THE C-TERMINAL REGION OF THE S COMPONENT OF STAPHYLOCOCCAL LEUKOCIDIN IS ESSENTIAL FOR THE BIOLOGICAL-ACTIVITY OF THE TOXIN

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 G(M1) (G(M1)), When excited at 280 nm, both S and LS2 exhibited intrinsic tryptophan fluorescence with an emission maximum at 318 nm. Upon binding to G(M1), the emission maximum of S underwent a blue shift to 310 nm, whereas no change in fluorescence took place on mixing G(M1) with LS2. We conclude that the C-terminal region of S is essential for its biological activity as well as for its binding to G(M1) and that this binding is accompanied by a conformational change of the S protein.