Recruitment and assembly of the mitochondrial membrane fission machinery

Mitochondria often form tubular networks that are remodeled by fission and fusion. Fission is essential for maintenance of mitochondrial morphology and distribution, and is important for neuronal function and development. Yeast mitochondrial fission requires the conserved, dynamin‐related GTPase, Dnm1. Dnm1 self assembles to form spirals that encircle mitochondrial tubules at sites of constriction and fission. Dnm1 targeting to mitochondria requires a membrane‐anchored protein called Fis1 and its binding partner, Mdv1. Although structural studies suggested that the N‐terminus of Mdv1 forms a clamp around the Fis1 cytoplasmic domain, these proteins do not form a simple heterodimer. Instead, our biophysical studies indicate that Mdv1 dimerizes via a coiled‐coil domain in the middle of the protein. Thus, the basic Dnm1 receptor on mitochondria is likely a heterotetramer composed of two Fis1 proteins bound to an Mdv1 homodimer. New data reveal that a short helix in Dnm1p is required for recruitment of dimeric Dnm1 from the cytoplasm to the C‐terminal beta propeller domain of Mdv1. Mutations in this Dnm1 helix do not impair self‐assembly. Rather, these mutations block Dnm1 binding to Mdv1, Dnm1 membrane recruitment, and fission complex assembly. These findings provide new information about the nature of the Dnm1 membrane receptor complex and the manner in which Dnm1 recognizes this receptor.