Second step of mRNA capping. Transfer of the GMP moiety of GTP to the 5'-diphosphate terminus of RNA via a covalent enzyme-GMP reaction intermediate. a 5'-end diphospho-ribonucleoside in mRNA + GTP + H(+) = a 5'-end (5'-triphosphoguanosine)-(ribonucleoside) in mRNA + diphosphate Optimum pH is 7.0. 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). Present with 279 molecules/cell in log phase SD medium. Belongs to the eukaryotic GTase family.