المؤلفون: Zhanna Lipatova, Nava Segev, S. F. Zhang, Valeriya Gyurkovska

مصطلحات موضوعية: medicine.anatomical_structure, Proteasome, Membrane protein, Chemistry, Endoplasmic reticulum, Lysosome, Autophagy, Unfolded protein response, medicine, Vacuole, Endoplasmic-reticulum-associated protein degradation, Cell biology

الوصف: Thirty percent of all cellular proteins are inserted into the endoplasmic reticulum (ER), which spans throughout the cytoplasm. Two well-established stress-induced pathways ensure quality control (QC) at the ER: ER-phagy and ER-associated degradation (ERAD), which shuttle cargo for degradation to the lysosome and proteasome, respectively. In contrast, not much is known about constitutive ER-phagy. We have previously reported that excess of integral-membrane proteins is delivered from the ER to the lysosome via autophagy during normal growth of yeast cells. Here, we characterize this pathway as constitutive ER-phagy. Constitutive and stress-induced ER-phagy share the basic macro-autophagy machinery including the conserved Atgs and Ypt1 GTPase. However, induction of stress-induced autophagy is not needed for constitutive ER-phagy to occur. Moreover, the selective receptors needed for starvation-induced ER-phagy, Atg39 and Atg40, are not required for constitutive ER-phagy and neither these receptors nor their cargos are delivered through it to the vacuole. As for ERAD, while constitutive ER-phagy recognizes cargo different from that recognized by ERAD, these two ER-QC pathways can partially substitute for each other. Because accumulation of membrane proteins is associated with disease, and constitutive ER-phagy players are conserved from yeast to mammalian cells, this process could be critical for human health.Author SummaryAccumulation of excess proteins can lead to their aggregation, which in turn can cause multiple disorders, notably neurodegenerative disease. Nutritional and endoplasmic-reticulum (ER) stress stimulate autophagy and ER-associated degradation (ERAD) to clear excess and misfolded proteins, respectively. However, not much is known about clearance of excess proteins during normal growth. We have previously shown that excess integral-membrane proteins are cleared from the ER by macro-autophagy during normal growth of yeast cells. Here we characterize this pathway as constitutive ER-phagy. While this pathway shares machinery of core Atgs and autophagosomes with nutritional stress-induced ER-phagy, it differs from the latter: It is independent of the stress response and of receptors needed for stress-induced ER-phagy, and while stress-induced ER-phagy is not discriminatory, constitutive ER-phagy has specific cargos. However, when constitutive ER-phagy fails, machinery specific to stress-induced ER-phagy can process its cargo. Moreover, constitutive ER-phagy is also not dependent on ER-stress or the unfolded protein response (UPR) stimulated by this stress, although its failure elicits UPR. Finally, constitutive ER-phagy and ERAD can partially process each other’s cargo upon failure. In summary, constitutive ER-phagy, which clears excess integral-membrane proteins from the ER during normal growth does not require nutritional or ER stress for its function.

الوصول الحر: https://explore.openaire.eu/search/publication?articleId=doi_________::74906c34f6bc5dbbc8360aa8e26775acTest
https://doi.org/10.1101/2020.10.21.349167Test

