Recently, the lab of Prof. YANG Yungui from Beijing Institute of Genomics, Chinese Academy of Sciences (CAS), in collaboration with Prof. LIU Feng from Institute of Zoology, CAS, revealed that mammalian wilms' tumour 1-associating protein (WTAP) is a regulatory subunit of the RNA N6-Methyladenosine (m6A) methyltransferase.

m6A, the most prevalent internal base modification in messenger RNA (mRNA), accounts for over 80% of all RNA base methylations in various species of higher eukaryotes. The occurrence of m6A is highly conserved with the consensus sequence RRACH (R=G or A; H=A, C or U) and is expected to play an essential role in RNA metabolism, but the mechanism underlying remains largely unknown.

The lab of prof. YANG in previous collaborative studies revealed FTO and ALKBH5 （Jia et al. Nature Chemical Biology 2011; Zheng et al. Molecular Cell 2013）as the two enzymes oxidatively demethylating m6A in mammalian RNAs. But compared to the demethylases, the discovery of the m6A methyltransferase components remains challenging and methyltransferase like 3 (METTL3) was the only subunit identified in the multicomplex that catalyzes m6A formation.

This study identified two new components of the human m6A methyltransferase complex, WTAP and methyltransferase like 14 (METTL14). WTAP interacts with METTL3 and METTL14 and is required for their localization into nuclear speckles enriched in pre-mRNA processing factors, also for catalytic activity of the m6A methyltransferase in vivo.

The majority of RNA bound by WTAP and METTL3 in vivo are mRNAs containing the consensus m6A motif. In the absence of WTAP, the RNA binding capability of METTL3 is strongly reduced, suggesting that WTAP may function to regulate accessibility of the m6A methyltransferase complex to mRNA target substrates.

Furthermore, transcriptomic analyses in combination with Photoactivatable-Ribonucleoside-Enhanced Crosslinking and Immunoprecipitation (PAR-CLIP) illustrate that WTAP and METTL3 regulate expression and alternative splicing of genes involved in RNA metabolic process and transcription.

Morpholino-mediated knockdown targeting WTAP and METTL3 in zebrafish embryos caused tissue differentiation defects and increased apoptosis. These findings provide strong evidence that WTAP may function as a regulatory subunit in the m6A methyltransferase complex and play a critical role in epitranscriptomic regulation of RNA metabolism.

The discovery and characterization of the RNA m6A methyltransferase complex, together with the recent advantages in the understanding of the demethylases FTO and ALKBH5, is an important step towards a more thorough understanding of the biological significance of the emerging epitranscriptomic marker m6A.

This research is supported by CAS, Ministry of Science and Technology and Natural Science Foundation of China，and published online in Cell Research on January 10, 2014.

WTAP binds to mRNA and recruits METT3/METTL14 to catalyze m6A formation at m6A consensus RRACH motif.

(Image by Professor YANG Yungui)