Properties of pores formed by cholesterol-dependent cytolysins and actinoporins

Planar lipid membrane (PLM) measurements allow direct observation of pores in model lipid membranes. This biophysical approach was very important for our understanding of how transmembrane pores are formed by cholesterol-dependent cytolysins (CDCs), a toxin family from pathogenic bacteria, and actinoporins, cytolysins from sea anemones. In this review we discuss current knowledge of pore formation by these two protein families and how the PLM approach revealed mechanisms by which these two unrelated protein families porate membranes. Interestingly, for both toxins, the protein portion constituting the pore walls has an alpha helical configuration in the secreted water soluble form. This structure is maintained for actinoporins in the membrane inserted configuration, while the pore of CDCs necessitates a drastic change in secondary structure, which transforms to beta hairpins in the membrane. Both proteins are able to form toroidal proteo-lipid pores.