Figure 1.

The macroautophagy process. Macroautophagy takes place in five main steps: (1) Induction occurs after a metabolic or therapeutic stress, and is mediated by a complex containing the ULK1 protein. (2) During the nucleation step, the formation of the phagophore (or isolation membrane) is initiated. This action is mainly triggered by a protein complex containing VPS34, a PtdIns3K of class III. (3) Elongation of the phagophore involves two «ubiquitin like» conjugation systems: the Atg12–Atg15 and the Atg8–PE, which are required for autophagosome formation. (4) Once formed, the autophagosome containing a cytosolic cargo will fuse with the lysosome, which triggers (5) the degradation of its content and the release of primary components in the cytosol for recycling. The lysosome can then be regenerated so that the process can start again.