Meiotic viral attenuation through an ancestral apoptotic pathway

Abstract

The programmed release of apoptogenic proteins from mitochondria is a core event of apoptosis, though ancestral roles of this phenomenon are not known. In mammals, one such apoptogenic protein is Endonuclease G (EndoG), a conserved nuclease that fragments the DNA of dying cells.In this work, we show that budding yeast executes meiotically programmed mitochondrial release of an EndoG homologue, Nuc1, during sporulation. In contrast to EndoG's ostensible pro-death function during apoptosis, Nuc1 mitochondrial release attenuates the cytosolic dsRNA mycovirus, Killer, protecting spores from a lethal accumulation of its encoded toxin. Our identification of cell-protective viral attenuation as a target of this rudimentary apoptotic pathway illuminates a primordial role for mitochondrial release of EndoG. One Sentence SummaryYeast sporulation induces release of mitochondrial endonuclease G to accomplish viral attenuation. Main TextMathematical models suggest that host-virus conflicts drove the evolution of programmed cell death (PCD) in single celled eukaryotes(1, 2). Unicellular yeast species harbor numerous cytosolic viruses that have no extracellular phase and are only vertically transmitted through cytoplasmic inheritance(3). The most comprehensively studied of these is Killer, a paired system of the L-A and M double stranded RNA (dsRNA) viruses in Saccharomyces cerevisiae. L-A produces a viral particle that houses and propagates M, which itself encodes a secreted toxin that kills neighboring uninfected cells and confers immunity to the host(3). Genetic studies reveal that Killer exists in clear conflict with its host, though how and if this relates to PCD or other developmental occurrences in yeast is not known(4).Yeast sporulation employs internal meiotic divisions and culminates in the development of spore progeny within the remnant of the mother cell(5). PCD of this remnant cell occurs as an intrinsic aspect of sporulation and is executed through vacuolar rupture, leading to mother cell autolysis(6, 7).Spores survive this process, called meiotic PCD, through coordinated development of their protective spore coats(6). Under conditions of reduced carbon, undeveloped meiotic nuclei are frequently swept up in meiotic PCD and their DNA is fragmented into nucleosomal ladders in a manner requiring NUC1, the yeast homolog of the EndoG family of mitochondrial nucleases (7). This finding is reminiscent of EndoG promoting DNA fragmentation of apoptotic cells following its mitochondrial release, though Strains and media. Standard S. cerevisiae genetic and strain manipulation techniques were used for strain construction and tetrad analysis (1). Refer to Table S3 for strains used in this paper. For tetrad dissections, sporulated strains were incubated in 3 mg/mL 20T Zymolyase (Seikagaku Glycobiology) for 15-20 minutes at room temperature and spread on 1% yeast extract, 2% peptone, 0.004% adenine, 2% dextrose (YPD) plates for dissecting.Plasmids. The p5472 NUC1 CEN/ARS URA3 MOBY plasmid was obtained from Brenda Andrews (2). Th...

Diverse innate immune factors protect yeast from lethal viral pathogenesis

“…These images revealed L-A dsRNA to accumulate in foci, reminiscent of "viral factory" sites of viral replication observed in human cells (Fig 1B and S1) 26 . Consistent with previous findings in other strain backgrounds, western blotting showed that Gag protein levels were elevated in nuc1∆ and ski3∆ mutants 12,18,27 , and we determined that nuc1∆ ski3∆ double mutants accumulated massively elevated Gag levels, confirming that NUC1 and SKI3 participate in separate antiviral pathways (Fig 2A).…”

“…We showed previously that L-A replication was held in check during sporulation by the mitochondrial DNA/RNA endonuclease Nuc1, a homolog of endonuclease G (EndoG) found in all eukaryotes and many prokaryotes 14 . EndoG is involved in mammalian programmed cell death that frequently occurs in virus infected cells, and programmed cell death is intrinsic to yeast sporulation [15][16][17][18] .…”

Gene-dependent yeast cell death pathway requires AP-3 vesicle trafficking leading to vacuole membrane permeabilization

“…autophagy, vesicle trafficking, cancer, neurodegeneration) and are ideal models for dissecting unicellular eukaryotic cell death. Many Saccharomyces cerevisiae genes have been reported to promote or inhibit death (Chaves et al, 2021), some with semblances to mammals (Fannjiang et al, 2004;Gao et al, 2019;Ivanovska and Hardwick, 2005). A common theme shared by the four bestcharacterized mammalian cell death pathways is membrane permeabilization carried out by pore-forming proteins such as BAX (apoptosis), MLKL (necroptosis), and gasdermins (pyroptosis), or by lipid peroxidation (ferroptosis), which typically mark a commitment or final step to cell death.…”

Infection by dsRNA viruses is associated with enhanced sporulation efficiency in Saccharomyces cerevisiae

“…The high variation in the sporulation efficiency of yeast carrying a virus could be determined by viral load, which plausibly influences host metabolic costs of endosymbiosis. Though not explored in this paper, the degree of coadaptation likely affects whether hosts have evolved stabilizing mechanisms, such as sanctions of viral load (Gao, Chau, Chowdhury, et al, 2019; Wickner & Edskes, 2015), in order to compensate for the metabolic costs of endosymbiosis.…”