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.
Subcellular locations: Nucleus
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.