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Published 7 July 2003. doi:10.1083/jcb.200302084
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© The Rockefeller University Press, 0021-9525/2003/7/59 $5.00
The Journal of Cell Biology, Volume 162, Number 1, 59-69


Article

Bax and Bak can localize to the endoplasmic reticulum to initiate apoptosis

Wei-Xing Zong1, Chi Li1, Georgia Hatzivassiliou1, Tullia Lindsten2, Qian-Chun Yu2, Junying Yuan3 and Craig B. Thompson1

1 Department of Medicine, Abramson Cancer Center, University of Pennsylvania, Philadelphia, PA 19104
2 Department of Pathology and Laboratory Medicine, Abramson Cancer Center, University of Pennsylvania, Philadelphia, PA 19104
3 Department of Cell Biology, Harvard Medical School, Boston, MA 02115

Address correspondence to Craig B. Thompson, Department of Cancer Biology, Abramson Cancer Center, 421 Curie Blvd., BRB II/III, 445, Philadelphia, PA 19104-6160. Tel.: (215) 746-5515. Fax: (215) 746-5511. E-mail: craig{at}mail.med.upenn.edu

Bax and Bak play a redundant but essential role in apoptosis initiated by the mitochondrial release of apoptogenic factors. In addition to their presence at the mitochondrial outer membrane, Bax and Bak can also localize to the ER. Agents that initiate ER stress responses can induce conformational changes and oligomerization of Bax on the ER as well as on mitochondria. In wild-type cells, this is associated with caspase 12 cleavage that is abolished in bax-/-bak-/- cells. In bax-/-bak-/- cells, introduction of Bak mutants selectively targeted to either mitochondria or the ER can induce apoptosis. However, ER-targeted, but not mitochondria-targeted, Bak leads to progressive depletion of ER Ca2+ and induces caspase 12 cleavage. In contrast, mitochondria-targeted Bak leads to enhanced caspase 7 and PARP cleavage in comparison with the ER-targeted Bak. These findings demonstrate that in addition to their functions at mitochondria, Bax and Bak also localize to the ER and function to initiate a parallel pathway of caspase activation and apoptosis.

Key Words: Bak; Bax; Ca2+; caspase 12; ER


W.-X. Zong and C. Li contributed equally to this work.

* Abbreviations used in this paper: BMH, bismaleimidohexane; cb5, cytochrome b5; HM, heavy membrane; IRES, internal ribosomal entry site; LM, light membrane; MEF, murine embryonic fibroblast; t-caspase, truncated caspase; UPR, unfolded protein response.


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