Protein Domain : tRNA (C5-cytosine) methyltransferase, NCL1 IPR023270

Type	Family
Description	RNA (C5-cytosine) methyltransferases (RCMTs) catalyse the transfer of a methyl group to the 5th carbon of a cytosine base in RNA sequences to produce C5-methylcytosine. RCMTs use the cofactor S-adenosyl-L-methionine (SAM) as a methyl donor [ ]. The catalytic mechanism of RCMTs involves an attack by the thiolate of a Cys residue on position 6 of the target cytosine base to form a covalent link, thereby activating C5 for methyl-group transfer. Following the addition of the methyl group, a second Cys residue acts as a general base in the beta-elimination of the proton from the methylated cytosine ring. The free enzyme is restored and the methylated product is released [].Numerous putative RCMTs have been identified in archaea, bacteria and eukaryota [ , ]; most are predicted to be nuclear or nucleolar proteins []. The Escherichia coli Ribosomal RNA Small-subunit Methyltransferase Beta (RSMB) FMU (FirMicUtes) represents the first protein identified and characterised as a cytosine-specific RNA methyltransferase. RSMB was reported to catalyse the formation of C5-methylcytosine at position 967 of 16S rRNA [, ].A classification of RCMTs has been proposed on the basis of sequence similarity [ ]. According to this classification, RCMTs are divided into 8 distinct subfamilies []. Recently, a new RCMT subfamily, termed RCMT9, was identified []. Members of the RCMT contain a core domain, responsible for the cytosine-specific RNA methyltransferase activity. This 'catalytic' domain adopts the Rossman fold for the accommodation of the cofactor SAM []. The RCMT subfamilies are also distinguished by N-terminal and C-terminal extensions, variable both in size and sequence [].The prototypical member of the Nuclear protein 1 RCMT subfamily, the S. cerevisiae NCL1 (also known as Trm4), has been demonstrated to methylate cytosine to C5-methylcytosine at positions 34, 40, 48 and 49 in different intron- containing tRNAs and tRNA precursors [ ]. Its human homologue, MISU/NSUN2, was found to catalyse the formation of C5-methylcytosine at position 34 of intron-containing pre-tRNAs []; it was not able to modify tRNAs at positions 48 or 49. It was also shown to be involved in Myc-mediated proliferation of cancer cells [].
Short Name	RCMT_NCL1