microautophagy

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/07/13 07:04:01」(JST)

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Microautophagy (unlike Macroautophagy and Chaperone-mediated autophagy) is the type of autophagic pathway which is mediated by direct lysosomal (mammals) or vacuolar (plants and fungi) engulfment of the cytoplasmic cargo. Cytoplasmic material is trapped in the lysosome/vacuole by the random process of membrane invagination. Microautophagic pathway is especially important for survival of cells under starvation, nitrogen deprivation or after teratment with rapamycin. Microautophagy generally is a non-selective process but there are 3 special cases of selective microautophagic pathway (micropexophagy, Piecemeal microautophagy of the nucleus, micromitophagy) which are activated under a specific conditions.^[1]

Functions of microautophagy

Microautophagy together with macroautophagy is necessary for nutrient recycling under starvation. Microautophagy due to degradation of lipids incorporated into vesicles regulates the composition of lysosomal/vacuolar membrane.^[1] Microautophagic pathway functions also as one of the mechanism of glycogen delivery into the lysosomes.^[2] This autophagic pathway engulfs multivesicular bodies formed after endocytosis therefore it plays role in membrane proteins turnover.^[3] Microautophagy is also connected with organellar size maintenance, composition of biological membranes, cell survival under nitrogen restriction, and the transition pathway from starvation-induced growth arrest to logarithmic growth.^[1]

Non-selective microautophagy

Non-selective microautophagic process can be dissected into 5 distinct steps. Majority of experiments were done on yeast (vacuolar invaginations) but the molecular principles seem to be more general ^[1]

Membrane invagination and autophagic tubes formation

Invagination is a constitutive process but it´s frequency is dramatically increased during periods of starvation. Invagination is a tubular process by which is formed the autophagic tube.^[4]

Formation of the autophagic tubes is mediated through Atg7-dependent ubiquitin-like conjugation (Ublc) or via vacuolar transporter chaperone (VTC) molecular complex which acts through calmodulin-dependent manner. Interestingly, calmodulin involvement in tube formation is calcium independent process.^[5]^[6]

Vesicle formation

The mechanism of vesicle formation is based on lateral sorting mechanism. Changed composition of membrane molecules (lipid enrichment in the autophagic tubes due to removal of transmembrane proteins) leads spontaneous vesicle formation via phase separation mechanism.^[4]

The process of microautophagic vesicle formation is similar to multivesicular bodies formation process ^[7]

Vesicle expansion and scission

Enlargement of vesicle is mediated by binding enzymes inside of unclosed vesicle. Basically, this process is reversal to endocytosis. Process follows by pich of the vesicle into the lysosomal/vacuolar lumen. This process is independent on SNARE proteins.^[8]

Vesicle degradation and recycling

Vesicle moves freely in the lumen and during the time is degraded by hydrolases (ec. Atg15p). Nutrients are then released by Atg22p.^[1]

Selective microautophagy

Process of non-selective microautophagy can be observed in all types of eucaryotic cells. On the other hand selective microautophagy is commonly observed in yeast cells. Three types of selective microautophagy selective microautophagy can be distinguished: micropexophagy, piecemeal microautophagy of the nucleus and micromitophagy ^[1]^[9]

