Dual roles of Atg8−PE deconjugation by Atg4 in autophagy

Abstract

Modification of target molecules by ubiquitin or ubiquitin-like (Ubl) proteins is generally reversible. Little is known, however, about the physiological function of the reverse reaction, deconjugation. Atg8 is a unique Ubl protein whose conjugation target is the lipid phosphatidylethanolamine (PE). Atg8 functions in the formation of double-membrane autophagosomes, a central step in the well-conserved intracellular degradation pathway of macroautophagy (hereafter autophagy). Here we show that the deconjugation of Atg8−PE by the cysteine protease Atg4 plays dual roles in the formation of autophagosomes. During the early stage of autophagosome formation, deconjugation releases Atg8 from non-autophagosomal membranes to maintain a proper supply of Atg8. At a later stage, the release of Atg8 from intermediate autophagosomal membranes facilitates the maturation of these structures into fusion-capable autophagosomes. These results provide new insights into the functions of Atg8−PE and its deconjugation.

Regulation of ATG8 membrane association by ATG4 in the parasitic protistToxoplasma gondii

“…In yeast, interfering with Atg8 deconjugation results in the mislocalization of Atg8 to the membrane of subcellular compartments other than autophagosomes, such as the ER, endosomes or the vacuole (yeast lysosomal compartment). [32][33][34] Depletion of TgATG4 leads to an increase in membraneassociated TgATG8, which potentially accumulates at the apicoplast prior to the loss of this organelle, or can be also found at vesicular structures that are devoid of apicoplast markers and could represent abnormal autophagic vesicles. It would be tempting to speculate that the multimembrane structures we observed by electron microscopy in TgATG4-depleted parasites are TgATG8-enriched, but we have not been able to demonstrate this by immuno-electron microscopy.…”

“…This suggests that the delipidation step of TgATG8 is equally important for an efficient function, as suggested for its role in forming autophagosomes in yeast. 32,33 TgATG4 is a very unusual enzyme with very large sequence insertions compared with most eukaryotic orthologs. As a consequence, we cannot use the structure information of the Atg4B-LC3 complex, 36 for example, to predict how it is interacting with TgATG8.…”

Limited and digestive proteolysis: crosstalk between evolutionary conserved pathways

“…Intriguingly, ATG8 is removed from the membrane of a mature autophagosome by the same ATG4 protease, which has the capacity to hydrolyze the amide bond between ATG8 and PE (Nair et al, 2012). The dual function of ATG4 is crucial for formation and elongation of the autophagic membrane (Feng et al, 2014), proper localization and recycling of ATG8 (Nakatogawa et al, 2012), and finally removal of ATG8 from the mature autophagosomes, which seems necessary for their subsequent fusion with a lytic compartment (Yu et al, 2012).…”

Structural biology of the macroautophagy machinery

“…During the later stages of autophagosome formation, Atg8 is deconjugated from PE by Atg4. This event is important for recycling other core Atg components and for completing the autophagosome biogenesis process (Nair et al, 2012;Nakatogawa et al, 2012;Yu et al, 2012). Interestingly, multiple Atg8 orthologs exist in mammalian cells, and these include LC3, GABARAP, and GATE-16.…”