The pore-forming toxin streptolysin O (SLO) can be used to reversibly permeabilize adherent and nonadherent cells, allowing delivery of molecules with up to 100 kDa mass to the cytosol. Using FITC-labeled albumin, 10 5 -10 6 molecules were estimated to be entrapped per cell. Repair of toxin lesions depended on Ca 2؉ -calmodulin and on intact microtubules, but was not sensitive to actin disruption or to inhibition of protein synthesis. Resealed cells were viable for days and retained the capacity to endocytose and to proliferate. The active domains of large clostridial toxins were introduced into three different cell lines. The domains were derived from Clostridium difficile B-toxin and Clostridium sordelli lethal toxin, which glycosylate small G-proteins, and from Clostridium botulinum C2 toxin, which ADP-ribosylates actin. After delivery with SLO, all three toxins disrupted the actin cytoskeleton to cause rounding up of the cells. Glucosylation assays demonstrated that G-proteins Rho and Ras were retained in the permeabilized cells and were modified by the respective toxins. Inactivation of these G-proteins resulted in reduced stimulus-dependent granule secretion, whereas ADP-ribosylation of actin by the C. botulinum C2-toxin resulted in enhanced secretion in cells. The presented method for introducing proteins into living cells should find multifaceted application in cell biology.protein delivery ͉ pore-forming toxin

Agents that deplete cells of K+ without grossly disrupting the plasma membrane were found to stimulate the cleavage of pro‐interleukin (IL)‐1 beta to mature IL‐1 beta. Agents examined in this study included staphylococcal alpha‐toxin and gramicidin, both of which selectively permeabilize plasma membranes for monovalent ions, the ionophores nigericin and valinomycin, and the Na+/K+ ATPase inhibitor ouabain. K+ depletion by brief hypotonic shock also triggered processing of pro‐IL‐1 beta. The central role of K+ depletion for inducing IL‐1 beta maturation was demonstrated in cells permeabilized with alpha‐toxin: processing of pro‐IL‐1 beta was totally blocked when cells were suspended in medium that contained high K+, but could be induced by replacing extracellular K+ with Na+, choline+ or sucrose. To test whether K+ flux might also be important in physiological situations, monocytes were stimulated with lipopolysaccharide (LPS) for 1‐2 h to trigger pro‐IL‐1 beta synthesis, and transferred to K(+)‐rich medium. This maneuver totally suppressed IL‐1 beta maturation. Even after 16 h, however, removal of K+ from the medium resulted in rapid processing and export of IL‐1 beta. Ongoing export of mature IL‐1 beta from cells stimulated with LPS for 2‐6 h could also be arrested by transfer to K(+)‐rich medium. Moreover, a combination of two K+ channel blockers inhibited processing of IL‐1 beta in LPS‐stimulated monocytes. We hypothesize that K+ movement and local K+ concentrations directly or indirectly influence the action of interleukin‐1 beta‐converting enzyme (ICE) and, possibly, of related intracellular proteases.

Staphylococcal alpha-toxin, streptolysin-O, and Escherichia coli hemolysin are well-studied prototypes of pore-forming bacterial cytotoxins. Each is produced as a water-soluble single-chain polypeptide that inserts into target membranes to form aqueous transmembrane pores. This review will compare properties of the three toxin prototypes, highlighting the similarities and also the differences in their structure, mode of binding, mechanism of pore formation, and the responses they elicit in target cells. Pore-forming toxins represent the most potent and versatile weapons with which invading microbes damage the host macroorganism.

Staphylococcus aureus alpha‐toxin is a hydrophilic polypeptide of 293 amino acids that produces heptameric transmembrane pores. During assembly, the formation of a pre‐pore precedes membrane permeabilization; the latter is linked to a conformational change in the oligomer. Here, 41 single‐cysteine replacement toxin mutants were thiol‐specifically labelled with the polarity‐sensitive fluorescent probe acrylodan. After oligomerization on membranes, only the mutants with acrylodan attached to residues in the sequence 118–140 exhibited a marked blue shift in the fluorescence emission maximum, indicative of movement of the fluorophore to a hydrophobic environment. Within this region, two functionally distinct parts could be identified. For mutants at positions 126–140, the shifts were partially reversed after membrane solubilization by detergents, indicating a direct interaction of the label with the membrane lipids. Membrane insertion of this sequence occurred together with the final pre‐pore to pore transition of the heptamer. Thus residues 126–140 constitute a transmembrane sequence in the pore. With labelled residues 118–124, pre‐pore assembly was the critical event to induce the spectral shifts, which persisted after the removal of membrane lipids and hence probably reflects protomer‐protomer contacts within the heptamer. Finally, a derivative of the mutant N121C yielded occluded pores which could be opened by reductive reversal of the modification. Therefore this residue probably lines the lumen of the pore.