Regulation of mitochondrial fission by MiD51

Abstract

Mitochondrial fission requires the recruitment of dynamin-related protein 1 (Drp1) to mitochondria and activation of its GTP-dependent scission function. In mammals, the recruitment of Drp1 is primarily regulated by the outer membrane receptors Mff, MiD49 and MiD51, while Fis1 plays a minor role. We recently demonstrated that these receptors can function independently of one another to recruit Drp1 and mediate mitochondrial division. Intriguingly, exogenous expression of MiD49 and MiD51 causes constitutive recruitment of Drp1 but inhibits its scission activity. With mitochondrial stress, however, these receptors can rapidly activate pre-recruited Drp1 and cause rapid mitochondrial fragmentation independently of Fis1 and Mff. This behavior suggests an ability to sense mitochondrial function, but the molecular mechanism is unknown. To better understand the role of the MiDs in regulating mitochondrial fission, we produced recombinant protein pertaining to the cytosolic domain of each MiD, and performed crystallographic screens. We obtained crystals for MiD51 that diffracted to high resolution, and we were able to determine its structure. MiD51 possesses a nucleotidyl transferase fold and uses a variant set of residues to tightly bind a ligand. We have performed a structure-function analysis of MiD51 by mutating structurally implicated residues and studying their importance in vivo and in vitro to activate Drp1. Our analysis reveals how MiD51 activates mitochondrial fission during mitochondrial stress.

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