This RNA methyltransferase domain is found in a wide range of ssRNA viruses, including Hordei-, Tobra-, Tobamo-, Bromo-, Clostero- and Caliciviruses. This methyltransferase is involved in mRNA capping. Capping of mRNA enhances its stability. This usually occurs in the nucleus. Therefore, many viruses that replicate in the cytoplasm encode their own. This is a specific guanine-7-methyltransferase domain involved in viral mRNA cap0 synthesis. Specificity for guanine 7 position is shown by NMR in and in vivo role in cap synthesis. Based on secondary structure prediction, the basic fold is believed to be similar to the common AdoMet-dependent methyltransferase fold. A curious feature of this methyltransferase domain is that it together with flanking sequences seems to have guanylyltransferase activity coupled to the methyltransferase activity. The domain is found throughout the so-called Alphavirus superfamily, (including alphaviruses and several other groups). It forms the defining, unique feature of this superfamily.
Jiyao W et al.(2023). "The conserved domain database in 2023.",