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Query: UMLS:C0038187 (starvation)
 24,951 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The vacuolar protein aminopeptidase I (API) uses a novel cytoplasm-to-vacuole targeting (Cvt) pathway. Complementation analysis of yeast mutants defective for cytoplasm-to-vacuole protein targeting (cvt) and autophagy (apg) revealed seven overlapping complementation groups between these two sets of mutants. In addition, all 14 apg complementation groups are defective in the delivery of API to the vacuole. Similarly, the majority of nonoverlapping cvt complementation groups appear to be at least partially defective in autophagy. Kinetic analyses of protein delivery rates indicate that autophagic protein uptake is induced by nitrogen starvation, whereas Cvt is a constitutive biosynthetic pathway. However, the machinery governing Cvt is affected by nitrogen starvation as targeting defects resulting from API overexpression can be rescued by induction of autophagy.
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PMID:Cytoplasm-to-vacuole targeting and autophagy employ the same machinery to deliver proteins to the yeast vacuole. 890 76

Autophagocytosis is a starvation-induced process responsible for transport of cytoplasmic proteins to the vacuole. In Saccharomyces cerevisiae, autophagy is characterized by the phenotypic appearance of autophagic vesicles inside the vacuole of strains deficient in proteinase yscB. The AUT1 gene, essential for autophagy, was isolated by complementation of the sporulation deficiency of a diploid aut1-1 mutant strain by a yeast genomic library and characterized. AUT1 is located on the right arm of chromosome XIV, 10 kb from the centromere, and encodes a protein of 310 amino acids, with an estimated molecular weight of 36 kDa. Cells carrying a chromosomal deletion of AUT1 are defective in the starvation-induced bulk flow transport of cytoplasmic proteins to the vacuole. aut1 null mutant strains are completely viable but show decreased survival rates during starvation. Homozygous delta aut1 diploid cells fail to sporulate. The selective cytoplasm-to-vacuole transport of aminopeptidase I is blocked in logarithmically growing and in starved delta autl cells. Deletion of the AUT1 gene had no obvious influence on secretion, fluid phase endocytosis, or vacuolar protein sorting. This supports the idea of autophagocytosis as being a novel route transporting proteins from the cytoplasm to the vacuole.
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PMID:AUT1, a gene essential for autophagocytosis in the yeast Saccharomyces cerevisiae. 902 85

The impact of severe starvation and refeeding on the intestinal mucosa of rats of different ages has been studied in a diet-controlled model. Structural and functional alterations of the small intestinal mucosa were assessed by standard parameters including mucosal protein, DNA content as well as maltase, sucrase and leucine aminopeptidase enzymatic activities. Decreases in mucosal mass, DNA, protein and leucine aminopeptidase activity in both the jejunum and ileum caused by starvation, diminished with age. The depression of disaccharidase activities increased with age in the jejunum but not in the ileum. Except for jejunal protein and leucine aminopeptidase activity, the recovery from starvation, after refeeding, was complete for the other parameters studied, regardless of age.
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PMID:Age-related response of the small intestine to severe starvation and refeeding in rats. 908 14

Stress conditions lead to a variety of physiological responses at the cellular level. Autophagy is an essential process used by animal, plant, and fungal cells that allows for both recycling of macromolecular constituents under conditions of nutrient limitation and remodeling the intracellular structure for cell differentiation. To elucidate the molecular basis of autophagic protein transport to the vacuole/lysosome, we have undertaken a morphological and biochemical analysis of this pathway in yeast. Using the vacuolar hydrolase aminopeptidase I (API) as a marker, we provide evidence that the autophagic pathway overlaps with the biosynthetic pathway, cytoplasm to vacuole targeting (Cvt), used for API import. Before targeting, the precursor form of API is localized mostly in restricted regions of the cytosol as a complex with spherical particles (termed Cvt complex). During vegetative growth, the Cvt complex is selectively wrapped by a membrane sac forming a double membrane-bound structure of approximately 150 nm diam, which then fuses with the vacuolar membrane. This process is topologically the same as macroautophagy induced under starvation conditions in yeast (Baba, M., K. Takeshige, N. Baba, and Y. Ohsumi. 1994. J. Cell Biol. 124:903-913). However, in contrast with autophagy, API import proceeds constitutively in growing conditions. This is the first demonstration of the use of an autophagy-like mechanism for biosynthetic delivery of a vacuolar hydrolase. Another important finding is that when cells are subjected to starvation conditions, the Cvt complex is now taken up by an autophagosome that is much larger and contains other cytosolic components; depending on environmental conditions, the cell uses an alternate pathway to sequester the Cvt complex and selectively deliver API to the vacuole. Together these results indicate that two related but distinct autophagy-like processes are involved in both biogenesis of vacuolar resident proteins and sequestration of substrates to be degraded.
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PMID:Two distinct pathways for targeting proteins from the cytoplasm to the vacuole/lysosome. 941 64

