press). Mgm1 and its orthologs exist in two forms of different lengths. To obtain new insights into their biogenesis and function, we have characterized these isoforms. The large isoform (l-Mgm1) contains an N-terminal putative transmembrane segment that is absent in the short isoform (sMgm1). The large isoform is an integral inner membrane protein facing the intermembrane space. Furthermore, the conversion of l-Mgm1 into s-Mgm1 was found to be dependent on Pcp1 (Mdm37/YGR101w) a recently identified component essential for wild type mitochondrial morphology. Pcp1 is a homolog of Rhomboid, a serine protease known to be involved in intercellular signaling in Drosophila melanogaster, suggesting a function of Pcp1 in the proteolytic maturation process of Mgm1. Expression of s-Mgm1 can partially complement the ⌬pcp1 phenotype. Expression of both isoforms but not of either isoform alone was able to partially complement the ⌬mgm1 phenotype. Therefore, processing of l-Mgm1 by Pcp1 and the presence of both isoforms of Mgm1 appear crucial for wild type mitochondrial morphology and maintenance of mitochondrial DNA.Mitochondria are dynamic structures. Their overall morphology and the shape of the inner membrane are highly variable (5). This is exemplified by a wide variety of mitochondrial ultrastructures observed in different organisms and tissues and by the dependence of these structures on the metabolic and genetic state of the cell. Mitochondrial dynamics has recently gained increasing attention, and quite a number of proteins affecting this structural diversity have been identified. Still, our understanding of the underlying molecular events is rather incomplete. Fusion and fission of mitochondrial membranes are processes that have to be balanced in order to maintain mitochondrial morphology (6). For example, when fusion of mitochondria is abolished, fission can still continue, and consequently mitochondria get progressively fragmented (reviewed in Ref. 7). The roles of a number of proteins such as the GTPase Fzo1 (8) and the dynamin-like GTPase Dnm1 (9) in fusion and fission are known. Less clear is the role of the mitochondrial dynamin-like protein Mgm1, which is another component essential for maintaining mitochondrial morphology in Saccharomyces cerevisiae.