Dynamin family members are large GTPases that are associated with diverse cellular processes, including clathrin-mediated endocytosis, fusion and fission of mitochondria, division of chloroplasts and peroxisomes, cell division, and resistance to viral infections. The founding member, dynamin, plays a direct role in endocytosis by assembling around the necks of clathrin-coated pits in a helical array. The self-assembly of dynamin into oligomers stimulates its GTPase activity, which is necessary for efficient fission during endocytosis. Recent evidence suggests that a three-bundle helix, near the G domain, undergoes a dramatic hydrolysis-dependent conformational change that may function as a dynamin powerstroke. To understand how the powerstroke propagates through the helical assembly and contributes to membrane fission, we solved the helical structure of a transition-state-defective dynamin mutant, K44A, using cryo-electron microscopy and image processing techniques. The 3-dimensional map of K44A is a 2-start helix with an inner luminal diameter of 3.7 nm, reaching the theoretical limit for spontaneous fission. Computational docking of G domains with the three bundle helix into the 3D map, reveals that a GTP ground state of dynamin is sufficient to achieve this ‘super-constricted’ state and shows how the 2-start helical arrangement generates the most efficient packing of dynamin around the membrane neck. Additional structural and biochemical studies are underway to unravel the global conformational changes that are needed to move beyond the final super-constricted pre-fission state.