page_title: Bcl-2 family
section_title: Function
hierarchical_section_title: Bcl-2 family / Function
context_page_description: The Bcl-2 Family consists of a number of evolutionarily-conserved proteins that share Bcl-2 homology domains. The Bcl-2 family is most notable for their regulation of apoptosis, a form of programmed cell death, at the mitochondrion. The Bcl-2 family proteins consists of members that either promote or inhibit apoptosis, and control apoptosis by governing mitochondrial outer membrane permeabilization, which is a key step in the intrinsic pathway of apoptosis. A total of 25 genes in the Bcl-2 family were identified by 2008.
context_section_description: Regulated cell death (apoptosis) is induced by events such as growth factor withdrawal and toxins. It is controlled by regulators, which have either an inhibitory effect on programmed cell death (anti-apoptotic) or block the protective effect of inhibitors (pro-apoptotic). Many viruses have found a way of countering defensive apoptosis by encoding their own anti-apoptosis genes preventing their target-cells from dying too soon.
  Bcl-x is a dominant regulator of programmed cell death in mammalian cells. The long form (Bcl-x(L), displays cell death repressor activity, but the short isoform (Bcl-x(S)) and the β-isoform (Bcl-xβ) promote cell death. Bcl-x(L), Bcl-x(S) and Bcl-xβ are three isoforms derived by alternative RNA splicing.
  There are a number of theories concerning how the Bcl-2 gene family exert their pro- or anti-apoptotic effect. An important one states that this is achieved by activation or inactivation of an inner mitochondrial permeability transition pore, which is involved in the regulation of matrix Ca²⁺, pH, and voltage. It is also thought that some Bcl-2 family proteins can induce (pro-apoptotic members) or inhibit (anti-apoptotic members) the release of cytochrome c into the cytosol which, once there, activates caspase-9 and caspase-3, leading to apoptosis. Although Zamzami et al. suggest that the release of cytochrome c is indirectly mediated by the PT pore on the inner mitochondrial membrane, strong evidence suggest an earlier implication of the MAC pore on the outer membrane.
  Another theory suggests that Rho proteins play a role in Bcl-2, Mcl-1 and Bid activation.  Rho inhibition reduces the expression of anti-apoptotic Bcl-2 and Mcl-1 proteins and increases protein levels of pro-apoptotic Bid but had no effect on Bax or FLIP levels.  Rho inhibition induces caspase-9 and caspase-3-dependent apoptosis of cultured human endothelial cells.
caption: Overview of signal transduction pathways involved in apoptosis.