Of septae/mitochondria 0 0,76 1,15 0,03 0,54 0,27 0,atp6-L247R atp6-L183RDatp12 Dcox2 rYeast cells of the indicated genotypes were fixed and analyzed by electron microscopy and mitochondria were analyzed for the presence of septae*, elongated and aligned inner membrane membranes that are connected to two boundary membranes and separate matrix compartments. All OXPHOS-deficient mitochondria, except atp6-L183R, display inner membrane septae. doi:10.1371/journal.pone.0049639.tenergetic requirements. The requirement of DYm for inner, and not outer membrane fusion, suggests that the observed fusion inhibition is related to the lower DYm in OXPHOS deficient cells. However, given the interdependence of respiration, ATP-synthesis and DYm, we cannot exclude 
that other parameters (like altered matrix ATP-levels [34]) also contribute to fusion inhibition. Surprisingly, fusion inhibition was not systematically associated to major alterations in mitochondrial distribution and morphology, implying that such fusion defects escape (and have escaped) detection in studies that were solely based on the analysis of mitochondrial morphology. Similarly, cells devoid of subunit e of the ATP-synthase (tim11/atp21), defective in ATP-synthase oligomerization, showed significant alterations of mitochondrial ultrastructure [35] that were not paralleled by defects in overall morphology or fusion [33]. The fact that major alterations in overall distribution and morphology were restricted to Datp6 and atp6-L247R strains, suggests that this phenotype is associated to the low levels of Atp6 protein rather than to a defect in fusion. In addition, it is interesting to note that among the mutants identified in the screen for altered mitochondrial distribution and morphology (n = 131), only 9 encoded OXPHOS-related proteins, and of those, 8 were components or assembly factors of ATP-synthase [13]. Further work is required to unravel the exact links between ATP-synthase and mitochondrial ultrastructure, morphology and/ or dynamics. In mammals, the inhibition of fusion by bioenergetic defects and/or loss of DYm is paralleled by fast and quantitative changes in the isoform-pattern of OPA1 [18,30]. We observed that, in yeast, the patterns of Mgm1-isoforms varied somewhat between strains and culture conditions, but that these variations did not correlate with the fusion capacity. We conclude that, in OXPHOS-deficient strains, fusion capacity was not lowered NT 157 through changes in the isoform pattern of Mgm1. The fact that fusion inhibition by dissipation of DYm was not associated to changes in the isoform pattern of Mgm1 further indicates that, in yeast, a factor other than Mgm1 requires DYm for inner membrane fusion. This points to differences in the properties and regulation of mitochondrial fusion and Mgm1/OPA1 in yeast and in mammals.and turnover. Current models of mitochondrial biogenesis and maintenance RE640 include the hypothesis (1) that defective mitochondria have a lower fusion capacity, (2) that this leads to their exclusion from the network of functional mitochondria and (3) that this facilitates their selective degradation by autophagy [8,18]. The dominant inhibition of fusion demonstrated in this work provides a mechanism for the exclusion of defective mitochondria (from the network of functional mitochondria) and thus for the selective degradation of mitochondria (and mutant mtDNA) by autophagy. Further work is required to elucidate the complex relationships between mitoc.Of septae/mitochondria 0 0,76 1,15 0,03 0,54 0,27 0,atp6-L247R atp6-L183RDatp12 Dcox2 rYeast cells of the indicated genotypes were fixed and analyzed by electron microscopy and mitochondria were analyzed for the presence of septae*, elongated and aligned inner membrane membranes that are connected to two boundary membranes and separate matrix compartments. All OXPHOS-deficient mitochondria, except atp6-L183R, display inner membrane septae. doi:10.1371/journal.pone.0049639.tenergetic requirements. The requirement of DYm for inner, and not outer membrane fusion, suggests that the observed fusion inhibition is related to the lower DYm in OXPHOS deficient cells. However, given the interdependence of respiration, ATP-synthesis and DYm, we cannot exclude that other parameters (like altered matrix ATP-levels [34]) also contribute to fusion inhibition. Surprisingly, fusion inhibition was not systematically associated to major alterations in mitochondrial distribution and morphology, implying that such fusion defects escape (and have escaped) detection in studies that were solely based on the analysis of mitochondrial morphology. Similarly, cells devoid of subunit e of the ATP-synthase (tim11/atp21), defective in ATP-synthase oligomerization, showed significant alterations of mitochondrial ultrastructure [35] that were not paralleled by defects in overall morphology or fusion [33]. The fact that major alterations in overall distribution and morphology were restricted to Datp6 and atp6-L247R strains, suggests that this phenotype is associated to the low levels of Atp6 protein rather than to a defect in fusion. In addition, it is interesting to note that among the mutants identified in the screen for altered mitochondrial distribution and morphology (n = 131), only 9 encoded OXPHOS-related proteins, and of those, 8 were components or assembly factors of ATP-synthase [13]. Further work is required to unravel the exact links between ATP-synthase and mitochondrial ultrastructure, morphology and/ or dynamics. In mammals, the inhibition of fusion by bioenergetic defects and/or loss of DYm is paralleled by fast and quantitative changes in the isoform-pattern of OPA1 [18,30]. We observed that, in yeast, the patterns of Mgm1-isoforms varied somewhat between strains and culture conditions, but that these variations did not correlate with the fusion capacity. We conclude that, in OXPHOS-deficient strains, fusion capacity was not lowered through changes in the isoform pattern of Mgm1. The fact that fusion inhibition by dissipation of DYm was 
not associated to changes in the isoform pattern of Mgm1 further indicates that, in yeast, a factor other than Mgm1 requires DYm for inner membrane fusion. This points to differences in the properties and regulation of mitochondrial fusion and Mgm1/OPA1 in yeast and in mammals.and turnover. Current models of mitochondrial biogenesis and maintenance include the hypothesis (1) that defective mitochondria have a lower fusion capacity, (2) that this leads to their exclusion from the network of functional mitochondria and (3) that this facilitates their selective degradation by autophagy [8,18]. The dominant inhibition of fusion demonstrated in this work provides a mechanism for the exclusion of defective mitochondria (from the network of functional mitochondria) and thus for the selective degradation of mitochondria (and mutant mtDNA) by autophagy. Further work is required to elucidate the complex relationships between mitoc.