Bifunctional mRNA-capping enzyme exhibiting RNA 5'-triphosphate monophosphatase activity in the N-terminal part and mRNA guanylyltransferase activity in the C-terminal part. Catalyzes the first two steps of cap formation: by removing the gamma-phosphate from the 5'-triphosphate end of nascent mRNA to yield a diphosphate end, and by transferring the GMP moiety of GTP to the 5'-diphosphate terminus of RNA via a covalent enzyme-GMP reaction intermediate. a 5'-end triphospho-ribonucleoside in mRNA + H2O = a 5'-end diphospho-ribonucleoside in mRNA + H(+) + phosphate a 5'-end diphospho-ribonucleoside in mRNA + GTP + H(+) = a 5'-end (5'-triphosphoguanosine)-(ribonucleoside) in mRNA + diphosphate Interacts with POLR2A (via C-terminus); this enhances guanylyltransferase activity. Binds (via GTase domain) to the elongating phosphorylated form of RNA polymerase II; can form direct interactions with the phosphorylated POLR2A C-terminal domain and indirect interactions via bound RNA (By similarity). Interacts with SUPT5H and RNMT. Interacts with HIV-1 Tat. (Microbial infection) Interacts with HIV-1 Tat. Isoform 1 and isoform 4 (at a lesser extent) are expressed in cerebrum, cerebellum, thyroid, lung, heart, liver, kidney, spleen, large intestine, testis, skin and muscle. Isoform 2 to isoform 4 lack mRNA 5'-guanylyltransferase activity due to disruptions of the GTase domain. In the N-terminal section; belongs to the non-receptor class of the protein-tyrosine phosphatase family. In the C-terminal section; belongs to the eukaryotic GTase family.