GABARAPs and LC3s have opposite roles in regulating ULK1 for autophagy induction

ULK1 (unc-51 like autophagy activating kinase 1) is the key mediator of MTORC1 signaling to macroautophagy/autophagy. ULK1 functions as a protein complex by interacting with ATG13, RB1CC1/FIP200, and ATG101. How the ULK1 complex is regulated to trigger autophagy induction remains unclear. In this study, we have determined roles of Atg8-family proteins (ATG8s) in regulating ULK1 activity and autophagy. Using human cells depleted of each subfamily of ATG8, we found that the GABARAP subfamily positively regulates ULK1 activity and phagophore and autophagosome formation in response to starvation. In contrast, the LC3 subfamily negatively regulates ULK1 activity and phagophore formation. By reconstituting ATG8-depleted cells with individual ATG8 members, we identified GABARAP and GABARAPL1 as positive and LC3B and LC3C as negative regulators of ULK1 activity. To address the role of ATG8 binding to ULK1, we mutated the LIR of endogenous ULK1 to disrupt the ATG8-ULK1 interaction by genome editing. The mutation drastically reduced the activity of ULK1, autophagic degradation of SQSTM1, and phagophore formation in response to starvation. The mutation also suppressed the formation and turnover of autophagosomes in response to starvation. Similar to the mutation of the ULK1 LIR, disruption of the ATG13-ATG8 interaction suppressed ULK1 activity and autophagosome formation. In contrast, RB1CC1 did not show any specific binding to ATG8s, and mutation of its LIR did not affect ULK1 activity. Together, this study demonstrates differential binding and opposite regulation of the ULK1 complex by GABARAPs and LC3s, and an important role of the ULK1- and ATG13-ATG8 interactions in autophagy induction.