Use of tagged proteins in protein purification. (a) Glutathione-S-transferase (GST) is a small enzyme that binds glutathione (a glutamate residue to which a Cys–Gly dipeptide is attached at the carboxyl carbon of the Glu side chain, hence the abbreviation GSH). (b) The GST tag is fused to the C-terminus of the protein by genetic engineering. The tagged protein is expressed in the cell and is present in the crude extract when the cells are lysed. The extract is subjected to chromatography through a matrix with immobilized glutathione. The GST-tagged protein binds to the glutathione, retarding its migration through the column, while the other proteins are washed through rapidly. The tagged protein is subsequently eluted with a solution containing elevated salt concentration or free glutathione.