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Title: Pro-apoptotic proteins released from the mitochondria regulate the protein composition and caspase-processing activity of the native Apaf-1/caspase-9 apoptosome complex.
Authors: Twiddy, D
Brown, DG
Adrain, C
Jukes, R
Martin, SJ
Cohen, GM
MacFarlane, M
Cain, K
First Published: 7-May-2004
Citation: J BIOL CHEM, 2004, 279 (19), pp. 19665-19682
Abstract: The apoptosome is a large caspase-activating ( approximately 700-1400 kDa) complex, which is assembled from Apaf-1 and caspase-9 when cytochrome c is released during mitochondrial-dependent apoptotic cell death. Apaf-1 the core scaffold protein is approximately 135 kDa and contains CARD (caspase recruitment domain), CED-4, and multiple (13) WD40 repeat domains, which can potentially interact with a variety of unknown regulatory proteins. To identify such proteins we activated THP.1 lysates with dATP/cytochrome c and used sucrose density centrifugation and affinity-based methods to purify the apoptosome for analysis by MALDI-TOF mass spectrometry. First, we used a glutathione S-transferase (GST) fusion protein (GST-casp9(1-130)) containing the CARD domain of caspase-9-(1-130), which binds to the CARD domain of Apaf-1 when it is in the apoptosome and blocks recruitment/activation of caspase-9. This affinity-purified apoptosome complex contained only Apaf-1XL and GST-casp9(1-130), demonstrating that the WD40 and CED-4 domains of Apaf-1 do not stably bind other cytosolic proteins. Next we used a monoclonal antibody to caspase-9 to immunopurify the native active apoptosome complex from cell lysates, containing negligible levels of cytochrome c, second mitochondria-derived activator of caspase (Smac), or Omi/HtrA2. This apoptosome complex exhibited low caspase-processing activity and contained four stably associated proteins, namely Apaf-1, pro-p35/34 forms of caspase-9, pro-p20 forms of caspase-3, X-linked inhibitor of apoptosis (XIAP), and cytochrome c, which was only bound transiently to the complex. However, in lysates containing Smac and Omi/HtrA2, the caspase-processing activity of the purified apoptosome complex increased 6-8-fold and contained only Apaf-1 and the p35/p34-processed subunits of caspase-9. During apoptosis, Smac, Omi/HtrA2, and cytochrome c are released simultaneously from mitochondria, and thus it is likely that the functional apoptosome complex in apoptotic cells consists primarily of Apaf-1 and processed caspase-9.
DOI Link: 10.1074/jbc.M311388200
ISSN: 0021-9258
Type: Journal Article
Appears in Collections:Published Articles, Dept. of Biochemistry

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