# Proteolytic cleavage activates the mitochondrial isoform of TOP3A

## TL;DR

A mitochondrial form of TOP3A is activated through proteolytic cleavage, which helps maintain mitochondrial DNA.

## Contribution

The study reveals that proteolytic cleavage of mitochondrial TOP3A enhances its DNA decatenation activity and enables its autonomous function in mitochondria.

## Key findings

- Mitochondrial TOP3A is cleaved by mitochondrial processing peptidase, removing ~90 amino acids from the C-terminus.
- Cleavage increases single-stranded DNA binding and decatenation activity of mitochondrial TOP3A.
- Mitochondrial TOP3A functions independently in mtDNA maintenance, uncoupled from nuclear BTRR complex interactions.

## Abstract

The TOP3A gene encodes two isoforms, one targeted to the nucleus and one to mitochondria. Nuclear TOP3A functions as part of the BTRR complex to resolve double Holliday junctions during homologous recombination, while the mitochondrial isoform separates hemicatenated daughter mitochondrial DNA (mtDNA) molecules following DNA replication. Here, we show that the mitochondrial isoform of TOP3A undergoes proteolytic cleavage by the mitochondrial processing peptidase, removing ~90 amino acids from the C-terminus. This cleavage enhances the enzyme’s biochemical properties, increasing single-stranded DNA binding and decatenation activity. Notably, all BTRR complex subunits, except TOP3A, are absent from mitochondria, suggesting that proteolytic processing enables TOP3A to function autonomously in mtDNA maintenance. We propose that this cleavage represents a post-import maturation step that tailors TOP3A to its mitochondrial context by uncoupling it from nuclear protein interactions and enhancing its catalytic efficiency.

Graphical Abstract