The Nrf3 Transcription Factor Is a Membrane-bound Glycoprotein Targeted to the Endoplasmic Reticulum through Its N-terminal Homology Box 1 Sequence

Transcription factor Nrf3 (NF-E2 p45-related factor 3) is targeted to the endoplasmic reticulum ( ER). Mouse Nrf3 is subject to proteolysis, Asn glycosylation, and deglycosylation reactions. It is synthesized as a similar to 96-kDa protein that is subsequently converted into isoforms of similar to 90, 80, and 70 kDa. In the ER, the similar to 90kDa glycoprotein is predominant and gives rise to similar to 80- and similar to 70- kDa isoforms. The similar to 90- and similar to 80- kDa polypeptides were observed in the nuclear envelope, whereas the similar to 70- kDa isoform was detected primarily in the nucleoplasm. Our experiments showed the N-terminal homology box 1 (NHB1, residues 12-31) is part of a tripartite signal peptide sequence, comprising n, h, and c regions. The h region ( residues 12-23) was demonstrated to target Nrf3 to the ER and is necessary for its Asn glycosylation. The n region ( residues 1-11) controlled the abundance of the similar to 90- kDa glycoprotein. The c region ( residues 24-39) was found to contain a signal peptidase cleavage site that is responsible for production of the similar to 90- kDa mature Nrf3 glycoprotein from a similar to 96-kDa precursor. We have found that Nrf3 is activated by the ER stressors tunicamycin and brefeldin A, and that NHB1 is required for this response. Amino acids between the c region and NHB2 ( residues 76-100) controlled the proteolytic processing of mouse Nrf3 into cleavage products of similar to 80- kDa ( glycated) and similar to 70- kDa (non-glycated); by contrast, human Nrf3 lacked a signal peptidase cleavage site between its c region and NHB2. Lastly, data are presented suggesting that the NHB2 sequence in mouse Nrf3 may regulate the topology of the transcription factor within the ER membrane.