Mechanism of membrane insertion of a multimeric beta-barrel protein: perfringolysin O creates a pore using ordered and coupled conformational changes.

Perfringolysin O, a bacterial cytolytic toxin, forms unusually large pores in cholesterol-containing membranes by the spontaneous insertion of two of its four domains into the bilayer. By monitoring the kinetics of domain-specific conformational changes and pore formation using fluorescence spectroscopy, the temporal sequence of domain-membrane interactions has been established. One membrane-exposed domain does not penetrate deeply into the bilayer and is not part of the actual pore, but is responsible for membrane recognition. This domain must bind to the membrane before insertion of the other domain into the bilayer is initiated. The two domains are conformationally coupled, even though they are spatially separated. Thus, cytolytic pore formation is accomplished by a novel mechanism of ordered conformational changes and interdomain communication.

The mechanism of membrane insertion for a cholesterol-dependent cytolysin: a novel paradigm for pore-forming toxins.

Perfringolysin O (PFO), a water-soluble monomeric cytolysin secreted by pathogenic Clostridium perfringens, oligomerizes and forms large pores upon encountering cholesterol-containing membranes. Whereas all pore-forming bacterial toxins examined previously have been shown to penetrate the membrane using a single amphipathic beta hairpin per polypeptide, cysteine-scanning mutagenesis and multiple independent fluorescence techniques here reveal that each PFO monomer contains a second domain involved in pore formation, and that each of the two amphipathic beta hairpins completely spans the membrane. In the soluble monomer, these transmembrane segments are folded into six alpha helices. The insertion of two transmembrane hairpins per toxin monomer and the major change in secondary structure are striking and define a novel paradigm for the mechanism of membrane insertion by a cytolytic toxin.

Identification of a membrane-spanning domain of the thiol-activated pore-forming toxin Clostridium perfringens perfringolysin O: an alpha-helical to beta-sheet transition identified by fluorescence spectroscopy.

Clostridium perfringens perfringolysin O (PFO or theta-toxin) is a cytolytic toxin that binds to cholesterol-containing membranes and then self-associates to spontaneously form aqueous pores of varying size in the bilayer. In this study, a membrane-spanning domain has been identified in PFO by a combination of fluorescence spectroscopic methods using the fluorescent dye N, N'-dimethyl-N-(iodoacetyl)-N'-(7-nitrobenz-2-oxa-1, 3-diazolyl)ethylenediamine (NBD) whose emission properties are sensitive to water. PFO was substituted with a single cysteine at most of the residues between amino acids K189 and N218, and then each cysteine was modified with NBD. Each purified NBD-labeled PFO was then bound to membranes, and the probe's environment was ascertained by measuring its fluorescence lifetime, emission intensity, and collisional quenching with either aqueous (iodide ions) or nonaqueous (nitroxide-labeled phospholipids) quenchers. Lifetime and intensity measurements revealed that the amino acid side chains in this region of the membrane-bound PFO polypeptide alternated between being in an aqueous or a nonaqueous environment. This pattern indicates that this portion of the membrane-bound PFO spans the membrane in an antiparallel beta-sheet conformation. The alternating exposure of these residues to the hydrophobic interior of the bilayer was demonstrated by their susceptibility to quenching by nitroxide moieties attached to phospholipid acyl chains. Residues K189-N218 therefore form a two-stranded, amphipathic beta-sheet in the membrane-bound PFO that creates a stable interface between the pore and the membrane. This same region packs as three short alpha-helices in the soluble, monomeric form of PFO, and therefore, the cholesterol-dependent conversion of PFO to a membrane-bound oligomer involves a major structural transition in which three alpha-helices unfold to form a membrane-spanning amphipathic beta-sheet.

Fluoresceinthiocarbamyl-insulin: a potential analytical tool for the assay of disulfide bond reduction.

We describe the synthesis of fluorescent derivatives of bovine pancreas insulin and its use as substrates of disulfide bond reduction in a spectrofluorometric assay. Amino groups of insulin were chemically modified with fluorescein isothiocyanate and proteins bearing one, two and three fluorescent groups were purified by ion-exchange chromatography. Upon incubation with dithiothreitol, di- and tri-fluoresceinthiocarbamyl-insulin evinced the highest and the lowest enhancement of fluorescence emission, whereas the mono-substituted protein had intermediate enhancement. Using di-fluoresceinthiocarbamyl-insulin, the reliability of this novel feature for the estimation of disulfide bond cleavage was assessed by (i) the separation of two fluorescent bands using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, (ii) the linear response of the fluorescence signal within a range from 0.04 to 1 microM, and (iii) the correlation of the rate of fluorescence enhancement with concentrations of dithiothreitol ranging from 0.1 to 5 mM. Moreover, di-fluoresceinthiocarbamyl-insulin was a sensitive oxidant when the catalytic capacity of thioredoxin and protein disulfide isomerase was analyzed in the presence of dithiothreitol or glutathione, as reductants. On this basis, di-fluoresceinthiocarbamyl-insulin constitutes an analytical tool to test the capacity of biochemical preparations in the reduction of disulfide bonds.

Di-fluoresceinthiocarbamyl-insulin: a fluorescent substrate for the assay of protein disulfide oxidoreductase activity.

We have developed a novel method for the continuous assay of protein disulfide oxidoreductase activity using as substrate bovine pancreas insulin in which both N-terminal amino groups are chemically modified with fluorescein isothiocyanate. The reduction of intercatenary disulfide bonds of di-fluoresceinthiocarbamyl-insulin with dithiothreitol initially lowers but subsequently enhances the emission intensity. In this biphasic kinetics, the rate of increase is sensitive enough for the estimation of Escherichia coli thioredoxin concentrations from 5 nM (0.06 microgram/ml) to 500 nM (6 micrograms/ml). Neither changes of pH over a range of 6.2 to 8.4 nor neutral salts (K+, Mg2+, and Ca2+) at concentrations lower than 100 mM affect this simple reaction system. Moreover, the fluorometric method is functional for measuring the reductive capacity of Brassica napus protein disulfide isomerase. Hence, a highly reproducible and accurate one-state assay for protein disulfide oxidoreductase activity not only greatly improves the sensitivity compared to the commonly used turbidimetric assay but also represents a reliable alternative to assays based on accessory enzymes or radiolabeled substrates.