Dynamin self-assembles into rings suggesting a mechanism for coated vesicle budding

DYNAMIN, a 100K member of the GTPase superfamily¹, is the mammalian homologue of the Drosophila shibire gene product^{2, 3}. Mutations in shibire cause a defect in endocytosis leading to accumulation of coated pits and deep imaginations at the plasma membrane of all tissues examined^{4, 5}. Similarly, invaginated coated pits accumulate in mammalian cells overexpressing dominant-negative mutants of dynamin, establishing that dynamin is required for the formation of ‘constricted’ coated pits and for coated vesicle budding⁶. Whether dynamin functions in the classic GTPase mode as a molecular switch to regulate events leading to coated vesicle budding or instead actively participates as a mechanochemical enzyme driving coated vesicle formation is unclear⁷. Here we show that dynamin spontaneously self-assembles into rings and stacks of interconnected rings, comparable in dimension to the ‘collars’ observed at the necks of invaginated coated pits that accumulate at synaptic terminals in shibire flies⁴. We propose that invaginated coated pits become constricted by the assembly of dynamin into rings around their necks. A concerted conformational change would then close the rings and pinch off the budding coated vesicles.