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Background
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Apoptosis plays a major role in normal organism development, tissue homeostasis, and removal of damaged cells. This process is regulated by the interplay of pro- and anti-apoptotic members of the B-cell lymphoma 2 (Bcl-2) family. The BH3-only (Bcl-2-homology domain 3 only) proteins are a pro-apoptotic subgroup of the Bcl-2 family that are critical initiators of lymphocyte apoptosis and other processes. Unlike the other members of the Bcl-2 family which contain at least two Bcl-2 homology (BH) domains, these proteins contain just one, the BH3 domain. Some of the members of this family, such as Hrk, may show little sequence homology to the others, but all BH-only proteins can bind to at least some pro-survival Bcl-2-like proteins through this BH3 domain and trigger apoptosis when overexpressed. For example, the proteins Bad, Bim and Bmf regulate apoptosis by forming heterodimers with the anti-apoptotic proteins Bcl-2 and Bcl-xL. Bim, Noxa, and PUMA are activated by p53 following DNA damage although the exact mechanisms are not entirely clear. Noxa may also be involved with the activation of Bik, a BH3-only protein whose localization in the endoplasmic reticulum (ER) suggests that Bik may play a role in ER-stress-induced apoptosis. While full-length Bid also promotes apoptosis when expressed, its activity is enhanced following cleavage by enzymes such as capsase-8, suggesting that Bid may play a key role in death-receptor induced apoptosis.
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