| Caspase-9 is an initiator caspase central to the intrinsic apoptosis pathway, acting at the apex of the mitochondrial death signaling cascade. Upon cytochrome c release from mitochondria, caspase-9 assembles into the Apaf-1 apoptosome. This process involves induced proximity oligomerization via Apaf-1 CARD domain interactions, leading to autocatalytic cleavage at Asp315 and formation of an active p35/p12 heterodimer. A secondary cleavage at Asp330 yields a p37 fragment, further amplifying effector caspase activation. The apoptosome-bound caspase-9 demonstrates over 100-fold enhanced activity due to allosteric stabilization of its active conformation, maintained by Apaf-1 even without continued dATP presence after assembly. This holoenzyme efficiently activates caspase-3 and caspase-7, initiating the executioner phase, which results in PARP cleavage, DNA fragmentation, and cytoskeletal collapse. Physiological inhibition occurs through Xiap, which binds the active site and BIR2 domain of processed caspase-9; however, caspase-3 feedback cleavage of Xiap relieves this inhibition, ensuring irreversible apoptosis commitment. Alternative splicing produces the dominant-negative caspase-9S isoform, which lacks catalytic activity and competes for apoptosome docking. Caspase-9 is essential for developmental sculpting, DNA damage surveillance, and stress-induced clearance in neurons and lymphocytes. It serves as the canonical marker for intrinsic apoptosis in research assays using fluorogenic substrates or dominant-negative mutants in flow cytometry. Caspase-9 deficiency leads to severe lymphoproliferation and autoimmunity, while APAF-1 methylation in cancer silences the pathway. |
Lane 1: Jurkat, Lane 2: Jurkat (staurosporine treated)
