Protein methylation study by click chemistry

Bioorthogonal Profiling of Protein Methylation Using Azido Derivative of S-Adenosyl-L-methionine, Minkui Luo, et al, J. Am. Chem. Soc. 2012, 134, 5909-5915
dx.doi.org/10.1021/ja2118333

A few key points:
1. Protein methytransferase transfers methyl group from  S-Adenosyl-L-methionine (SAM) to specific proteins.
2. Selective mutation of a transferase allows its acceptance of modified azido-SAM. 
3. Instead of methyl group, the azido-R group was transferred to specific proteins in cells with above SAM transferase.
4. Copper-free Click chemistry and MS reveal the identity of the specific methylated protein.
5. Therfore, methyltransferase - target protein link is established.