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The Major Histocompatibility Complex class I (MHC-I) molecules present antigenic peptides (AP) to CD8+T cells for self versus non-self recognition. Loading of AP on MHC-I takes place in the endoplasmic reticulum (ER), upon shuttling of cytoplasmic AP substrates to the ER. Understanding of this process has been influenced by the view that MHC-I antigens are produced from the proteasomal degradation of cellular proteins. Recent observations on the intronic and untranslated region-derived peptides as well as on the non-AUG translation products presented on the MHC-I open the possibility that antigenic peptides can derive from pre-spliced mRNAs translated in the nuclear compartment. In this brief report, we show that a fraction of human MHC-I molecules (human leukocyte antigens type A, HLA-A) is present in the nuclei of cells, in the proximity of histone H2B. With this finding, we hope to initiate a new direction of research on the nuclear role of MHC-I and ask whether the loading of antigens can take place in the nuclear compartment.

The Nrf2 pathway is an essential defense pathway in a cell. It responds to oxidative and electrophilic stress via derepression of Nrf2 from Keap1-Cul3-mediated degradation, accumulation of Nrf2 in the nucleus and transcriptional activation of a number of detoxifying and cell protective Nrf2 target genes. Here we report that normal and cancer cells also express the N-terminally truncated Nrf2 isoform (ΔN-Nrf2), which originates from an alternative promoter. Co-immunoprecipitation together with molecular dynamics simulation showed that the binding between ΔN-Nrf2 and Keap1 is impaired, resulting in the much higher stability of this form. ΔN-Nrf2 is retained in the cytoplasm in response to electrophilic stress, indicating that it does not regulate transcription under the same stress stimuli as the full-length Nrf2. Altogether this data suggests that Nrf2 has other functions in cells than transcriptional activation of genes, which most probably rely on the protein-protein interactions in the cytoplasm. The regulation between these functions takes place on the level of transcription.Significance StatementThis work signifies the importance of alternative transcription in assigning the function to the produced protein. Nrf2 transcripts produced from the second promoter of the Nrf2 gene give rise to the N-terminally truncated Nrf2 form (ΔN-Nrf2), which is retained in the cytoplasm upon stress, thus it has a different role in cells than transcriptional regulation. ΔN-Nrf2 is resistant to the Keap1-Cul3 degradation pathway and is highly expressed in all tested cell types. This work points to the new, cytoplasmic role of Nrf2 in cells, determined at the level of transcription.