The enzyme, found in higher eukaryotes including insects and vertebrates, and their viruses, methylates the ribose of the nucleoside at the second transcribed position of mRNAs and snRNAs. This methylation is known as cap2. The human enzyme can also methylate mRNA molecules where the upstream purine ribonucleotide is not methylated (see EC 2.1.1.57, cap1 methyltransferase), but with lower efficiency [2].
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S-adenosyl-L-methionine + a 5′-(N(7)-methyl 5′-triphosphoguanosine)- (2′-O-methyl-purine-ribonucleotide)-(ribonucleotide)-[mRNA] = S-adenosyl- L-homocysteine + a 5′-(N(7)-methyl 5′-triphosphoguanosine)-(2′-O-methyl- purine-ribonucleotide)-(2′-O-methyl-ribonucleotide)-[mRNA].
