The Heptameric Prepore of a Staphylococcal α-Hemolysin Mutant in Lipid Bilayers Imaged by Atomic Force Microscopy

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Abstract
We have used atomic force microscopy to study the oligomeric state of a genetically engineered mutant of staphylococcal α-hemolysin (αHL-H5) that can be arrested as a “prepore” assembly intermediate. AFM images of αHL-H5 on supported bilayers of a fluid-phase lipid, egg-yolk phosphatidylcholine (egg-PC), under conditions that lock αHL-H5 into the prepore state, clearly show a heptameric structure for many individual oligomers. The central dent of the prepore has a diameter of 3.2 ± 0.2 nm. The distance between the centers of mass of neighboring subunits is 2.8 ± 0.3 nm. The heptamer has an average diameter of 8.9 ± 0.6 nm. These results support a recently proposed pathway for the assembly of α-hemolysin.