We report the purification from a rat natural killer (RNK) large granular lymphocyte leukemia of a 32-kD granule protein that induces rapid DNA fragmentation and apoptosis. The protein, which we have called "fragmentin," was capable of causing DNA from intact YAC-1 cells to be cleaved into oligonucleosomal-sized fragments and producing severe chromatin condensation within 1 h. Amino acid sequence of tryptic peptides indicated that fragmentin was highly homologous to the NK and T cell granule serine proteases RNK protease 1 and mouse cytotoxic T cell protease I (CCPI)/granzyme B. Preincubation with the serine esterase inhibitor 3,4-dichloroisocoumarin blocked fragmentin-induced DNA damage, but had no effect on cytolysin. Fragmentin activity against four lymphoma target cells was completely dependent on the presence of cytolysin. Fragmentin produced rapid membrane damage as well as DNA fragmentation at nonlytic cytolysin doses, suggesting that fragmentin activity was not limited to its effects on the nucleus. Fragmentin and cytolysin activity were completely inhibited by EGTA, indicating the process was Ca2+ dependent. A role for cytolysin in endocytosis of fragmentin was suggested by the observation that treatment of YAC-1 with cytochalasin B or sodium azide and 2-deoxyglucose blocked DNA fragmentation but not cytolysin activity. A 30-kD N alpha-CBZ-L-lysine thiobenzyl esterase, which copurified with fragmentin, was inactive on its own but was able to synergistically amplify the DNA damage induced by fragmentin in the presence of cytolysin. Fragmentin activity was not dependent on protein synthesis, as cycloheximide treatment of YAC-1 cells did not prevent DNA damage. We postulate that fragmentin is the molecular mediator of NK cell-mediated DNA fragmentation and apoptosis.

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