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Autophagy is a cellular process in which proteins and organelles are delivered to and degraded in lysosomes or vacuoles. Microautophagy and macroautophagy differ considerably in their membrane dynamics. In macroautophagy, the newly generated isolation membrane sequesters a portion of cytoplasm first, whereas in microautophagy, vacuolar membranes engulf and sequester the target components directly and formation of the isolation membrane has been believed not to occur (Klionsky and Ohsumi [1999]. Annu. Rev. Cell Dev. Biol. 15, 1-32). Nevertheless, multiple gene products necessary for microautophagic processes were shown to overlap with APG/AUT (recently renamed as ATG) genes required for macroautophagy (Müller et al., [2000] J. Cell Biol. 151, 519-528; Stromhaug et al., [2001] J. Biol. Chem. 276, 42422-42435; Mukaiyama et al., [2002] Genes Cells 7, 75.90). The manuscript by Mukaiyama et al. in this issue of Molecular Biology of the Cell (pages 58-70) reports that a novel membrane structure is formed during micropexophagy (i.e., microautophagic degradation of peroxisomes, observed in the yeast Pichia pastoris). (Upper left): After the onset of micropexophagy, a membrane-bound flattened sac (arrow heads) is newly formed and attaches to one of the peroxisomes in the cluster at the far side of the invaginating vacuole. (Upper right panel): To the newly formed membrane structure, a ubiquitin-like protein Paz2 (renamed as PpAtg8) fused with fluorescent protein (GFP-Paz2) is localized. (Lower left panel): Just before contact is made by opposing vacuolar membranes (red FM 4-64 fluorescence), GFP-Paz2 fluorescence is observed exclusively between them. During the micropexophagic process, YFP-Paz2 fluorescence disappears at the time point indicated (arrow head), concomitant with an appearance of a cup-shaped FM 4-64 fluorescence. We suggest that the cup-shaped membrane structure is formed as a "micropexophagy-specific membrane apparatus" and that it mediates the fusion between opposing vacuolar membranes to sequester peroxisomes from the cytosol. P, peroxisome: V, vacuole.

-Yasuyoshi Sakai