Figure 4.

A speculative model of caspase activation in non-apoptotic roles. IAPs could be bound to ProCaspase-9/apotosome complex, activated Caspase-9 and/or activated Caspase-3. The transient and fast activation of caspases in non-apoptotic roles suggests a clutch-like role of IAPs where active caspases are stalled ready to be released and used. In ‘direct drive’ mode, the formation of the apoptosome might overwhelm the IAP clutch and push the system into a slow but more extensive accumulation of active caspases. caspase-3*, activated caspase-3; caspase-9*, activated caspase-9; APAF-1, apoptotic protease-activating factor 1; IAP, inhibitors of apoptosis; IBP, IAP-binding motif proteins