METTL16 exerts an mA-independent function to facilitate translation and tumorigenesis.

METTL16 has recently been identified as an RNA methyltransferase responsible for the deposition of N-methyladenosine (mA) in a few transcripts. Whether METTL16 methylates a large set of transcripts, similar to METTL3 and METTL14, remains unclear. Here we show that METTL16 exerts both methyltransferase activity-dependent and -independent functions in gene regulation. In the cell nucleus, METTL16 functions as an mA writer to deposit mA into hundreds of its specific messenger RNA targets. In the cytosol, METTL16 promotes translation in an mA-independent manner. More specifically, METTL16 directly interacts with the eukaryotic initiation factors 3a and -b as well as ribosomal RNA through its Mtase domain, thereby facilitating the assembly of the translation-initiation complex and promoting the translation of over 4,000 mRNA transcripts. Moreover, we demonstrate that METTL16 is critical for the tumorigenesis of hepatocellular carcinoma. Collectively, our studies reveal previously unappreciated dual functions of METTL16 as an mA writer and a translation-initiation facilitator, which together contribute to its essential function in tumorigenesis.