ENDOU-1-induced cytoplasmic HnRNPA3 recognizes m6A methylation on the upstream reading frame of human CHOP transcripts to achieve maximal CHOP translation.

Translation of the coding sequence for human C/EBP homologous protein (CHOP) is highly regulated by its upstream open reading frame (uORFchop). ENDOU-1 was shown to play positive roles in mediating CHOP translation through suppressing uORFchop-mediated translational inhibition (uORFchop-MTI) during ER stress. However, when we screened for other RNA-binding proteins putatively engaged in the same inhibitory activity, we found that HnRNPA3 associated with zebrafish Endouc and its mammalian orthologue ENDOU-1 and could also be involved in uORFchop-MTI in a p-eIF2α-independent manner. A time course experiment showed that the dynamic patterns of ENDOU-1, HnRNPA3 and CHOP were similarly fluctuant and positively correlated during ER stress. Moreover, overexpression of ENDOU-1 induced the increase of HnRNPA3 expression and caused a shift of HnRNPA3 from the nucleus to the cytoplasm. The resultant cytoplasmic HnRNPA3 served as a reader protein to recognize the N6-methyladenosine site on human uORFchop cassette methylated by Wilms’ tumor 1-associating protein. Collectively, we suggest that HnRNPA3 is a positive effector of ENDOU-1 in an m6A-dependent manner, allowing it to reach maximal CHOP translation during ER stress.