Dynamin is a large, extended protein consisting of five domains. We have determined the 2.0 Angstrom resolution crystal structure of a human dynamin 1-derived minimal GTPase-GED (for GTPase effector domain) fusion protein. This fusion protein dimerizes in the presence of the transition state mimic GDP-aluminum fluoride. The structure reveals dynamin's catalytic machinery and explains how assembly-stimulated GTP hydrolysis is achieved through G domain dimerization. Particularly intriguing was the identification of a sodium ion in the active site, suggesting that dynamin uses a cation to compensate for the developing negative in the transition state and providing a rationale for the inability to previously implicate a more-usual arginine finger. The structure of this fusion domain allows us to provide a model for the role of dimerization during dynamin-catalyzed membrane fission. Recently, we have determined the structure of the GTPase-GED fusion with a ground state GTP analog. The structure showed a surprising reorientation of a part of the molecule. We believe that movement may have significance in transmitting conformational information toward the membrane. Durieux, A.C., Prudhon, B., Guicheney, P., and Bitoun, M. (2010) J. Mol. Med. 88, 339-350.