Autophagy is a well regulated and critical intracellular catabolic process involving lysosome-dependent degradation of undesired cytosolic components, such as dysfunctional organelles, invasive pathogens, and bulk protein aggregates, for cellular homeostasis and/or adaptation to various stresses in mammals. As a unique type of adaptor proteins, autophagy receptors play pivotal roles in selective autophagy processes. They can serve as bridging adaptors to specifically link the autophagic substrates with autophagosomes, thereby mediating the subsequent selective autophagy processes of the targeting substrates. The dysfunctions of autophagy receptors are associated with many human diseases, such as Crohn’s disease and neurodegenerative diseases. NDP52 is a crucial autophagy receptor in mammals, which plays important roles in the selective autophagy of invading pathogens such as Salmonella enterica Typhimurium (xenophagy), and depolarized mitochondria (mitophagy). The recruitment of ULK and TBK1 complexes by NDP52 is essential for the initiation of autophagosome formation during NDP52-mediated selective autophagy, and is mediated by the interactions of NDP52 with adaptor proteins RB1CC1 and NAP1. However, the molecular mechanism governing the association of NDP52, RB1CC1 and NAP1 remains elusive. Moreover, the recruited TBK1 kinase can directly phosphorylate NDP52 and regulate its autophagic function in selective autophagy, but the precise downstream effects induced by this TBK1-mediated phosphorylation of NDP52 are still largely unknown.

 

Recently, researchers from Shanghai Institute of Organic Chemistry, CAS have systematically characterized the interaction between NDP52 and RB1CC1, and discovered, for the first time, that there are dual interaction sites between NDP52 and RB1CC1. In addition to the NDP52 SKICH/RB1CC1 coiled-coil interaction, the LIR motif of NDP52 can directly bind to RB1CC1 Claw domain, like that of NAP1 FIR motif. Furthermore, they demonstrated that NAP1 FIR is essential for the formation of stable NDP52/RB1CC1/NAP1 ternary complex. Subsequently, they elucidated the detailed binding mechanism and uncovered the key determinants for the interactions of RB1CC1 with NDP52 and NAP1 by solving the crystal structure of NDP52 SKICH/RB1CC1 coiled-coil complex and the related NAP1 FIR/RB1CC1 Claw complex. Importantly, the determined NAP1 FIR/RB1CC1 Claw complex structure together with relevant biochemical analyses also uncovers a general binding mode shared by many FIR motifs for interacting with RB1CC1 Claw. Finally, they elucidated that NDP52 LIR and NAP1 FIR can both selectively bind to ATG8 family proteins, and the TBK1-mediated phosphorylation of NDP52 LIR can promote the interactions of NDP52 LIR with ATG8 family proteins. They further determined the NAP1 FIR/GABARAP complex structure, and revealed that RB1CC1 Claw and ATG8 family proteins are competitive in binding to NAP1 FIR and NDP52 LIR. In summary, their findings provide mechanistic insights into the interactions of NDP52, NAP1 with RB1CC1 as well as ATG8 family proteins, and establish a new paradigm by which autophagy receptor interacts with RB1CC1 and NAP1 to recruit the autophagy-initiating ULK and TBK1 complexes for autophagosome biogenesis in selective autophagy.

 

The research findings titled “Structural and biochemical advances on the recruitment of the autophagy-initiating ULK and TBK1 complexes by autophagy receptor NDP52” have been published in the journal Science Advances (https://advances.sciencemag.org/content/7/33/eabi6582).

 

Pan Lifeng Ph.D.Professor

Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences

Ling Ling Road 345 Shanghai 200032 China

Tel: 0086-21-54925561

Email: panlf@sioc.ac.cn