Mitochondrial Fission is Mediated by Conformational Changes in the Dynamin-related Protein, Dnm1

Abstract

DNM1, a dynamin-related protein, plays an essential role in mitochondrial fission in yeast. Dnm1 contains a GTPase, middle and GTPase-effector domains, similar to dynamin, but lacks the Pleckstrin homology (PH) and proline-rich domains that are essential for dynamin function during endocytosis. Previous studies show that Dnm1 forms spiral structures larger than dynamin (∼110 nm for Dnm1 vs. 50 nm for dynamin), and this increased diameter coincides with the size of mitochondrial constriction sites observed in vivo (1). Additionally, Dnm1 drives membrane tubulation and readily organizes into a helical array in the presence of liposomes. We have solved the three-dimensional structure of Dnm1-lipid tubes using single-particle helical reconstruction methods. These tubes have an overall diameter of ∼125 nm with close to 30 repeating subunits per turn. We observe no interaction between the Dnm1 protein and the lipid bilayer, consistent with the lack of a PH domain in Dnm1. The organization of the Dnm1 oligomer is similar to dynamin-lipid tubes, but significant differences exist that mediate an altered conformational change. When GTP is added to dynamin tubes, the helical array constricts from 50 nm to 40 nm (2). Dnm1 tubes, which have an initial diameter of ∼110 nm, rapidly constrict to less than 60 nm when GTP is added, and the protein quickly dissociates from the lipid bilayer. This work demonstrates that Dnm1 plays an active role in constricting the mitochondrial membrane during fission in a GTP-dependant manner. When compared with dynamin structures, Dnm1 provides a better understanding of the basic structural features essential for membrane fission.1. E. Ingerman et al., J Cell Biol 170, 1021 (2005).2. Y. J. Chen, et al., Nat Struct Mol Biol 11, 574 (2004).