Mitochondria Associated Degradation Pathway (MAD) Function Beyond the Outer Membrane

Abstract

The mitochondria-associated degradation (MAD) pathway mediates ubiquitination and removal of mitochondrial outer membrane (MOM) proteins, and targets them for degradation by the proteasome. We find that MAD, but not other mitochondrial control pathways including macroautophagy, mitophagy, or mitochondrial chaperones and proteases, is critical for yeast cellular fitness under conditions of paraquat (PQ) induced chronic, low-level oxidative stress in mitochondria. Specifically, inhibition of MAD results in increased PQ-induced defects in growth and mitochondrial quality, and reduced chronological lifespan. In light of this, we tested whether MAD function in mitochondrial proteostasis extends beyond the known substrates in MOM. Our mass spectrometry analysis revealed possible MAD substrates in different mitochondrial compartments. We confirmed that 2 matrix proteins, Kgd1p and Pim1p, are MAD substrates. Our studies reveal a broader function for MAD in mitochondrial protein surveillance beyond the MOM, and a major role in cellular and mitochondrial fitness in response to chronic, low level oxidative stress in mitochondria.