| Description |
The morphology and function of mitochondria, the energy producing organelles of the eukaryotic cell, determine the fate of diverse cellular processes such as metabolic demand, embryonic development, and cell death. The cell uses a dedicated protein based machinery to divide mitochondria for distribution to daughter cells and to ensure faithful distribution of the mitochondrial genome. As a consequence, the impact of this division machinery applies to all processes that are influenced by the mitochondrial network. In mammals, this machinery is composed of the soluble protein Dynamin-Related Protein 1 and its membrane protein receptors Mitochondrial Dynamics 49kDa/51kDa (MiD49/51) and Mitochondrial Fission Factor (Mff). Drp1 forms assemblies that encircle the mitochondria. In addition, it binds and hydrolyzes the nucleotide guanosine triphosphate (GTP) which is thought to provide the necessary energy for mitochondrial membrane fission. Although the importance of these proteins to the mitochondrial fission process is established, the mechanisms by which the receptor proteins recruit and mediate Drp1 activity are unknown. In this dissertation, I present functional and structural analyses of the interaction between Drp1 and its receptor proteins. In Chapter 3, I was a part of a study that showed that each single receptor can recruit Drp1 to the membrane of the mitochondria and cause mitochondrial fission. In Chapter 4, I extended this finding and used cryogenic electron microscopy (cryo-EM) to determine structures of Drp1 bound to MiD49. These structures help us visualize Drp1 conformations in the recruited state on the mitochondrion and establish the role of nucleotide binding and hydrolysis on Drp1 activity. Specifically, we find that Drp1 assumes an extended conformational state upon nucleotide binding. This state enables Drp1 to bind to MiD49 and polymerize into filaments that are structurally suited to encircle mitochondrial tubules. Furthermore, the addition of GTP to this structure induces receptor dissociation and conversion to a ringlike state that is suited to constrict mitochondria. The dimensions of this ring-like state correspond to the Drp1 mediated constrictions observed in human cell cultures. Taken together, this work helps us understand the functional context for multiple receptors in mitochondrial fission and enables us to visualize the conformational dynamics of Drp1 required for its engagement with receptor proteins. |
