In enzymology, a mRNA guanylyltransferase is an enzyme that catalyzes the chemical reaction Thus, the two substrates of this enzyme are GTP and (5')ppPur-mRNA, whereas its two products are diphosphate and G(5')pppPur-mRNA. This enzyme belongs to the family of transferases, specifically those transferring phosphorus-containing nucleotide groups (nucleotidyltransferases).op The systematic name of this enzyme class is GTP:mRNA guanylyltransferase. Other names in common use include mRNA capping enzyme, messenger RNA guanylyltransferase, and Protein 2.

Structural studies

As of late 2007, 5 structures have been solved for this class of enzymes, with PDB accession codes, , , , and.