RESUMO
Caspase-8 is the main initiator caspase in death receptor-induced apoptosis. Procaspase-8 is activated at the death-inducing signaling complex (DISC). Previous studies suggested a two-step model of procaspase-8 activation. The first cleavage step occurs between the protease domains p18 and p10. The second cleavage step takes place between the prodomain and the large protease subunit (p18). Subsequently, the active caspase-8 heterotetramer p18(2)-p10(2) is released into the cytosol, starting the apoptotic signaling cascade. In this report, we have further analyzed procaspase-8 processing upon death receptor stimulation directly at the DISC and in the cytosol. We have found an alternative sequence of cleavage events for procaspase-8. We have demonstrated that the first cleavage can also occur between the prodomain and the large protease subunit (p18). The resulting cleavage product, p30, contains both the large protease subunit (p18) and the small protease subunit (p10). p30 is further processed to p10 and p18 by active caspases. Furthermore, we show that p30 can sensitize cells toward death receptor-induced apoptosis. Taken together, our data suggest an alternative mechanism of procaspase-8 activation at the DISC.
Assuntos
Caspase 8/metabolismo , Fragmentos de Peptídeos/metabolismo , Transdução de Sinais/fisiologia , Anticorpos Monoclonais/metabolismo , Apoptose/fisiologia , Proteína Reguladora de Apoptosis Semelhante a CASP8 e FADD/genética , Proteína Reguladora de Apoptosis Semelhante a CASP8 e FADD/metabolismo , Caspase 8/genética , Caspases/genética , Caspases/metabolismo , Ativação Enzimática , Humanos , Células Jurkat , Fragmentos de Peptídeos/genética , Estrutura Terciária de Proteína , Receptores de Morte Celular/genética , Receptores de Morte Celular/metabolismo , Receptor fas/genética , Receptor fas/metabolismoRESUMO
Stimulation of CD95 (APO-1/Fas) by its natural ligand CD95L (APO-1L/FasL) leads to the formation of the death-inducing signaling complex. Here we report that upon CD95 stimulation in several T and B cell lines, a novel signaling complex is formed, which we term complex II. Complex II is composed of the death effector domain proteins as follows: procaspase-8a/b, three isoforms of c-FLIP (c-FLIP(L), c-FLIP(S), c-FLIP(R)), and FADD. Notably, complex II does not contain CD95. Based on our findings we suggest that CD95 signaling includes two steps. The first step involves formation of the death-inducing signaling complex at the cell membrane. The second step involves formation of the cytosolic death effector domain protein-containing complex that may play an important role in amplification of caspase activation.
