Heptameric structures of two alpha-hemolysin mutants imaged with in situ atomic force microscopy

Atomic force microscopy has been used to study self-assembled structures of two a-hemolysin mutants. For a mutant (alpha HL-H5) that was locked into t he prepore state on fluid phase egg-PC membranes, we visualized, for the fi rst time, heptameric prepores and showed that the 7-fold axis in the prepor e lies perpendicular to the membrane surface. For another mutant (TCM) with the transmembrane domain, the self-assembled oligomer that assumes the con formation of the fully assembled pore is also a heptamer. These results sho w that heptamers are the preferred oligomerization state of alpha-hemolysin . (C) 1999 Wiley-Liss, Inc.