ADARs (adenosine deaminases acting on RNA) are editing enzymes that convert adenosine (A) to inosine (I) in duplex RNA, a modification that has wide-ranging consequences on RNA function including altering miRNA recognition sites, redirecting RNA splicing and changing the meaning of specific codons in mRNA. Recent work has demonstrated a causal link between altered RNA editing and human disease. However, our understanding of the ADAR reaction mechanism, origin of editing site selectivity and the effect of disease-causing mutations was limited by the lack of high-resolution structural data for complexes of ADARs bound to RNA. This presentation will describe the combined chemical biology/structural biology approach used to solve this problem wherein we used RNA bearing a nucleoside analog to trap the reaction intermediate allowing for crystallization of the complexes. Solving the structures of the complexes uncovered ADARs’ use of a unique base-flipping mechanism well-suited for modifying duplex RNA, revealed an ADAR-specific RNA-binding loop near the enzyme active site and explained flanking sequence preferences. In addition, our results provide a structural framework for understanding the effects of ADAR mutations associated with human disease.
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