The heptameric prepore of a staphylococcal alpha-hemolysin mutant in lipid bilayers imaged by atomic force microscopy

We have used atomic force microscopy to study the oligomeric state of a genetically engineered mutant of staphylococcal alpha-hemolysin (alphaHL-H5) that can be arrested as a "prepore" assembly intermediate. AFM images of alphaHL-H5 on supported bilayers of a fluid-phase lipid, egg-yolk phosphatidylcholine (egg-PC), under conditions that lock alphaHL-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 alpha-hemolysin.