Cloning and characterization of three human cDNAs encoding mRNA (guanine-7-)-methyltransferase, an mRNA cap methylase

The mRNA cap structure is synthesized by a series of reactions catalyzed by capping enzyme and mRNA (guanine-7-)-methyltransferase. mRNA (guanine-7-)-methyltransferase catalyzes the methylation of GpppN- at the guanine N7 position, which is an essential step for gene expression in eukaryotic cells. Here we isolated three human cDNAs encoding mRNA (guanine-7-)-methyltransferase termed hCMT1a, hCMT1b and hCMT1c. hCMT1a and hCMT1b encode 476 and 504 amino acids, respectively, and differ only at the region coding for the C-terminal portion of the enzyme after amino acid residue 465. The third cDNA hCMT1c seems to encode the same polypeptide as hCMT1a, however, the 3'-noncoding region of hCMT1c contains sequences corresponding to part of the C-terminal coding and noncoding regions of hCMT1b thus consisting of a mosaic of hCMT1a and hCMT1b. RT-PCR showed that all 3 types of mRNAs were expressed in every tissue examined. Comparison of the deduced amino acid sequences with those of other viral and cellular enzymes showed the regions which are highly conserved among mRNA (guanine-7-)-methyltransferases. The recombinant hCMT1a expressed in E. coli exhibited mRNA (guanine-7-)-methyltransferase activity. On the other hand, neither mRNA (guanine-7-)-methyltransferase nor mRNA (nucleoside-2'-O-)-methyltransferase activity was detected with the recombinant hCMT1b protein. Although the biological significance of the expression of these three mRNA (guanine-7-)-methyltransferase mRNA species remains unknown at present, the nucleotide sequences suggest that they are produced by alternative RNA splicing.