Editors' ChoiceApoptosis

Defining Death

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Science Signaling  18 Aug 2009:
Vol. 2, Issue 84, pp. ec279
DOI: 10.1126/scisignal.284ec279

Receptors such as CD95 (also called Fas or Apo1) can cause cell death by assembling a complex of signaling proteins known as the death-inducing signaling complex or DISC. In this complex, inactive procaspase-8 monomers are thought to be brought into close proximity such that they can cleave each other and be activated toward other substrates that induce apoptosis, like the executioner caspase, caspase-3. To explore this regulatory mechanism in more detail, Hughes et al. analyzed activation of procaspase-8 in a reconstituted simplified DISC system comprising recombinant intracellular domain of CD95, FADD (Fas-associated death domain protein), and full-length wild-type or mutant procaspase-8 added to Jurkat cell lysates. The CD95 construct was active because it was prepared as a fusion protein with glutathione S-transferase, which resulted in its dimerization. Analysis of a procaspase-8 mutant that could not be cleaved showed that the mutant protein had limited activity and only cleaved proteins that were in high concentration because of localization to the DISC, other procaspase-8 molecules, or FADD. Cleavage of substrates like procaspase 3 or Bid appeared to require cleavage of procaspase-8. When expressed in cells, the procaspase-8 mutant did not support CD-95–induced cell death. Under some circumstances, CD95 can promote cell survival rather than cause apoptosis. The authors propose that the DISC may function as a two-stage switch in which initial signaling by active but uncleaved dimers of procaspase-8 may produce a survival signal, whereas further generation of the cleaved fully active form of caspase-8 is required to modify substrates that cause the cell to undergo cell death.

M. A. Hughes, N. Harper, M. Butterworth, K. Cain, G. M. Cohen, M. MacFarlane, Reconstitution of the death-inducing signaling complex reveals a substrate switch that determines CD95-mediated death or survival. Mol. Cell 35, 265–279 (2009). [Online Journal]