Transcription Factor Nrf1 Is Topologically Repartitioned across Membranes to Enable Target Gene Transactivation through Its Acidic Glucose-Responsive Domains

Abstract

The membrane-bound Nrf1 transcription factor regulates critical homeostatic and developmental genes. The conserved N-terminal homology box 1 (NHB1) sequence in Nrf1 targets the cap‘n’collar (CNC) basic basic-region leucine zipper (bZIP) factor to the endoplasmic reticulum (ER), but it is unknown how its activity is controlled topologically within membranes. Herein, we report a hitherto unknown mechanism by which the transactivation activity of Nrf1 is controlled through its membrane-topology. Thus after Nrf1 is anchored within ER membranes, its acidic transactivation domains (TADs), including the Asn/Ser/Thr-rich (NST) glycodomain situated between acidic domain 1 (AD1) and AD2, are transiently translocated into the lumen of the ER, where NST is glycosylated in the presence of glucose to yield an inactive 120-kDa Nrf1 glycoprotein. Subsequently, portions of the TADs partially repartition across membranes into the cyto/nucleoplasmic compartments, whereupon an active 95-kDa form of Nrf1 accumulates, a process that is more obvious in glucose-deprived cells and may involve deglycosylation. The repartitioning of Nrf1 out of membranes is monitored within this protein by its acidic-hydrophobic amphipathic glucose-responsive domains, particularly the Neh5L subdomain within AD1. Therefore, the membrane-topological organization of Nrf1 dictates its post-translational modifications (i.e. glycosylation, the putative deglycosylation and selective proteolysis), which together control its ability to transactivate target genes.

Transcription factor Nrf1 is negatively regulated by its O‐GlcNAcylation status

“…1B, middle panel ). It should be herein noted that distinct TCF11/hNrf1 isoforms were separated in the pH 8.9 Laemmli SDS–PAGE containing 8% polyacrylamide, and therefore their electrophoretic mobility on the pH 8.9 SDS–PAGE gel is obviously different from the equivalent mobility exhibited on the pH 7.0 NuPAGE containing 4–12% polyacrylamide [36–38]).…”

Mechanisms and functions of Nrf2 signaling in Drosophila

“…Nrf1 is attached to the ER membrane via its NH 2 -terminally localized N-terminal domain (NTD) (Wang and Chan, 2006; Zhang et al, 2006). As opposed to other membrane-tethered transcription factors such as SREBP or ATF6 the activation of Nrf1 does not appear to involve signal regulated intramembrane proteolysis, but rather a change in its posttranslational glycosylation status (Zhang et al, 2007; Zhang et al, 2014b). It has been suggested that deglycolsylation of Nrf1 changes the topology and the biochemical function of transcription activation domains ultimately resulting in target gene activation (Zhang et al, 2014b).…”

“…Indeed, Nrf1, like its more intensely studied cousin Nrf2, controls the transcription of a battery of antioxidant and stress defense genes. Nrf1 activity increases in response to oxidative stress and glucose depletion (Chen et al, 2003; Ohtsuji et al, 2008; Zhang et al, 2014b). In addition, signaling through the TorC1 complex can stimulate the expression of proteasome subunit genes by a mechanism that relies on Nrf1 but is Nrf2 independent (Zhang et al, 2014a; Zhang et al, 2014b).…”

“…Nrf1 activity increases in response to oxidative stress and glucose depletion (Chen et al, 2003; Ohtsuji et al, 2008; Zhang et al, 2014b). In addition, signaling through the TorC1 complex can stimulate the expression of proteasome subunit genes by a mechanism that relies on Nrf1 but is Nrf2 independent (Zhang et al, 2014a; Zhang et al, 2014b). Nrf1 deficiency in mice causes developmental lethality (Chan et al, 1998; Chen et al, 2003).…”

“…The N-terminal domains (NTD) present in Nrf1, Nrf3, CncC, and CncO contain the NHB1/2 subdomains that are believed to play an important role in targeting these proteins to the endoplasmic reticulum (Zhang et al, 2014b). The NEH2 domains contain the DIDLID/DLG and the ETGE motifs that are essential for Keap1 binding as well as Keap1-mediated degradation of these proteins (Fukutomi et al, 2014; Katoh et al, 2005).…”