Keap1 is a redox-regulated substrate adaptor protein for a Cul3-dependent ubiquitin ligase complex

The bZIP transcription factor Nrf2 controls a genetic program that protects cells from oxidative damage and maintains cellular redox homeostasis. keap1, a BTB-Kelch protein, is the major upstream regulator of Nrf2 and controls both the subcellular localization and steady-state levels of Nrf2. In this report, we demonstrate that keap1 functions as a substrate adaptor protein for a Cul3-dependent E3 ubiquitin ligase complex. keap1 assembles into a functional E3 ubiquitin ligase complex with Cul3 and Rbx1 that targets multiple lysine residues located in the N-terminal Neh2 domain of Nrf2 for ubiquitin conjugation both in vivo and in vitro. keap1-dependent ubiquitination of Nrf2 is inhibited following exposure of cells to quinone-induced oxidative stress and sulforaphane, a cancer-preventive isothiocyanate. A mutant keap1 protein containing a single cysteine-to-serine substitution at residue 151 within the BTB domain of keap1 is markedly resistant to inhibition by either quinone-induced oxidative stress or sulforaphane. Inhibition of keap1-dependent ubiquitination of Nrf2 correlates with decreased association of keap1 with Cul3. Neither quinone-induced oxidative stress nor sulforaphane disrupts association between keap1 and Nrf2. Our results suggest that the ability of keap1 to assemble into a functional E3 ubiquitin ligase complex is the critical determinant that controls steady-state levels of Nrf2 in response to cancer-preventive compounds and oxidative stress.