المؤلفون: Nava Segev

المصدر: PLoS Genetics, Vol 16, Iss 3, p e1008631 (2020)
PLoS Genetics

مصطلحات موضوعية: Cancer Research, Physiology, AMP-Activated Protein Kinases, QH426-470, Biochemistry, 0302 clinical medicine, Fluorescence Microscopy, AMP-activated protein kinase, Medicine and Health Sciences, Homeostasis, Proteasome endopeptidase complex, Genetics (clinical), 0303 health sciences, Microscopy, biology, Cell Death, Organic Compounds, Monosaccharides, Eukaryota, Light Microscopy, Cell biology, Chemistry, Immunoblot Analysis, medicine.anatomical_structure, Cell Processes, Physical Sciences, Cellular Structures and Organelles, Research Article, Proteasome Endopeptidase Complex, Autophagic Cell Death, Carbohydrates, Molecular Probe Techniques, Research and Analysis Methods, 03 medical and health sciences, Lysosome, medicine, Genetics, Microautophagy, Molecular Biology Techniques, Molecular Biology, Ecology, Evolution, Behavior and Systematics, 030304 developmental biology, Endosomal Sorting Complexes Required for Transport, Endoplasmic reticulum, Organic Chemistry, Chemical Compounds, Organisms, Fungi, Biology and Life Sciences, Proteins, Protein Complexes, Proteasomes, Cell Biology, Yeast, Glucose, Proteasome, Cytoplasm, Vacuoles, biology.protein, Lysosomes, Physiological Processes, 030217 neurology & neurosurgery

الوصف: The ubiquitin-proteasome system regulates numerous cellular processes and is central to protein homeostasis. In proliferating yeast and many mammalian cells, proteasomes are highly enriched in the nucleus. In carbon-starved yeast, proteasomes migrate to the cytoplasm and collect in proteasome storage granules (PSGs). PSGs dissolve and proteasomes return to the nucleus within minutes of glucose refeeding. The mechanisms by which cells regulate proteasome homeostasis under these conditions remain largely unknown. Here we show that AMP-activated protein kinase (AMPK) together with endosomal sorting complexes required for transport (ESCRTs) drive a glucose starvation-dependent microautophagy pathway that preferentially sorts aberrant proteasomes into the vacuole, thereby biasing accumulation of functional proteasomes in PSGs. The proteasome core particle (CP) and regulatory particle (RP) are regulated differently. Without AMPK, the insoluble protein deposit (IPOD) serves as an alternative site that specifically sequesters CP aggregates. Our findings reveal a novel AMPK-controlled ESCRT-mediated microautophagy mechanism in the regulation of proteasome trafficking and homeostasis under carbon starvation.
Author summary Protein homeostasis is critical for maintaining organismal health. The cellular dysfunction caused by accumulation and aggregation of aberrant proteins or other normally short-lived proteins is associated with aging and many human diseases, including neurodegenerative disorders, diabetes, and various types of cancer. The eukaryotic ubiquitin-proteasome system regulates numerous cellular processes and through selective protein degradation helps maintain cellular protein homeostasis under normal growth conditions. However, hundreds of cellular granules or condensates are formed during carbon starvation in yeast cells, including proteasome storage granules (PSGs). PSGs result from a massive relocation of proteasomes from the nucleus to the cytoplasm under these conditions. However, how cells regulate proteasome homeostasis under these conditions remains largely unknown. Here, we demonstrate that AMPK (AMP-activated protein kinase), a master cellular energy regulator, drives ESCRT (endosomal sorting complexes required for transport)-dependent microautophagy of aberrant proteasomes. This allows rapid re-mobilization of functional proteasomes from PSGs upon glucose refeeding. Previous studies had identified classical macroautophagy as a means of degrading proteasomes during starvation. Our work shows that direct uptake of proteasomes into the vacuole (lysosome) by microautophagy is a major means of proteasome elimination under limiting glucose conditions.

الوصول الحر: https://explore.openaire.eu/search/publication?articleId=doi_dedup___::3f7b24da07c39bca56371981d7aa4875Test
https://doaj.org/article/2a488a1e85c24a68be52664af1becbc9Test

المؤلفون: Zhanna Lipatova, Nava Segev

مصطلحات موضوعية: medicine.anatomical_structure, Membrane protein, Lysosome, Endoplasmic reticulum, Organelle, Autophagy, Reticulophagy, medicine, Unfolded protein response, Biology, Endoplasmic-reticulum-associated protein degradation, Cell biology

