Bcl-2 family member Bcl-G is not a proapoptotic protein.

The three major subgroups of the Bcl-2 family, including the prosurvival Bcl-2-like proteins, the proapoptotic Bcl-2 homology (BH)3-only proteins and Bax/Bak proteins, regulate the mitochondrial apoptotic pathway. In addition, some outliers within the Bcl-2 family do not fit into these subgroups. One of them, Bcl-G, has a BH2 and a BH3 region, and was proposed to trigger apoptosis. To investigate the physiological role of Bcl-G, we have inactivated the gene in the mouse and generated monoclonal antibodies to determine its expression. Although two isoforms of Bcl-G exist in human, only one is found in mice. mBcl-G is expressed in a range of epithelial as well as in dendritic cells. Loss of Bcl-G did not appear to affect any of these cell types. mBcl-G only binds weakly to prosurvival members of the Bcl-2 family, and in a manner that is independent of its BH3 domain. To understand what the physiological role of Bcl-G might be, we searched for Bcl-G-binding partners through immunoprecipitation/mass spectroscopy and yeast-two-hybrid screening. Although we did not uncover any Bcl-2 family member in these screens, we found that Bcl-G interacts specifically with proteins of the transport particle protein complex. We conclude that Bcl-G most probably does not function in the classical stress-induced apoptosis pathway, but rather has a role in protein trafficking inside the cell.

Detection of Bcl-2 family member Bcl-G in mouse tissues using new monoclonal antibodies.

Bcl-G is an evolutionarily conserved member of the Bcl-2 family of proteins that has been implicated in regulating apoptosis and cancer. We have generated monoclonal antibodies that specifically recognise mouse Bcl-G and have used these reagents to analyse its tissue distribution and subcellular localisation using western blotting, immunohistochemistry and immunofluorescence. We found that Bcl-G predominantly resides in the cytoplasm and is present in a wide range of mouse tissues, including the spleen, thymus, lung, intestine and testis. Immunohistochemical analyses revealed that Bcl-G is expressed highly in mature spermatids in the testis, CD8(+) conventional dendritic cells (DCs) in hematopoietic tissues and diverse epithelial cell types, including those lining the gastrointestinal and respiratory tracts. The Bcl-G monoclonal antibodies represent new tools for studying this protein, using a variety of techniques, including immunoprecipitation and flow cytometry.

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?