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 Optimum pH is 6.5-8.5. Heterodimer (PubMed:6389537, PubMed:3029058). The mRNA-capping enzyme is composed of two separate chains alpha and beta, respectively a mRNA guanylyltransferase and an mRNA 5'-triphosphate monophosphatase (PubMed:6389537, PubMed:3029058, PubMed:12788946). Present with 3900 molecules/cell in log phase SD medium. Belongs to the fungal TPase family.