Functionally Distinct Double-stranded RNA-binding Domains Associated with Alternative Splice Site Variants of the Interferon-inducible Double-stranded RNA-specific Adenosine Deaminase

The double-stranded RNA-specific adenosine deaminase (ADAR) is an interferon-inducible RNA-editing enzyme implicated in the site-selective deamination of adenosine to inosine in viral RNAs and cellular pre-mRNAs. We have isolated and characterized human genomic clones of the ADAR gene and cDNA clones encoding splice site variants of the ADAR protein. Southern blot and sequence analyses revealed that the gene spans about 30 kilobase pairs and consists of 15 exons. The codon phasing of the splice site junctions of exons 3, 5, and 7 that encode the three copies of the highly conserved RNA-binding R-motif (RI, RII, and RIII) was exactly conserved and identical to those R-motif exons of the interferon-inducible RNA-dependent protein kinase. Alternative splice site variants of the 1226-amino acid ADAR-a protein, designated b and c, were identified that differed in exons 6 and 7. ADAR-b was a 5'-splice site variant that possessed a 26-amino acid deletion within exon 7; ADAR-c was a 3'-splice site variant that possessed an additional 19-amino acid deletion within exon 6. The wild-type ADAR-a, -b, and -c proteins all possessed comparable double-stranded RNA-specific adenosine deaminase activity. However, mutational analysis of the R-motifs revealed that the exon 6 and 7 deletions of ADAR-b and -c variants altered the functional importance of each of the three R-motifs.