Autophagy

Under stress conditions, such as starvation, deficiency, or infection, cells dispose of obsolete or worn-out organelles through a "self-eating" process called autophagy.

Autophagy occurs when other disposal mechanisms cannot degrade large substances such as whole organelles, macromolecules, and protein aggregates.

Autophagy begins when membrane vesicles of unknown origin fuse to form a crescent-shaped structure that grows and surrounds cytoplasmic cargo to create a double membraned autophagosome.

The outer membrane of the autophagosome contains transmembrane protein markers, such as ATG9, that target it to the lysosome.

SNARE proteins on the outer membrane of the autophagosome mediate membrane fusion with the lysosome to form an autolysosome.

Lysosomal lipases and proteases digest the inner membrane of the autophagosome and its contents. Amino acid permeases on the outer membrane allow the transport of free amino acids back to the cytosol, where they are used to synthesize new proteins.

After the degradation is complete, only the residual body remains, which can be eliminated by exocytosis or retained in the cytosol indefinitely as lipofuscin granule.