Phosphorylation by protein kinase A disassembles the caspase-9 core

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• Caspases are critical cysteine proteases regulating apoptosis.
• Phosphorylation is a key mechanism controlling caspase activity.
• Caspase-9 activation is a crucial step in the apoptotic cascade.

Purpose of the Study:

• To investigate the regulatory role of PKA-mediated phosphorylation on caspase-9 activity.
• To elucidate the molecular mechanism by which PKA phosphorylation inhibits caspase-9 activation.
• To identify novel regulatory pathways controlling apoptosis.

Main Methods:

• Site-directed mutagenesis to target specific phosphorylation sites on caspase-9.
• In vitro kinase assays using PKA and caspase-9.
• Analysis of caspase-9 self-processing and substrate binding.
• Biophysical techniques to study caspase-9 subunit interactions and aggregate formation.

Main Results:

• PKA phosphorylates caspase-9 at three sites, including the functionally important Ser-183 residue.
• Ser-183 phosphorylation prevents caspase-9 self-processing and blocks substrate binding.
• Phosphorylation at Ser-183 induces disassembly of caspase-9 subunits, forming 20nm aggregates.
• This phosphorylation-induced disassembly and aggregation represent a novel regulatory mechanism for caspase-9.

Conclusions:

• PKA-mediated phosphorylation of caspase-9 at Ser-183 provides an upstream inhibitory block to apoptosis.
• The two-stage mechanism involving blocked processing and subunit disassembly offers a new perspective on caspase regulation.
• This regulatory mechanism may be conserved across other caspases, highlighting its potential significance.