The Molecular Basis of Human ALKBH3 Mediated RNA N1‐methyladenosine (m1A) Demethylation

Abstract N 1 ‐methyladenosine (m 1 A) is a prevalent post‐transcriptional RNA modification, and the distribution and dynamics of the modification play key epitranscriptomic roles in cell development. At present, the human AlkB Fe(II)/α‐ketoglutarate‐dependent dioxygenase family member ALKBH3 is the only known mRNA m 1 A demethylase, but its catalytic mechanism remains unclear. Here, we present the structures of ALKBH3‐oligo crosslinked complexes obtained with the assistance of a synthetic antibody crystallization chaperone. Structural and biochemical results showed that ALKBH3 utilized two β‐hairpins (β4‐loop‐β5 and β′‐loop‐β′′) and the α2 helix to facilitate single‐stranded substrate binding. Moreover, a bubble‐like region around Asp194 and a key residue inside the active pocket (Thr133) enabled specific recognition and demethylation of m 1 A‐ and 3‐methylcytidine (m 3 C)‐modified substrates. Mutation of Thr133 to the corresponding residue in the AlkB Fe(II)/α‐ketoglutarate‐dependent dioxygenase family members FTO or ALKBH5 converted ALKBH3 substrate selectivity from m 1 A to N 6 ‐methyladenosine (m 6 A), as did Asp194 deletion. Our findings provide a molecular basis for understanding the mechanisms of substrate recognition and m 1 A demethylation by ALKBH3. This study is expected to aid structure‐guided design of chemical probes for further functional studies and therapeutic applications.