A molecular, phylogenetic and functional study of the dADAR mRNA truncated isoform during Drosophila embryonic development reveals an editing-independent function

Adenosine Deaminases Acting on RNA (ADARs) have been studied in many animal phyla, where they have been shown to deaminate specific adenosines into inosines in duplex mRNA regions. In Drosophila, two isoform classes are encoded, designated full-length (contains the editase domain) and truncated (lacks this domain). Much is known about the full-length isoform, which plays a major role in regulating functions of voltage-gated ion channel proteins in the adult brain. In contrast, almost nothing is known about the functional significance of the truncated isoform. In situ hybridization shows that both isoform mRNA classes are maternally derived and transcripts for both localize primarily to the developing central nervous system. Quantitative RT-PCR shows that about 35% of all dADAR mRNA transcripts belong to the truncated class in embryos. 3'-RACE results show that abundance of the truncated isoform class is developmentally regulated, with a longer transcript appearing after the mid-blastula transition. 3'-UTR sequences for the truncated isoform have been determined from diverse Drosophila species and important regulatory regions including stop codons have been mapped. Western analysis shows that both mRNA isoform classes are translated into protein during embryonic development, as full-length variant levels gradually diminish. The truncated protein isoform is present in every Drosophila species studied, extending over a period spanning about 40 × 10⁶ years, implying a conserved function. Previous work has shown that a dADAR protein isoform binds to the evolutionarily conserved rnp-4f pre-mRNA stem-loop located in the 5'-UTR to regulate splicing, while no RNA editing was observed, suggesting the hypothesis that it is the non-catalytic truncated isoform which regulates splicing. To test this hypothesis, we have utilized RNAi technology, the results of which support the hypothesis. These results demonstrate a novel, non-catalytic function for the truncated dADAR protein isoform in Drosophila embryonic development, which is very likely evolutionarily conserved.