BH3-only proteins and their roles in programmed cell death.

The Bcl-2 family of proteins controls the mitochondrial pathway to apoptosis. It consists of pro-survival and pro-apoptotic members, and their interactions decide whether apoptogenic factor confined to the mitochondrial intermembrane space can leak to the cytosol. Despite the intense efforts to understand the molecular mechanisms that lead to the permeabilization of the mitochondrial membrane, this particular issue remains a matter of intense controversy. It is well accepted that pro-apoptotic Bax and Bak are directly responsible for the damage to the mitochondria, but pro-survival family members prevent them from doing so. It is also accepted that stress signals activate selected Bcl-2 homology (BH)3-only proteins. But do these BH3-only proteins bind and activate Bax and Bak directly, or do they inhibit the pro-survival family members?