Abstract
Macroautophagy/autophagy is a conserved lysosome-dependent metabolic recycling pathway. ULK1 plays an essential role in autophagy initiation through a complex formed with ATG13, RB1CC1/FIP200, and ATG101 in mammalian cells. However, while autophagy is triggered by nutrient starvation where it is essential for cell survival, such conditions lead to the rapid degradation of ULK1, indicating that autophagy must be tightly controlled. Nevertheless, the precise mechanisms regulating the ULK1 complex are still largely unknown. Here we reveal the critical roles played by two novel ULK1 complex binding proteins in autophagy regulation, TRIM27 and STK38L. We show that basal autophagy is maintained through TRIM27-mediated ubiquitination and proteasomal degradation of ULK1, whereas under starvation conditions, excessive autophagy is restrained by the combined actions of TRIM27 and STK38L. TRIM27 ubiquitinates and activates STK38L which in turn phosphorylates ULK1, delivering ULK1 in a permissive state for hyper-ubiquitination by TRIM27. Thus, TRIM27 and STK38L kinase act in concert as a rheostat to control ULK1 levels. We further demonstrate increased basal autophagy in <i>trim27</i> knockout mice and establish physiological relevance in the context of breast cancer. Our study highlights the STK38L-TRIM27-ULK1 axis as a potential treatment avenue to explore for activating autophagy in various disease states.