Bcl-xES, a BH4- and BH2-containing antiapoptotic protein, delays Bax oligomer formation and binds Apaf-1, blocking procaspase-9 activation.

Bcl-2 family members either negatively or positively regulate the apoptotic threshold of cells. Bcl-xES (extra short), a novel Bcl-x member, possesses a unique combination of BH4 and BH2 domains as well as a COOH-terminal hydrophobic transmembrane anchor domain. Bcl-xES contains sequences of hydrophobic alpha-6 helices but lacks sequences of alpha-5 helices, suggesting that it does not have pore channel-forming activity but functions uniquely as a trapping protein. mRNA expression analysis by reverse transcriptase-polymerase chain reaction and RNase protection assay reveal that Bcl-xES is expressed in a variety of human cancer cell lines and human tumors, including bone marrow from patients with acute lymphoblastic leukemia. Bcl-xES expression is much less pronounced in some specimens of normal human tissues, including the breast, ovary, testis and lung. Stable, transfected human B lymphoma Namalwa variant cells expressing Bcl-xES were derived to investigate its role in apoptosis. Bcl-xES had a preventive effect on cell death induced by tumor necrosis factor-alpha and various concentrations of anticancer drugs, including camptothecin, etoposide and cisplatin. Its protective action on cell death was correlated with the inhibition of mitochondrial cytochrome c release and caspase activation. In a yeast two-hybrid system, Bcl-xES interacted with most Bcl-2 family members, including those containing only a BH3 domain, and with the Ced-4 homolog Apaf-1. Co-immunoprecipitation and gel filtration chromatography experiments suggest that Bcl-xES delays drug-induced apoptosis by disturbing the formation of Bax oligomers and preventing cytochrome c release, but also by interacting with Apaf-1 and inhibiting procaspase-9 activation, thus averting the apoptogenic proteolytic caspase cascade and cell death.