掲載種別：研究論文（学術雑誌）  

Lysosome/vacuole-mediated intracellular degradation pathways, collectively known as autophagy, play crucial roles in the maintenance and regulation of various cellular functions. However, little is known about the relationship between different modes of autophagy. In the budding yeast Saccharomyces cerevisiae, nitrogen starvation triggers both macronucleophagy and micronucleophagy, in which nuclear components are degraded via macroautophagy and microautophagy, respectively. We previously revealed that Atg39-mediated macronucleophagy is important for cell survival under nitrogen starvation; however, the underlying mechanism remains unknown. Here, we reveal that defective Atg39-mediated macronucleophagy leads to the hyperactivation of micronucleophagy, resulting in the excessive transport of various nuclear components into the vacuole. Micronucleophagy occurs at the nucleus-vacuole junction (NVJ). We show that nuclear membrane proteins localized to the NVJ, including Nvj1, which is responsible for micronucleophagy, are degraded via macronucleophagy. Therefore, defective Atg39-mediated macronucleophagy results in the accumulation of Nvj1, which contributes to micronucleophagy enhancement. Blocking micronucleophagy almost completely suppresses cell death caused by the absence of Atg39, whereas enhanced micronucleophagy correlates with death in Atg39-mutant cells under nitrogen starvation. These results suggest that macronucleophagy modulates micronucleophagy in order to prevent the excess removal of nuclear components, thereby maintaining nuclear and cellular homeostasis during nitrogen starvation.

DOI： 10.1038/s41467-024-55045-9

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Rockefeller University Press  

DOI： 10.1083/jcb.202103178

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掲載種別：研究論文（学術雑誌）  

The endoplasmic reticulum (ER) is selectively degraded by autophagy (ER-phagy) through proteins called ER-phagy receptors. In Saccharomyces cerevisiae, Atg40 acts as an ER-phagy receptor to sequester ER fragments into autophagosomes by binding Atg8 on forming autophagosomal membranes. During ER-phagy, parts of the ER are morphologically rearranged, fragmented, and loaded into autophagosomes, but the mechanism remains poorly understood. Here we find that Atg40 molecules assemble in the ER membrane concurrently with autophagosome formation via multivalent interaction with Atg8. Atg8-mediated super-assembly of Atg40 generates highly-curved ER regions, depending on its reticulon-like domain, and supports packing of these regions into autophagosomes. Moreover, tight binding of Atg40 to Atg8 is achieved by a short helix C-terminal to the Atg8-family interacting motif, and this feature is also observed for mammalian ER-phagy receptors. Thus, this study significantly advances our understanding of the mechanisms of ER-phagy and also provides insights into organelle fragmentation in selective autophagy of other organelles.

DOI： 10.1038/s41467-020-17163-y

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掲載種別：研究論文（学術雑誌）  

Macroautophagy (hereafter referred to as autophagy) degrades various intracellular constituents to regulate a wide range of cellular functions, and is also closely linked to several human diseases. In selective autophagy, receptor proteins recognize degradation targets and direct their sequestration by double-membrane vesicles called autophagosomes, which transport them into lysosomes or vacuoles. Although recent studies have shown that selective autophagy is involved in quality/quantity control of some organelles, including mitochondria and peroxisomes, it remains unclear how extensively it contributes to cellular organelle homeostasis. Here we describe selective autophagy of the endoplasmic reticulum (ER) and nucleus in the yeast Saccharomyces cerevisiae. We identify two novel proteins, Atg39 and Atg40, as receptors specific to these pathways. Atg39 localizes to the perinuclear ER (or the nuclear envelope) and induces autophagic sequestration of part of the nucleus. Atg40 is enriched in the cortical and cytoplasmic ER, and loads these ER subdomains into autophagosomes. Atg39-dependent autophagy of the perinuclear ER/nucleus is required for cell survival under nitrogen-deprivation conditions. Atg40 is probably the functional counterpart of FAM134B, an autophagy receptor for the ER in mammals that has been implicated in sensory neuropathy. Our results provide fundamental insight into the pathophysiological roles and mechanisms of 'ER-phagy' and 'nucleophagy' in other organisms.

