Dnm1 self-assembles into spirals tailored to fit mitochondria.

Dynamin-related proteins regulate membrane dynamics throughout the cell. Ingerman et al. report on page 1021 that these proteins form large polymers that are structurally tailored to fit their individual jobs.

The canonical protein in the family, dynamin-1, constricts the necks of clathrin-coated pits during endocytosis. In the process, it assembles into rings that are 50 nm in diameter. By contrast, Ingerman et al. saw that Dnm1, a yeast protein required for mitochondrial fission, assembles into rings that are 109 nm across. That size closely matches the diameter of mitochondrial constrictions in vivo.

As in dynamin-1 assembly, GTP binding is required for Dnm1 ring formation. In the absence of GTP, Dnm1 assembles into long, slightly curved filaments, whereas in the presence of a nonhydrolyzable GTP analogue, it forms the spirals used in fission. There is a substantial lag between the addition of GTP and hydrolysis, which is a pattern of hydrolysis seen in filament proteins, such as actin, that have a rate-limiting nucleation step.It is possible that once the spirals form around a mitochondrion, Dnm1, together with two other fission proteins, Mdv1 and Fis1, simply pinches the organelle in two. However, although GTP binding is required for formation of the constricting ring, GTP hydrolysis is not. Therefore, the team speculates that a separate hydrolysis step, which would follow self-assembly, is required to generate a specific force that clips off both outer and inner membranes of the organelle. They are starting to set up an in vitro system to test that model.