The structure of Staphylococcus aureus alpha-toxin: effects of trypsin treatment.

Staphylococcus aureus alpha-toxin was treated with trypsin, which inactivates the toxin. Two-dimensional crystals of the modified protein were produced on preformed lipid layers. The projection structure obtained by electron crystallographic analysis of a large number of crystals showed tetragonal p4 symmetry and a resolution of approximately 12 A. The fragments of the toxin, 17 and 18 kDa large, were arranged in a way resembling those observed earlier for the native protein (Olofsson et al., J. Mol. Biol. 214, 299-306, 1990). However, after trypsin treatment the stain-deficient region corresponding to one alpha-toxin monomer shows two separated subdomains of similar size. This separation is probably related to the inability of the modified toxin to undergo the conformational change thought to be essential for the membrane-damaging effect.