RNA methyltransferases involved in 5' cap biosynthesis

Byszewska, Magdalena; Smietanski, Miroslaw; Purta, Elzbieta; Bujnicki, Janusz M

The enzyme responsible for the trimethylguanosine (m 2,2,7 G, TMG) synthesis was first identified in yeast and named yTgs1. 64 The Tgs enzymes of budding and fission yeast and Giardia are relatively small polypeptides (239-315 amino acids) consisting of little more than an RFM methyltransferase catalytic domain (Fig. 4) , whereas metazoan Tgs1 proteins are much larger, because they include an N-terminal extension not found in lower eukaryotes. 65, 66 Tgs1 activity is strictly dependent on prior cap0 (m 7 G) methylation, thereby restricting its activity to RNAs that were already methylated by cap0 methyltransferase. 67 Similar substrate requirements are characteristic for the Giardia Tgs2 enzyme. 68 Interestingly, in contrast to Tgs1 methyltransferases able to catalyze 2 sequential N2 methylation steps leading to TMG cap formation, Tgs2 activity is apparently limited to a single round of N2 methylation, resulting in the synthesis of a 2,7dimethylguanosine (m 2,7 G) product. Bioinformatics analyses predicted that the Tgs enzymes are related to a large group of RFM enzymes that act on exocyclic amine groups in nucleic acid bases, including m 6 A, m 4 C, and m 2 G and have a characteristic NPPY-like motif at the active site. 66 The crystal structure of the active C-terminal methyltransferase domain of the human TGS enzyme bound to a minimal substrate m 7 GTP as well as the reaction product SAH has been reported, confirming these predictions and revealing the atomic details of these protein-ligand interactions. 69, 70 Other methyltransferases involved in cap-specific base modifications