Mrx6 binds the Lon protease Pim1 N-terminal domain to confer selective substrate specificity and regulate mtDNA copy number.

Mitochondrial DNA (mtDNA) copy number regulation remains incompletely understood, despite its importance in cellular function. In Saccharomyces cerevisiae, Mrx6 belongs to the Pet20-domain-containing protein family, consisting of Mrx6, Pet20, and Sue1. Notably, absence of Mrx6 leads to increased mtDNA copy number. Here, we identify the C-terminus of Mrx6 as essential for its stability and interaction with the mitochondrial matrix protein Mam33. Deletion of Mam33 mimics the effect of Mrx6 loss, resulting in elevated mtDNA copy number. Bioinformatics, mutational analyses, and immunoprecipitation studies corroborate that a subcomplex of Mam33 and Mrx6 trimers interacts with the substrate recognition domain of the conserved mitochondrial Lon protease Pim1 through a bipartite domain in the Pet20 domain of Mrx6. Loss of Mrx6, its paralog Pet20, Mam33, or mutations disrupting the interaction between Mrx6 and Pim1 stabilize key proteins required for mtDNA maintenance, the RNA polymerase Rpo41 and the HMG-box-containing protein Cim1. We propose that Mrx6, alongside Pet20 and Mam33, regulates mtDNA copy number by modulating substrate degradation through Pim1. Additionally, Mrx6 loss alters Cim1's function, preventing the detrimental effect on mtDNA maintenance observed upon Cim1 overexpression. The presence of three Pet20-domain proteins in yeast implies broader roles of Lon protease substrate recognition beyond mtDNA regulation.