Aup1p, a Yeast Mitochondrial Protein Phosphatase Homolog, Is Required for Efficient Stationary Phase Mitophagy and Cell Survival

Abstract

Autophagy is a catabolic membrane-trafficking process that occurs in all eukaryotic cells and leads to the hydrolytic degradation of cytosolic material in the vacuolar or lysosomal lumen. Mitophagy, a selective form of autophagy targeting mitochondria, is poorly understood at present. Several recent reports suggest that mitophagy is a selective process that targets damaged mitochondria, whereas other studies imply a role for mitophagy in cell death processes. In a screen for protein phosphatase homologs that functionally interact with the autophagy-dedicated protein kinase Atg1p in yeast, we have identified Aup1p, encoded by Saccharomyces cerevisiae reading frame YCR079w. Aup1p is highly similar to a family of protein phosphatase homologs in animal cells that are predicted to localize to mitochondria based on sequence analysis. Interestingly, we found that Aup1p localizes to the mitochondrial intermembrane space and is required for efficient mitophagy in stationary phase cells. Viability studies demonstrate that Aup1p is required for efficient survival of cells in prolonged stationary phase cultures, implying a pro-survival role for mitophagy under our working conditions. Our data suggest that Aup1p may be part of a signal transduction mechanism that marks mitochondria for sequestration into autophagosomes.Mitochondria perform numerous essential physiological functions in all eukaryotic cells. Apart from their role in oxidative phosphorylation and fatty acid oxidation, they are also essential for biosynthesis of central building blocks such as amino acids and nucleotides. At the same time, mitochondria are a threat to cellular well-being. Mitochondria are a major source of reactive oxygen species in cells. In addition, disruption of mitochondrial compartmentalization results in leakage of cytochrome c and other cytotoxic factors, and mitochondria with defective chemiosmotic coupling can cause an energy drain on the cell. Accumulation of mitochondrial genetic variation and mitochondrial damage are widely considered to underlie many age-related metabolic diseases and late-onset genetic disorders (1, 2). It is commonly postulated that in normal cells defective mitochondria are broken down in the lysosomal compartment through autophagy, and inability to clear defective mitochondria is thought to underlie numerous pathological conditions (3, 4).Autophagy is a set of catabolic membrane trafficking mechanisms that allow import of cytosolic material into the vacuole/ lysosome. The best understood form of autophagy is macroautophagy, in which intracellular membranes of undetermined origin engulf cytosolic material to form a double or multi-bilayer membrane bound intermediate called the autophagosome (reviewed in Refs. 3 and 5-8). This intermediate then goes on to fuse with the vacuole/lysosome, releasing a single-bilayer bound vesicle called an autophagic body into the lumen of the lytic compartment where it is broken down, releasing the cytosol-derived material for further degradation to biosynthetic building blocks. Cl...

The machinery of macroautophagy

“…In addition, the absence of the mitochondrial outer membrane protein Uth1 impairs the induction of autophagy by rapamycin treatment or starvation [131], suggesting its potential role as a ligand [118]. Ptc6/Aup1, a yeast mitochondrial protein phosphatase homolog, is required for post-logarithmic phase mitophagy [132], and the function of Ptc6 in mitophagy may be based on its regulation of Rtg3-dependent transcription [133].…”

Autophagy: for better or for worse

“…Although the mechanisms involved in selective organelle removal are not well understood, specific autophagy-receptors seem to play a role. In yeast, two mitochondria-associated proteins, Uth1p and Aup1, mark mitochondria for sequestration in autophagosomes [47,48]. Similarly, in mammals, BNIP3L (BCL2/adenovirus E1B 19 kDa protein-interacting protein 3-like)/ Nix is indispensable for mitochondria elimination during reticulocyte maturation [49].…”

Essential role for Nix in autophagic maturation of erythroid cells

“…28) and Aup1p (ref. 29), have been shown to regulate mitochondrial autophagy in yeast. Here, we provide an example of specific regulation of mitochondrial autophagy in mammalian cells.…”