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Numerous stimuli can induce autophagy – for example hypoxia and nutrient starvation – which enable the cell to free vital amino acids to re-use as well as clear aberrant structures or organelles. There are three types of autophagy: Macroautophagy, microautophagy, and chaperone-mediated autophagy.
Autophagy involves a series of sequential steps that start with initiation/sequestration, continue with fusion with lysosomes, ultimately leading to degradation.
Initiation of autophagy occurs when a double membrane forms within the cytoplasm that non-selectively engulfs proteins and structures. Autophagy-mediated degradation of specific proteins and organelles can occur via targeting proteins.
The early double membrane structure begins as the phagophore and matures into the autophagosome when an enclosed vesicle is formed. Numerous proteins are involved in the initiation and development of the autophagosome, these are illustrated below.
Autophagy concludes by the fusion of the autophagosome with the lysosome, where the cytoplasmic constituents are degraded by lysosomal hydrolases.