The three-dimensional structure of caspase-8: an initiator enzyme in apoptosis

Background: In the initial stages of Fas-mediated apoptosis the cysteine pr otease caspase-8 is recruited to the cell receptor as a zymogen (procaspase -8) and is incorporated into the death-signalling complex. Procaspase-8 is subsequently activated leading to a cascade of proteolytic events, one of t hem being the activation of caspase-3, and ultimately resulting in cell des truction, Variations in the substrate specificity of different caspases hav e been reported. Results: We report here the crystal structure of a complex of the activated human caspase-8 (proteolytic domain) with the irreversible peptidic inhibi tor Z-Glu-Val-Asp-dichloromethylketone at 2.8 Angstrom resolution. This is the first structure of a representative of the long prodomain initiator cas pases and of the group III substrate specificity class. The overall protein architecture resembles the caspase-1 and caspase-3 folds, but shows distin ct structural differences in regions forming the active site. In particular , differences observed in subsites S-3, S-4 and the loops involved in inhib itor interactions explain the preference of caspase-8 for substrates with t he sequence (Leu/Val)-Glu-X-Asp, Conclusions: The structural differences could be correlated with the observ ed substrate specificities of caspase-1, caspase-3 and caspase-8, as determ ined from kinetic experiments. This information will help us to understand the role of the Various caspases in the propagation of the apoptotic signal . The information gained from this investigation should be useful for the d esign of specific inhibitors.