Table 1.
Overview of MTase classification
| Name | Structure | Exemplary crystal structure |
|---|---|---|
| Class I Rossmann-Fold MTases (264,286–288) | Seven β-sheets flanked by three α-helices on each side forming an αβα-sandwich | nsp10-nsp16 methyltransferase complex SARS-CoV-2 PDB 6YZ1 (264) |
| ClassII(21) | Eight central β-sheets framed by several α-helices at both ends including a characteristic central, antiparallel β-sheet | MetH methionine synthase E. coli K12 PDB 1MSK (21) |
| Class III (22) | Two αβ-clusters each containing five central β-sheets surrounded by four α-helices | CbiF cobalt-precorrin-4 transmethylase Priestia megaterium PDB 1CBF (22) |
| Class IV SPOUT (SpoU-TrmD) MTases (26,289) | Characteristic knot structure at the C terminus; six-stranded parallel β-sheet surrounded by seven α-helices | RlmB 2′-O-methyltransferase E. coli PDB: 1GZ0 (289) |
| Class V SET-domain containing proteins (23,290) | Combination of three β-sheet formations each consisting of three β-strands surrounding the remarkable ‘pseudoknot’ at the C terminus | SET7/9 lysine methyltransferase H. sapiens PDB: 1MT6 (23) |
| Class VI Transmembrane MTases (291,292) | Five transmembrane α-helices and a cofactor-binding pocket enclosed within a highly conserved C-terminal catalytic subdomain comprising an α-helix. | Isoprenylcysteine carboxyl methyltransferase (ICMT), Methanosarcina acetivorans PDB: 4A2N (291) |
| ClassVII TIM barrel MTases (293,294) | Partial (α/β)6 TIM barrel or full TIM barrel in some cases: a β-sheet is anchored inside by peripheral α-helices, on top the [4Fe-4S] cluster. | MiaB radical methylthiotransferase Bacteroides uniformis PDB: 7MJV (294) |
| ClassVIII(295,296) | Six-stranded anti-parallel β-barrel, β-sheets are connected by numerous extended loops. | TrmO/YaeB tRNA methyltransferase Archaeoglobus fulgidus PDB: 2NV4 (295,296) |
| Class IX (297) | Two four-stranded antiparallel β-sheets forming a concave surface with five α-helices at the reverse side of the protein. | SsTaw3 tRNA-yW N-4 Methyltransferases Saccharolobus solfataricus PDB: 1TLJ (297) |
The table includes information about characteristic structural features and examples of enzyme structures.