Autophagy is a bulk degradation system induced by cellular stresses such as nutrient starvation. Its function relies on the formation of double-membrane vesicles called autophagosomes. Unlike other organelles that appear to stably exist in the cell, autophagosomes are formed on demand, and once their formation is initiated, it proceeds surprisingly rapidly. ~ Japanese cytologists Shusaku Shibutani & Tamotsu Yoshimori
The cytoplasm of a cell is a busy place. Most cellular activities happen within that voluminous, gelatinous space. A vast population of proteins and other macromolecules bustle about. Waste is discarded as work proceeds. Old proteins expire on the spot.
Autophagy is the cleanup process: hauling out cytoplasmic sludge to keep the cell running efficiently. Reusable bits, such as amino acids, are recycled. All eukaryotic cells employ autophagy.
Amino acid starvation or other trigger sends a complex of proteins to form a cup-shaped container in the cytoplasm. This phagophore expands to engulf the targeted trash.
A phagophore’s open end closes to form a double-membraned vesicle. An autophagosome is born.
Yeast autophagosomes are 0.3–0.9 µm. Mammal autophagosomes range from 0.5–1.5 µm: large enough to engulf a worn-out organelle.
An autophagosome fuses with a lysosome (in animals) or a vacuole (in plants, fungi, yeast, and other microbes). Then the contents inside the autophagosome and its inner membrane are degraded by hydrolases: enzymes that catalyze molecular breakdown via hydrolysis. The entire process takes 15 minutes or less.
Autophagy is an essential cellular process. Cells become cesspools if not for autophagy.
Lively autophagy is promoted by a healthy lifestyle, particularly curtailed food consumption. Chronic overeating promotes autophagy breakdown, as the body has a surfeit of material from which to readily draw cellular building blocks.
The vitality of autophagy is a principal cellular mechanism in the aging process. Its dysfunction causes innumerable diseases, including diabetes, heart problems, tumors, and intelligence system degradation.
Cancer cells know how important autophagy is. They sometimes invoke autophagy to save themselves from treatments intended to starve them to death.