Gag3p, an outer membrane protein required for fission of mitochondrial tubules

Mitochondrial morphology and function depend on MGM1, a Saccharomyces cerev isiae gene encoding a dynamin-like protein of the mitochondrial outer membr ane. Here, we show that mitochondrial fragmentation and mitochondrial genom e loss caused by lesions in MGM1 are suppressed by three novel mutations, g ag1, gag2, and gag3 (for glycerol-adapted growth). Cells with any of the ga g mutations displayed aberrant mitochondrial morphology characterized by el ongated, unbranched tubes and highly fenestrated structures. Additionally, each of the gag mutations prevented mitochondrial fragmentation caused by l oss of the mitochondrial fusion factor, Fzo1p, or by treatment of cells wit h sodium azide. The gag1 mutation mapped to DNM1 that encodes a dynamin-rel ated protein required for mitochondrial fission. GAG3 encodes a novel WD40- repeat protein previously found to interact with Dnm1p in a two-hybrid assa y. Gag3p was localized to mitochondria where it was found to associate as a peripheral protein on the cytosolic face of the outer membrane. This assoc iation requires neither the DNM1 nor GAG2 gene products. However, the local ization of Dnm1p to the mitochondrial outer membrane is substantially reduc ed by the gag2 mutation, but unaffected by loss of Gag3p. These results ind icate that Gag3p plays a distinct role on the mitochondrial surface to medi ate the fission of mitochondrial tubules.