الوصف: In autophagy a cellular recycling process, excess, or damaged cellular components are shuttled to the lysosome for degradation and their resultant building blocks can be reused. Macro-autophagy, but not micro-autophagy, occurs via autophagosomes and is dependent on a conserved set of autophagy-specific proteins termed Atgs. While autophagy has been studied extensively under stress conditions, it also occurs under normal growth conditions. The endoplasmic reticulum (ER), which spans throughout the cell, is the site where newly synthesized membrane proteins and lipids are inserted into the membrane before being transported to other organelles. A rigorous quality control (QC) mechanism termed ER-associated degradation (ERAD) ensures that only properly folded proteins leave the ER, and autophagy was proposed as a backup mechanism to ERAD. Ten years ago, selective autophagy of the ER, termed ER-phagy or reticulophagy, induced by ER stress, was described in yeast. However, later it became clear that this process is independent of Atgs, and therefore belongs to the micro-autophagy type. Recently, two types of macro-ER-phagy were characterized in yeast and mammalian cells: starvation induced and QC. Here, we summarize what is currently known about the different types of ER-phagy, highlight differences between them, and discuss future questions.

الوصول الحر: https://explore.openaire.eu/search/publication?articleId=doi_________::b9247534795f364b68e6f3006a7313c0Test
https://doi.org/10.1016/b978-0-12-812146-7.00010-xTest

المؤلفون: Nava Segev, Zhanna Lipatova

المصدر: PLoS Genetics
PLoS Genetics, Vol 11, Iss 7, p e1005390 (2015)

مصطلحات موضوعية: Cancer Research, Protein Folding, Saccharomyces cerevisiae Proteins, lcsh:QH426-470, Autophagy-Related Proteins, Vacuole, Saccharomyces cerevisiae, Endoplasmic-reticulum-associated protein degradation, Biology, Endoplasmic Reticulum, Lysosome, Genetics, medicine, Autophagy, Animals, Molecular Biology, Integral membrane protein, Genetics (clinical), Ecology, Evolution, Behavior and Systematics, Membrane Proteins, Endoplasmic Reticulum-Associated Degradation, Endoplasmic Reticulum Stress, Cell biology, lcsh:Genetics, medicine.anatomical_structure, Membrane protein, rab GTP-Binding Proteins, Proteolysis, Unfolded protein response, Rab, Lysosomes, Research Article

الوصف: The endoplasmic-reticulum quality-control (ERQC) system shuttles misfolded proteins for degradation by the proteasome through the well-defined ER-associated degradation (ERAD) pathway. In contrast, very little is known about the role of autophagy in ERQC. Macro-autophagy, a collection of pathways that deliver proteins through autophagosomes (APs) for degradation in the lysosome (vacuole in yeast), is mediated by autophagy-specific proteins, Atgs, and regulated by Ypt/Rab GTPases. Until recently, the term ER-phagy was used to describe degradation of ER membrane and proteins in the lysosome under stress: either ER stress induced by drugs or whole-cell stress induced by starvation. These two types of stresses induce micro-ER-phagy, which does not use autophagic organelles and machinery, and non-selective autophagy. Here, we characterize the macro-ER-phagy pathway and uncover its role in ERQC. This pathway delivers 20–50% of certain ER-resident membrane proteins to the vacuole and is further induced to >90% by overexpression of a single integral-membrane protein. Even though such overexpression in cells defective in macro-ER-phagy induces the unfolded-protein response (UPR), UPR is not needed for macro-ER-phagy. We show that macro-ER-phagy is dependent on Atgs and Ypt GTPases and its cargo passes through APs. Moreover, for the first time the role of Atg9, the only integral-membrane core Atg, is uncoupled from that of other core Atgs. Finally, three sequential steps of this pathway are delineated: Atg9-dependent exit from the ER en route to autophagy, Ypt1- and core Atgs-mediated pre-autophagsomal-structure organization, and Ypt51-mediated delivery of APs to the vacuole.
Author Summary ER-quality control (ERQC) ensures delivery of “native” proteins through the secretory pathway. Currently, ER-associated degradation (ERAD), which delivers misfolded proteins for degradation by the proteasome, is considered a major ERQC pathway, with autophagy as its backup. Until now, the role of autophagy, which shuttles cellular components for degradation in the lysosome through autophagosomes (APs), in ERQC was ill defined. Recently, the process of ER degradation induced by ER stress was defined as micro-ER-phagy, which does not require autophagic machinery and does not pass through APs. Here, we characterize the macro-ER-phagy pathway, which delivers excess membrane proteins for degradation in the lysosome, as a novel ERQC pathway. This pathway functions in the absence of cellular or ER stress and can be further induced by overexpression of a single integral-membrane protein. Unlike the micro-ER-phagy pathway, the marco-ER-phagy pathway requires core autophagy-specific proteins, Atgs, and Ypt/Rab GTPases. In addition, for the first time, the function of the only membrane core Atg, Atg9, was uncoupled from that of the other core Atgs. Whereas Atg9 plays a role in the assembly of ER-to-autophagy membranes (ERAM), other core Atgs and Ypt1 assemble the Atg-protein complex on ERAM to form the pre-autophagosomal structure.

