Abstract. Yeast cells with the mdm/0 mutation possess giant spherical mitochondria and are defective for mitochondrial inheritance. The giant mitochondria display classical features of mitochondrial ultrastructure, yet they appear incapable of movement or division. Genetic analysis indicated that the mutant phenotypes resulted from a single nuclear mutation, and the isolated MDMIO gene restored wild-type mitochondrial distribution and morphology when introduced into mutant cells. MDMIO encodes a protein of 56.2 kD located in the mitochondrial outer membrane. Depletion of Mdml0p from cells led to a condensation of normally extended, tubular mitochondria into giant spheres, and reexpression of the protein resulted in a rapid restoration of normal mitochondrial morphology. These results demonstrate that Mdml0p can control mitochondrial morphology, and that it plays a role in the inheritance of mitochondria. MITOCHONDRIAL inheritance is an essential component of cell proliferation (Yaffe, 1991b). This inheritance requires the growth and division of preexisting mitochondria and the distribution of mitochondria between daughter cells before cell division (Attardi and Schatz, 1988). Cytoskeletal elements have been implicated in the positioning and movement of mitochondria (Heggeness et al., 1978;Chen, 1988;McConnell and Yaffe, 1992;Drubin et al., 1993), but mechanisms underlying mitochondrial inheritance are poorly understood.Mitochondria are usually found as snakelike tubules, widely distributed in the eukaryotic cytoplasm (Tzagoloff, 1982;Bereiter-Hahn, 1990). Often, these tubules are interconnected into extended mitochondrial reticula (Stevens, 1981;Chen, 1988). Microscopic studies of live cells have revealed that these mitochondrial networks are extremely dynamic, with tubular processes undergoing frequent fragmentation, branching, and fusion, as well as redistribution within the cytoplasm (Bereiter-Hahn, 1990; Koning et ai., 1993). In addition to the dynamic properties of mitochondria found in most types of cells, certain pathological conditions lead to gross changes in mitochondrial morphology, including the development of giant mitochondria (Tandler and Hoppel, 1986;Inagaki et al., 1992). The molecular bases for these changes in mitochondrial morphology and distribution in both normal and diseased cells are unknown.We recently described several Saccharomyces cerevisiae mutants defective for mitochondrial inheritance (McConnell et al., 1990). These mitochondrial distribution and morphology (mdm) I mutants were identified by screening collections of temperature-sensitive strains by fluorescence microscopy for cells that failed to transfer mitochondria into daughter buds during incubation at the nonpermissive temperature. Analysis of two of these mutant strains led to the identification of a novel cytoskeletal component that mediates mitochondrial inheritance Yaffe, 1992, 1993), and it indicated a role for unsaturated fatty acids in mitochondrial movement (Stewart and Yaffe, 1991). Here, we describe a new mdm m...

Saccharomyces cerevisiae cells lacking the MDM12 gene product display temperature-sensitive growth and possess abnormally large, round mitochondria that are defective for inheritance by daughter buds. Analysis of the wild-type MDM12 gene revealed its product to be a 31-kD polypeptide that is homologous to a protein of the fission yeast Schizosaccharomyces pombe. When expressed in S. cerevisiae, the S. pombe Mdm12p homolog conferred a dominant-negative phenotype of giant mitochondria and aberrant mitochondrial distribution, suggesting partial functional conservation of Mdm12p activity between budding and fission yeast. The S. cerevisiae Mdm12p was localized by indirect immunofluorescence microscopy and by subcellular fractionation and immunodetection to the mitochondrial outer membrane and displayed biochemical properties of an integral membrane protein. Mdm12p is the third mitochondrial outer membrane protein required for normal mitochondrial morphology and distribution to be identified in S. cerevisiae and the first such mitochondrial component that is conserved between two different species.