The possibility that single-cell organisms undergo programmed cell death has been questioned in part because they lack several key components of the mammalian cell death machinery. However, yeast encode a homolog of human Drp1, a mitochondrial fission protein that was shown previously to promote mammalian cell death and the excessive mitochondrial fragmentation characteristic of apoptotic mammalian cells. In support of a primordial origin of programmed cell death involving mitochondria, we found that the Saccharomyces cerevisiae homolog of human Drp1, Dnm1, promotes mitochondrial fragmentation/degradation and cell death following treatment with several death stimuli. Two Dnm1-interacting factors also regulate yeast cell death.

To promote cell survival, the antiapoptotic factor Bcl-xL both inhibits Bax-induced mitochondrial outer membrane permeabilization and stabilizes mitochondrial inner membrane ion flux and thus overall mitochondrial energetic capacity.

Summary Loss or duplication of chromosome segments can lead to further genomic changes associated with cancer. However, it is not known if only a select subset of genes is responsible for driving further changes. To determine if perturbation of any given gene in a genome suffices to drive subsequent genetic changes, we analyzed the yeast knockout collection for secondary mutations of functional consequence. Unlike wild type, most gene knockout strains were found to have one additional mutant gene affecting nutrient responses and/or heat-stress-induced cell death. Moreover, independent knockouts of the same gene often evolved mutations in the same secondary gene. Genome sequencing identified acquired mutations in several human tumor suppressor homologs. Thus, mutation of any single gene may cause a genomic imbalance with consequences sufficient to drive adaptive genetic changes. This complicates genetic analyses, but is a logical consequence of losing a functional unit originally acquired under pressure during evolution.

Genetic mutations affecting mitochondrial fission and fusion proteins cause human neurological disorders, but are assumed to be well tolerated in yeast. The conserved mitochondrial fission protein Dnm1/Drp1 is required for normal mitochondrial division, but also promotes cell death in mammals and yeast. Fis1, an outer mitochondrial membrane-anchored receptor for Dnm1/Drp1, also can promote cell death in mammals, but appears to have prosurvival activity in yeast. Here we report that deletion of the FIS1 gene in yeast consistently results in acquisition of a secondary mutation that confers sensitivity to cell death. In several independently derived FIS1 knockouts, tiling arrays and genomic sequencing identified the secondary mutation as a premature termination in the same stressresponse gene, WHI2. The WHI2 mutation rescues the mitochondrial respiratory defect (petite formation) caused by FIS1 deficiency, but also causes a failure to suppress cell growth during aminoacid deprivation. Thus, loss of Fis1 drives the selection for specific compensatory mutations that confer defective growth control and cell death regulation, characteristic of human tumor cells. The important long-term survival function of Fis1 that is compensated by WHI2 mutation appears to be independent of fission factor Dnm1/Drp1 and its adaptor Mdv1, but may be mediated through a second adaptor Caf4, as WHI2 is also mutated in a CAF4 knockout. KeywordsFis1; WHI2; programmed cell death; mitochondrial fission; apoptosis; Drp1Mitochondria are dynamic organelles that continuously undergo fission and fusion for organelle maintenance. 1 Mitochondrial fission and fusion events are mediated by distinct molecular complexes that are characterized best in the budding yeast Saccharomyces cerevisiae. 2 Deletion or depletion of the small, membrane-anchored fission protein Fis1 from yeast or mammalian cells results in a more connected mitochondrial network. Yeast Fis1 forms a complex with dynamin-related 1 (Dnm1), a dynamin-like GTPase conserved in animals (Drp1/DLP1) and plants (ADL2b) through either of two WD40-repeat adaptor proteins Mdv1 or its paralog Caf4. 2-4 Mammalian Drp1 facilitates normal synaptic activity in neurons, and a mutation in human Drp1 is linked to severe neurological deficits. 5-7 In addition to its critical Here we report that yeast Fis1 has a long-term survival function, as de novo deletion of the FIS1 gene in S. cerevisiae selects for chromosome abnormalities and mitochondrial deficits, but does not immediately result in sensitivity to cell death. Rather, the acute sensitivity to cell death is due to secondary mutations in whiskey 2 (WHI2), a stress-response gene, 23 that specifically arose in FIS1 knockouts but not in the mitochondrial fission-defective knockouts of DNM1 or MDV1. This process in yeast may share similarities to human tumorigenesis, where one mutation selects for additional mutations that confer loss of growth control despite the accompanying sensitivity to cell death induced by toxic chemicals such as anticancer...