m6A RNA Degradation Products Are Catabolized by an Evolutionarily Conserved N6-Methyl-AMP Deaminase in Plant and Mammalian Cells

N-6-methylated adenine (m(6)A) is the most frequent posttranscriptional modification in eukaryotic mRNA. Turnover of RNA generates N-6-methylated AMP (N-6-mAMP), which has an unclear metabolic fate. We show that Arabidopsis thaliana and human cells require an N-6-mAMP deaminase (ADAL, renamed MAPDA) to catabolize N-6-mAMP to inosine monophosphate in vivo by hydrolytically removing the aminomethyl group. A phylogenetic, structural, and biochemical analysis revealed that many fungi partially or fully lack MAPDA, which coincides with a minor role of N(6)A-RNA methylation in these organisms. MAPDA likely protects RNA from m(6)A misincorporation. This is required because eukaryotic RNA polymerase can use N-6-mATP as a substrate. Upon abrogation of MAPDA, root growth is slightly reduced, and the N-6-methyladenosine, N-6-mAMP, and N-6-mATP concentrations are increased in Arabidopsis. Although this will potentially lead to m(6)A misincorporation into RNA, we show that the frequency is too low to be reliably detected in vivo. Since N-6-mAMP was severalfold more abundant than N-6-mATP in MAPDA mutants, we speculate that additional molecular filters suppress the generation of N-6-mATP. Enzyme kinetic data indicate that adenylate kinases represent such filters being highly selective for AMP versus N-6-mAMP phosphorylation. We conclude that a multilayer molecular protection system is in place preventing N-6-mAMP accumulation and salvage.