Regulatory subunit of the WMM N6-methyltransferase complex, a multiprotein complex that mediates N6-methyladenosine (m6A) methylation of some adenosine residues of some mRNAs and plays a role in the efficiency of mRNA splicing and processing and mRNA stability. Required for accumulation of METTL3 and METTL14 to nuclear speckle. Acts as a mRNA splicing regulator. Regulates G2/M cell-cycle transition by binding to the 3' UTR of CCNA2, which enhances its stability. Impairs WT1 DNA-binding ability and inhibits expression of WT1 target genes.