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Abstract: In eukaryotic cells, nutrient starvation induces the bulk degradation of cellular materials; this process is called autophagy. In the yeast Saccharomyces cerevisiae, most of the ATG (autophagy) genes are involved in not only the process of degradative autophagy, but also a biosynthetic process, the cytoplasm to vacuole (Cvt) pathway. In contrast, the ATG17 gene is required specifically in autophagy. To better understand the function of Atg17, we have performed a biochemical characterization of the Atg17 protein. We found that the atg17Δ mutant under starvation condition was largely impaired in autophagosome formation and only rarely contained small autophagosomes, whose size was less than one-half of normal autophagosomes in diameter. Two-hybrid analyses and coimmunoprecipitation experiments demonstrated that Atg17 physically associates with Atg1-Atg13 complex, and this binding was enhanced under starvation conditions. Atg17-Atg1 binding was not detected in atg13Δ mutant cells, suggesting that Atg17 interacts with Atg1 through Atg13. A point mutant of Atg17, Atg17C24R, showed reduced affinity for Atg13, resulting in impaired Atg1 kinase activity and significant defects in autophagy. Taken together, these results indicate that Atg17-Atg13 complex formation plays an important role in normal autophagosome formation via binding to and activating the Atg1 kinase.

Abstract: Selective autophagy of damaged or redundant organelles is an important mechanism for maintaining cell homeostasis. We found previously that endoplasmic reticulum (ER) stress in the yeast Saccharomyces cerevisiae causes massive ER expansion and triggers the formation of large ER whorls. Here, we show that stress-induced ER whorls are selectively taken up into the vacuole, the yeast lysosome, by a process termed ER-phagy. Import into the vacuole does not involve autophagosomes but occurs through invagination of the vacuolar membrane, indicating that ER-phagy is topologically equivalent to microautophagy. Even so, ER-phagy requires neither the core autophagy machinery nor several other proteins specifically implicated in microautophagy. Thus, autophagy of ER whorls represents a distinct type of organelle-selective autophagy. Finally, we provide evidence that ER-phagy degrades excess ER membrane, suggesting that it contributes to cell homeostasis by controlling organelle size.

Abstract: Autophagy is a degradative pathway conserved among eukaryotes. It is a major routefor degradation of long-lived proteins and entire organelles, such as peroxisomes. Atg26,a sterol glucosyltransferase, is specifically required for micro- and macropexophagy, butnot for starvation-induced bulk autophagy in Pichia pastoris. Here we study the requirementof Saccharomyces cerevisiaeAtg26 in the Cvt pathway, nonspecific autophagy andpexophagy. Our results show that the S. cerevisiae atg26∆strain is not defective inprApe1 maturation, macroautophagy or peroxisome degradation, in contrast to the situationseen in Pichia pastoris. These studies highlight the importance of examining mutants inmultiple organisms. INTRODUCTION Eukaryotic cells utilize autophagy to adapt to environmental changes such as alterationsin available nutrients or other types of stress. Microautophagy and macroautophay are twomechanisms for sequestering cytoplasmic components and organelles. 1 Microautophagyinvolves the engulfment of cytoplasm and organelles by the lysosomal/vacuolar membranefollowed by their internalization and degradation. Macroautophagy is a vesicular processinvolving the formation of double-membrane vesicles called autophagosomes thatsequester cytosol and organelles. The autophagosomes then fuse with the lysosome/vacuole and release the inner single-membrane vesicle or autophagic body into the lysosome/vacuole lumen, resulting in the breakdown and subsequent recycling of the cargo.Although these processes are generally considered to be nonselective, there are selectivetypes of autophagy, such as the cytoplasm to vacuole targeting (Cvt) pathway and pexophagy.The Cvt pathway is a biosynthetic pathway for delivery of the resident vacuolar hydrolasesaminopeptidase I (Ape1) and α-mannosidase (Ams1).