RBM33 is a unique m6A RNA-binding protein that regulates ALKBH5 demethylase activity and substrate selectivity

Regulation of RNA substrate selectivity of m6A demethylase ALKBH5 remains elusive. Here, we identify RNA-binding motif protein 33 (RBM33) as a previously unrecognized m6A-binding protein that plays a critical role in ALKBH5-mediated mRNA m6A demethylation of a subset of mRNA transcripts by forming a complex with ALKBH5. RBM33 recruits ALKBH5 to its m6A-marked substrate and activates ALKBH5 demethylase activity through the removal of its SUMOylation. We further demonstrate that RBM33 is critical for the tumorigenesis of head-neck squamous cell carcinoma (HNSCC). RBM33 promotes autophagy by recruiting ALKBH5 to demethylate and stabilize DDIT4 mRNA, which is responsible for the oncogenic function of RBM33 in HNSCC cells. Altogether, our study uncovers the mechanism of selectively demethylate m6A methylation of a subset of transcripts during tumorigenesis that may explain demethylation selectivity in other cellular processes, and we showed its importance in the maintenance of tumorigenesis of HNSCC. [Display omitted] •RBM33 binds m6A and regulates ALKBH5-mediated m6A demethylation selectivity•RBM33 activates ALKBH5 demethylase activity via suppression of ALKBH5 SUMOylation•RBM33/ALKBH5 modulates m6A-mediated DDIT4 mRNA decay and DDIT4/TXNIP-based autophagy•Suppression of RBM33/ALKBH5 activity inhibits HNSCC and primary patient cell growth Yu et al. identify RBM33 as a new m6A-binding protein that forms a complex with ALKBH5 and mediates m6A demethylation of selected transcripts by regulating ALKBH5 substrate accessibility and activity. In HNSCC cells, highly expressed RBM33 promotes HNSCC tumorigenesis by stabilizing DDIT4 in an m6A-dependent manner, thereby inducing autophagy.