Macroautophagy sequestrates damaged organelles and unused long-lived proteins in a double-membrane vesicle, called an ''autophagosome'' or ''autophagic vacuole (AV)'', inside the cell. Autophagosomes form from the elongation of small membrane structures known as ''autophagosome precursors''.
The formation of autophagosomes is initiated by class III phosphoinositide 3-kinase and autophagy-related gene (Atg) 6 (also known as Beclin-1).
In addition, two further systems are involved, composed of the ubiquitin-like protein Atg8 (known as LC3 in mammalian cells) and the Atg4 protease on the one hand and the Atg12-Atg5-Atg16 complex on the other.
The outer membrane of the autophagosome fuses in the cytoplasm with a lysosome to form an ''autolysosome'' or ''autophagolysosome'' where their contents are degraded via acidic lysosomal hydrolases.
Microautophagy, on the other hand, happens when lysosomes directly engulf cytoplasm by invaginating, protrusion, and/or septation of the lysosomal limiting membrane.
In Chaperone-mediated autophagy, or CMA, only those proteins that have a consensus peptide sequence get recognized by the binding of a hsc70-containing chaperone/co-chaperone complex.
This CMA substrate/chaperone complex then moves to the lysosomes, where the CMA receptor lysosome-associated membrane protein type-2A (LAMP-2A) recognizes it; the protein is unfolded and translocated across the lysosome membrane assisted by the lysosomal hsc70 on the other side. CMA differs from macroautophagy and microautophagy in two main ways:
- The substrates are translocated across the lysosome membrane on a one-by-one basis, whereas in the macroautophagy and microautophagy the substrates are engulfed or sequestered in-bulk.
- CMA is very selective in what it degrades and can degrade only certain proteins and not organelles.
Autophagy is part of everyday normal cell growth and development wherein mTOR plays an important regulatory role.
This article is licensed under the Creative Commons Attribution-ShareAlike License.
It uses material from the Wikipedia article on
All material adapted used from Wikipedia is available under the terms of the
Creative Commons Attribution-ShareAlike License.
Wikipedia® itself is a registered trademark of the Wikimedia Foundation, Inc.