The projection structure of alpha-toxin from Staphylococcus aureus in human platelet membranes as analyzed by electron microscopy and image processing.

Most strains of Staphylococcus aureus produce alpha-toxin, a 33-kDa membrane active protein which is considered to be an important virulence factor of this bacterium. When alpha-toxin interacts with membranes an oligomeric from of the toxin can be seen by electron microscopy as characteristic ring structures in the membrane. A two-dimensional study of these annular structures, incorporated in membranes of human platelets, was performed, introducing a partly new method for rotational alignment of individual particles. It is shown that the averaged oligomer consists of six subunits. At neutral pH the outer diameter of the ring is about 75 A. The stain-filled pore or cavity in the center has a diameter of about 25 A. The size of the hexamer is increased if the pH is lowered.