Catalytic subunit of the mRNA-capping methyltransferase RNMT:RAMAC complex that methylates the N7 position of the added guanosine to the 5'-cap structure of mRNAs (PubMed:10347220, PubMed:11114884, PubMed:22099306, PubMed:27422871, PubMed:9705270, PubMed:9790902). Binds RNA containing 5'-terminal GpppC (PubMed:11114884). {ECO:0000269|PubMed:10347220, ECO:0000269|PubMed:11114884, ECO:0000269|PubMed:22099306, ECO:0000269|PubMed:27422871, ECO:0000269|PubMed:9705270, ECO:0000269|PubMed:9790902}. This is the function of RNMT (RNA guanine-7 methyltransferase, Ensembl gene identifier ENSG00000101654).