Bifunctional mRNA-capping enzyme exhibiting RNA 5'-triphosphatase 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.
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.
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.