The role of DAXX in the selective autophagy receptor SQSTM1/p62 phase condensation

Abstract

Macroautophagy (‘autophagy’ hereafter) is a lysosome-dependent degradation system for intracellular protein quality control. Autophagy can selectively remove cargo materials, which is dependent on cargo receptors. SQSTM1/p62 (‘p62’ hereafter) is one of the best characterised receptors mediating the autophagic degradation of misfolded proteins, and its body formation is essential for its cargo recognition and clearance. Mounting evidence has revealed that membraneless compartments, participating in various biological processes, are assembled via liquid-liquid phase separation (LLPS). These phase-separated condensates ensure that cellular activities occur in a spatiotemporally controlled manner and provide a method for concentrating and segregating cellular components. Liquid-like non-membrane condensates present dynamic properties and can exchange macromolecules with surroundings. The formation of non-membrane-bound p62 bodies is mediated by liquid-liquid phase separation. It has been yet poorly characterised the regulation of p62 liquid-liquid phase separation, and the molecular details of the assembly and functions of p62 bodies remain elusive. This study aims to identify a mechanism of it. Via an unbiased yeast two-hybrid screening, DAXX is found a p62 binding protein. Further experiments confirmed that cytoplasmic DAXX promotes p62 puncta formation and drives p62 liquid phase condensation. In vitro experiments also confirmed that DAXX promotes p62 phase condensation. Evidence has suggested the critical role of the p62-Keap1-Nrf2 pathway in combating the oxidative stress and maintaining the cellular redox homeostasis. To examine whether DAXX has an effect on redox homeostasis, Reactive Oxygen Species (ROS) assays and Dual-luciferase assays were applied, and results showed that the DAXX-induced p62 phase condensation promotes p62 recruitment of Keap1 and subsequent Nrf2-mediated stress responses. Collectively, this study suggests that cytoplasmic DAXX drives p62 phase condensation and regulates cellular redox homeostasis, providing a mechanistic insight into p62 phase separation and the prosurvival function of DAXX. Further investigation would reveal if DAXX-induced p62 phase condensation is implicated in ROS-relevant human diseases.