Dynamin, best studied for its role in clathrin-mediated endocytosis, is the prototypical member of a multi-domain GTPase family involved in fission and remodeling of multiple organelles. Dynamin alone can catalyze fission of membrane tubules and vesicle formation from planar lipid templates, and thus constitutes a minimal fission apparatus. Through extensive structure/function analyses using in vitro and in vivo assays we have identified the conformational changes and biochemical requirements for dynamin-catalyzed membrane fission. More recently, based on available structural information we have engineered conformationally trapped dynamin molecules and examined their ability to drive distinct stages in membrane remodeling and scission. Together these studies suggest a two-stage model for dynamin-catalyzed fission. In stage one, mechanochemical activities of assembled dynamin helices and wedging activity due to two hydrophobic loops in the membrane-interacting pleckstrin homology domain (PHD) constrict the underlying membrane. GTPase driven cycles of assembly/disassembly restrict and optimize the length of the assembled helical collars enforcing highly localized curvature stress. In the GTP-hydrolysis transition-state, the wedging activity of the tightly packed PHDs is increased and together they create a catalytic center that guides lipid remodeling and drives the formation of a hemi-fission intermediate. Resolution of this intermediate requires GTP hydrolysis and loosening of the otherwise stabilizing protein scaffold. Twisting and/or expansion of the scaffold likely also contribute to fission in stage two. Although dynamin is a self-contained, elegant fission machine, dynamin-catalyzed fission remains an inefficient and stochastic process that is presumably aided in vivo by coat and other curvature generating endocytic accessory proteins. Many current and former lab members including Hanna Damke, Dale Warnock, Jenny Hinshaw, Rajesh Ramachandran, Josh Chappie, Thomas Pucadyil, Ya-Wen Liu, Sylvia Neumann, Juha-Pekka Mattila and Saipraveen Srinivasan, as well as outstanding collaborators, including Vadim Frolov, Anna Shnyrova and Fred Dyda have contributed to these studies.
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