MICROSCOPY Vol.45▶No.2 2010
â– Feature Articles: Autophagy: Fusion of Morphology and Molecular Biology

Mechanism of Mitophagy in Yeast

Noriko Okamoto, Koji Okamoto and Yoshinori Ohsumi

Abstract: Autophagy-dependent degradation of mitochondria is a fundamental process conserved from yeast to humans. In contrast to starvation-induced, nonselective autophagy responsible for nutrient recycling, selective autophagy, which involves particular cues and receptors, mediates breakdown specific to mitochondria. Although numerous studies highlight that mitochondria-specific autophagy (mitophagy) contributes to mitochondrial homeostasis, the molecular mechanisms underlying this selective clearance process are poorly understood. Our fluorescence and electron microscopy analyses reveal that a substantial fraction of mitochondria are exclusively sequestered, and transported to the vacuole, a lytic compartment, in yeast cells at post-log phase under respiratory conditions. This phenomenon requires Atg11 but not Atg17, proteins acting as scaffolds crucial for selective and nonselective autophagy pathways, respectively. Using a genome-wide visual screen, we identified Atg32, a protein essential for mitophagy in yeast. During respiratory growth, Atg32 is highly expressed, likely in response to oxidative stress, and anchored to the surface of mitochondria. We also demonstrate that Atg32 interacts with Atg8 and Atg11, proteins critical for recognition of cargo receptors. Notably, Atg32 contains WXXI/L/V, a conserved motif that serves as a binding site for the Atg8 family members. We propose that Atg32 is a transmembrane receptor that directs the autophagy machinery to mitochondria.

Key words: autophagy, mitochondria, yeast, quality control, oxidative stress