D Dynamin family members are large GTPases involved in membrane fission and fusion events throughout the cell. The founding member, dynamin, plays a major role in vesiculation events during endocytosis, synaptic membrane recycling, and membrane trafficking. The current model predicts dynamin wraps around the necks of coated pits and upon GTP hydrolysis dynamin constricts and disassociates from the lipid, which then leads to membrane fission. In support of this model, purified dynamin self-assembles into spirals around lipid, generating dynamin-lipid tubes that constrict, twist and fall off upon GTP addition. To determine the conformational changes that occur during GTP hydrolysis, we calculated 3D maps of dynamin by cryo-electron microscopy methods. Here we present our latest 3D map of ΔPRD-dynamin (resolution of 12 A) with three crystal structures docked into our map, the GMP-PCP GG domain (GTPase domain-GED fragment), the stalk domain from another dynamin family member, MxA, and the PH domain from dynamin. Based on the docking results, we predict the location and interactions between the domains. In addition, comparison between the GTP-bound state (GMP-PCP) and transition state (GDP.AlF4-) within the GG construct suggests that the conformational change induced by GTP hydrolysis drives a large swing of the BSE (bundle signaling element). We predict that the BSE movement is dynamin's power stroke that results in a significant twist and constriction of the underlying lipid bilayer leading to membrane fission. Recently, we have calculated a 3D map of full-length dynamin in a further constricted state, with a resolution of ∼15A. The inner lumenal diameter of this structure is ∼2-4 nm, a range that is compatible with spontaneous lipid fusion. Currently we are docking the crystal structures into our K44A-dyn map to identify changes within dynamin domains that leads to maximum constriction and ultimately membrane fission.