A complementary microscopy analysis of Sticholysin II crystals on lipid films: Atomic force and transmission electron characterizations

A new crystal form of the cytotoxin Sticholysin II (StnII) formed on lipid monolayers of 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) has been characterized by transmission electron microscopy (TEM) and by tapping mode atomic force microscopy (AFM) under nearly physiological conditions. Both approaches show the existence of single- and double-layered 2D crystals possessing hexagonal symmetry and unit cell dimensions of a = b = 10 nm and γ = 120°. However, single-layered StnII crystals could only be analysed by TEM and double-layered crystals by AFM. Considering the previously known atomic structure of native StnII and that of a tetrameric assembly, a model is proposed for this new crystal form in which StnII conserves its monomeric state upon interaction with the lipid monolayer. These results are in agreement with the existence of the so called M2 state of the actinoporins.