Autophagy is a bulk protein degradation process that is induced by starvation. The control mechanism for induction of autophagy is not well understood. We found that Tor, a phosphatidylinositol kinase homologue, is involved in the control of autophagy in the yeast, Saccharomyces cerevisiae. When rapamycin, an inhibitor of Tor function, is added, autophagy is induced even in cells growing in nutrient-rich medium. A temperature-sensitive tor mutant also leads to induction of autophagy at a nonpermissive temperature. These results indicate that Tor negatively regulates the induction of autophagy. Tor is the first molecule that is identified as a pivotal player in the starvation-signaling pathway of autophagy. Furthermore, we found that a high concentration of cAMP is inhibitory for induction of autophagy. APG gene products are involved in autophagy induced by starvation. Autophagy was not induced in apg mutants in the presence of rapamycin, indicating that the site of action of Tor is upstream of those of Apg proteins. In nutrient-rich medium, Apg proteins are involved also in the transport of aminopeptidase I from the cytosol to the vacuole. Tor may act to switch Apg function between autophagy and transport of aminopeptidase I.
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PMID:Tor, a phosphatidylinositol kinase homologue, controls autophagy in yeast. 946 83

Proper functioning of organelles necessitates efficient protein targeting to the appropriate subcellular locations. For example, degradation in the fungal vacuole relies on an array of targeting mechanisms for both resident hydrolases and their substrates. The particular processes that are used vary depending on the available nutrients. Under starvation conditions, macroautophagy is the primary method by which bulk cytosol is sequestered into autophagic vesicles (autophagosomes) destined for this organelle. Molecular genetic, morphological, and biochemical evidence indicates that macroautophagy shares much of the same cellular machinery as a biosynthetic pathway for the delivery of the vacuolar hydrolase, aminopeptidase I, via the cytoplasm-to-vacuole targeting (Cvt) pathway. The machinery required in both pathways includes a novel protein modification system involving the conjugation of two autophagy proteins, Apg12p and Apg5p. The conjugation reaction was demonstrated to be dependent on Apg7p, which shares homology with the E1 family of ubiquitin-activating enzymes. In this study, we demonstrate that Apg7p functions at the sequestration step in the formation of Cvt vesicles and autophagosomes. The subcellular localization of Apg7p fused to green fluorescent protein (GFP) indicates that a subpopulation of Apg7pGFP becomes membrane associated in an Apg12p-dependent manner. Subcellular fractionation experiments also indicate that a portion of the Apg7p pool is pelletable under starvation conditions. Finally, we demonstrate that the Pichia pastoris homologue Gsa7p that is required for peroxisome degradation is functionally similar to Apg7p, indicating that this novel conjugation system may represent a general nonclassical targeting mechanism that is conserved across species.
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PMID:Apg7p/Cvt2p is required for the cytoplasm-to-vacuole targeting, macroautophagy, and peroxisome degradation pathways. 1023 48

In nutrient-rich, vegetative conditions, the yeast Saccharomyces cerevisiae transports a resident protease, aminopeptidase I (API), to the vacuole by the cytoplasm to vacuole targeting (Cvt) pathway, thus contributing to the degradative capacity of this organelle. When cells subsequently encounter starvation conditions, the machinery that recruited precursor API (prAPI) also sequesters bulk cytosol for delivery, breakdown, and recycling in the vacuole by the autophagy pathway. Each of these overlapping alternative transport pathways is specifically mobilized depending on environmental cues. The basic mechanism of cargo packaging and delivery involves the formation of a double-membrane transport vesicle around prAPI and/or bulk cytosol. Upon completion, these Cvt and autophagic vesicles are targeted to the vacuole to allow delivery of their lumenal contents. Key questions remain regarding the origin and formation of the transport vesicle. In this study, we have cloned the APG9/CVT7 gene and characterized the gene product. Apg9p/Cvt7p is the first characterized integral membrane protein required for Cvt and autophagy transport. Biochemical and morphological analyses indicate that Apg9p/Cvt7p is localized to large perivacuolar punctate structures, but does not colocalize with typical endomembrane marker proteins. Finally, we have isolated a temperature conditional allele of APG9/CVT7 and demonstrate the direct role of Apg9p/Cvt7p in the formation of the Cvt and autophagic vesicles. From these results, we propose that Apg9p/Cvt7p may serve as a marker for a specialized compartment essential for these vesicle-mediated alternative targeting pathways.
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PMID:Apg9p/Cvt7p is an integral membrane protein required for transport vesicle formation in the Cvt and autophagy pathways. 1066 73

