METTL4 catalyzes m6Am methylation in U2 snRNA to regulate pre-mRNA splicing
N6-methylation of 2-O-methyladenosine (Am) in RNA occurs in eukaryotic cells to generate N6,2-O-dimethyladenosine (m6Am). Identification of the methyltransferase responsible for m6Am catalysis has accelerated studies on the function of m6Am in RNA processing. While m6Am is generally found in the first transcribed nucleotide of mRNAs, the modification is also found internally within U2 snRNA. However, the writer required for catalyzing internal m6Am formation had remained elusive. By sequencing transcriptome-wide RNA methylation at single-base-resolution, we identified human METTL4 as the writer that directly methylates Am at U2 snRNA position 30 into m6Am. We found that METTL4 localizes to the nucleus and its conserved methyltransferase catalytic site is required for U2 snRNA methylation. By sequencing human cells with overexpressed Mettl4, we determined METTL4s in vivo target RNA motif specificity. In the absence of Mettl4 in human cells, U2 snRNA lacks m6Am thereby affecting a subset of splicing events that exhibit specific features such as overall 3 splice-site weakness with certain motif positions more affected than others. This study establishes that METTL4 methylation of U2 snRNA regulates splicing of specific pre-mRNA transcripts.