Piecemeal microautophagy of the nucleus: Genetic and morphological traits

Krick, Roswitha; Mühe, Yvonne; Prick, Tanja; Bredschneider, Monika; Bremer, Sebastian; Wenzel, Dirk; Eskelinen, Eeva‐Liisa; Thumm, Michael

doi:10.4161/auto.5.2.7639

Cited by 50 publications

(63 citation statements)

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“…Upon nutrient depletion or rapamycin treatment NVJs proliferate and subsequently undergo engulfment into the lumen of the vacuole, leading to the eventual degradation of the engulfed nuclear membranes (6). To test whether the interaction interfaces between Vac8p and Nvj1p revealed by our tVac8p-tNvj1p crystal structure are also critical for mediating PMN, yeast cells expressing EGFP-conjugated wild-type Nvj1p or mutant Nvj1p were subjected to rapamycin treatment, and PMNmediated engulfment of Nvj1p-EGFP into the vacuole was analyzed by fluorescence microscopy.…”

Section: Resultsmentioning

confidence: 99%

“…PMN is a selective autophagic recycling process stimulated by carbon or nitrogen starvation through the target of rapamycin signaling pathway in Saccharomyces cerevisiae. Upon nutrient starvation, the region of the nucleus in the vicinity of NVJs invaginates into the vacuolar lumen and forms a bleb-like structure, which is released as a vesicle and eventually degraded by vacuolar hydrolases (5,6). Because PMN occurs at the NVJ sites, their formation is essential for the initiation of the PMN process (7).…”

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confidence: 99%

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Mechanistic insight into the nucleus–vacuole junction based on the Vac8p–Nvj1p crystal structure

Jeong

Park

Kim

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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Formation of the nucleus-vacuole junction (NVJ) is mediated by direct interaction between the vacuolar protein Vac8p and the outer nuclear endoplasmic reticulum membrane protein Nvj1p. Herein we report the crystal structure of Vac8p bound to Nvj1p at 2.4-Å resolution. Vac8p comprises a flexibly connected N-terminal H1 helix followed by 12 armadillo repeats (ARMs) that form a right-handed superhelical structure. The extended 80-Å-long loop of Nvj1p specifically binds the highly conserved inner groove formed from ARM1−12 of Vac8p. Disruption of the Nvj1p-Vac8p interaction results in the loss of tight NVJs, which impairs piecemeal microautophagy of the nucleus in Saccharomyces cerevisiae. Vac8p cationic triad (Arg276, Arg317, and Arg359) motifs interacting with Nvj1p are also critical to the recognition of Atg13p, a key component of the cytoplasm-to-vacuole targeting (CVT) pathway, indicating competitive binding to Vac8p. Indeed, mutation of the cationic triad abolishes CVT of Ape1p in vivo. Combined with biochemical data, the crystal structure reveals a Vac8p homodimer formed from ARM1, and this self-association, likely regulated by the flexible H1 helix and the C terminus of Nvj1p, is critical for Vac8p cellular functions. M embrane contact sites (MCSs) between subcellular compartments play pivotal roles in cellular processes such as cooperative lipid biosynthesis, ion homeostasis, and interorganellar trafficking of molecules in eukaryotic cells (1-3). MCSs are physically formed through dynamic and direct interactions between proteins that are located in two distinct subcompartments. The nucleus-vacuole junction (NVJ), one of the best-characterized MCSs, is a physical contact site between perinuclear and vacuolar membranes and mediates essential cellular processes such as piecemeal microautophagy of the nucleus (PMN) and lipid biosynthesis in yeast (4, 5). PMN is a selective autophagic recycling process stimulated by carbon or nitrogen starvation through the target of rapamycin signaling pathway in Saccharomyces cerevisiae. Upon nutrient starvation, the region of the nucleus in the vicinity of NVJs invaginates into the vacuolar lumen and forms a bleb-like structure, which is released as a vesicle and eventually degraded by vacuolar hydrolases (5, 6). Because PMN occurs at the NVJ sites, their formation is essential for the initiation of the PMN process (7). NVJs are also involved in lipid metabolism by recruiting the two lipid-modifying enzymes, oxystereol-binding proteins homology (Osh1p) involved in nonvesicular lipid trafficking and the enoyl-CoA reductase Tsc13p that mediates the synthesis of verylong-chain fatty acids (8-11).Previous studies revealed that NVJs are generated by forming tight interactions between Vac8p, a vacuolar membrane protein, and Nvj1p, a nuclear membrane protein (12). Vac8p was initially found as a key player in vacuole inheritance by cooperating with Vac17p, actin, profilin, and Myo2p (13). In addition, Vac8p has a crucial role in mediating the processing of aminopeptidase I through in...

