The cytotoxic T cell proteinase granzyme B does not activate interleukin-1 beta-converting enzyme

Murine granzyme B (cytotoxic cell proteinase-1 (CCP1)) is a member of a family of seven serine proteases found in cytoplasmic granules of cytotoxic T lymphocytes (CTLs). Evidence has suggested that it is involved in target cell DNA fragmentation during CTL-mediated cytotoxicity, although intracellular substrates for granzyme B have not yet been identified. The substrate specificity of granzyme B, requiring an aspartic acid residue at site P1, is unique among eukaryotic serine proteases and is shared with only one other known eukaryotic protease, interleukin-1 beta-converting enzyme (ICE). ICE is responsible for processing pro-interleukin-1 beta to produce biologically active interleukin-1 beta and is itself synthesized as an inactive precursor. Recent evidence has suggested a role for ICE in programmed cell death, which led to a model for CTL-mediated cytotoxicity. In this proposal granzyme B activates ICE in the target cell by proteolytically processing the ICE precursor, resulting in active ICE heterodimer that induces apoptosis in the target cell. We have isolated the cDNA encoding murine ICE and generated in vitro translated ICE precursor. Using lysates from COS cells expressing granzyme B we show that ICE precursor is not a substrate for granzyme B and propose an alternate mechanism for CTL-mediated cytotoxicity.