Interactions, localization, and phosphorylation of the m⁶A generating METTL3-METTL14-WTAP complex

N⁶-methyladenine (m⁶A) is found on many eukaryotic RNAs including mRNAs. m⁶A modification has been implicated in mRNA stability and turnover, localization, or translation efficiency. A heterodimeric enzyme complex composed of METTL3 and METTL14 generates m⁶A on mRNAs. METTL3/14 is found in the nucleus where it is localized to nuclear speckles and the splicing regulator WTAP is required for this distinct nuclear localization pattern. Although recent crystal structures revealed how the catalytic MT-A70 domains of METTL3 and METTL14 interact with each other, a more global architecture including WTAP and RNA interactions has not been reported so far. Here, we used recombinant proteins and mapped binding surfaces within the METTL3/14-WTAP complex. Furthermore, we identify nuclear localization signals and identify phosphorylation sites on the endogenous proteins. Using an in vitro methylation assay, we confirm that monomeric METTL3 is soluble and inactive while the catalytic center of METTL14 is degenerated and thus also inactive. In addition, we show that the C-terminal RGG repeats of METTL14 are required for METTL3/14 activity by contributing to RNA substrate binding. Our biochemical work identifies characteristic features of METTL3/14-WTAP and reveals novel insight into the overall architecture of this important enzyme complex.