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Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

Mechanistic insight into the nucleus–vacuole junction based on the Vac8p–Nvj1p crystal structure

Jeong

Park

Kim

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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“…The involvement of both these proteins in lipid biosynthesis suggests that lipids may play a crucial role in PMN. 44 It has been suggested by Thumm and colleagues 27,28 that the Atg machinery might mediate the formation of the vacuolar protrusions around the nuclear cargo, its terminal engulfment and the final fusion stages of vacuolar membrane extensions by processes similar to those used for micropexophagy in P. pastoris (see below).…”

Section: Microautophagy-mediated Degradation Pathways In Mammalian Cellsmentioning

confidence: 99%

“…Subsequently, these vesicles and their contents are degraded by the resident vacuolar hydrolases. 9,10,27,28 In addition to the requirement of PMN-specific proteins (e.g., Vac8 and Nvj1), 9,29 PMN requires all protein components of the core Atg machinery, 27,28 which is defined as Atg1-Atg10, Atg12-Atg16, Atg18 and Atg22, 43 plus additional Atg-specific proteins, including Atg11 and Atg17. The fusion of the vacuolar extensions during PMN is not a classical homotypic fusion event, nevertheless some components of the homotypic vacuole fusion machinery including Sec17, Sec18, Vam7, Vam3, Ypt7, Vps33, Vps39 and Vps41 are needed for efficient PMN, although others (e.g., Vtc1-Vtc4, Vph1) are not.…”

mentioning

confidence: 99%

“…To date, this process has only been described in the yeast S. cerevisiae and has been termed PMN 9 or micronucleophagy. 27,28 It is a multistep process driven by the formation of nucleus-vacuole (NV) junctions formed through interactions between Vac8 in the vacuolar membrane and Nvj1 in the outer nuclear membrane. Nitrogen or carbon starvation as well as rapamycin (a pharmacological inducer of autophagy) treatment promotes the budding of NV junctions into the vacuolar lumen eventually leading to the scission of the nuclear and vacuolar membranes concomitant with the sequestration of a portion of the nucleus.…”

mentioning

confidence: 99%

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Microautophagy in mammalian cells: Revisiting a 40-year-old conundrum

Mijaljica¹,

Prescott²,

Devenish³

2011

Autophagy

470

327

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Emerging lysosomal pathways for quality control at the endoplasmic reticulum

2019

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Protein misfolding occurring in the endoplasmic reticulum (ER) might eventually lead to aggregation and cellular distress, and is a primary pathogenic mechanism in multiple human disorders. Mammals have developed evolutionary‐conserved quality control mechanisms at the level of the ER. The best characterized is the ER‐associated degradation (ERAD) pathway, through which misfolded proteins translocate from the ER to the cytosol and are subsequently proteasomally degraded. However, increasing evidence indicates that additional quality control mechanisms apply for misfolded ER clients that are not eligible for ERAD. This review focuses on the alternative, ERAD‐independent, mechanisms of clearance of misfolded polypeptides from the ER. These processes, collectively referred to as ER‐to‐lysosome–associated degradation, involve ER‐phagy, microautophagy or vesicular transport. The identification of the underlying molecular mechanisms is particularly important for developing new therapeutic approaches for human diseases associated with protein aggregate formation.

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Piecemeal microautophagy of the nucleus: Genetic and morphological traits

Cited by 50 publications

References 18 publications

Mechanistic insight into the nucleus–vacuole junction based on the Vac8p–Nvj1p crystal structure

Mechanistic insight into the nucleus–vacuole junction based on the Vac8p–Nvj1p crystal structure

Microautophagy in mammalian cells: Revisiting a 40-year-old conundrum

Emerging lysosomal pathways for quality control at the endoplasmic reticulum

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