METTL3

In Toxoplasma gondii, METTL3 is the catalytic writer of an N6-methyladenosine (m6A) methyltransferase complex that couples mRNA methylation to pre-mRNA 3'-end processing and is essential for parasite replication (PMID:34324585). METTL3 acts together with WTAP to deposit m6A marks that are preferentially positioned near the 3'-boundary of transcripts, where they are recognized by YTH-domain reader proteins that are integral components of the cleavage and polyadenylation machinery (PMID:34324585). Loss of METTL3 diminishes m6A marks, disrupts mRNA 3'-end formation, and completely arrests replication, with knockdown of WTAP and the YTH reader YTH1 phenocopying the 3'-end processing defect (PMID:34324585). Beyond this writer-complex-to-reader axis linking m6A to 3'-end processing, no further mechanistic detail has been characterized in the available corpus.

1. 2021 Medium

   Establishing whether m6A writing is functionally required in Toxoplasma, this work showed METTL3 is a component of the m6A methyltransferase complex whose loss abolishes m6A marks and blocks parasite replication.

   Evidence Knockdown loss-of-function with m6A mapping and replication phenotyping in T. gondii

   PMID:34324585

   Open questions at the time

   • Catalytic activity of METTL3 not directly demonstrated in vitro
   • Direct RNA substrates not resolved from indirect replication effects
2. 2021 Medium

   To define the molecular consequence of m6A loss, METTL3 knockdown was shown to cause defective mRNA 3'-end formation, linking m6A deposition to pre-mRNA cleavage and polyadenylation.

   Evidence METTL3 knockdown with transcript 3'-end analysis, phenocopied by WTAP and YTH1 knockdown in T. gondii

   PMID:34324585

   Open questions at the time

   • Direct mechanism by which m6A marks recruit the 3'-end processing machinery not resolved
   • Transcript specificity of the 3'-end defect not defined
3. 2021 Medium

   To connect writer to reader, METTL3 was shown to act with WTAP to place m6A near mRNA 3'-boundaries, marks that are read by YTH-domain proteins integral to the cleavage and polyadenylation machinery.

   Evidence Complex component identification, m6A transcriptome mapping, Co-IP/interaction studies and knockdown of YTH readers in T. gondii

   PMID:34324585

   Open questions at the time

   • Stoichiometry and architecture of the METTL3-WTAP complex not determined
   • Direct physical contact between YTH readers and cleavage/polyadenylation factors not structurally defined

• Whether METTL3's role in 3'-end processing is conserved beyond Toxoplasma and how m6A-marked transcripts are mechanistically routed to specific processing outcomes remain open.
• No structural model of the METTL3 complex
• Individual mRNA targets whose 3'-end processing depends on METTL3 not enumerated
• Catalytic mechanism not reconstituted in vitro