The study identifies WTAP and METTL14 as new components of the mammalian m6A methyltransferase complex, with WTAP acting as a regulatory subunit essential for m6A methyltransferase activity. The complex, termed WMM, includes WTAP, METTL3, and METTL14. WTAP interacts with METTL3 and METTL14, facilitating their localization in nuclear speckles and catalytic activity. The majority of RNAs bound by WTAP and METTL3 are mRNAs containing the m6A motif. WTAP is required for the RNA-binding capability of METTL3, suggesting it regulates the recruitment of the m6A methyltransferase complex to mRNA targets. Transcriptomic analyses and PAR-CLIP data show that WTAP and METTL3 regulate gene expression and alternative splicing. Knockdown of WTAP or METTL3 in zebrafish embryos leads to tissue differentiation defects and increased apoptosis, indicating their critical role in RNA metabolism. The WMM complex is essential for tissue differentiation in zebrafish, highlighting its importance in epitranscriptomic regulation. The study provides evidence that WTAP functions as a regulatory subunit in the m6A methyltransferase complex, playing a critical role in RNA metabolism.The study identifies WTAP and METTL14 as new components of the mammalian m6A methyltransferase complex, with WTAP acting as a regulatory subunit essential for m6A methyltransferase activity. The complex, termed WMM, includes WTAP, METTL3, and METTL14. WTAP interacts with METTL3 and METTL14, facilitating their localization in nuclear speckles and catalytic activity. The majority of RNAs bound by WTAP and METTL3 are mRNAs containing the m6A motif. WTAP is required for the RNA-binding capability of METTL3, suggesting it regulates the recruitment of the m6A methyltransferase complex to mRNA targets. Transcriptomic analyses and PAR-CLIP data show that WTAP and METTL3 regulate gene expression and alternative splicing. Knockdown of WTAP or METTL3 in zebrafish embryos leads to tissue differentiation defects and increased apoptosis, indicating their critical role in RNA metabolism. The WMM complex is essential for tissue differentiation in zebrafish, highlighting its importance in epitranscriptomic regulation. The study provides evidence that WTAP functions as a regulatory subunit in the m6A methyltransferase complex, playing a critical role in RNA metabolism.