Ugo1 is distinctly required for both mitochondrial outer and inner membrane fusion. (A) A schematic representation of the in vitro mitochondrial outer membrane fusion assay is shown. The graph is a comparison of outer membrane in vitro fusion efficiency at the nonpermissive temperature (37°C) of mitochondria isolated from UGO1, ugo1^ts, fzo1-1, and mgm1-5 strains. The two classes of fusion mutants are represented by gray and black bars for mitochondrial outer membrane (MOM) and mitochondrial inner membrane (MIM) fusion deficient, respectively. The absolute values of the mean of three independent experiments (percentage of total mitochondria, n > 350) are UGO1, 5.2; ugo1-1, 2.5; ugo1-2, 2.9; ugo1-3, 3.2; ugo1-4, 5.3; ugo1-5, 5.9; fzo1-1, 2.4; and mgm1-5, 4.9. (B) Representative electron micrograph of outer membrane–fused mitochondria observed in samples subjected to in vitro fusion at the nonpermissive temperature. The graph compares the abundance of outer membrane–fused mitochondria observed in electron micrographs. The mitochondria were isolated from UGO1, ugo1^ts, fzo1-1, and mgm1-5 strains and subjected to mitochondrial outer membrane (S1) in vitro fusion conditions at the nonpermissive temperature (37°C). The absolute values of the mean of three independent experiments (percentage of total mitochondria, n > 350) are UGO1, 7; ugo1-1, 3.3; ugo1-2, 3.2; ugo1-3, 2.7; ugo1-4, 7.9; ugo1-5, 5.6; fzo1-1, 0.8; and mgm1-5, 7.5. White bars show wild-type control strains. Data are represented as mean ± SEM. Bar, 0.5 µm.