Selective isotope labeling of recombinant proteins in Escherichia coli

Selective stable-isotope labeling is a useful technique to study structures of proteins, especially intrinsically disordered proteins, by nuclear magnetic resonance spectroscopy. Here, we describe a simple method for amino acid selective isotope labeling of recombinant proteins in E. coli. This method only requires addition of an excess of unlabeled amino acids and, if necessary, enzyme inhibitors to the culture medium. Its efficiency has been demonstrated even in labeling with glutamine or glutamate that is easily converted to other amino acid types by the metabolic pathways of E. coli.