Detection of mitophagy in mammalian cells, mice, and yeast

The GET pathway serves to activate Atg32-mediated mitophagy by ER targeting of the Ppg1-Far complex

Mitophagy removes defective or superfluous mitochondria via selective autophagy. In yeast, the pro-mitophagic protein Atg32 localizes to the mitochondrial surface and interacts with the scaffold protein Atg11 to promote degradation of mitochondria. Although Atg32-Atg11 interactions are thought to be stabilized by Atg32 phosphorylation, how this posttranslational modification is regulated remains obscure. Here, we show that cells lacking the guided entry of the tail-anchored protein (GET) pathway exhibit reduced Atg32 phosphorylation and Atg32-Atg11 interactions, which can be rescued by additional loss of the ER-resident Ppg1-Far complex, a multi-subunit phosphatase negatively acting in mitophagy. In GET-deficient cells, Ppg1-Far is predominantly localized to mitochondria. An artificial ER anchoring of Ppg1-Far in GET-deficient cells significantly ameliorates defects in Atg32-Atg11 interactions and mitophagy. Moreover, disruption of GET and Msp1, an AAA-ATPase that extracts non-mitochondrial proteins localized to the mitochondrial surface, elicits synthetic defects in mitophagy. Collectively, we propose that the GET pathway mediates ER targeting of Ppg1-Far, thereby preventing dysregulated suppression of mitophagy activation.

Mitochondrial dynamics and degradation in the oleaginous yeast Lipomyces starkeyi

Emerging evidence implicates the vital role of mitochondria in lipid consumption and storage, highlighting the intimate link between energy production and saving. Although formation of giant lipid droplets, which is the key hallmark of the oleaginous yeast Lipomyces starkeyi, appears to be regulated in response to changes in mitochondrial shape and metabolism, technical limitations of genetic manipulation have become an obstacle to uncover the mitochondrial behavior in this nonconventional yeast. Here, we established an L. starkeyi strain stably expressing a fluorescent marker for monitoring mitochondrial morphology and degradation and found that mitochondria are mostly fragmented in L. starkeyi cells under fermentable, nonfermentable, and nitrogen depletion conditions. Notably, a fraction of mitochondria-specific fluorescent signals was localized to the vacuole, a lytic organelle in yeast, indicating degradation of mitochondria in those cells. This possible catabolic event was more predominant in cells under nutrient-poor conditions than that in cells under nutrient-rich conditions, concomitantly with lipid droplet formation. Collectively, our studies provide a new tool to investigate mitochondrial dynamics in L. starkeyi and decipher the potential role of mitochondrial degradation in lipid metabolism.

The protein N-terminal acetyltransferase A complex contributes to yeast mitophagy via promoting expression and phosphorylation of Atg32

Mitophagy is an evolutionarily conserved catabolic process that selectively degrades damaged or superfluous mitochondria via autophagy. Although mitophagy is considered to be critical to maintain cellular homeostasis, detailed mechanisms of mitophagy remain largely unknown. In the budding yeast Saccharomyces cerevisiae, the protein N-terminal acetyltransferase A (NatA) complex is important for transcriptional induction of the pro-mitophagic factor Atg32 and efficient degradation of mitochondria under prolonged respiratory conditions. Overexpression of Atg32 only partially recovers mitophagy in cells lacking NatA, raising the possibility that NatA may contribute to mitophagy via additional mechanisms. Here we demonstrate that Atg32 phosphorylation, which is required for facilitating mitophagy, is altered in respiring NatA-deficient cells. Hyperphosphorylation of Atg32 partially rescues mitophagy in cells lacking NatA. Notably, mitophagy is mostly restored in NatA-null cells overexpressing hyperphosphorylated Atg32. Loss of NatA does not impair the interaction of phosphorylated Atg32 with Atg11, a scaffold protein critical for selective autophagy, suggesting that NatA-dependent Atg32 phosphorylation promotes mitophagy independently of Atg32-Atg11 interactions. We propose that NatA-mediated protein N-terminal acetylation acts in Atg32 expression and phosphorylation to drive mitophagy.