Structure analysis of the conserved methyltransferase domain of human trimethylguanosine synthase TGS1

Methyltransferases play an important role in the post-transcriptional maturation of most ribonucleic acids. The modification of spliceosomal UsnRNAs includes N2-dimethyl­ation of the m⁷G cap catalyzed by trimethylguanosine synthase 1 (TGS1). This 5'-cap hypermethylation occurs during the biogenesis of UsnRNPs as it initiates the m₃G cap-dependent nuclear import of UsnRNPs. The conserved methyltransferase domain of human TGS1 has been purified, crystallized and the crystal structure of this domain with bound substrate m⁷GpppA was solved by means of multiple-wavelength anomalous dispersion. Crystal structure analysis revealed that m⁷GpppA binds via its adenosine moiety to the structurally conserved adenosylmethionine-binding pocket, while the m⁷ guanosine remains unbound. This unexpected binding only occurs in the absence of AdoMet and suggests an incomplete binding pocket for the m⁷G cap which is caused by the N-­terminal truncation of the protein. These structural data are consistent with the finding that the crystallized fragment of human TGS1 is catalytically inactive, while a fragment that is 17 amino acids longer exhibits activity.