Role of the BH3 (Bcl-2 homology 3) domain in the regulation of apoptosis and Bcl-2-related proteins.

The Bcl-2 family of proteins play a prominent role in the regulation of apoptosis. From the initial identification of bcl-2 as an oncogene in follicular lymphoma through genetic studies in Caenorhabditis elegans to recent functional studies focusing on the importance of mitochondrial events in cell death signalling, the members of this protein family continue to be implicated in pivotal decision points regarding the survival of the cell. The family can be divided into two classes: those such as Bcl-2 and Bcl-xL that suppress cell death, and others, such as Bak and Bax, that appear to promote apoptosis. The Bcl-2 family is characterized by specific regions of homology termed Bcl-2 homology (BH1, BH2, BH3, BH4) domains, which are critical to the function of these proteins, including their impact on cell survival and their ability to interact with other family members and regulatory proteins. The identification of the BH3 domain as a potent mediator of cell death has led to the emergence of an additional family of proapoptotic proteins (such as Bad, Bik, Bid and Hrk) that share identity with Bcl-2 only within this death domain. These BH3-only proteins may be part of a regulatory network serving to integrate cell survival and death signals, an assertion that is supported by the identification of a BH3-only protein, Egl-1, as part of the central core of cell death signalling in C. elegans. While the mechanism of action of the BH3-only proteins remains unclear, recent studies on the regulation of critical protein-protein interactions and activity of Bad by phosphorylation in response to growth factor signalling suggest that the active state of BH3-only proteins may be regulated by post-translational modification. Additional modes of regulation, such as transcriptional, translational and subcellular localization, are also likely to be important.