Fis1p and Caf4p, but not Mdv1p, determine the polar localization of Dnm1p clusters on the mitochondrial surface.

The mitochondrial division machinery consists of the large dynamin-related protein Dnm1p (Drp1/Dlp1 in humans), and Fis1p, Mdv1p and Caf4p. Proper assembly of Dnm1p complexes on the mitochondrial surface is crucial for balanced fission and fusion events. Using quantitative confocal microscopy, we show that Caf4p is important for the recruitment of Dnm1p to the mitochondria. The mitochondrial Dnm1p assemblies can be divided into at least two morphologically distinguishable fractions. A small subset of these assemblies appear to be present as Dnm1p-spirals (or rings) that encircle tubule constrictions, with seldom more than seven turns. A larger fraction of the Dnm1p assemblies is primarily present at one side of the mitochondrial tubules. We show that a majority of these mitochondria-associated Dnm1p clusters point towards the cell cortex. This polarized orientation is abolished in fis1Delta and caf4Delta yeast cells, but is maintained in mdv1Delta cells and after disruption of the actin cytoskeleton. This study suggests that Caf4p plays a key role in determining the polarized localization of those Dnm1p clusters that are not immediately involved in the mitochondrial fission process.