The function of short OPA1 forms is linked to mitochondrial fission. (A) Mitochondria fuse in Yme1^−/− cells. Mitochondrial fusion was analyzed as in Fig. 2 using matrix-PA-GFP. (B) Quantification of mitochondrial fusion in Yme1l^−/− cells after 60 min. Results are represented in the form of a box plot (see Fig. 2). WT versus Yme1l^−/− cells, P = 0.14. (C) Expression of S-OPA1 forms in Yme1l^−/−Oma1^−/− cells induces mitochondrial fragmentation. Mitochondrial morphology was assessed in Yme1l^−/−Oma1^−/− MEFs expressing Flag-tagged variants of rat S-OPA1-Flag or S-OPA1^K301A-Flag (bottom) harboring a mutation in the GTPase domain of OPA1. (D) Quantification of three independent experiments (error bars indicate mean ± SD). n ≥ 100; **, P ≥ 0.01. (E) Mitochondrial fusion occurs in fragmented mitochondria of WT and Yme1l^−/−Oma1^−/− cells expressing S-OPA1-Flag. (F) Quantification of mitochondrial fusion after 60 min. Results are shown in the form of a box plot (see Fig. 2). Shown are WT/Yme1l^−/−Oma1^−/− cells versus WT/Yme1l^−/−Oma1^−/− cells expressing rat S-OPA1. Bars, 15 µm.