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. Author manuscript; available in PMC: 2015 Feb 2.
Published in final edited form as: Nature. 2015 Jan 15;517(7534):302–310. doi: 10.1038/nature14190

Figure 4. Nutrients and Autophagy.

Figure 4

Autophagy serves as an internal source of stored nutrients under conditions of nutrient limitation. Two main regulatory inputs for autophagy are AMPK and mTORC1. Autophagy initiation can be promoted by the activation of ULK1 via AMPK-dependent phosphorylation during low ATP:AMP ratio. mTORC1 is activated by growth factors at the outer lysosomal surface if cellular AAs and glucose have recruited mTORC1 via the action of the Rag GTPases. Once activated, mTORC1 inhibits ULK1 and Atg13 by phosphorylation. Hence, low nutrients promote autophagy by the inhibition of mTORC1. Autophagy starts with the engulfment of cellular constituents: glycogen, lipids from lipid droplets, soluble proteins, ribosomes or organelles in a double membrane structure that then fuses with lysosomes, where the enzymatic breakdown occurs. The products of autophagy, basic nutrients (sugars, lipids, amino acid, and nucleosides), are then exported into the cytoplasm, where may be used as a source of energy, or re-used for anabolism.