DOI： 10.1038/nature14506

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掲載種別：研究論文（学術雑誌）  

In selective autophagy, degradation targets are specifically recognized, sequestered by the autophagosome, and transported into the lysosome or vacuole. Previous studies delineated the molecular basis by which the autophagy machinery recognizes those targets, but the regulation of this process is still poorly understood. In this paper, we find that the highly conserved multifunctional kinase Hrr25 regulates two distinct selective autophagy-related pathways in Saccharomyces cerevisiae. Hrr25 is responsible for the phosphorylation of two receptor proteins: Atg19, which recognizes the assembly of vacuolar enzymes in the cytoplasm-to-vacuole targeting pathway, and Atg36, which recognizes superfluous peroxisomes in pexophagy. Hrr25-mediated phosphorylation enhances the interactions of these receptors with the common adaptor Atg11, which recruits the core autophagy-related proteins that mediate the formation of the autophagosomal membrane. Thus, this study introduces regulation of selective autophagy as a new role of Hrr25 and, together with other recent studies, reveals that different selective autophagy-related pathways are regulated by a uniform mechanism: phosphoregulation of the receptor-adaptor interaction.

DOI： 10.1083/jcb.201402128

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DOI： 10.15252/embr.202255192

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Recent studies have revealed that even the nucleus can be degraded by selective macroautophagy (hereafter macronucleophagy). In Saccharomyces cerevisiae, the nuclear envelope (NE) protein Atg39 acts as a macronucleophagy receptor that mediates sequestration of nucleus-derived double-membrane vesicles (NDVs) into phagophores. The outer and inner membranes of these NDVs are derived from the outer and inner nuclear membranes (ONM and INM), respectively, and the lumen contains nucleoplasmic material. Little was known about the mechanisms underlying macronucleophagy, including how the two nuclear membranes are coordinately deformed to generate NDVs and what nuclear components are preferentially loaded into or rather eliminated from NDVs. We found that Atg39 links the ONM and INM through the ONM-embedded transmembrane domain and INM-associated amphipathic helices (APHs). These APHs are important for Atg39 anchoring to the NE and autophagosome formation-coupled Atg39 clustering in the NE. In addition, the overaccumulation of Atg39 in the NE caused NE protrusion toward the cytoplasm depending on the APHs. These results allowed us to propose the mechanism by which Atg39 conducts NDV formation in coordination with autophagosome formation during macronucleophagy.

DOI： 10.1080/15548627.2022.2069957

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Reticulophagy (or ER-phagy) is a type of selective autophagy that targets the endoplasmic reticulum (ER). In the process of reticulophagy, part of the ER is fragmented and packed within autophagosomes. However, the underlying mechanism that induces this local remodeling of ER subdomains was poorly understood. Our recent study showed that in the budding yeast Saccharomyces cerevisiae the reticulophagy receptor Atg40 plays an important role in ER remodeling beyond its role as a tether between the ER and the phagophore [1]. Atg40 has an ability to generate positive membrane curvature through the reticulon-like domain and locally forms a super assemblage though its binding to Atg8 at ER-phagophore contacts. These Atg40 assemblages cause folding of the ER subdomains to allow them to be efficiently packed into autophagosomes. Furthermore, our structural analysis identified an evolutionarily conserved short helix that assists strong Atg8-binding of reticulophagy receptors.

DOI： 10.1080/15548627.2020.1831801

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Autophagy targets various intracellular components ranging from proteins and nucleic acids to organelles for their degradation in lysosomes or vacuoles. In selective types of autophagy, receptor proteins play central roles in target selection. These proteins bind or localize to specific targets, and also interact with Atg8 family proteins on forming autophagosomal membranes, leading to the efficient sequestration of the targets by the membranes. Our recent study revealed that yeast cells actively degrade the endoplasmic reticulum (ER) and even part of the nucleus via selective autophagy under nitrogen-deprived conditions. We identified novel receptors, Atg39 and Atg40, specific to these pathways. Here, we summarize our findings on 'reticulophagy' (or 'ER-phagy') and 'nucleophagy', and discuss key issues that remain to be solved in future studies.

DOI： 10.1080/15548627.2015.1106665

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DOI： 10.1080/15384101.2015.1084206

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