The Apaf-1•procaspase-9 apoptosome complex functions as a proteolytic-based molecular timer

Abstract

During stress-induced apoptosis, the initiator caspase-9 is activated by the Apaf-1 apoptosome and must remain bound to retain significant catalytic activity. Nevertheless, in apoptotic cells the vast majority of processed caspase-9 is paradoxically observed outside the complex. We show herein that apoptosome-mediated cleavage of procaspase-9 occurs exclusively through a CARD-displacement mechanism, so that unlike the effector procaspase-3, procaspase-9 cannot be processed by the apoptosome as a typical substrate. Indeed, procaspase-9 possessed higher affinity for the apoptosome and could displace the processed caspase-9 from the complex, thereby facilitating a continuous cycle of procaspase-9 recruitment/activation, processing, and release from the complex. Owing to its rapid autocatalytic cleavage, however, procaspase-9 per se contributed little to the activation of procaspase-3. Thus, the Apaf-1 apoptosome functions as a proteolytic-based 'molecular timer', wherein the intracellular concentration of procaspase-9 sets the overall duration of the timer, procaspase-9 autoprocessing activates the timer, and the rate at which the processed caspase-9 dissociates from the complex (and thus loses its capacity to activate procaspase-3) dictates how fast the timer 'ticks' over.

Pathways related to mitochondrial dysfunction in cartilage of endemic osteoarthritis patients in China

“…Sequential activation of caspases plays a central role in the execution-phase of cell apoptosis. APAF1 is related to caspase 9 and commits a cell to apoptosis [16,17]. Combining this evidence with that from our previous study [7] shed light on the role of mitochondria-related genes and their contribution to the mitochondria-mediated chondrocyte apoptosis underlying the pathogenesis of KBD.…”

The Caspase-8 Homolog Dredd Cleaves Imd and Relish but Is Not Inhibited by p35

“…Perhaps, more similar to our results with Dredd, activation of caspase-8 does not require interdomain cleavage in certain non-apoptotic roles, such as LPS-induced B-lymphocyte proliferation or when complexed with FLIP L (31,32). Caspase-9 is another example of initiator caspase that does not require interdomain cleavage for its activation and apoptotic function (33)(34)(35). Like Caspase-9, unprocessed Dredd is likely to function as part of a multimeric complex in the Imd signaling pathway.…”

Cellular Mechanisms Controlling Caspase Activation and Function

“…The remarkably similar phenotypes of the Apaf-1 -/ -and caspase-9 -/ -mice suggest that caspase-9 is indeed dependent on this Apaf-1-based complex for its activation (Cecconi et al 1998;Hakem et al 1998;Kuida et al 1998). Recent data suggest that each apoptosome backbone recruits and activates only two caspase-9 molecules, creating a 7:2 ratio between Apaf-1 and caspase-9 within the apoptosome (Malladi et al 2009). Active caspase-9 cleaves and activates downstream effector caspases, such as caspase-3 (Slee et al 1999) (Fig.…”

“…Cleavage of caspase-9 does, however, have a variety of implications on its regulation, including the generation of a new epitope that is required for subsequent caspase-9 inhibition by XIAP. Additionally, Bratton and colleagues have suggested a model whereby cleavage of caspase-9 lowers its affinity for the apoptosome (relative to procaspase-9), promoting its replacement by new incoming procaspase-9 molecules recruited to the Apaf-1 caspase recruitment domains (CARDs) for activation (Malladi et al 2009). …”