RNA editing of AZIN1 coding sites is catalyzed by ADAR1 p150 after splicing

Adenosine-to-inosine RNA editing is catalyzed by nuclear ADAR1 p110 and ADAR2, and cytoplasmic ADAR1 p150 in mammals, all of which recognize double-stranded RNAs (dsRNAs) as targets. Although its frequency is quite rare, RNA editing occurs in coding regions, which alters protein functions by exchanging amino acid sequences, and is therefore physiologically significant. In general, such coding sites are edited by ADAR1 p110 and ADAR2 prior to splicing, given that the corresponding exon forms a dsRNA structure with an adjacent intron. We previously found that RNA editing at two coding sites of antizyme inhibitor 1 (AZIN1) is sustained in Adar1 p110/Aadr2 double knockout mice. However, the molecular mechanisms underlying RNA editing of AZIN1 remain unknown. Here, we showed that Azin1 editing levels were increased upon type I interferon treatment, which activated Adar1 p150 transcription, in mouse Raw264.7 cells. Azin1 RNA editing was observed in mature mRNA but not precursor mRNA. Furthermore, we revealed that the two coding sites were editable only by ADAR1 p150 in both mouse Raw264.7 and human HEK293T cells. This unique editing was achieved by forming a dsRNA structure with a downstream exon after splicing and the intervening intron suppressed RNA editing. Therefore, deletion of a nuclear export signal from ADAR1 p150, shifting its localization to the nucleus, decreased Azin1 editing levels. Finally, we demonstrated that Azin1 RNA editing was completely absent in Adar1 p150 knockout mice. Thus, these findings indicate that RNA editing of AZIN1 coding sites is exceptionally catalyzed by ADAR1 p150 after splicing.