Eiyama, A. | bioRxiv Science

bioRxiv2024

ER-associated degradation by Doa10 restrains mitophagy in yeast

Degradation of defective or superfluous mitochondria via mitophagy, a specialized form of selective autophagy, is important for maintaining mitochondrial quality and quantity. In yeast, the pro-mitophagic factor Atg32 is transcriptionally upregulated upon oxidative stress and anchored to the mitochondrial surface, where it acts as a molecular signal to initiate efficient degradation of mitochondria. However, how the protein levels of Atg32 are regulated post-translationally remains enigmatic. Here we show that the endoplasmic reticulum (ER) serves as a hub to govern Atg32 protein turnover. We found that the ER-associated degradation (ERAD) E3 ligase Doa10 interacts with Atg32, leading to its degradation by the proteasome. Furthermore, we show that Atg32 is destined for the ER in a manner dependent on the GET (guided entry of tail-anchored proteins) pathway, which mediates the delivery of tail-anchored (TA) proteins to the ER where Atg32 is potentially recognized by Doa10. Notably, Doa10 deficiency increased Atg32 levels and enhanced mitophagy under respiratory conditions, thus determining that ERAD serves as a brake on mitophagy.

cell biology↗

bioRxiv2020

Disturbed intramitochondrial phosphatidic acid transport impairs cellular stress signaling

Lipid transfer proteins of the Ups1/PRELID1 family facilitate the transport of phospholipids across the intermembrane space of mitochondria in a lipid-specific manner. Heterodimeric complexes of yeast Ups1/Mdm35 or human PRELID1/TRIAP1 shuttle phosphatidic acid (PA) synthesized in the endoplasmic reticulum (ER) to the inner membrane, where it is converted to cardiolipin (CL), the signature phospholipid of mitochondria. Loss of Ups1/PRELID1 proteins impairs the accumulation of CL and broadly affects mitochondrial structure and function. Unexpectedly and unlike yeast cells lacking the cardiolipin synthase Crd1, Ups1 deficient yeast cells exhibit glycolytic growth defects, pointing to functions of Ups1-mediated PA transfer beyond CL synthesis. Here, we show that the disturbed intramitochondrial transport of PA in ups1{Delta} cells leads to altered phospholipid composition of the ER membrane, independent of disturbances in CL synthesis. The impaired flux of PA into mitochondria is associated with the increased synthesis of phosphatidylcholine (PC) and a reduced phosphatidylethanolamine (PE)/PC ratio in the ER of ups1{Delta} cells which suppresses the unfolded protein response (UPR). Moreover, we observed inhibition of TORC1 signaling in these cells. Activation of either UPR by ER protein stress or of TORC1 signaling by disruption of its negative regulator, the SEACIT complex, increased cytosolic protein synthesis and restored glycolytic growth of ups1{Delta} cells. These results demonstrate that PA influx into mitochondria is required to preserve ER membrane homeostasis and that its disturbance is associated with impaired glycolytic growth and cellular stress signaling.

cell biology↗