Type One Protein Phosphatase Regulates Fixed-Carbon Starvation-Induced Autophagy by Dephosphorylating ATG13a to Facilitate ATG1a-ATG13a Formation in Arabidopsis

ABSTRACT Autophagy, a conserved pathway which carries out the bulk degradation of cytoplasmic material in eukaryotic cells, is critical in plant physiology and development. It is tightly regulated by ATG13, a core component of ATG1 kinase complex which initiates autophagy. Although it has been reported that ATG13 is dephosphorylated immediately after nutrient starvation, the phosphatase regulating this process is poorly understood. Here, we demonstrated that the septuple mutant ( topp-7m ) and octuple mutant ( topp-8m ) of type one protein phosphatase (TOPP) exhibited significantly reduced tolerance to fixed-carbon (C) starvation due to compromised autophagy activity. Genetic analysis placed TOPP upstream of autophagy. Interestingly, ATG13a was found to be an interactor of TOPP. And TOPP directly dephosphorylated ATG13a in vitro and in vivo . Meanwhile, eighteen phosphorylation sites of ATG13a were identified by LC-MS. Mimic dephosphorylation of ATG13a at these 18 sites significantly promoted autophagy and increased the atg13ab mutant tolerance to fixed-C starvation. Further study showed that the dephosphorylation of ATG13a facilitated ATG1a-ATG13a complex formation. Consistently, the recruitment of ATG13a for ATG1a was markedly inhibited in topp-7m-1 . In addition, TOPP-controlled dephosphorylation of ATG13a boosted ATG1a phosphorylation. Taken together, our study reveals the crucial role of TOPP in regulating autophagy by stimulating the formation of ATG1a-ATG13a complex through dephosphorylating ATG13a in Arabidopsis .