FLIPL induces caspase-8 activity in the absence of interdomain caspase-8 cleavage and alters substrate specificity

摘要

Caspase-8 is an initiator caspase that is activated by death receptors to initiate the extrinsic pathway of apoptosis. Caspase-8 activation involves dimerization and subsequent interdomain autoprocessing of caspase-8 zymogens, and recently published work has established that elimination of the autoprocessing site of caspase-8 abrogates its pro-apoptotic function while leaving its proliferative function intact. The observation that the developmental abnormalities of caspase-8 deficient mice are shared by mice lacking the dimerization adapter FADD or the caspase paralog FLIPL has led to the hypothesis that FADD-dependent formation of heterodimers between caspase-8 and FLIPL could mediate the developmental role of caspase-8. Using an inducible dimerization system we demonstrate that cleavage of the catalytic domain of caspase-8 is crucial for its activity in the context of activation by homodimerization. However, we find that use of FLIPL as a partner for caspase-8 in dimerization-induced activation rescues the requirement for intersubunit linker proteolysis in both protomers. Moreover, before processing, caspase-8 in complex with FLIPL does not generate a fully active enzyme, but an attenuated species able to process only select natural substrates. Based on these results we propose a mechanism of caspase-8 activation by dimerization in the presence of FLIPL, as well as a mechanism of caspase-8 functional divergence in apoptotic and non-apoptotic pathways.