Crystal structure of Rv1220c, a SAM-dependent O-methyltransferase from Mycobacterium tuberculosis

Rv1220c from Mycobacterium tuberculosis is annotated as an O-methyltransferase (MtbOMT). Currently, no structural information is available for this protein. Here, the crystal structure of MtbOMT refined to 2.0 Å resolution is described. The structure reveals the presence of a methyltransferase fold and shows clear electron density for one molecule of S-adenosylmethionine (SAM), which was apparently bound by the protein during its production in Escherichia coli. Although the overall structure of MtbOMT resembles the structures of O-methyltransferases from Cornybacterium glutamicum, Coxiella burnetti and Alfa alfa, differences are observed in the residues that make up the active site. Notably, substitution of Asp by His164 seems to abrogate metal binding by MtbOMT. A putative catalytic His–Asp pair located in the vicinity of SAM is absolutely conserved in MtbOMT homologues from all species of Mycobacterium, suggesting a conserved function for this protein.