Staphylococcal leukocidin nd gamma-hemolysin consist of LukF and LukS for leukocidin and LukF and HIg2 for gamma-hemolysin. In this report, we identify the minimum segment responsible for the LukS-specific function of leukocidin. After chemical analysis and homology study of the amino acid sequence of the C-terminal region between LukS and HIg2, we found a unique 5-residue sequence (IKRST246)-K-242-R-243-S-244-T-245 in LukS in which the 4-residue KRST is identical with that of the phosphorylated segment of a protein phosphorylated by protein kinase A. To elucidate whether the 5-residue segment is essential for the LukS function, we created plasmids containing a series of mutant genes corresponding to the 5-residue sequence and expressed them in Escherichia coli. The mutant proteins were purified and assayed for their leukocytolytic activity with LukF. The mutant MLS-TS, in which the T-246 in the 5-residue sequence was replaced by S, showed leukocidin activity 10 times higher than that of the intact LukS. However, neither mutant MLS-TY nor MLS-TA, in which T-246 was replaced by Y or A, respectively, showed leukocidin activity. The 5-residue segment was found to be deleted in HIg2. The mutant of HIg2, in which the 5-residue segment was inserted at the position that the segment is deleted, showed leukocidin activity. The boiled LukS, MLS-TS, and MHS-Z were strongly phosphorylated with [gamma-P-32]ATP in the presence of protein kinase A in a cell-free system. Thus, we conclude that the 5-residue segment (IKRST246)-K-242-R-243-S-244-T-245 is the pivotal segment of LukS responsible for the LukS function of staphylococcal leukocidin. (C) 1997 Federation of European Biochemical Societies.