The APOBEC deaminase family plays a crucial role in a diverse array of biological functions by catalyzing the conversion of Cytidine (C) to Uridine (U) on RNA and/or DNA.1, 2, 3 The first instance of C-to-U RNA editing was documented in the ApoB mRNA at position 6666, leading to the creation of an early stop codon and the production of two distinct ApoB protein isoforms: ApoB100 and ApoB48, which are implicated in lipid and cholesterol metabolism.4, 5, 6, 7 This RNA editing event is observed in the small intestine of humans and in both the small intestine and liver of mice.8 APOBEC1 (A1) is responsible for this ApoB RNA editing. A1 requires a cofactor to form a so-called “editosome” complex to show obviously detectible RNA editing activity. The first cofactor identified is called A1 complement factor (A1CF), which is shown to interact with A1 and regulate lipid uptake and metabolism.9, 10, 11 A second cofactor, RNA-Binding-Motif-protein-47 (RBM47), has later been identified to enable A1 RNA editing.12, 13, 14, 15 The possibility of the existence of other RNA binding protein cofactors for A1 has been proposed.16 Both A1CF and RBM47 contain conserved RNA-recognition motifs (RRM) that directly interact with RNAs.17 These proteins are conserved across multiple species and are involved in various biological functions.18, 19, 20 While A1CF and RBM47 are homologous proteins and share many RNA substrates for A1 editing, they also showed different substrate specificity and editing efficiency when paired with A1 in both in vitro and in vivo studies.15, 21

In addition to A1CF and RBM47, another homologous RNA binding protein called SYNCRIP (Synaptotagmin Binding Cytoplasmic RNA Interacting Protein) is reported to inhibit A1-mediated ApoB RNA editing possibly by binding to and sequestering A1CF, A1, or ApoB RNA.22 This finding suggests that the A1-mediated RNA editing activities may be regulated by more cellular cofactors in different ways than previously anticipated. In addition to ApoB RNA, A1 and cofactors are shown in recent years to edit some other cellular RNAs in different types of cells,23, 24, 25, 26 indicating that A1-mediated RNA editing may have a much broader range of yet-to-be-characterized biological functions.

In this study, we have identified RBM46 (RNA-binding-motif-protein-46) as a novel cofactor of A1 for RNA editing and have characterized its essential functional domains for enabling A1 to perform RNA editing activity. Using the low-error circular RNA seq technique,27 we have also identified cellular RNA substrates for A1/RBM46 editosome, and have characterized the critical RNA sequence features as the preferred substrate for A1/RBM46 and other A1 editosomes. The identification of RBM46 as a cofactor for A1-mediated RNA editing expands our understanding of the A1-mediated RNA editing mechanism and provides new insights into the biological functions of A1 and its cofactors.