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J. Biol. Chem., Vol. 281, Issue 28, 19676-19687, July 14, 2006

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Nrf1 Is Targeted to the Endoplasmic Reticulum Membrane by an N-terminal Transmembrane Domain

INHIBITION OF NUCLEAR TRANSLOCATION AND TRANSACTING FUNCTION^*

From the Department of Laboratory Medicine and Pathology, University of California, Irvine, California 92697

Expression of antioxidant and phase 2 xenobiotic metabolizing enzyme genes is regulated through cis-acting sequences known as antioxidant response elements. Transcriptional activation through the antioxidant response elements involves members of the CNC (Cap `n' Collar) family of basic leucine zipper proteins including Nrf1 and Nrf2. Nrf2 activity is regulated by Keap1-mediated compartmentalization in the cell. Given the structural similarities between Nrf1 and Nrf2, we sought to investigate whether Nrf1 activity is regulated similarly to Nrf2. Nrf1 also resides normally in the cytoplasm of cells. Cytoplasmic localization however, is independent of Keap1. Colocalization analysis using green fluorescent protein-tagged Nrf1 and subcellular fractionation of endogenous Nrf1 and fusion proteins indicate that Nrf1 is primarily a membrane-bound protein localized in the endoplasmic reticulum. Membrane targeting is mediated by the N terminus of the Nrf1 protein that contains a predicted transmembrane domain, and deletion of this domain resulted in a predominantly nuclear localization of Nrf1 that significantly increased the activation of reporter gene expression. Treatment with tunicamycin, an endoplasmic reticulum stress inducer, caused an accumulation of a smaller form of Nrf1 that correlated with detection of Nrf1 in the nucleus by biochemical fractionation and immunofluorescent analysis. These results suggest that Nrf1 is normally targeted to the endoplasmic reticulum membrane and that endoplasmic reticulum stress may play a role in modulating Nrf1 function as a transcriptional activator.

Received for publication, March 24, 2006 , and in revised form, May 8, 2006.

^* This work was supported by National of Institutes of Health Grant CA91907 (to J. Y. C.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Dept. of Laboratory Medicine and Pathology, University of California, Irvine, D440 Medical Sciences 1, Irvine, CA 92697-4800. Tel.: 949-824-9605; Fax: 949-824-4455; E-mail: jchan{at}uci.edu.

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