Abstract

Mitochondrial dynamics resulting from competing membrane fusion and fission reactions are required for normal cellular function in eukaryotes. Mgm1p, a dynamin-related protein, is a key component in yeast mitochondrial fusion and is evolutionarily conserved. Previous studies suggest that Mgm1p mediates mitochondrial inner membrane fusion in a manner similar to that of other dynamin proteins that use GTP hydrolysis and oligomerization to induce structural changes in lipid bilayers; however, a direct demonstration of these activities has yet to be presented. Here we show that purified Mgm1p forms low-order oligomers that are dependent on protein concentration, suggesting a dynamic and reversible interaction. We further demonstrate that Mgm1p has GTPase activity and kinetic properties consistent with a mechanoenzyme and with a role in inner membrane mitochondrial fusion. Mutations of key residues in conserved motifs of the GTPase domain show markedly reduced or diminished GTPase activity. A mutation in the GTPase effector domain, involved in assembly and assembly-stimulated GTP hydrolysis, has basal GTPase activity similar to that of wild-type Mgm1p but has a weaker propensity to form oligomers. Finally, our data indicate that Mgm1p interacts specifically with negatively charged phospholipids found in mitochondrial membranes, and point mutations in the predicted lipid-binding domain abrogate these interactions. These findings suggest the presence of a putative lipid-binding domain, providing insight into how this protein mediates inner membrane fusion. Together, these data indicate that Mgm1p mediates fusion through oligomerization, GTP hydrolysis, and lipid binding in a manner similar to those of other dynamin mechanoenzymes.