Responsible for methylating the 5'-cap structure of mRNAs. a 5'-end triphospho-(purine-ribonucleoside) in mRNA + H2O = a 5'-end diphospho-(purine-ribonucleoside) in mRNA + H(+) + phosphate a 5'-end diphospho-ribonucleoside in mRNA + GTP + H(+) = a 5'-end (5'-triphosphoguanosine)-(ribonucleoside) in mRNA + diphosphate a 5'-end (5'-triphosphoguanosine)-(ribonucleoside) in mRNA + S-adenosyl-L-methionine = a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-ribonucleoside in mRNA + S-adenosyl-L-homocysteine mRNA processing; mRNA capping. In the N-terminal section; belongs to the dsDNA virus mRNA guanylyltransferase family. In the C-terminal section; belongs to the class I-like SAM-binding methyltransferase superfamily. mRNA cap 0 methyltransferase family.