Orms (l-Mgm1; s-Mgm1) has been proposed to link mitochondrial bioenergetics and dynamics [31]. The selective inhibition of inner membrane fusion, and the lower DYm, prompted us to investigate whether the abundance or the isoform-pattern of Mgm1 were altered in OXPHOS deficient cells. Cells were grown in glucose or in galactose containing medium (conditions when mitochondrial biogenesis is repressed or not) and the isoform pattern of Mgm1 was analyzed by Westernblot. We observed that all strains contained similar amounts and isoform patterns of Mgm1. However, s-Mgm1 was slightly lower in ATP-synthase mutants and significantly higher in Dcox2 or r0 cells (Fig. 6B, C). Next we analyzed the isoform pattern in wild-type cells treated (or not) with valinomycin, a condition leading to the dissipation of DYm and to severe fusion inhibition (Fig. 1). Western-blot analysis revealed that the isoform pattern of Mgm1 was not significantly altered (Fig. 6A). The fact that fusion inhibition by defective OXPHOS or dissipation of DYm is not associated to a particular pattern of Mgm1-isoforms suggests that, in yeast, bioenergetic modulation of inner membrane fusion is not (solely) mediated by Mgm1-processing.Selective Inhibition of Inner Membrane Fusion Alters Mitochondrial UltrastructureThe fact that, in MedChemExpress Tubastatin A OXPHOS-deficient cells, fusion defects were not systematically associated to alterations of mitochondrial distribution and morphology (Supp. Fig. S3) led us to investigate mitochondrial ultrastructure. Mitochondrial outer and inner membranes can fuse in separate reactions [14,15], but most mitochondrial encounters result in the coordinated fusion of outer and inner membranes [16]. The selective inhibition of inner membrane fusion in ts-mutants of Mgm1 [15], or upon dissipation of the inner membrane potential [14], is accompanied by the appearance of unfused, elongated and aligned inner membranes (septae) 
that are connected to boundary membranes and separate matrix compartments (cf. Fig. 1C, D). In the mitochondria of wildtype yeast, cristae membranes are relatively short and connected to one boundary membrane (Fig. 7: WT). In the mitochondria of OXPHOS-deficient cells, we observed elongated aligned inner membranes that were connected to two mitochondrial boundaries and separated matrix compartments within mitochondria (Fig. 7, Table 3). In cells carrying the atp6-L183R mutation, elongated and aligned inner membranes were not observed at 28uC (Fig. 7, Table 3), but at 36u, when levels of Atp6 and of Indolactam V chemical information assembled ATPsynthase are lowered [32]. The similarity of elongated inner membranes in OXPHOS deficient mitochondria (Fig. 7) and in mitochondria with inhibited inner membrane fusion ([14,15] and Fig. 3C, D) suggest that their appearance is associated to the specific inhibition of inner membrane fusion and can serve as a hallmark for such fusion defects.Figure 
3. Deletion or mutation of OXPHOS genes inhibits mitochondrial fusion. Cells expressing matrix-targeted mtGFP or mtRFP were conjugated and the proportion of zygotes with Total (T), Partial (P) or No 16574785 fusion (N) was determined by fluorescence microscopy after the indicated times (A ) or after 4 hours (D). A: Fusion in strains devoid of mitochondrial COX2 (Dcox2) or mitochondrial DNA (r0). B: Fusion in strains with defects in ATP-synthase genes (Datp6, atp6-L183R, atp6-L247R, Datp12). C, D: Comparison of total fusion as a function of time (C) or of Total, Partial and No fusion after 4 hours (D) in wild-typ.Orms (l-Mgm1; s-Mgm1) has been proposed to link mitochondrial bioenergetics and dynamics [31]. The selective inhibition of inner membrane fusion, and the lower DYm, prompted us to investigate whether the abundance or the isoform-pattern of Mgm1 were altered in OXPHOS deficient cells. Cells were grown in glucose or in galactose containing medium (conditions when mitochondrial biogenesis is repressed or not) and the isoform pattern of Mgm1 was analyzed by Westernblot. We observed that all strains contained similar amounts and isoform patterns of Mgm1. However, s-Mgm1 was slightly lower in ATP-synthase mutants and significantly higher in Dcox2 or r0 cells (Fig. 6B, C). Next we analyzed the isoform pattern in wild-type cells treated (or not) with valinomycin, a condition leading to the dissipation of DYm and to severe fusion inhibition (Fig. 1). Western-blot analysis revealed that the isoform pattern of Mgm1 was not significantly altered (Fig. 6A). The fact that fusion inhibition by defective OXPHOS or dissipation of DYm is not associated to a particular pattern of Mgm1-isoforms suggests that, in yeast, bioenergetic modulation of inner membrane fusion is not (solely) mediated by Mgm1-processing.Selective Inhibition of Inner Membrane Fusion Alters Mitochondrial UltrastructureThe fact that, in OXPHOS-deficient cells, fusion defects were not systematically associated to alterations of mitochondrial distribution and morphology (Supp. Fig. S3) led us to investigate mitochondrial ultrastructure. Mitochondrial outer and inner membranes can fuse in separate reactions [14,15], but most mitochondrial encounters result in the coordinated fusion of outer and inner membranes [16]. The selective inhibition of inner membrane fusion in ts-mutants of Mgm1 [15], or upon dissipation of the inner membrane potential [14], is accompanied by the appearance of unfused, elongated and aligned inner membranes (septae) that are connected to boundary membranes and separate matrix compartments (cf. Fig. 1C, D). In the mitochondria of wildtype yeast, cristae membranes are relatively short and connected to one boundary membrane (Fig. 7: WT). In the mitochondria of OXPHOS-deficient cells, we observed elongated aligned inner membranes that were connected to two mitochondrial boundaries and separated matrix compartments within mitochondria (Fig. 7, Table 3). In cells carrying the atp6-L183R mutation, elongated and aligned inner membranes were not observed at 28uC (Fig. 7, Table 3), but at 36u, when levels of Atp6 and of assembled ATPsynthase are lowered [32]. The similarity of elongated inner membranes in OXPHOS deficient mitochondria (Fig. 7) and in mitochondria with inhibited inner membrane fusion ([14,15] and Fig. 3C, D) suggest that their appearance is associated to the specific inhibition of inner membrane fusion and can serve as a hallmark for such fusion defects.Figure 3. Deletion or mutation of OXPHOS genes inhibits mitochondrial fusion. Cells expressing matrix-targeted mtGFP or mtRFP were conjugated and the proportion of zygotes with Total (T), Partial (P) or No 16574785 fusion (N) was determined by fluorescence microscopy after the indicated times (A ) or after 4 hours (D). A: Fusion in strains devoid of mitochondrial COX2 (Dcox2) or mitochondrial DNA (r0). B: Fusion in strains with defects in ATP-synthase genes (Datp6, atp6-L183R, atp6-L247R, Datp12). C, D: Comparison of total fusion as a function of time (C) or of Total, Partial and No fusion after 4 hours (D) in wild-typ.
