N6-methyladenosine (m6A) is the most prevalent and reversible internal modification in mammalian messenger and non-coding RNAs. We report here that human METTL14 catalyzes m6A RNA methylation. Together with METTL3, the only previously known m6A methyltransferase, these two proteins form a stable heterodimer core complex of METTL3-14 that functions in cellular m6A deposition on mammalian nuclear RNAs. WTAP, a mammalian splicing factor, can interact with this complex and affect this methylation. N6-methyladenosine (m6A) is a conserved internal modification found in almost all eukaryotic nuclear RNAs1–3, as well as in the viral RNA that replicates inside host nuclei4. The discoveries of functionally significant demethylases that reverse this methylation5,6, together with the recently revealed m6A distributions in mammalian transcriptomes7,8, strongly indicate regulatory functions of this dynamic modification. The m6A modification is post-transcriptionally installed by a multi-component N6-adenosine methyltransferase (MT) complex yet to be fully identified and characterized. Of a ~200 kD MT complex isolated from mammalian cell nuclear extract that exhibits methyltransferase activity, only a 70 kD protein was identified and named MT-A70 or METTL3 (methyltransferase like 3)9. The knockdown of METTL3 led to apoptosis of human HeLa Users may view, print, copy, download and text and datamine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use: http://www.nature.com/authors/editorial_policies/license.html#terms To whom correspondence should be addressed: chuanhe@uchicago.edu. 2These authors contributed equally to this work Author Contributions C. H. conceived the project. J. L. and Y. Y. designed and performed most experiments. D. H. and Z. L. performed high-throughput sequencing data analyses. X.W. and Y. F. helped perform PAR-CLIP experiment, biochemistry assay, and data analysis. L. Z. and M. Y. assisted in expressing recombinant proteins in insect cells. G. J. and W. C. participated in the subcloning. X. D. participated in nuclear extract separation. Q. D. synthesized the d3-m6A standard for LC-MS/MS analysis. J. L., Y. Y., and C. H. wrote the manuscript. Competing financial interests The authors declare no competing financial interests. Supplementary information is available in the online version of the paper. The PAR-CLIP and m6A-seq data were deposited in the Gene Expression Omnibus (http://www.ncbi.nlm.nih.gov/geo) under accession number GEO46705. Reprints and permissions information is available online at http://www.nature.com/reprints/index.html. HHS Public Access Author manuscript Nat Chem Biol. Author manuscript; available in PMC 2014 August 01. Published in final edited form as: Nat Chem Biol. 2014 February ; 10(2): 93–95. doi:10.1038/nchembio.1432. A uhor M anscript