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   » » Wiki: Microautophagy
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Microautophagy is one of the three common forms of , but unlike and chaperone-mediated autophagy, it is mediated—in by action or in and by action—by direct engulfment of the cargo. Cytoplasmic material is trapped in the lysosome/vacuole by a random process of invagination.

The microautophagic pathway is especially important for survival of cells under conditions of starvation, deprivation, or after treatment with . Generally a non-selective process, there are three special cases of a selective microautophagic pathway: micropexophagy, piecemeal microautophagy of the nucleus, and micromitophagy, all which are activated only under a specific conditions.


Functions of microautophagy
Microautophagy together with is necessary for nutrient recycling under starvation. Microautophagy due to degradation of incorporated into vesicles regulates the composition of / membrane. Microautophagic pathway functions also as one of the mechanism of delivery into the . This autophagic pathway engulfs multivesicular bodies formed after therefore it plays role in membrane proteins turnover. Microautophagy is also connected with size maintenance, composition of biological membranes, cell survival under nitrogen restriction, and the transition pathway from starvation-induced growth arrest to logarithmic growth.


Non-selective microautophagy
Non-selective microautophagic process can be dissected into 5 distinct steps. The majority of experiments were done in (vacuolar invaginations) but the molecular principles seem to be more general.


Membrane invagination and autophagic tubes formation
is a constitutive process but its frequency is dramatically increased during periods of starvation. Invagination is a process by which is formed the tube.

Formation of the tubes is mediated through Atg7-dependent -like conjugation (Ublc) or via vacuolar transporter chaperone (VTC) molecular complex which acts through -dependent manner. Calmodulin involvement in tube formation is independent process.


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

The process of microautophagic vesicle formation is similar to multivesicular bodies formation process


Vesicle expansion and scission
Enlargement of vesicle is mediated by binding inside of unclosed vesicle. Basically, this process is reversal to . Process follows by pich of the vesicle into the lysosomal/vacuolar lumen. This process is independent on proteins.


Vesicle degradation and recycling
Vesicle moves freely in the lumen and during the time is degraded by (ec. Atg15p). Nutrients are then released by Atg22p.


Selective microautophagy
Process of non-selective microautophagy can be observed in all types of . On the other hand, selective microautophagy is commonly observed in cells. Three types of selective microautophagy selective microautophagy can be distinguished: micropexophagy, piecemeal microautophagy of the nucleus and micromitophagy

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