First step of mRNA capping. Converts the 5'-triphosphate end of a nascent mRNA chain into a diphosphate end. a 5'-end triphospho-ribonucleoside in mRNA + H2O = a 5'-end diphospho-ribonucleoside in mRNA + H(+) + phosphate Heterodimer. The mRNA-capping enzyme is composed of two separate chains alpha and beta, respectively a mRNA guanylyltransferase and an mRNA 5'-triphosphate monophosphatase. Belongs to the fungal TPase family.