Division versus fusion: Dnm1p and Fzo1p antagonistically regulate mitochondrial shape

In yeast, mitochondrial division and fusion are highly regulated during gro wth, mating and sporulation, yet the mechanisms controlling these activitie s are unknown, Using a novel screen, we isolated mutants in which mitochond ria lose their normal structure? and instead form a large network of interc onnected tubules, These mutants, which appear defective in mitochondrial di vision, all carried mutations in DNM1, a dynamin-related protein that local izes to mitochondria. We also isolated mutants containing numerous mitochon drial fragments, These mutants were defective in FZO1, a gene previously sh own to be required for mitochondrial fusion. Surprisingly, we found that in dnm1 fzo1 double mutants, normal mitochondrial shape is restored, Inductio n of Dnm1p expression in dnm1 fzo1 cells caused rapid fragmentation of mito chondria. We propose that dnm1 mutants are defective in the mitochondrial d ivision, an activity antagonistic to fusion. Our results thus suggest that mitochondrial shape is normally controlled by a balance between division an d fusion which requires Dnm1p and Fzo1p, respectively.