Caspase-9 (casp-9) is an initiator caspase and plays a central role in activating apoptotic cell death. Control of all caspases is tightly regulated by a series of phosphorylation events enacted by several different kinases. Caspase-9 is the most heavily phosphorylated of all caspases， with phosphorylation of at least 11 distinct residues in all three caspase-9 domains by nine kinases. Caspase-9 phosphorylation by the non-receptor tyrosine kinase c-Abl at Tyr-153 reportedly leads to caspase-9 activation. All other phosphorylation events on caspases have been shown to block proteolytic function by a number of mechanisms， so we sought to unravel the molecular mechanism of the putative caspase-9 activation by ... More
Caspase-9 (casp-9) is an initiator caspase and plays a central role in activating apoptotic cell death. Control of all caspases is tightly regulated by a series of phosphorylation events enacted by several different kinases. Caspase-9 is the most heavily phosphorylated of all caspases， with phosphorylation of at least 11 distinct residues in all three caspase-9 domains by nine kinases. Caspase-9 phosphorylation by the non-receptor tyrosine kinase c-Abl at Tyr-153 reportedly leads to caspase-9 activation. All other phosphorylation events on caspases have been shown to block proteolytic function by a number of mechanisms， so we sought to unravel the molecular mechanism of the putative caspase-9 activation by phosphorylation. Surprisingly， we observed no evidence for Tyr-153 phosphorylation of caspase-9 or in cells， suggesting that Tyr-153 is not phosphorylated by c-Abl. Instead， we identified a new site for c-Abl-mediated phosphorylation， Tyr-397. This residue is adjacent to the caspase-9 active site but， as a member of the second shell， not a residue that directly contacts substrate. Our results further indicate that Tyr-397 is the dominant site of c-Abl phosphorylation both and upon c-Abl activation in cells. Of note， phosphorylation at this site inhibits caspase-9 activity， and the bulk of the added phosphate moiety appeared to directly block substrate binding. c-Abl plays both proapoptotic and prosurvival roles， and our findings suggest that c-Abl's effects on caspase-9 activity promote the prosurvival mode.
