Bcl 2 Family Proteins


 Creative BioMart Bcl 2 Family Proteins Product List
 Bcl 2 Family Proteins Background

Bcl-2 family members play a pivotal role in the intrinsic (mitochondrial) pathway of apoptosis. The “founding” protein, Bcl-2, was first discovered in human B cell follicular lymphoma cells carrying the t(14; 18) chromosomal translocation. Most lymphomas of this type have the breakpoint located in the Bcl-2 gene. Over-expression of this protein was later discovered to prolong cell survival by blocking apoptosis. Bcl-2 members possess at least one of four conserved a-helical regions known as Bcl-2 homology domains (BH1-4). Based on their function, the members can be divided into two groups, the pro-survival and pro-apoptotic members. The pro-survival members can inhibit apoptosis triggered by a wide variety of stimuli and they mostly contain all four BH domains. The pro-apoptotic members can be divided into “multi-domain” and “BH3 only” members. The multi-domain proteins resemble Bcl-2, and contain BH1-3 domains. In contrast, the other pro-apoptotic members contain only the BH3 domain that is essential for killing. Structural analysis of Bcl-XL revealed that BH1-3 domains assemble with a hydrophobic groove that can accommodate the BH3 domain of proapoptotic members. Commitment to life or death often is determined by opposing members of the Bcl-2 family. Pro- and anti-apoptotic proteins can heterodimerize and compromise one another's action, and the balance of their corresponding concentration may be the key in determining whether cell death occurs.

The anti-apoptotic members include proteins such as Bcl-2, Bc1-XL, and Mcl-1. Bcl-2 protects against diverse cytotoxic insults that can trigger apoptosis such as starvation of growth factors, loss of cell attachment to extracellular matrix, Fas-stimulation and cytotoxic T-cell killing. Over-expression of Bcl-2 protein, conferring a survival advantage, is frequently found in human cancers such as B-cell lymphomas and breast cancer. Although Bcl-2 -/- mice develop normally, accelerated lymphocyte death in thymus and spleen, distorted small intestine and neuronal disease have been observed.

BH3-only members can engage pro-survival proteins by the interaction of BH3 domains. The pro-apoptotic activity of BH3-only members is kept in check by either transcriptional control or post-translational modification. They serve as sensors for initiating the intrinsic apoptotic pathway in response to selective stimuli. For example, Bid is engaged through the activation of death receptors, while Noxa and Puma respond to DNA damage. Bim and Bad can be activated by multiple stimuli including growth factor deprivation, detachment from the cell matrix, chemotherapeutic agents or UV treatments. Studies using knockout mice of BH3-only proteins in certain types of cells confer resistance to selective apoptotic stimuli. For example, loss of Bim renders lymphocytes resistant to paclitaxel, ionomycin and cytokine deprivation induced apoptosis while loss of Bad in mammary epithelial cells confers some resistance to withdrawal of epidermal growth factor. Moreover, Noxa-deficient cells are partially resistant to DNA-damaged induced apoptosis. Overall, the redundancy of BH3-only proteins creates a robust control system that integrates responses to different stimuli.

After BH3-only proteins sense death stimuli, they need to activate Bax and Bak to initiate commitment to apoptosis. The first pro-apoptotic homolog, Bax, was first described as a protein that counteracted the pro-survival function of Bcl-2. Overexpression of Bax or the addition of purified recombinant Bax accelerates apoptosis. In healthy cells, Bax exists as a monomer in the cytosol or loosely associated with membranes. Death stimuli cause a conformational change, allowing exposure of the hydrophobic grove that is otherwise hindered by the C-terminal helix of Bax. This is immediately followed by the translocation and insertion of Bax proteins into the mitochondrial outer membrane as oligomers. Inactive Bak exists as an integral membrane protein in mitochondria. Bak is also induced to undergo conformational changes and oligomerization in response to apoptotic signals. Bax-deficient mice display hyperplasia of thymocytes and B cells as well as abnormalities in the development of the male reproductive system. Bak null mice show no developmental defects. However, when Bak- and Bax-deficient mice are intercrossed, a more marked phenotype is seen in the double-knockout. Fewer than 10% of the animals survive into adulthood, and those that do display multiple developmental defects. Furthermore, the double knockout cells are resistant to multiple death stimuli. It is evident from this that the combined pro-apoptotic functions of Bax and Bak are crucial for normal tissue development.

Bax and Bak in concert are an essential gateway for activation of caspases in the intrinsic apoptotic pathway, but there are debates as to how BH3-only proteins lead to the activation of Bax and Bak. The direct binding model suggests that the “activators” of BH3-only proteins such as Bid and Bim can directly activate Bax and Bak. Bad or Bik act as “sensitizers,” that sequester the pro-survival proteins and allow unbound Bax and Bak to oligomerize. Inconsistent observations for binding of endogenous Bid and Bim to Bax or Bak lead to the suggestion that they operate at a “hit-and-run” fashion. The displacement model suggests that BH3-only proteins can displace the binding of Bax or Bak to pro-survival proteins that sequester their active forms. Chen et al recently demonstrated the differential affinity of BH3-only proteins for pro-survival proteins. Using peptides mapped to BH3 sequence of BH3-only proteins, it was shown that certain molecules such as Puma and Bim can bind to all pro survival proteins. Bad can counteract Bcl-2 and Bcl-XL but not Mcl-1 while Noxa complements only by interacting with Mcl-1. By engineering Noxa to enhance promiscuous binding to all pro-survival proteins, effective killing occurred. This was further supported by the observation of Willies et al showing that both Noxa and Bad are required to neutralize Mcl-1 and Bcl-XL respectively to drive efficient apoptosis mediated by Bak. A recent report showed that Bax and Bak can mediate apoptosis without discernable association with the putative BH3-only activators (Bim, Bid, and Puma), even in cells with no Bim or Bid and reduced Puma while others showed mitochondrial permeabilization relies on BH3-only proteins engaging pro-survival Bcl-2 relatives and not Bak. These results further support the notion that BH3-only proteins induce apoptosis at least primarily by engaging the multiple pro-survival relatives guarding Bax and Bak.