A redox-independent stress response mediated by phase-separated SQSTM1/p62.

The KEAP1 (kelch like ECH associated protein 1)- NFE2L2/NRF2 (NFE2 like bZIP transcription factor 2) pathway is a major antioxidative stress pathway that contributes to cellular homeostasis. KEAP1 acts as a sensor and attenuates degradation of the transcription factor NRF2, which induces gene expression for a network of enzymes involved in the antioxidant response. When cells are exposed to various electrophiles and reactive oxidative species, they modify one or more selective cysteine residues in KEAP1, resulting in conformational changes that disable its NRF2-inhibitory function. In addition to this redox-dependent pathway, SQSTM1/p62 (sequestosome 1), which is a selective autophagy receptor for ubiquitinated proteins and a driver of liquid-liquid phase separation (LLPS) upon binding to ubiquitinated proteins, competitively inhibits the binding between KEAP1 and NRF2, thereby disabling the NRF2-repressive function of KEAP1. Our study showed that phase-separated SQSTM1/p62 bodies are phosphorylated by ULK1 (Unc-51 like autophagy activating kinase 1) and that KEAP1 is retained in the SQSTM1/p62 body, resulting in NRF2-activation in a redox-independent manner.