The Role of Dynamin Family Members in Membrane Fission

Abstract

The dynamin family of proteins are required for numerous membrane fission and remodeling events throughout the eukaryotic cell. Dynamin is involved in the final stages of fission during clathrin‐mediated endocytosis, caveolae internalization, and vesiculation from the Golgi and recycling endosome, while Drp1 (dynamin‐related protein 1) is necessary for mitochondrial division. During membrane fission, dynamin or Drp1 is believed to wrap around the constricted site to facilitate vesiculation or organelle division. In support of this model purified dynamin has been shown to self‐assemble into spirals (50 nm diameter) and readily form dynamin‐lipid tubes, which constrict and fragment upon addition of GTP. The three‐dimensional structures of dynamin in the constricted and non‐constricted states were solved by cryo‐electron microscopy methods. Placement of the GTPase and Pleckstrin Homology crystal structures into the cryo‐EM densities revealed a twisting motion that suggests a corkscrew model for dynamin constriction. We are currently examining the structure and function of the Drp1 homologue in yeast, Dnm1, to determine if a common mechanism of action exists among dynamin family members. In collaboration with Dr. Jodi Nunnari (UC Davis), we have shown that Dnm1 assembles into large spirals with a diameter of ~110 nm, which is a similar diameter to the observed mitochondrial constriction sites seen in vivo. Dnm1 also assembles onto lipid and forms Dnm1 decorated tubes that constrict significantly upon GTP addition. A preliminary 3D map of Dnm1‐lipid tubes reveals a slightly different architecture to dynamin. For example, the Dnm1 map lacks a strong inner radial density near the lipid bilayer, which correlates well with the absence of a pleckstrin homology domain in Dnm1. Overall, these results suggest that although dynamin family members share common mechanochemical properties, the structure of each member vary to fit their unique function.