Redox-mediated activation of ATG3 promotes ATG8 lipidation and autophagy progression in Chlamydomonas

Autophagy is one of the main degradative pathways used by eukaryotic organisms to eliminate useless or damaged intracellular material in order to maintain cellular homeostasis under stress conditions. Mounting evidence indicates a strong interplay between the generation of ROS and the activation of autophagy. Although a tight redox regulation of autophagy has been shown in several organisms including microalgae, the molecular mechanisms underlying this control remain poorly understood. In this study, we have performed an in-depth in vitro and in vivo redox characterization of ATG3, an E2-activating enzyme involved in ATG8 lipidation and autophagosome formation, from two evolutionary distant unicellular model organisms: the green microalga Chlamydomonas reinhardtii and the budding yeast Saccharomyces cerevisiae. Our results indicated that ATG3 activity from both organisms is subjected to redox regulation since these proteins require reducing equivalents to transfer ATG8 to the phospholipid phosphatidylethanolamine. We established the catalytic Cys of ATG3 as redox target in algal and yeast proteins, and showed that the oxidoreductase thioredoxin efficiently reduces ATG3. Moreover, in vivo studies revealed that the redox state of ATG3 from Chlamydomonas reinhardtii undergoes profound changes in the absence of photoprotective carotenoids, a stress condition that activates autophagy in algae.