References

^ ^a ^b ^c ^d ^e ^f Li, WW; Li, J; Bao, JK (Apr 2012). "Microautophagy: lesser-known self-eating.". Cellular and molecular life sciences : CMLS 69 (7): 1125–36. PMID 22080117. Cite uses deprecated parameters (help)
^ Takikita, S; Myerowitz, R; Zaal, K; Raben, N; Plotz, PH (Apr 2009). "Murine muscle cell models for Pompe disease and their use in studying therapeutic approaches.". Molecular genetics and metabolism 96 (4): 208–17. PMID 19167256. Cite uses deprecated parameters (help)
^ Saksena, S; Emr, SD (Feb 2009). "ESCRTs and human disease.". Biochemical Society transactions 37 (Pt 1): 167–72. PMID 19143624. Cite uses deprecated parameters (help)
^ ^a ^b Müller, O; Sattler, T; Flötenmeyer, M; Schwarz, H; Plattner, H; Mayer, A (Oct 30, 2000). "Autophagic tubes: vacuolar invaginations involved in lateral membrane sorting and inverse vesicle budding.". The Journal of cell biology 151 (3): 519–28. PMID 11062254. Cite uses deprecated parameters (help)
^ Doelling, JH; Walker, JM; Friedman, EM; Thompson, AR; Vierstra, RD (Sep 6, 2002). "The APG8/12-activating enzyme APG7 is required for proper nutrient recycling and senescence in Arabidopsis thaliana.". The Journal of biological chemistry 277 (36): 33105–14. PMID 12070171. Cite uses deprecated parameters (help)
^ Uttenweiler, A; Schwarz, H; Mayer, A (Sep 30, 2005). "Microautophagic vacuole invagination requires calmodulin in a Ca2+-independent function.". The Journal of biological chemistry 280 (39): 33289–97. PMID 16055436. Cite uses deprecated parameters (help)
^ Sattler, T; Mayer, A (Oct 30, 2000). "Cell-free reconstitution of microautophagic vacuole invagination and vesicle formation.". The Journal of cell biology 151 (3): 529–38. PMID 11062255. Cite uses deprecated parameters (help)
^ Tian, Y; Li, Z; Hu, W; Ren, H; Tian, E; Zhao, Y; Lu, Q; Huang, X; Yang, P; Li, X; Wang, X; Kovács, AL; Yu, L; Zhang, H (Jun 11, 2010). "C. elegans screen identifies autophagy genes specific to multicellular organisms.". Cell 141 (6): 1042–55. PMID 20550938. Cite uses deprecated parameters (help)
^ Mijaljica, D; Prescott, M; Devenish, RJ (Jul 2011). "Microautophagy in mammalian cells: revisiting a 40-year-old conundrum.". Autophagy 7 (7): 673–82. PMID 21646866. Cite uses deprecated parameters (help)

English Journal

Direct evidence of intracrine angiotensin II signaling in neurons.

Deliu E1, Brailoiu GC, Eguchi S, Hoffman NE, Rabinowitz JE, Tilley DG, Madesh M, Koch WJ, Brailoiu E.
American journal of physiology. Cell physiology.Am J Physiol Cell Physiol.2014 Apr;306(8):C736-44. doi: 10.1152/ajpcell.00131.2013. Epub 2014 Jan 8.
The existence of a local renin-angiotensin system (RAS) in neurons was first postulated 40 years ago. Further studies indicated intraneuronal generation of ANG II. However, the function and signaling mechanisms of intraneuronal ANG II remained elusive. Since ANG II type 1 receptor (AT1R) is the majo
PMID 24401846

Interorganelle interactions and inheritance patterns of nuclei and vacuoles in budding yeast meiosis.

Tsai IT1, Lin JL2, Chiang YH1, Chuang YC3, Liang SS4, Chuang CN2, Huang TN2, Wang TF5.
Autophagy.Autophagy.2014 Feb;10(2):285-95. doi: 10.4161/auto.27192. Epub 2013 Dec 4.
Many of the mechanisms by which organelles are inherited by spores during meiosis are not well understood. Dramatic chromosome motion and bouquet formation are evolutionarily conserved characteristics of meiotic chromosomes. The budding yeast bouquet genes (NDJ1, MPS3, CSM4) mediate these movements
PMID 24345927

The multifunctional autophagy pathway in the human malaria parasite, Plasmodium falciparum.

Cervantes S1, Bunnik EM2, Saraf A3, Conner CM2, Escalante A2, Sardiu ME3, Ponts N2, Prudhomme J2, Florens L3, Le Roch KG2.
Autophagy.Autophagy.2014 Jan;10(1):80-92. doi: 10.4161/auto.26743. Epub 2013 Nov 11.
Autophagy is a catabolic pathway typically induced by nutrient starvation to recycle amino acids, but can also function in removing damaged organelles. In addition, this pathway plays a key role in eukaryotic development. To date, not much is known about the role of autophagy in apicomplexan parasit
PMID 24275162

Japanese Journal

The TOR and EGO protein complexes orchestrate microautophagy in yeast

DUBOULOZ F.
Mol. Cell 19, 15-26, 2005
NAID 30010675233

オルガネラ間トポロジーのダイナミズムペルオキソーム・ミクロオートファジー

阪井康能
バイオサイエンスとインダストリー = Bioscience & industry 56(10), 35-36, 1998-10-01
NAID 10007296970

Lipid Mobilization and Acid Phosphatase Activity in Lytic Compartments during Conidium Dormancy and Appressorium Formation of Colletotrichum graminicola.

Schadeck Ruth Janice Guse,Leite Breno,Buchi Dorly de Freitas
Cell Structure and Function 23(6), 333-340, 1998
… Lipid bodies, stored in the ungerminated conidia cytoplasm, were internalized by vacuoles in a process analogous to microautophagy and were apparently digested inside them. …
NAID 130003513094