الوصول الحر: https://explore.openaire.eu/search/publication?articleId=doi_dedup___::357c6b08b28cea638db23e5d0d1ef5c8Test
http://europepmc.org/articles/PMC4504476Test

المؤلفون: Zhanna Lipatova, Jonathan W. Mulholland, Nava Segev, Ankur H. Shah, Jane J. Kim

المصدر: Molecular Biology of the Cell

مصطلحات موضوعية: Saccharomyces cerevisiae Proteins, Golgi Apparatus, GTPase, Saccharomyces cerevisiae, Biology, Endoplasmic Reticulum, 03 medical and health sciences, symbols.namesake, 0302 clinical medicine, Lysosome, Gene Expression Regulation, Fungal, medicine, Autophagy, Molecular Biology, 030304 developmental biology, 0303 health sciences, Endoplasmic reticulum, Membrane Proteins, Cell Biology, Articles, Golgi apparatus, Cell biology, Transport protein, Protein Transport, medicine.anatomical_structure, rab GTP-Binding Proteins, Membrane Trafficking, Mutation, Proteolysis, symbols, Unfolded protein response, Rab, Lysosomes, 030217 neurology & neurosurgery

الوصف: Ypt1 GTPase, in the context of an autophagy-specific module, regulates ER-phagy. Because Ypt1 is a known regulator of ER-to-Golgi transport, this means that a single Ypt/Rab can regulate two alternative transport steps from one compartment, the ER, to two different destinations, the Golgi and the autophagy pathway.
Accumulation of misfolded proteins on intracellular membranes has been implicated in neurodegenerative diseases. One cellular pathway that clears such aggregates is endoplasmic reticulum autophagy (ER-phagy), a selective autophagy pathway that delivers excess ER to the lysosome for degradation. Not much is known about the regulation of ER-phagy. The conserved Ypt/Rab GTPases regulate all membrane trafficking events in eukaryotic cells. We recently showed that a Ypt module, consisting of Ypt1 and autophagy-specific upstream activator and downstream effector, regulates the onset of selective autophagy in yeast. Here we show that this module acts at the ER. Autophagy-specific mutations in its components cause accumulation of excess membrane proteins on aberrant ER structures and induction of ER stress. This accumulation is due to a block in transport of these membranes to the lysosome, where they are normally cleared. These findings establish a role for an autophagy-specific Ypt1 module in the regulation of ER-phagy. Moreover, because Ypt1 is a known key regulator of ER-to-Golgi transport, these findings establish a second role for Ypt1 at the ER. We therefore propose that individual Ypt/Rabs, in the context of distinct modules, can coordinate alternative trafficking steps from one cellular compartment to different destinations.

الوصول الحر: https://explore.openaire.eu/search/publication?articleId=doi_dedup___::1484d74c6534006210bb0a7ffbde1d9fTest
https://pubmed.ncbi.nlm.nih.gov/23924895Test