Permeabilization Pore-forming toxins

Bhakdi, S, Weller U, Walev I et al. (1993) A guide to the use of pore-forming toxins tor controlled permeabilization of cell membranes. In Med Microbiol Immunol 182 167-175. 

Permeabilized cells allow the study of intracellular processes in situ under conditions which are believed to be close to the physiological situation in intact cells. Permeabilization by bacterial pore-forming toxins, alpha-toxin and streptolysin O (SLO) is now a widely accepted approach in the functional analysis of intracellular organelles. 

Transmitter uptake into secretory vesicles is an ATP-dependent process. So far these studies have been restricted to isolated secretory vesicles, where intracellular substances necessary for regulation may be lost during purification. Neuroendocrine cells or synaptosomes permeabilized with both pore-forming toxins can be used to study the regulation of transmitter storage without the necessity of purifying secretory vesicles. 

Based on the results of a-LTX mutagenesis, strong correlation exists between pore formation and stimulation of Ca2+-dependent exocytosis from neuroendocrine cells. However, in some experiments with chromaffin cells, a-LTX action does not involve Ca2+ entry (Michelena et al. 1997). In addition, a-LTX sensitizes chromaffin cells to Ca2+ even when the cells are permeabilized and toxin pores should have no effect this involves protein kinase C (PKC) activation (Bittner and Holz 2000). Furthermore, the ability of a-LTXN4C to induce Ca2+-dependent exocytosis without forming pores implicates a stimulating mechanism other than pore formation. 

Structural homologies between PFTs and other toxins have not been identified. However, the process of membrane permeabilization may be operative in many cases where proteins have to escape from an intracellular compartment. Well known examples are diphtheria toxin, the neurotoxins and anthrax toxin. Specific domains in many intracel-lularly active toxins have in fact been shown to produce pores in artificial lipid bilayers, and membrane permeabilization is thought to form the basis for translocation of the active moieties from the late endo-some to the cytoplasm (reviewed in Montecucco et ai, 1994). The molecular mechanism of this translocation remains obscure. In the... 

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