A cleaved METTL3 potentiates the METTL3-WTAP interaction and breast cancer progression.

-methyladenosine (mA) methylation of RNA by the methyltransferase complex (MTC), with core components including METTL3-METTL14 heterodimers and (WTAP), contributes to breast tumorigenesis, but the underlying regulatory mechanisms remain elusive. Here, we identify a novel cleaved form METTL3a (residues 239-580 of METTL3). We find that METTL3a is required for the METTL3-WTAP interaction, RNA mA deposition, as well as cancer cell proliferation. Mechanistically, we find that METTL3a is essential for the METTL3-METTL3 interaction, which is a prerequisite step for recruitment of WTAP in MTC. Analysis of mA sequencing data shows that depletion of METTL3a globally disrupts mA deposition, and METTL3a mediates mammalian target of rapamycin (mTOR) activation via mA-mediated suppression of TMEM127 expression. Moreover, we find that METTL3 cleavage is mediated by proteasome in an mTOR-dependent manner, revealing positive regulatory feedback between METTL3a and mTOR signaling. Our findings reveal METTL3a as an important component of MTC, and suggest the METTL3a-mTOR axis as a potential therapeutic target for breast cancer.