Phosphorylation by protein kinase A disassembles the caspase-9 core.

Caspases， the cysteine proteases which facilitate the faithful execution of apoptosis， are tightly regulated by a number of mechanisms including phosphorylation. In response to cAMP， PKA phosphorylates caspase-9 at three sites preventing caspase-9 activation， and suppressing apoptosis progression. Phosphorylation of caspase-9 by PKA at the functionally relevant site Ser-183 acts as an upstream block of the apoptotic cascade， directly inactivating caspase-9 by a two-stage mechanism. First， Ser-183 phosphorylation prevents caspase-9 self-processing and directly blocks substrate binding. In addition， Ser-183 phosphorylation breaks the fundamental interactions within the caspase-9 core， promoting disassembly of the large and small subunits. This occurs despite Ser-183 being a surface residue distal from the interface between the large and small subunits. This phosphorylation-induced disassembly promotes the formation of ordered aggregates around 20 nm in diameter. Similar aggregates of caspase-9 have not been previously reported. This two-stage regulatory mechanism for caspase-9 has likewise not been reported previously but may be conserved across the caspases.