CRYO-TEM DIRECT-IMAGING OF α-HEMOLYSIN INTERACTION WITH PHOSPHOLIPID MEMBRANES
Alpha-hemolysin (αHL) is a 33.2 kDa protein that is secreted from bacteria, such as Staphylococcus aureus. The protein monomer self-assembles into hexa- or heptameric structure that forms a transmembrane channel. The ability to control membrane permeability with αHL is of great importance in biological studies and in biotechnological assays. The oligomer structure is mushroom shaped with three distinct domains: an extracellular cap domain, a rim domain on the outer leaflet of the membrane, and a stem, which forms the transmembrane amphiphilic b-barrel pore. We used cryogenic transmission electron microscopy (cryo-TEM) to study αHL-induced pore formation in the lipid bilayer of DOPC liposomes. We show that αHL complexes form pores mainly in smaller, higher curvature liposomes, while larger liposomes remain intact. Moreover, we find that the external parts of αHL complexes attract each other, forming clusters of several liposomes.