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Background
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Methyltransferase displays a cytoplasmic capping enzyme activity. This function is necessary since all viral RNAs are synthesized in the cytoplasm, and host capping enzymes are restricted to the nucleus. The enzymatic reaction involves a covalent link between 7-methyl-GMP and the methyltransferase, whereas eukaryotic capping enzymes form a covalent complex only with GMP. Methyltransferase catalyzes transfer of a methyl group from S-adenosylmethionine to GTP and GDP to yield m7GTP or m7GDP. GMP, GpppG, and GpppA were poor substrates for the methyltransferase. This enzyme also displays guanylyltransferase activity to form a covalent complex, methyltransferase-m7GMP, from which 7-methyl-GMP is transferred to the mRNA to create the cap structure. Cap analogs such as m7GTP, m7GDP, et2m7GMP, and m2et7GMP inhibit the methyltransferase reaction (By similarity).
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