The Staphylococcal toxin leukocidin consists of two protein components, F and S. From a culture medium of Staphylococcus aureus RIMD 310925, we isolated a truncated form of S (LS2), of which the C-terminal 17-residue segment is missing. Unlike intact S, LS2 showed neither leukocytolytic activity in the presence of F nor affinity for monosialoganglioside GM1 (GM1). When excited at 280 nm, both S and LS2 exhibited intrinsic tryptophan fluorescence with an emission maximum at 318 nm. Upon binding to GM1, the emission maximum of S underwent a blue shift to 310 nm, whereas no change in fluorescence took place on mixing GM1 with LS2. We conclude that the C-terminal region of S is essential for its biological activity as well as for its binding to GM1 and that this binding is accompanied by a conformational change of the S protein.
