the protozoan, Blepharisma japonicum, form ion-permeable channels in planar lipid bilayer membranes

Blepharismins are polycyclic quinones found in the pigment granules of the ciliated protozoan, Blepharisma. Exposure to purified blepharismins results in lethal damage to several other ciliates. We here report that, at cytotoxic concentrations, blepharismins formed cation-selective channels in planar phospholipid bilayer membranes. The channels formed in a diphytanoylphosphatidylcholine bilayer had a K(+)/Cl(-) permeability ratio of 6.6:1. Single channel recordings revealed the conductance to be quite heterogeneous, ranging from 0.2 to 2.8 nS in solutions containing 0.1 M KCl, possibly reflecting different states of aggregation of blepharismin. Our observations suggest that channel formation is a cytotoxic mechanism of blepharismin's action against predatory protozoa.

Holins: the protein clocks of bacteriophage infections

Two proteins, an endolysin and a holin, are essential for host lysis by bacteriophage. Endolysin is the term for muralytic enzymes that degrade the cell wall; endolysins accumulate in the cytosol fully folded during the vegetative cycle. Holins are small membrane proteins that accumulate in the membrane until, at a specific time that is "programmed" into the holin gene, the membrane suddenly becomes permeabilized to the fully folded endolysin. Destruction of the murein and bursting of the cell are immediate sequelae. Holins control the length of the infective cycle for lytic phages and so are subject to intense evolutionary pressure to achieve lysis at an optimal time. Holins are regulated by protein inhibitors of several different kinds. Holins constitute one of the most diverse functional groups, with >100 known or putative holin sequences, which form >30 ortholog groups.

Measuring pore formation by Bcl-2 family proteins

Two methods for assaying Bcl-2 protein family-induced solute efflux from liposomes have been outlined. They utilize either ion-selective electrodes to follow ion efflux or fluorescence to monitor changes in fluorescence of the liposome-encapsulated dye SPQ or carboxyfluorescein. Both methods provide a simple means of determining protein activity. These methods do not have the capability to detect either single-channel conductivity or ion selectivity, but they indicate whether the bulk of the protein population is inducing solute efflux. Although in in vivo significance of Bcl-2 protein family pore formation remains to be determined, in vitro measurements of channel activity should provide a means to determine whether a given protein preparation has activity and whether mutations have an adverse effect on channel formation.

Beticolins, nonpeptidic, polycyclic molecules produced by the phytopathogenic fungus Cercospora beticola, as a new family of ion channel-forming toxins

Beticolins are toxins produced by Cercospora beticola, a phytopathogenic fungus responsible for the leaf spot disease of sugar beet. They form a family of 20 nonpeptidic compounds (named B0 to B19) that share the same polycyclic skeleton but differ by isomeric configuration (ortho- or para-) and by a variable residue R (bridging two carbons in one of the six cycles). It has been previously shown that B0 assembles itself into a multimeric structure and forms ion channels into planar lipid bilayers (C. Goudet, A.-A. Very, M.-L. Milat, M. Ildefonse, J.-B. Thibaud, H. Sentenac, and J.-P. Blein, Plant J. 14:359-364, 1998). In the present work, we investigate pore formation by three ortho-beticolins, B0, B2, and B4, and their related (i.e., same R) para-isomers, B13, B1, and B3, respectively, using planar lipid bilayers. All beticolins were able to form ion channels with multiple conductance states, although the type of cyclization (ortho- or para-) and residue (R) result in variations of channel conductance and ionic permeability, respectively. Channel formation by beticolins is likely to be involved in the biological activity of these toxins.

Membrane channel formation by antimicrobial protegrins

Protegrins are small, arginine- and cysteine-rich, beta-sheet peptides with potent activity against bacteria, fungi, and certain enveloped viruses. We report that protegrins form weakly anion-selective channels in planar phospholipid bilayers, induce potassium leakage from liposomes and form moderately cation-selective channels in planar lipid membranes that contain bacterial lipopolysaccharide. The disruption of microbial membranes may be a central attribute related to the host defense properties of protegrins.

Applications of S-layers

The wealth of information existing on the general principle of S-layers has revealed a broad application potential. The most relevant features exploited in applied S-layer research are: (i) pores passing through S-layers show identical size and morphology and are in the range of ultrafiltration membranes; (ii) functional groups on the surface and in the pores are aligned in well-defined positions and orientations and accessible for binding functional molecules in very precise fashion; (iii) isolated S-layer subunits from many organisms are capable of recrystallizing as closed monolayers onto solid supports at the air-water interface, on lipid monolayers or onto the surface of liposomes. Particularly their repetitive physicochemical properties down to the subnanometer scale make S-layers unique structures for functionalization of surfaces and interfaces down to the ultimate resolution limit. The following review focuses on selected applications in biotechnology, diagnostics, vaccine development, biomimetic membranes, supramolecular engineering and nanotechnology. Despite progress in the characterization of S-layers and the exploitation of S-layers for the applications described in this chapter, it is clear that the field lags behind others (e.g. enzyme engineering) in applying recent advances in protein engineering. Genetic modification and targeted chemical modification would allow several possibilities including the manipulation of pore permeation properties, the introduction of switches to open and close the pores, and the covalent attachment to surfaces or other macromolecules through defined sites on the S-layer protein. The application of protein engineering to S-layers will require the development of straightforward expression systems, the development of simple assays for assembly and function that are suitable for the rapid screening of numerous mutants and the acquisition of structural information at atomic resolution. Attention should be given to these areas in the coming years.