Sugar transport by the bacterial phosphotransferase system. Radioactive and electron paramagnetic resonance labeling of the Salmonella typhimurium phosphocarrier protein (HPr) at the NH2-terminal methionine.

Investigation of the complex interactions between the different proteins of the bacterial phosphotransferase system (PTS) would be greatly facilitated by the availability of specifically labeled proteins, such as the low molecular weight phosphocarrier HPr. Salmonella typhimurium HPr contains two methionine residues, one at the NH2 terminus, and the other at amino acid residue 80. The method of Link and Stark (Link, T. P., and Stark, G. R. (1968) J. Biol. Chem 243, 1082-1088) was used to alkylate HPr with [3H]methyl iodide, and with the EPR probe, 3-[(2-bromoacetamido)methyl]-2,-2,5,5-tetramethyl-l-pyrrolidinyloxyl. The radiolabeled HPr was purified to apparent homogeneity, and was as active in phosphorylation assays in vitro as native HPr. The EPR-labeled HPr was isolated in better than 95%