CRYSTALLINE LAYERS AND 3-DIMENSIONAL STRUCTURE OF STAPHYLOCOCCUS-AUREUS ALPHA-TOXIN

Interaction of the pore-forming protein α-toxin fromStaphylococcus aureus with lipid components from platelet membranes induces crystal formation of the toxin oligomers. Structure analysis of crystalline areas in either sodium phosphotungstic acid or a sodium phosphotungstic acid/glucose mixture has been performed with electron microscopy and image processing. Ordered domains extending up to a few micrometers were observed, particularly after application of α-toxin to pre-formed lipid layers. The crystals, showing tetragonal symmetry, formed either separate two-dimensional sheets or three-dimensional piles of layers. The corresponding unit cell parameter of the single layer wasa = b = 109.4A ° (standard deviation 2.1A °,n = 21). Incubation of the toxin with intact membranes or extracted lipids as well as application of the lipid layer technique resulted in congruous crystalline properties. The projected averaged α-toxin oligomer shows cyclic symmetry with a stain-filled space in the centre. The bulk of the three-dimensional model consists of four asymmetric protein units forming a ring. In addition, a small domain covers the central cavity at the face of the protein opposite to the underlying lipid. The conditions under which the tetragonal arrays are formed on the lipid layers suggest that the α-toxin molecule is in a conformation binding to a hydrophobic surface rather than fully inserted into a lipid bilayer. © 1990 Academic Press Limited.