# Mitochondrial fission during mitophagy requires both inner and outer mitofissins

## TL;DR

The paper discovers a new protein, Mfi2, that helps break apart mitochondria during mitophagy from the outer mitochondrial membrane, working alongside another protein, Atg44, from the inner membrane.

## Contribution

The study identifies Mfi2 as a novel outer membrane mitofissin required for mitophagy, revealing a dual mechanism involving both inner and outer mitofissins.

## Key findings

- Mfi2 is a mitochondrial outer membrane-resident mitofissin required for mitophagy.
- Mfi2 and Dnm1 independently facilitate mitochondrial fission during mitophagy.
- Mfi2 exhibits Atg44-like mitofissin activity in vivo and in vitro.

## Abstract

Mitophagy maintains mitochondrial homeostasis through the selective degradation of damaged or excess mitochondria. Recently, we identified mitofissin/Atg44, a mitochondrial intermembrane space-resident fission factor, which directly acts on lipid membranes and drives mitochondrial fission required for mitophagy in yeast. However, it remains unclear whether mitofissin is sufficient for mitophagy-associated mitochondrial fission and whether other factors act from outside mitochondria. Here, we identify a mitochondrial outer membrane-resident mitofissin-like microprotein required for mitophagy, and we name it mitofissin 2/Mfi2 based on the following results. Overexpression of an N-terminal Atg44-like region of Mfi2 induces mitochondrial fragmentation and partially restores mitophagy in atg44Δ cells. Mfi2 binds to lipid membranes and mediates membrane fission in a cardiolipin-dependent manner in vitro, demonstrating its intrinsic mitofissin activity. Coarse-grained molecular dynamics simulations further support the stable interaction of Mfi2 with cardiolipin-containing bilayers. Genetic analyses reveal that Mfi2 and the dynamin-related protein Dnm1 independently facilitate mitochondrial fission during mitophagy. Thus, Atg44 and Mfi2, two mitofissins with distinct localizations, are required for mitophagy-associated mitochondrial fission.

Mfi2 is a mitochondrial outer membrane-resident mitofissin required for mitophagy. Mfi2 and the dynamin-related protein Dnm1 promote mitophagy-associated mitochondrial fission from outside the mitochondria, together with the inner mitofissin Atg44.

Mfi2 is a mitochondrial outer membrane microprotein required for mitophagy.Mfi2 exhibits an Atg44-like mitofissin activity in vivo and in vitro.Mfi2 associates with the mitochondrial membrane in a cardiolipin-dependent manner.Mfi2 and Dnm1 independently facilitate mitochondrial fission during mitophagy.

Mfi2 is a mitochondrial outer membrane microprotein required for mitophagy.

Mfi2 exhibits an Atg44-like mitofissin activity in vivo and in vitro.

Mfi2 associates with the mitochondrial membrane in a cardiolipin-dependent manner.

Mfi2 and Dnm1 independently facilitate mitochondrial fission during mitophagy.

Mfi2 is a mitochondrial outer membrane-resident mitofissin required for mitophagy. Mfi2 and the dynamin-related protein Dnm1 promote mitophagy-associated mitochondrial fission from outside the mitochondria, together with the inner mitofissin Atg44.