Key words: aegerolysin, sphingomyelin, cholesterol, pleurotolysin B
Aegerolysins are β-structured proteins sharing the following common features: low isoelectric points, similar molecular weights (15–17 kDa), and stability in a wide pH range. They have been isolated from fungi, bacteria and plants and they all bind to lipid receptors in biological membranes.
Biological membranes are composed of thousands of species of proteins and lipids. While for the proteins, the diverse set of functions are largely known, the roles of several thousand different species of lipids are still not exactly clear. This is due to the lack of suitable probes for detection and visualization of lipids.
Ostreolysin A (OlyA), an aegerolysin protein from the edible mushroom Pleurotus ostreatus, has been found to interact selectively with membranes rich in cholesterol and sphingomyelin. Fluorescently tagged OlyA labels cholesterol/sphingomyelin domains in artificial membrane systems and in membranes of mammalian cells. This staining is abolished by pretreatment with either with methyl-β-cyclodextrin or sphingomyelinase. OlyA is therefore a promising tool for labeling cholesterol/sphingomyelin domains and for monitoring their turnover inside the cells.
Binding of OlyA to cholesterol/sphingomyelin domains is essential for recruitment of the membrane attack complex/perforin-domain containing 59-kDa protein pleurotolysin B (PlyB) to cell membranes. OlyA/PlyB proteins form multimeric binary pore-complexes that are permeable to solutes. The cytotoxic effect of OlyA/PlyB was evaluated on T24 invasive and RT4 noninvasive human urothelial cells, and on normal porcine urothelial (NPU) cells, showing a selective cytotoxicity towards both cancer cell lines. This cytotoxicity was assigned to the higher cholesterol synthesis, and to the enrichment of the membranes of these cells in cholesterol/sphingomyelin domains.