Apoptosis, a genetically controlled process of cell death, plays a central role in metazoan development and homeostasis (61,74). The apoptotic program is highly conserved during evolution, and striking similarities have been observed in the cell death programs of rodents, mammalian cells, Drosophila melanogaster, and Caenorhabditis elegans (28,81,85,91). In normally proliferating cells, the apoptotic program is actively suppressed or inactivated; however, withdrawal or inhibition of the apoptosis suppressor mechanisms triggers apoptotic pathways (16,37,86). One mechanism by which apoptotic gene products may be prevented from executing their effect may involve direct interaction with specific proteins that act to attenuate the function of the apoptotic activators or effectors. The interaction between antiapoptotic protein Bcl-2 and proapoptotic protein Bax illustrates this point: whether a cell undergoes apoptosis or not is dependent upon the relative levels of these two proteins; an excess of Bax will trigger apoptosis, whereas an excess of Bcl-2 will prevent apoptosis (38,66,68,89,90). Identification of other such antiapoptotic and proapoptotic protein pairs that dictate the survival of cells should enhance our understanding of the apoptosis process.Apoptosis is characterized by cell membrane blebbing, chromatin condensation, changes in nuclear architecture, and oligonucleosome-length DNA fragmentation (87, 88). The process of apoptotic cell death is triggered by diverse stimuli such as cytokines, withdrawal of growth factors, DNA damage, expression of oncogenes or immediate-early genes, and fluctuations in the levels of Bcl-2 family members (3,13,16,37,47,48,54,71,78,86,89). Certain apoptotic stimuli can sequentially activate the basal cell death machinery composed of initiator, amplifier, and effector proteases belonging to the interleukin-1-converting enzyme (ICE) subfamily or an ICE-related family (9,17,21,41,46,91). Downstream targets of these proteases include the ICE subfamily proteases themselves; nuclear enzymes poly(ADP-ribose) polymerase and DNA-dependent protein kinase, which are involved in DNA repair; the nuclear protein U1 ribonucleoprotein and nuclear lamins; and cytoplasmic components such as protein kinase C␦ and cytoskeleton components such as actin (cited in reference 21). However, it is unclear whether any of these cellular components are directly linked to the morphological changes associated with apoptosis.Prostate tissue, which is composed of androgen-dependent and -independent cells (8, 58), provides an excellent model system for studying apoptosis. Androgen ablation in animals leads to an elevation of intracellular calcium that subsequently results in apoptosis of the androgen-dependent but not of the androgen-independent prostatic cells (11,(42)(43)(44). However, apoptosis can be induced in androgen-independent cell cultures by artificially upregulating intracellular calcium with calcium ionophores (53, 69) or with thapsigargin (TG) (22), an