PHAPI, CAS, and Hsp70 Promote Apoptosome Formation by Preventing Apaf-1 Aggregation and Enhancing Nucleotide Exchange on Apaf-1

Howard Hughes Medical Institute, University of Texas Southwestern Medical Center at Dallas, 5323 Harry Hines Boulevard, Dallas, TX 75390, USA.
Molecular cell (Impact Factor: 14.02). 05/2008; 30(2):239-47. DOI: 10.1016/j.molcel.2008.03.014
Source: PubMed


During apoptosis, cytochrome c is released from mitochondria to the cytosol, where it binds Apaf-1. The Apaf-1/cytochrome c complex then oligomerizes either into heptameric caspase-9-activating apoptosome, which subsequently activates caspase-3 and caspase-7, or bigger inactive aggregates, depending on the availability of nucleotide dATP/ATP. A tumor suppressor protein, PHAPI, enhances caspase-9 activation by promoting apoptosome formation through an unknown mechanism. We report here the identification of cellular apoptosis susceptibility protein (CAS) and heat shock protein 70 (Hsp70) as mediators of PHAPI activity. PHAPI, CAS, and Hsp70 function together to accelerate nucleotide exchange on Apaf-1 and prevent inactive Apaf-1/cytochrome c aggregation. CAS expression is induced by multiple apoptotic stimuli including UV irradiation. Knockdown of CAS by RNA interference (RNAi) in cells attenuates apoptosis induced by UV light and causes endogenous Apaf-1 to form aggregates. These studies indicated that PHAPI, CAS, and Hsp70 play an important regulatory role during apoptosis.

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    • "During apoptosis, the levels of nucleotides decrease significantly to allow cytochrome c-Apaf-1 interaction, which initiate nucleotide exchange by nucleotide exchange factor consisting of PHAP-1, CAS and Hsp70 (Chandra et al., 2006; Kim et al., 2008). The hydrolysis of bound nucleotide during nucleotide exchange triggers the formation of apoptosome and activates caspases (Kim et al., 2008), however, later studies suggest that ATP hydrolysis may not be required for apoptosome formation (Reubold et al., 2009). It is not clear how prosurvival and proapoptotic function of Hsp70 are regulated but further studies are required to understand/resolve the dual function of Hsp70. "
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