Inositol hexakisphosphate Functions as a Cofactor and Modulator of ADAR1 Activity
Adenosine-to-inosine (A-to-I) RNA editing, catalyzed by adenosine deaminases acting on RNA (ADARs), is a key post-transcriptional modification that regulates RNA splicing, stability, and translation. Dysregulation of ADAR activity caused by mutations in ADAR1 leads to Aicardi-Goutières syndrome (AGS), an autoimmune disorder characterized by aberrant activation of Melanoma differentiation-associated protein 5 (MDA5) by self RNA and excessive type I interferon production. Despite its biological and clinical relevance, the regulation of ADAR1 activity remains incompletely understood. Here, we show that ADAR1 protein levels and RNA editing activity in mammalian cells critically depend on the cofactor inositol hexakisphosphate (IP6). Using Inositol-pentakisphosphate 2-kinase (IPPK)-knockout cells, next-generation sequencing (NGS), and a cell-permeable IP6 prodrug (Pro-IP6), we demonstrate that IP6 depletion drastically reduces global RNA editing, while supplementation with Pro-IP6 restores and even enhances editing levels. In vitro ADAR1 translation and RNA editing assays revealed that IP6 contributes to the folding and full catalytic activity of ADAR1, and that inositol pentakisphosphate (1,3,4,5,6-IP5) can partially substitute IP6 as a cofactor. Molecular dynamics simulations and biochemical analyses identified the C6-phosphate of IP6 as a critical determinant of ADAR1 catalytic efficiency, functioning within a hydrogen-bonding network that indirectly governs Zn²?-ion positioning through interactions with key residues, including K1039 and N907. Notably, the AGS-associated N907S mutation impairs RNA editing, by altering IP6 coordination and introducing more dynamic situation in the hydrogen-bonding network of IP6 and Zn²?-ion. Together, these findings identify IP6 as an essential cofactor and regulator of ADAR1 activity. Through a network of interactions and the availability of IP6, this may represent a new strategy for therapeutically controlling RNA editing in the disease. Overall design: RNAseq of HeLa WT and HeLa KO IPPK cells  treated or untreated with Pro-IP6 for 24 h. Three independent experiments.