Fig. 2.

Conjugation systems involved in autophagy. The first conjugation system mediates the formation of the conjugate Atg12/Atg5. Atg12 is activated by the E1-like enzyme Atg7, and then Atg12 is transferred to the E2-like enzyme Atg10. Finally, Atg12 is covalently linked to a specific lysine of Atg5. Then, the conjugate Atg12/Atg5 interacts non-covalently with Atg16 (Atg16L in mammals) to trigger homo-oligomerization leading to a macromolecular complex necessary for the formation of autophagosomes. This structure is associated with the outer side of the autophagosomes in formation and dissociates from the membranes before the autophagosome completion. The second conjugation system results in the formation of the Atg8/PE (phosphatidyl–ethanolamin) conjugate. Atg8 (LC3 in mammals) is first proteolysed by Atg4, activated by the E1-like enzyme Atg7, and then transferred to the E2-like enzyme Atg3. Finally, Atg8 is covalently linked to PE. This conjugate is present in both sides of the autophagosomes and seems fundamental for its completion. The two conjugation systems are connected: The Atg5/Atg12-Atg16 complex is necessary for the formation of the second conjugate. Over-expression of Atg10 facilitates the maturation of MAP-LC3, over-expression of Atg3 facilitates the conjugation of Atg12 to Atg5, and excess amount of the Atg12-Atg5 conjugate inhibits the MAP-LC3 maturation