The Apaf-1*procaspase-9 apoptosome complex functions as a proteolytic-based molecular timer.
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.
Malladi
Srinivas
S
Division of Pharmacology and Toxicology, College of Pharmacy, The University of Texas at Austin, Austin, TX 78712-0125, USA.
Challa-Malladi
Madhavi
M
Fearnhead
Howard O
HO
Bratton
Shawn B
SB
D000818:Animals; D017209:Apoptosis; D053478:Apoptosomes / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D053477:Apoptotic Protease-Activating Factor 1 / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D053148:Caspase 3 / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D053453:Caspase 9 / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D002460:Cell Line; D003001:Cloning, Molecular; D004789:Enzyme Activation; D006801:Humans; D051379:Mice; D009154:Mutation
D053478:Apoptosomes; D053477:Apoptotic Protease-Activating Factor 1; D053148:Caspase 3; D053453:Caspase 9