Granzyme B mimics apical caspases - Description of a unified pathway for trans-activation of executioner caspase-3 and -7

Granzyme B (GrB) is predicted to trigger apoptosis by activating preferred caspases, but the zymogens that are directly processed by the granzyme and the requirements for these interactions remain unclarified. We examined thi s dilemma by comparing the kinetics and pattern of GrB-mediated activation of the executioner caspase-7 in vitro and in vivo. GrB rapidly activates pr ocaspase-7 in vitro by cleaving between the large and small subunits leavin g the propeptide intact. During GrB-mediated apoptosis, the caspase-7 prope ptide is removed and cleavage occurs between the subunits, Strikingly, casp ase-7 is unprocessed in caspase-3-deficient MCF-7 cells exposed to GrB bud is rapidly activated when the cells are solubilized. Transfection with casp ase-3 restores the removal of the caspase-7 propeptide and the capacity of GrB to subsequently activate the caspase. The data suggest that GrB activat es caspase-3, which then removes the propeptide of caspase-7 allowing activ ation by GrB, Thus GrB initiates the death pathway by processing the access ible caspase-3, and the caspase-7 propeptide regulates transactivation of t he zymogen by granzyme. As a consequence, two proteases, caspase-3 and GrB, are required to activate procaspase-7.