Identification of a phosphorylation site on Ulk1 required for genotoxic stress-induced alternative autophagy

Alternative autophagy is an autophagy-related protein 5 (Atg5)-independent type of macroautophagy. Unc51-like kinase 1 (Ulk1) is an essential initiator not only for Atg5-dependent canonical autophagy but also for alternative autophagy. However, the mechanism as to how Ulk1 differentially regulates both types of autophagy has remained unclear. In this study, we identify a phosphorylation site of Ulk1 at Ser(746), which is phosphorylated during genotoxic stress-induced alternative autophagy. Phospho-Ulk1(746) localizes exclusively on the Golgi and is required for alternative autophagy, but not canonical autophagy. We also identify receptor-interacting protein kinase 3 (RIPK3) as the kinase responsible for genotoxic stress-induced Ulk1(746) phosphorylation, because RIPK3 interacts with and phosphorylates Ulk1 at Ser(746), and loss of RIPK3 abolishes Ulk1(746) phosphorylation. These findings indicate that RIPK3-dependent Ulk1(746) phosphorylation on the Golgi plays a pivotal role in genotoxic stress-induced alternative autophagy. Unlike canonical macroautophagy, alternative autophagy does not require the factors Atg5 and Atg7, but does require Ulk1. Here the authors show that phosphorylation of Ulk1 at Ser(746) by RIPK3 is required for alternative autophagy initiation at the Golgi in response to genotoxic stress.