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J. Cell Biol.,
Volume 144, Number 2, January 25, 1999 281-292






* Molecular Cell Biology Laboratory, Department of Biology, National University of Ireland, Maynooth, Co. Kildare, Ireland; Exit of cytochrome c from mitochondria into
the cytosol has been implicated as an important step in
apoptosis. In the cytosol, cytochrome c binds to the
CED-4 homologue, Apaf-1, thereby triggering Apaf-1-mediated activation of caspase-9. Caspase-9 is thought to propagate the death signal by triggering other
caspase activation events, the details of which remain
obscure. Here, we report that six additional caspases
(caspases-2, -3, -6, -7, -8, and -10) are processed in cell-free extracts in response to cytochrome c, and that
three others (caspases-1, -4, and -5) failed to be activated under the same conditions. In vitro association
assays confirmed that caspase-9 selectively bound to
Apaf-1, whereas caspases-1, -2, -3, -6, -7, -8, and -10 did
not. Depletion of caspase-9 from cell extracts abrogated cytochrome c-inducible activation of caspases-2,
-3, -6, -7, -8, and -10, suggesting that caspase-9 is required for all of these downstream caspase activation
events. Immunodepletion of caspases-3, -6, and -7 from cell extracts enabled us to order the sequence of
caspase activation events downstream of caspase-9 and
reveal the presence of a branched caspase cascade.
Caspase-3 is required for the activation of four other
caspases (-2, -6, -8, and -10) in this pathway and also participates in a feedback amplification loop involving
caspase-9.
Division of Cellular Immunology, La Jolla Institute for Allergy and Immunology, San Diego, California 92121; § Department of
Molecular and Experimental Medicine, The Scripps Research Institute, La Jolla, California 92037;
The Burnham Institute, La
Jolla, California 92037; ¶ Departments of Biochemistry and Molecular Biology, Merck Frosst Centre for Therapeutic Research,
Pointe Claire-Dorval, Quebec, H9R 4P8, Canada; and ** Center for Apoptosis Research and The Kimmel Cancer Institute,
Jefferson Medical College, Philadelphia, Pennsylvania 19107
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