Yeast can undergo cell death accompanied by cellular markers of apoptosis. However, orthologs of classical mammalian apoptosis regulators appeared to be missing from the yeast genome, challenging a common mechanism of yeast and mammalian apoptosis. Here we investigate Yor197w, a yeast protein with structural homology to mammalian caspases, and demonstrate caspase-like processing of the protein. Hydrogen peroxide treatment induces apoptosis together with a caspase-like enzymatic activity in yeast. This response is completely abrogated after disruption and strongly stimulated after overexpression of Yor197w. Yor197w also mediates the death process within chronologically aged cultures, pointing to a physiological role in elimination of overaged cells. We conclude that Yor197w indeed functions as a bona fide caspase in yeast and propose the name Yeast Caspase-1 (YCA1, gene YCA1).

During the past years, yeast has been successfully established as a model to study mechanisms of apoptotic regulation. However, the beneficial effects of such a cell suicide program for a unicellular organism remained obscure. Here, we demonstrate that chronologically aged yeast cultures die exhibiting typical markers of apoptosis, accumulate oxygen radicals, and show caspase activation. Age-induced cell death is strongly delayed by overexpressing YAP1, a key transcriptional regulator in oxygen stress response. Disruption of apoptosis through deletion of yeast caspase YCA1 initially results in better survival of aged cultures. However, surviving cells lose the ability of regrowth, indicating that predamaged cells accumulate in the absence of apoptotic cell removal. Moreover, wild-type cells outlast yca1 disruptants in direct competition assays during long-term aging. We suggest that apoptosis in yeast confers a selective advantage for this unicellular organism, and demonstrate that old yeast cells release substances into the medium that stimulate survival of the clone.

Apoptosis is a highly coordinated cellular suicide program crucial for metazoan health and diseases. Although its increasing importance in cancer, neurodegenerative disorders and AIDS led to intense research and a better understanding of apoptosis, many details of its regulation or the apoptotic phenotypes are poorly understood. The complex regulatory network and the often contradictory results obtained with human cell lines made application of an easier model system desirable. Apoptosis in yeast promises to provide a better understanding of the genetics of apoptosis. During the past 2 years, scientists were successful in identifying new cell-death regulators of humans, plants and fungi using Saccharomyces cerevisiae. The finding of apoptotic phenotypes, even in protists, suggests that apoptosis developed in unicellular organisms long before the evolutionary separation between fungi, plants and metazoan animals occurred.

Yeast cells deleted for the SRO7/SOP1 encoded tumor suppressor homologue show increased sensitivity to NaCl stress. On exposure to growth-inhibiting NaCl concentrations, sro7⌬ mutants display a rapid loss in viability that is associated with markers of apoptosis: accumulation of reactive oxygen species, DNA breakage, and nuclear fragmentation. Additional deletion of the yeast metacaspase gene YCA1 prevents the primary fast drop in viability and diminishes nuclear fragmentation and DNA breakage. We also observed that NaCl induced loss in viability of wild-type cells is Yca1p dependent. However, a yeast strain deleted for both SRO7 and its homologue SRO77 exhibits NaCl-induced cell death that is independent on YCA1. Likewise, sro77⌬ single mutants do not survive better after additional deletion of the YCA1 gene, and both sro77⌬ and sro77⌬yca1⌬ mutants display apoptotic characteristics when exposed to growth-inhibiting salinity, suggesting that yeast possesses Yca1p-independent pathway(s) for apoptosis-like cell death. The activity of Yca1p increases with increasing NaCl stress and sro7⌬ mutants achieve levels that are higher than in wild-type cells. However, mutants lacking SRO77 do not enhance caspase activity when subject to NaCl stress, suggesting that Sro7p and Sro77p exert opposing effects on the cellular activity of Yca1p. INTRODUCTIONThe physiological cell death process of apoptosis is a morphologically distinct form of cellular suicide (Kerr, 2002), designed to remove potentially threatening or undesired cells (Vaux and Korsmeyer, 1999;Beers and McDowell, 2001). In animal cells, apoptosis occurs in an ordered series of event and is associated with activation of the programmed cell death machinery (Wyllie et al., 1980;Joza et al., 2002). Inappropriate regulation of apoptosis is linked to the pathogenesis of many human diseases, such as AIDS, cancer, autoimmune, and neurodegenerative disorders (Uren and Vaux, 1996). The process of programmed cell death can be triggered by a vast array of stimuli, and a complex network of regulators and effectors coordinates the process (Beers and McDowell, 2001;Joza et al., 2002). At the end of almost all apoptotic scenarios, a constellation of morphological characteristics emerge, including chromatin condensation, DNA fragmentation, flipping of phosphatidylserine to the outer leaflet of the plasma membrane, and breakage of the cells to small membrane-enclosed vesicles, so called apoptotic bodies (Wyllie et al., 1980). These structural features constitute the cytological hallmarks of apoptosis.The finding that cellular suicide occurs in most, if not all, multicellular organisms raises the question on how the mechanisms of apoptosis have evolved. Do these mechanisms have a common origin and have these processes first evolved in single-celled organisms? For unicellular organisms exposed to harsh conditions, cellular suicide may serve to propagate the genome to future generations if some of the members of a population are sacrificed to promote survival of others (Engelberg-Kulk...