Structural Insights of Outer Membrane Protein Methyltransferases from Rickettsia

The observation of correlation between hypermethylation of rickettsial outer membrane protein B (OmpB) and the bacterial virulence has suggested the importance of an enzymatic system for the methylation of OmpB. Two protein lysine methyltransferases (rPKMT1 and rPKMT2) were found to methylate recombinant OmpB fragments through the incorporation of radioactively labeled methyl group from S‐adenosylmethionine (SAM). The two proteins share 45 % identity. LC/MS‐MS analyses showed both rPKMT1 and rPKMT2 methylated at numerous lysine residues of OmpB, but rPKMT1 catalyzed mono‐, di‐ and tri‐methylation of lysine, while rPKMT2 catalyzed exclusively trimethylation of lysine. We used X‐ray crystallography to investigate the structure of the lysine methyltransferases. The crystal structures of rPKMT1 and rPKMT2 and the respective complexes with SAM were determined. These are the first reported crystal structures of outer membrane protein methyltransferases. The crystal structures of rPKMT1 and rPKMT2 are superimposable. Both methyltransferases are dimeric, which was confirmed using dynamic light scattering. Each monomer consists of dimerization, SAM binding, middle, and C‐terminal domains. Comparison of the two structures revealed a number of structural insights of the unusual enzymatic methylation of OmpB.