The ULK1 effector BAG2 regulates autophagy initiation by modulating AMBRA1 localization

Autophagy initiation is regulated by the ULK1 kinase complex. To gain insights into functions of the holo-complex, we generated a deep interactome by combining affinity purification- and proximity labeling-mass spectrometry of all four complex members: ULK1, ATG13, ATG101, and RB1CC1/FIP200. Under starvation conditions, the ULK1 complex interacts with several protein and lipid kinases and phosphatases, implying the formation of a signalosome. Interestingly, several selective autophagy receptors also interact with ULK1, indicating the activation of selective autophagy pathways by nutrient starvation. One effector of the ULK1 complex is the HSC/HSP70 co-chaperone BAG2, which regulates the subcellular localization of the VPS34 lipid kinase complex member AMBRA1. Depending on the nutritional status, BAG2 has opposing roles. In growth conditions, the unphosphorylated form of BAG2 sequesters AMBRA1, attenuating autophagy induction. In starvation conditions, ULK1 phosphorylates BAG2 on Ser31, which supports the recruitment of AMBRA1 to the ER membrane, positively affecting autophagy. [Display omitted] •ULK1 complex interacts with protein and lipid kinases and phosphatases forming a signalosome•In starvation, ULK1 complex members interact with selective autophagy receptors (SARs)•ULK1 complex interacts and phosphorylates the HSP70 co-chaperone BAG2•BAG2 has opposing roles in autophagy regulation by modulating ER localization of AMBRA1 Sankar et al. generated a deep ULK1-complex interactome using all four complex members as baits. They define the HSP70 co-chaperone BAG2 as bona fide ULK1 target that has opposing roles in autophagy regulation. Unphosphorylated BAG2 sequesters AMBRA1, attenuating autophagy, whereas phosphorylated BAG2 supports ER recruitment of AMBRA1, positively affecting autophagy.