In silico and proteomic analysis of protein methyltransferase CheR from Bacillus subtilis

Protein methyltransferase (CheR) catalyzes the methylation of the cytosolic domain of the membrane bound chemotaxis receptors, and plays a pivotal role in the chemotactic signal transduction pathway in bacteria. Crystal structure of CheR is available only from the gram-negative bacterium Salmonella typhimurium (StCheR), which contain a catalytic C-terminal domain, encompassing a β-subdomain, connected via a linker to the N-terminal domain. The structural-functional similitude between CheR of the gram-negative and the gram-positive bacteria remains obscure. We investigated CheR, from a gram-positive bacterium, Bacillus subtilis (BsCheR), and have identified the functional roles of its N-terminal domain, by using the in silico molecular modeling and docking approach along with mass spectrophotometry and sequence analysis. The structural studies established that the N-terminal domain directly bound to S-Adenosyl-l-homocysteine (SAH). Structural and sequence analyses revealed that the α2 helix of the N-terminal domain was involved in the recognition of the methylation site of the chemotactic receptor. Additionally, immunoblot analysis showed that the purified BsCheR was phosphorylated. Further, mass spectrometry studies detected the phosphorylation at Thr3 position in the N-terminal domain of BsCheR. Phosphorylation of BsCheR suggested a regulatory role of the N-terminal domain, analogous to its antagonistic enzyme, the chemotaxis-specific methylesterase (CheB).