Phospholipid Association Is Essential for Dynamin-related Protein Mgm1 to Function in Mitochondrial Membrane Fusion

Mgm1, the yeast ortholog of mammalian OPA1, is a key component in mitochondrial membrane fusion and is required for maintaining mitochondrial dynamics and morphology. We showed recently that the purified short isoform of Mgm1 (s-Mgm1) possesses GTPase activity, self-assembles into low order oligomers, and interacts specifically with negatively charged phospholipids (Meglei, G., and McQuibban, G. A. (2009) Biochemistry 48, 1774–1784). Here, we demonstrate that s-Mgm1 binds to a mixture of phospholipids characteristic of the mitochondrial inner membrane. Binding to physiologically representative lipids results in ∼50-fold stimulation of s-Mgm1 GTPase activity. s-Mgm1 point mutants that are defective in oligomerization and lipid binding do not exhibit such stimulation and do not function in vivo. Electron microscopy and lipid turbidity assays demonstrate that s-Mgm1 promotes liposome interaction. Furthermore, s-Mgm1 assembles onto liposomes as oligomeric rings with 3-fold symmetry. The projection map of negatively stained s-Mgm1 shows six monomers, consistent with two stacked trimers. Taken together, our data identify a lipid-binding domain in Mgm1, and the structural analysis suggests a model of how Mgm1 promotes the fusion of opposing mitochondrial inner membranes.Mitochondrial dynamics have been implicated in neurodegenerative diseases such as dominant optic atrophy and Parkinson disease (1, 2). Mitochondrial morphology is regulated by balanced membrane fusion and fission reactions that are orchestrated by members of the highly conserved dynamin-related protein family (3). Dynamin-related proteins are large GTPases that can self-assemble and promote membrane remodeling (4, 5). We have shown previously that the dynamin-related protein Mgm1 has GTPase activity, self-assembles into low order oligomers, and binds to negatively charged phospholipids (6). Mgm1 exists as two isoforms in the mitochondria; l-Mgm1² is anchored to the IM via a transmembrane domain, and s-Mgm1 is peripherally associated with the IM and also found in the intermembrane space. s-Mgm1 results from the regulated cleavage by the mitochondrial rhomboid protease (7, 8). It was shown recently that both isoforms are essential but have distinct roles in mitochondrial membrane fusion whereby only s-Mgm1 requires its GTPase activity (9). It is proposed that l-Mgm1 serves as a receptor for s-Mgm1 to mediate fusion of opposing membranes upon GTP hydrolysis. Here, we provide molecular data indicating that lipid binding of s-Mgm1 is required for proper membrane fusion. Furthermore, structural analysis of s-Mgm1 assembled onto liposomes suggests a model whereby stacked trimers of s-Mgm1 on opposing membranes would facilitate fusion.