A novel Apaf-1–independent putative caspase-2 activation complex

Abstract

CVaspase activation is a key event in apoptosis execution. In stress-induced apoptosis, the mitochondrial pathway of caspase activation is believed to be of central importance. In this pathway, cytochrome c released from mitochondria facilitates the formation of an Apaf-1 apoptosome that recruits and activates caspase-9. Recent data indicate that in some cells caspase-9 may not be the initiator caspase in stress-mediated apoptosis because caspase-2 is required upstream of mitochondria for the release of cytochrome c and other apoptogenic factors. To determine how caspase-2 is activated, we have studied the formation of a complex that mediates caspase-2 activation. Using gel filtration analysis of cell lysates, we show that caspase-2 is spontaneously recruited to a large protein complex independent of cytochrome c and Apaf-1 and that recruitment of caspase-2 to this complex is sufficient to mediate its activation. Using substrate-binding assays, we also provide the first evidence that caspase-2 activation may occur without processing of the precursor molecule. Our data are consistent with a model where caspase-2 activation occurs by oligomerization, independent of the Apaf-1 apoptosome.

Cellular Mechanisms Controlling Caspase Activation and Function

“…It is activated similarly to other initiator caspases via induced proximity dimerization and autocatalytic processing within an adaptor protein complex, and it appears to function, at least under some circumstances, upstream of the mitochondria (Baliga et al 2004). The current model for caspase-2 activation involves two adaptor proteins, PIDD ( p53-induced protein with a death domain) and RAIDD (RIP-associated ICH-1/CED-3 homologous proteins with a death domain) (Duan and Dixit 1997;Read et al 2002;Tinel and Tschopp 2004). Together, these proteins form the PIDDosome complex, which consists of five PIDDs, seven RAIDDs, and seven caspase-2 molecules .…”

Caspase function in programmed cell death

“…Although caspase-2 was one of the first apoptotic caspases discovered, its physiological function remains a matter of considerable debate and somewhat of an enigma. Caspase-2 contains a CARD that facilitates dimerisation of procaspase-2 molecules, 97,98 interaction with CARD-containing protein RAIDD 99 or recruitment to large multiprotein complexes, 100 events that trigger procaspase-2 activation. Caspase-2 is recruited into a protein complex similar to the Apaf-1/caspase-9 apoptosome.…”

Caspase substrates

“…However, the inherent cleavage site specificity of caspases have been applied to make active site probes that show some degree of selectivity for individual caspases, 10 and when these are coupled with selective antisera the identity of active caspases in complex mixtures is more readily disclosed. [11][12][13][14] Specificity for protein substrates Exosites. Having a preferred cleavage site specificity is not sufficient for proteolysis of natural proteins; appropriate presentation of the cleavage site seems critical for efficient hydrolysis.…”