Interactions of the BcI-2 protein with itself and other members of the Bcl-2 family, including Bcl-X-L, Bcl-X-S, Mci-i, and Bax, were explored with a yeast twohybrid system. Fusion proteins were created by linking BcI-2 family proteins to a LexA DNA-binding domain or a B42 trans-activation domain. Protein-protein interactions were examined by expression of these fusion proteins in Saccharomyces cerevisiae having a lacZ ((-galactosidase) gene under control of a LexA-dependent operator. This approach gave evidence for Bcl-2 protein homodimerization. Bcl-2 also interacted with Bcl-X-L and Mcl-i and with the dominant inhibitors Bax and Bcl-X-S. Bd-X-L displayed the same pattern of combinatorial interactions with Bd-2 family proteins as Bc1-2. Use of deletion mutants of Bc-2 suggested that BcI-2 homodimerization involves interactions between two distinct regions within the Bcl-2 protein, since a LexA protein containing Bcl-2 amino acids 83-218 mediated functional interactions with a B42 fusion protein contaiing Bcl-2 amino acids 1-81 but did not complement a B42 fusion protein containing BcI-2 amino acids 83-218. In contrast to LexA/Bcl-2 fusion proteins, expression of a LexA/Bax protein was lethal to yeast. This cytotoxicity could be abrogated by B42 fusion proteins containing BcI-2, Bcl-X-L, or Mci-i but not those containing Bcl-X-S (an alternatively spliced form of Bcl-X that lacks a well-conserved 63-amino acid region). The findings suggest a model whereby Bax and Bcl-X-S differentially regulate Bcd-2 function, and indicate that requirements for Bcl-2/Bax heterodimerization may be different from those for Bcl-2/Bcl-2 homodimerization.The bcl-2 gene becomes dysregulated in a wide variety of human cancers and contributes to neoplastic cell expansion by prolonging cell survival rather than by accelerating rates of cellular proliferation. Specifically, bcl-2 blocks programmed cell death, a physiological process that normally ensures a homeostatic balance between cell production and cell turnover in most tissues with self-renewal capacity and which often involves characteristic changes in cell morphology termed apoptosis. In fact, Bc1-2 can prevent or delay apoptosis induced by a wide variety of stimuli, including growth factor deprivation, alterations in Ca2+, free radicals, cytotoxic lymphokines, some types of viruses, radiation, and most chemotherapeutic drugs, suggesting that this oncoprotein controls a common final pathway involved in cell death regulation (reviewed in refs. 1 and 2).The mechanism by which Bcl-2 prevents cell death remains enigmatic, as the predicted amino acid sequence of the 26-kDa human Bcl-2 protein (239 aa) has no significant homology with other proteins whose biochemical activity is known. Recently, however, Bcl-2 has been shown to interact with a low molecular weight GTPase member of the Ras family, p23-R-Ras (3), and also can be coimmunoprecipitated with the serine/threonine-specific protein kinase Raf-1 (4). Thus, Bcl-2 may somehow regulate a signal transduction pathway involving...

The BCL-2 gene was first discovered because of its involvement in the t(14;18) chromosomal translocations commonly found in lymphomas, which result in deregulation of BCL-2 gene expression and cause inappropriately high levels of Bcl-2 protein production. Expression of the BCL-2 gene can also become altered in human cancers through other mechanisms, including loss of the p53 tumor suppressor which normally functions as a repressor of BCL-2 gene expression in some tissues. Bcl-2 is a blocker of programmed cell death and apoptosis that contributes to neoplastic cell expansion by preventing cell turnover caused by physiological cell death mechanisms, as opposed to accelerating rates of cell division. Overproduction of the Bcl-2 protein also prevents cell death induced by nearly all cytotoxic anticancer drugs and radiation, thus contributing to treatment failures in patients with some types of cancer. Several homologs of Bcl-2 have recently been discovered, some of which function as inhibitors of cell death and others as promoters of apoptosis that oppose the actions of the Bcl-2 protein. Many of these Bcl-2 family proteins can interact through formation of homo- and heterotypic dimers. In addition, several nonhomologous proteins have been identified that bind to Bcl-2 and that can modulate apoptosis. These protein-protein interactions may eventual serve as targets for pharmacologically manipulating the physiological cell death pathway for treatment of cancer and several other diseases.