Mitochondrial structure and distribution are regulated by division and fusion events. Mitochondrial division is regulated by Dnm1/Drp1, a dynamin-related protein that forms helices around mitochondria to mediate fission. Little is known about what determines sites of mitochondrial fission within the mitochondrial network. The ER and mitochondria exhibit tightly coupled dynamics and have extensive contacts. We tested whether ER plays a role in mitochondrial division. We found that mitochondrial division occurred at positions where ER tubules contacted mitochondria and mediated constriction prior to Drp1 recruitment. Thus, ER tubules may play an active role in defining the position of mitochondrial division sites.

Mitochondria are derived from eubacteria; however, in most eukaryotes, novel mechanisms for the propagation of this organelle and its genome have evolved. This review focuses on what is currently known about the novel molecular machines that divide and fuse mitochondria.

Statement MITO-MAP, a high-density genetic interaction map in budding yeast, identifies a mitochondrial inner membrane–associated complex that promotes normal mitochondrial membrane organization and morphology.

Close proximities between organelles have been described for decades. However, only recently a specific field dealing with organelle communication at membrane contact sites has gained wide acceptance, attracting scientists from multiple areas of cell biology. The diversity of approaches warrants a unified vocabulary for the field. Such definitions would facilitate laying the foundations of this field, streamlining communication and resolving semantic controversies. This opinion, written by a panel of experts in the field, aims to provide this burgeoning area with guidelines for the experimental definition and analysis of contact sites. It also includes suggestions on how to operationally and tractably measure and analyze them with the hope of ultimately facilitating knowledge production and dissemination within and outside the field of contact-site research.

Mitochondria are dynamic organelles that undergo cycles of fission and fusion. The yeast dynamin-related protein, Dnm1, has been localized to sites of mitochondrial division. Using cryo-electron microscopy (cryo-EM), we have determined the three-dimensional structure of Dnm1 in a GTP-bound state. The 3D map reveals a unique helical assembly for Dnm1 when compared with dynamin, a protein involved in vesicle scission during endocytosis. We also show that upon GTP hydrolysis Dnm1 constricts liposomes and subsequently dissociates from the lipid bilayer. The magnitude of Dnm1 constriction is substantially larger than the decrease in diameter previously reported for dynamin. We postulate that the larger conformational change is mediated by a flexible Dnm1 structure that has limited interaction with the underlying bilayer. Together, our structural studies support a mechanochemical role for Dnm1 during mitochondrial division.