The vacuole/lysosome performs a central role in degradation. Proteins and organelles are transported to the vacuole by selective and non-selective pathways. Transport to the vacuole by autophagy is the primary mode for degradation of cytoplasmic constituents under starvation conditions. Autophagy overlaps mechanistically and genetically with a biosynthetic pathway termed Cvt (Cytoplasm-to-vacuole targeting) that operates under vegetative conditions to transport the resident vacuolar hydrolase aminopeptidase I (API). API import has been dissected to reveal the action of a novel mechanism that transports cargo within double-membrane vesicles. Recent work has uncovered molecular components involved in autophagy and the Cvt pathway.
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PMID:Transport of proteins to the yeast vacuole: autophagy, cytoplasm-to-vacuole targeting, and role of the vacuole in degradation. 1090 74

Autophagy is a degradative pathway by which cells sequester nonessential, bulk cytosol into double-membrane vesicles (autophagosomes) and deliver them to the vacuole for recycling. Using this strategy, eukaryotic cells survive periods of nutritional starvation. Under nutrient-rich conditions, autophagy machinery is required for the delivery of a resident vacuolar hydrolase, aminopeptidase I, by the cytoplasm to vacuole targeting (Cvt) pathway. In both pathways, the vesicle formation process requires the function of the starvation-induced Aut7 protein, which is recruited from the cytosol to the forming Cvt vesicles and autophagosomes. The membrane binding of Aut7p represents an early step in vesicle formation. In this study, we identify several requirements for Aut7p membrane association. After synthesis in the cytosol, Aut7p is proteolytically cleaved in an Aut2p-dependent manner. While this novel processing event is essential for Aut7p membrane binding, Aut7p must undergo additional physical interactions with Aut1p and the autophagy (Apg) conjugation complex before recruitment to the membrane. Lack of these interactions results in a cytosolic distribution of Aut7p rather than localization to forming Cvt vesicles and autophagosomes. This study assigns a functional role for the Apg conjugation system as a mediator of Aut7p membrane recruitment. Further, we demonstrate that Aut1p, which physically interacts with components of the Apg conjugation complex and Aut7p, constitutes an additional factor required for Aut7p membrane recruitment. These findings define a series of steps that results in the modification of Aut7p and its subsequent binding to the sequestering transport vesicles of the autophagy and cytoplasm to vacuole targeting pathways.
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PMID:Membrane recruitment of Aut7p in the autophagy and cytoplasm to vacuole targeting pathways requires Aut1p, Aut2p, and the autophagy conjugation complex. 1114 20

Three overlapping pathways mediate the transport of cytoplasmic material to the vacuole in Saccharomyces cerevisiae. The cytoplasm to vacuole targeting (Cvt) pathway transports the vacuolar hydrolase, aminopeptidase I (API), whereas pexophagy mediates the delivery of excess peroxisomes for degradation. Both the Cvt and pexophagy pathways are selective processes that specifically recognize their cargo. In contrast, macroautophagy nonselectively transports bulk cytosol to the vacuole for recycling. Most of the import machinery characterized thus far is required for all three modes of transport. However, unique features of each pathway dictate the requirement for additional components that differentiate these pathways from one another, including at the step of specific cargo selection.We have identified Cvt9 and its Pichia pastoris counterpart Gsa9. In S. cerevisiae, Cvt9 is required for the selective delivery of precursor API (prAPI) to the vacuole by the Cvt pathway and the targeted degradation of peroxisomes by pexophagy. In P. pastoris, Gsa9 is required for glucose-induced pexophagy. Significantly, neither Cvt9 nor Gsa9 is required for starvation-induced nonselective transport of bulk cytoplasmic cargo by macroautophagy. The deletion of CVT9 destabilizes the binding of prAPI to the membrane and analysis of a cvt9 temperature-sensitive mutant supports a direct role of Cvt9 in transport vesicle formation. Cvt9 oligomers peripherally associate with a novel, perivacuolar membrane compartment and interact with Apg1, a Ser/Thr kinase essential for both the Cvt pathway and autophagy. In P. pastoris Gsa9 is recruited to concentrated regions on the vacuole membrane that contact peroxisomes in the process of being engulfed by pexophagy. These biochemical and morphological results demonstrate that Cvt9 and the P. pastoris homologue Gsa9 may function at the step of selective cargo sequestration.
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PMID:Cvt9/Gsa9 functions in sequestering selective cytosolic cargo destined for the vacuole. 1130 18


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