Structural basis of Atg conjugation systems essential for autophagy

Autophagy is an evolutionarily-conserved, intracellular degradation system for which two ubiquitin-like modifiers, Atg8 and Atg12, play essential roles. After processed by Atg4, the exposed C-terminal glycine of Atg8 is activated by Atg7 (E1) and is then transferred to Atg3 (E2), and is finally conjugated with a phospholipid, phosphatidylethanolamine (PE) through an amide bond. Whereas, Atg12 is activated by the same E1, Atg7, without processing, and is then transferred to Atg10 (E2), and is finally conjugated with Atg5 through an isopeptide bond. Atg12-Atg5 conjugates, together with Atg16, function as an E3-like enzyme to facilitate the conjugation reaction between Atg8 and PE. During autophagy, Atg8-PE conjugates play a critical role in selective cargo recognition in addition to autophagosome formation. We determined the structures of all these Atg proteins and their complexes mainly by X-ray crystallography, and performed structure-based biochemical analyses on them [1,2]. These studies established the molecular mechanisms of Atg8 and Atg12 modification reactions that have many unique features compared with canonical ubiquitin-like systems. Furthermore, we found a conserved motif named the Atg8-family interacting motif (AIM), through which Atg8 recognizes specific cargoes and selectively incorporates them into autophagosomes for degradation [3].

Structure basis for E2-E3 interaction in the plant Atg conjugation system

Autophagy is an intracellular degradation system conserved from yeast to mammal that isolates the cellular materials and organelles into a double membrane vesicle, termed an autophagosome, for degradation. Autophagosome formation depends on Atg8 and Atg12 conjugation systems, which produce Atg8-phosphatidylethanolamine (PE) conjugate as a final product that plays a crucial role for promoting autophagy. Another conjugate, Atg12-Atg5, functions as E3 and promotes transfer of Atg8 from Atg3 (E2) to PE. Here, we identified the minimum binding region of Atg3 for Atg12 by in vitro pull-down assay using Arabidopsis thaliana (At) homologs and crystallized the AtAtg12b-AtAtg3 peptide complex. The obtained crystal (P64, a = 128.5, b = 128.5, c = 163.2 Å) diffracted X-rays to 3.2 Å resolution, and its structure was determined by the molecular replacement method. AtAtg12b forms a swapping dimer in the crystal. The side-chain of AtAtg3 Met157 is bound deeply to the hydrophobic pocket of AtAtg12b consisting of Phe30, Val41, and Phe44. The importance of AtAtg3 Met157 for AtAtg12b binding was confirmed by mutational analysis. These data establish the basis for E2-E3 interaction in the plant Atg system.