Autophagy

Autophagy, or autophagocytosis, is a process of organelle degradation that takes place inside the cell. It is executed by lysosomes and is part of everyday normal cell growth and development. Its main purpose is to maintain a balance between biogenesis (production) of cell structures, and their degradation and turnover (see Bruce Alberts et al.: Molecular biology of the cell, 4th edition, Garland Publishing 2002, NCBI Bookshelf [1]). For example, a liver-cell mitochondrion lasts around ten days before it is degraded and its contents are reused.

Autophagy also plays a major role in the destruction of bacteria and unecessary proteins that have begun to aggregate within a cell and may potentially cause problems. It is especially prominent in insects that undergo complete metamorphosis; larval tissue is recycled to become appendages in an adult insect.

The rate of autophagy increases when the cell is subjected to food deprivation, and also when it receives stimuli that result in organelle proliferation.

When autophagy involves the total destruction of the cell, it is called autophagic cell death (also known as cytoplasmic cell death). This is one of the main types of programmed cell death. As such, it is a regulated process of cell death in a multicellular organism, or in a colony of individual cells such as yeast (see Daniel J. Klionsky and Scott D. Emr: "Autophagy as a Regulated Pathway of Cellular Degradation", Science Vol. 290 p.1717, 1 Dec. 2000 [2]).

Autophagy can be further separated into two types: macroautophagy and microautophagy. Macroautophagy involves the formation of a membrane containing target materials moving into the lysosome/vacuole while microautophagy is the invagination of the lysosome/vacuole of target materials. Although both micro and macroautophagy serve as fundamental functions in plants and metazoa (multicellular animals), as well as in other eukaryotes such as slime moulds and yeast.

Cytoplasmic autophagy is different from other methods of protein degradation, i.e. Polyubiquitination

Further reading

• D. J. Klionsky, Editor: Autophagy (Landes Bioscience, Georgetown, Tx, 2004)