Inhibitors of apoptosis (IAPs) are a family of proteins that bear baculoviral IAP repeats (BIRs) and regulate apoptosis in vertebrates and Drosophila melanogaster. The yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe both encode a single IAP, designated BIR1 and bir1, respectively, each of which bears two BIRs. In rich medium, BIR1 mutant S. cerevisiae underwent normal vegetative growth and mitosis. Under starvation conditions, however, BIR1 mutant diploids formed spores inefficiently, instead undergoing pseudohyphal differentiation. Most spores that did form failed to survive beyond two divisions after germination. bir1 mutant S. pombe spores also died in the early divisions after spore germination and became blocked at the metaphase͞ anaphase transition because of an inability to elongate their mitotic spindle. Rather than inhibiting caspase-mediated cell death, yeast IAP proteins have roles in cell division and appear to act in a similar way to the IAPs from Caenorhabditis elegans and the mammalian IAP Survivin.Apoptosis is implemented by a mechanism highly conserved among metazoans. A number of mammalian cell death proteins resemble those from insects and nematodes both in structure and function, and some, such as members of the Bcl-2, caspase, and inhibitor of apoptosis (IAP) families are able to act in heterologous organisms (1), suggesting an ancient origin of the effector and control mechanisms of cell death.Although examples of cell-suicide mechanisms have been described in single-celled organisms (reviewed in ref.2), it is not yet known whether any similarity exists between the mechanisms of cell death in metazoans and unicellular organisms. In single-celled organisms, homologs of cell death molecules may be involved in cell death or may have unrelated roles. In either case, analysis of such proteins may reveal clues as to how they function. Although no homologs of Bcl-2 or caspases have been identified within single-celled organisms, by using low-stringency database searches we have identified homologs of IAP proteins in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe (3). We designated these genes BIR1 and bir1, respectively, because they bear a pair of BIR (baculovirus IAP repeat) motifs (4) within their amino termini.IAP proteins are a family of cell death inhibitors identified in baculoviruses, where they prevent defensive apoptosis of the host cell and thereby promote viral replication (5). Function of these baculoviral IAPs is conserved, because they also were able to inhibit apoptosis of mammalian cells (6), and most of the cellular homologs of IAPs identified in Drosophila and vertebrates are also cell death inhibitors (7-10). Nevertheless, it is possible that some IAPs have other functions, as at least one IAP from Caenorhabditis elegans is probably not involved in apoptosis but is required for cytokinesis during the first cell divisions after fertilization (11).Here we describe the phenotype of BIR1 and bir1 deletion mutants and localization of the prot...

How Bcl-2 and its pro-survival relatives prevent activation of the caspases that mediate apoptosis is unknown, but they appear to act through the caspase activator apoptosis protease–activating factor 1 (Apaf-1). According to the apoptosome model, the Bcl-2–like proteins preclude Apaf-1 activity by sequestering the protein. To explore Apaf-1 function and to test this model, we generated monoclonal antibodies to Apaf-1 and used them to determine its localization within diverse cells by subcellular fractionation and confocal laser scanning microscopy. Whereas Bcl-2 and Bcl-xL were prominent on organelle membranes, endogenous Apaf-1 was cytosolic and did not colocalize with them, even when these pro-survival proteins were overexpressed or after apoptosis was induced. Immunogold electron microscopy confirmed that Apaf-1 was dispersed in the cytoplasm and not on mitochondria or other organelles. After the death stimuli, Bcl-2 and Bcl-xL precluded the release of the Apaf-1 cofactor cytochrome c from mitochondria and the formation of larger Apaf-1 complexes, which are steps that presage apoptosis. However, neither Bcl-2 nor Bcl-xL could prevent the in vitro activation of Apaf-1 induced by the addition of exogenous cytochrome c. Hence, rather than sequestering Apaf-1 as proposed by the apoptosome model, Bcl-2–like proteins probably regulate Apaf-1 indirectly by controlling upstream events critical for its activation.